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Published in ''Computer Methods in Applied Mechanics and Engineering'', Vol. 194 (21-24), 2406-2443, 2005
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DOI: 10.1016/j.cma.2004.07.039
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== SUMMARY ==
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A family of rotation-free three node triangular shell elements is presented. The simplest element of the family is based on an assumed constant curvature field expressed in terms of the nodal deflections of a patch of four elements and a constant membrane field computed from the standard linear interpolation of the displacements within each triangle. An enhanced version of the element is obtained by using a quadratic interpolation of the geometry in terms of the six patch nodes. This allows to compute an assumed linear membrane strain field which improves the in-plane behaviour of the original element. A simple and economic version of the element using a single integration point is presented. The efficiency of the different rotation-free shell triangles is demonstrated in many examples of application including linear and non linear analysis of shells under static and dynamic loads, the inflation and de-inflation of membranes and a sheet stamping problem.
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==1 INTRODUCTION==
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Triangular shell elements are very useful for the solution of large scale shell problems such as those occurring in many practical engineering situations. Typical examples are the analysis of shell roofs under static and dynamic loads, sheet stamping processes, vehicle dynamics and crash-worthiness situations. Many of these problems involve high geometrical and material non linearities and time changing frictional contact conditions. These difficulties are usually increased by the need of discretizing complex geometrical shapes. Here the use of shell triangles and non-structured meshes becomes a critical necessity. Despite recent advances in the field <span id='citeF-1'></span>[[#cite-1|[1]]]&#8211;<span id='citeF-6'></span>[[#cite-6|[6]]] there are not so many simple shell triangles which are capable of accurately modelling the deformation of a shell structure under arbitrary loading conditions.
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A promising line to derive simple shell triangles is to use the nodal displacements as the only unknown for describing the shell kinematics. This idea goes back to the original attempts to solve thin plate bending problems using finite difference schemes with the deflection as the only nodal variable <span id='citeF-7'></span>[[#cite-7|[7]]]&#8211;<span id='citeF-9'></span>[[#cite-9|[9]]].
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In past years some authors have derived a number of thin plate and shell triangular elements free of rotational degrees of freedom (d.o.f.) based on Kirchhoff's theory [10]&#8211;<span id='citeF-27'></span>[[#cite-27|[27]]]. In essence all methods attempt to express the curvatures field over an element in terms of the displacements of a collection of nodes belonging to a patch of adjacent elements. Oñate and Cervera [14] proposed a general procedure of this kind combining finite element and finite volume concepts for deriving thin plate triangles and quadrilaterals with the deflection as the only nodal variable and presented a simple and competitive rotation-free three d.o.f. triangular element termed BPT (for Basic Plate Triangle). These ideas were extended and formalized in <span id='citeF-29'></span>[[#cite-29|29]] to derive a number of rotation-free thin plate and shell triangles. The basic ingredients of the method are a mixed Hu-Washizu formulation, a standard discretization into three node triangles, a linear finite element interpolation of the displacement field within each triangle and a finite volume type approach for computing constant curvature and bending moment fields within appropriate non-overlapping control domains. The so called cell-centered and cell-vertex triangular domains yield different families of rotation-free plate and shell triangles. Both the BPT plate element and its extension to shell analysis (termed BST for Basic Shell Triangle) can be derived from the cell-centered formulation. Here the control domain is an individual triangle. The constant curvatures field within a triangle is computed in terms of the displacements of the six nodes belonging to the four elements patch formed by the chosen triangle and the three adjacent triangles. The cell-vertex approach yields a different family of rotation-free plate and shell triangles. Details of the derivation of both rotation-free triangular shell element families can be found in <span id='citeF-29'></span>[[#cite-29|[21]]].
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An extension of the BST element to the non linear analysis of shells was implemented in an explicit dynamic code by Oñate ''et al.'' [26] using an updated lagrangian formulation and a hypo-elastic constitutive model. Excellent numerical results were obtained for non linear dynamics of shells involving frictional contact situations and sheet stamping problems [18,19,20,26].
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A large strain formulation for the BST element using a total lagrangian description was presented by Flores and Oñate [24]. A recent extension of this formulation is based on a quadratic interpolation of the geometry of the patch formed by the BST element and the three adjacent triangles [27]. This yields a linear displacement gradient field over the element from which linear membrane strains and  constant curvatures  can be computed within the BST element.
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In this paper the formulation of the BST element is revisited using an assumed strain approach. The content of the paper is the following. First some basic concepts of the formulation of the original BST element using an assumed constant curvature field are given. Next, the basic equations of the non linear thin shell theory chosen based on a total lagrangian description are presented. Then the non linear formulation of the BST element is presented. This is based on an assumed constant membrane field derived from the linear displacement interpolation and an assumed constant curvature field expressed in terms of the displacements of the nodes of the four element patch using a finite volume type approach. An enhanced version of the BST element is derived using an assumed linear field for the membrane strains and an assumed constant curvature field. Both assumed fields are obtained from the quadratic interpolation of the patch geometry following the ideas presented in [27]. Details of the derivation of the tangent stiffness matrix needed  for a quasi-static implicit solution are given for both the BST and EBST elements. An efficient version of the  EBST element using one single quadrature point for integration of the tangent matrix is  presented. An explicit  scheme adequate for dynamic analysis is  briefly described.
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The efficiency and accuracy of the standard and enhanced versions of the BST element is validated in a number of examples of application including linear and non linear analysis of shells under static and dynamic loads, the inflation and de-inflation of membranes and a sheet stamping problem.
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==2 FORMULATION OF THE BASIC PLATE TRIANGULAR USING AN ASSUMED CONSTANT CURVATURE FIELD==
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Let us consider a patch of four plate three node triangles (Figure [[#img-1|1]]). The nodes 1, 2, and 3 in the main central triangle (M) are marked with circles while the external nodes in the patch (nodes 4, 5 and 6) are marked with squares. Mid side points in the central triangle are also marked with smaller squares. Kirchhoff's thin plate theory will be assumed to hold. The deflection is linearly interpolated within each three node triangle in the standard finite element manner as
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|-
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| 
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{| style="text-align: left; margin:auto;" 
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|-
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| style="text-align: center;" | <math>w=\sum _{i=1}^{3}L_{i}^{e}w_{i}^{e} </math>
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| style="width: 5px;text-align: right;" | (1)
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where <math display="inline">L_{i}^{e}</math> are the linear shape functions of the three node triangle, <math display="inline">w_{i}^{e}</math> are nodal deflections and superindex <math display="inline">e</math> denotes element values.
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<div id='img-1'></div>
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{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
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|-
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|[[Image:draft_Samper_226033773-fig1.png|200px|Patch of three node triangular elements including the central triangle (M) and three adjacent triangles (1, 2 and 3)]]
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|- style="text-align: center; font-size: 75%;"
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| colspan="1" | '''Figure 1:''' Patch of three node triangular elements including the central triangle (M) and three adjacent triangles (1, 2 and 3)
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The curvature field within the central triangle can be expressed in terms of a constant assumed curvature field as
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<span id="eq-2"></span>
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
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|-
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| 
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{| style="text-align: left; margin:auto;" 
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|-
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| style="text-align: center;" | <math>{\boldsymbol \kappa }=\left\{ \begin{array}{c} \kappa _{xx}\\ \kappa _{yy}\\ \kappa _{xy} \end{array} \right\} =\hat{\boldsymbol \kappa } </math>
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| style="width: 5px;text-align: right;" | (2)
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where <math display="inline">{\boldsymbol \kappa }</math> is the curvature vector and <math display="inline">\hat{\boldsymbol \kappa }</math> is the assumed constant curvature field defined as
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<span id="eq-3"></span>
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{| style="text-align: left; margin:auto;" 
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|-
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| style="text-align: center;" | <math>\hat{\boldsymbol \kappa }={\frac{1}{A_{M}}}\int \int _{A_{M}}\left[ -{\frac{\partial ^{2} w}{\partial x^{2}}}-,{\frac{\partial ^{2}w}{\partial y^{2}}},-2{\frac{\partial ^{2}w}{\partial x\partial y}}\right] ^{T}dA </math>
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| style="width: 5px;text-align: right;" | (3)
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where <math display="inline">A_{M}</math> is the area of the central triangle in Figure [[#img-1|1]].
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Integrating by parts Eq.([[#eq-3|3]]) and substituting the resulting expression for <math display="inline">\hat{\boldsymbol \kappa }</math> into Eq.([[#eq-2|2]]) gives the constant curvature field within the element as
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<span id="eq-4"></span>
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>{\boldsymbol \kappa }={\frac{1}{A_{M}}}   {\displaystyle \oint _{\Gamma _{M}}}  \left[ \begin{array}{cc} -n_{x} & 0\\ 0 & -n_{y}\\ -n_{y} & -n_{x} \end{array} \right] \left\{ \begin{array}{c} \dfrac{\partial w}{\partial x}\\ \dfrac{\partial w}{\partial y} \end{array} \right\} d\Gamma </math>
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| style="width: 5px;text-align: right;" | (4)
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where <math display="inline">\Gamma _{M}</math> is the boundary of the central triangle.
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Eq.([[#eq-4|4]]) defines the assumed constant curvature field within the central triangle in terms of the deflection gradient along the sides of the triangle. Eq.([[#eq-4|4]]) can be found to be equivalent to the standard conservation laws used in finite volume procedures as described in [28,29].
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The computation of the line integral in Eq.([[#eq-4|4]]) poses a difficulty as the deflection gradient is discontinuous along the element sides. A simple method to overcome this problem is to compute the deflection gradient at the element sides as the average values of the gradient contributed by the two triangles sharing the side <span id='citeF-29'></span>[[#cite-29|[21,29]]]. Following this idea the constant curvature field with the element is computed as
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<span id="eq-5"></span>
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>{\boldsymbol \kappa }={\frac{1}{A_{M}}}\sum _{j=1}^{3}{\frac{l_{j}^{M}}{2}}\left[ \begin{array}{cc} -n_{x}^{j} & 0\\ 0 & -n_{y}^{j}\\ -n_{y}^{j} & -n_{x}^{j} \end{array} \right] ^{M}\left[ {\nabla }L_{i}^{M}w_{i}^{M}+{\nabla }L_{i}^{j}w_{i} ^{j}\right] =\mathbf{B}_{b}\mathbf{w}^{p} </math>
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| style="width: 5px;text-align: right;" | (5)
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where <math display="inline">\mathbf{w}^{p}=[w_{1},w_{2},w_{3},w_{4},w_{5},w_{6}]^{T}</math> is the deflection vector of the six  nodes in the patch. In Eq.([[#eq-5|5]]) the sum extends over the three sides of the central element <math display="inline">M</math>, <math display="inline">l_{j}^{M}</math> are the lengths of the element sides and superindexes <math display="inline">M</math> and <math display="inline">j</math> refer to the central triangle and to each of the adjacent elements, respectively. The standard sum convention for repeated indexes is used.
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Note that the constant curvature field is expressed in terms of the six nodes of the four element patch linked to the element <math display="inline">M</math>. The expression of the <math display="inline">3\times{6}</math> <math display="inline">\mathbf{B}_{b}</math> matrix can be found in [14,21].
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The virtual work expression is written as
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<span id="eq-6"></span>
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>\int \int _{A}\delta{\boldsymbol \kappa }^{T}\mathbf{m}\,dA=\int \int _{A}\delta w\,q\,dA </math>
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| style="width: 5px;text-align: right;" | (6)
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where '''m''' is the bending moment field related to the curvature by the standard constitutive equations
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>\mathbf{m}=[M_{xx},M_{yy},M_{xy}]^T = \mathbf{D}_{b}{\boldsymbol \kappa }\quad ,\quad \mathbf{D}_{b}={\frac{Eh^{3} }{(1-\nu ^{2})}}\left[ \begin{array}{ccc} 1 & \nu & 0\\ \nu & 1 & 0\\ 0 & 0 & \frac{1-\nu }{2} \end{array} \right] </math>
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| style="width: 5px;text-align: right;" | (7)
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In Eqs.(6) and (7) <math display="inline">h</math> is the plate thickness, <math display="inline">E</math> is the Young's modulus, <math display="inline">\nu </math> is the Poisson's ratio, <math display="inline">\delta{\boldsymbol \kappa }</math> and <math display="inline">\delta w</math> are the virtual curvatures and the virtual deflection, respectively, and <math display="inline">q</math> is a distributed vertical load.
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Substituting the approximation for the vertical deflection and the assumed constant curvature field into ([[#eq-6|6]]) leads to the standard linear system of equations
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>\mathbf{K}\mathbf{w}=\mathbf{f}</math>
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| style="width: 5px;text-align: right;" | (8)
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where the stiffness matrix <math display="inline">\mathbf{K}</math> and the equivalent nodal force <math display="inline">\mathbf{f}</math> can be found by assembly of the element contributions given by
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>\mathbf{K}^{e}=\int \int _{A^{e}}\mathbf{B}_{b}^{T}\mathbf{D}_{b}\mathbf{B}_{b}dA </math>
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| style="width: 5px;text-align: right;" | (9)
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>\mathbf{f}^{e}=\int \int _{A^{e}}q\left\{ \begin{array}{c} L_{1}^e\\ L_{2}^e\\ L_{3}^e \end{array} \right\} dA </math>
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| style="width: 5px;text-align: right;" | (10)
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Note that <math display="inline">\mathbf{K}^{e}</math> is  a <math display="inline">6\times{6}</math> matrix, whereas <math display="inline">\mathbf{f}^{e}</math> has the same structure than for the standard linear triangle.
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The explicit form of <math display="inline">\mathbf{K}^{e}</math> and <math display="inline">\mathbf{f}^{e}</math> can be found in [14].
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The resulting Basic Plate Triangle (BPT) has one degree of freedom per node and a wider bandwidth than the standard three node triangles as each triangular element is linked to its three neighbours through Eq.([[#eq-5|5]]).
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Examples of the good performance of the BPT element for analysis of thin plates can be found in [14,21]. The extension of the BPT element to the analysis of shells yields the Basic Shell Triangle (BST) <span id='citeF-29'></span>[[#cite-29|[29]]]. Different applications of the BST element to linear and non linear analysis of shells are reported in [14,18&#8211;21,24,26,27].
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The ideas used to derive the BPT element will now be extended to derive two families of Basic Shell Triangles using a total lagrangian description.
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==3 BASIC THIN SHELL EQUATIONS USING A TOTAL LAGRANGIAN FORMULATION==
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===3.1 Shell kinematics===
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A summary of the most relevant hypothesis related to the kinematic behaviour of a thin shell are presented. Further details may be found in the wide literature dedicated to this field [8,9].
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Consider a shell with undeformed middle surface occupying the domain <math display="inline">\Omega ^{0}</math> in <math display="inline">R^{3}</math> with a boundary <math display="inline">\Gamma ^{0}</math>. At each point of the middle surface a thickness <math display="inline">h^{0}</math> is defined. The positions <math display="inline">\mathbf{x}^{0}</math> and <math display="inline">\mathbf{x}</math> of a point in the undeformed and the deformed configurations can be respectively written as a function of the coordinates of the middle surface <math display="inline">{\boldsymbol \varphi }</math> and the normal <math display="inline">\mathbf{t}_{3}</math> at the point as
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{| style="text-align: left; margin:auto;" 
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| style="text-align: right;" | <math>\mathbf{x}^{0}\left( \xi _{1},\xi _{2},\zeta \right)  </math>
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| <math>  ={\boldsymbol \varphi }^{0}\left( \xi _{1},\xi _{2}\right) +\lambda \mathbf{t}_{3}^{0}</math>
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| style="width: 5px;text-align: right;" | (11)
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| style="text-align: right;" | <math> \mathbf{x}\left( \xi _{1},\xi _{2},\zeta \right)  </math>
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| <math>  ={\boldsymbol \varphi }\left( \xi  _{1},\xi _{2}\right) +\zeta \lambda \mathbf{t}_{3} </math>
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| style="width: 5px;text-align: right;" | (12)
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where <math display="inline">\xi _{1},\xi _{2}</math> are curvilinear local coordinates defined over the middle surface of the shell, and <math display="inline">\zeta </math> is the distance in the undeformed configuration of the point to the middle surface. The product <math display="inline">\zeta \lambda </math> is the distance of the point to the middle surface measured on the deformed configuration. This implies a constant strain in the normal direction associated to the parameter <math display="inline">\lambda </math> relating the thickness at the present and initial configurations, i.e.
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>\lambda =\frac{h}{h^{0}} </math>
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| style="width: 5px;text-align: right;" | (13)
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A convective coordinate system is defined at each point as
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>\mathbf{g}_{i}\left( \mathbf{\xi }\right) =\frac{\partial \mathbf{x}}{} {\partial \xi _{i}}\qquad i=1,2,3 </math>
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| style="width: 5px;text-align: right;" | (14)
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{| style="text-align: left; margin:auto;" 
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| style="text-align: left;" | <math>\mathbf{g}_{\alpha }\left( \mathbf{\xi }\right)</math> <math> =\frac{\partial \left( \mathbf{\boldsymbol \varphi }\left( \xi _{1},\xi _{2}\right) +\zeta \lambda \mathbf{t}_{3}\right) }{\partial \xi _{\alpha }}={\boldsymbol \varphi }_{^{\prime }\alpha }+\zeta \left( \lambda \mathbf{t}_{3}\right) _{^{\prime }\alpha }\quad \alpha=1,2</math>
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| style="width: 5px;text-align: right;" | (15)
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{| style="text-align: left; margin:auto;" 
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| style="text-align: left;" | <math> \mathbf{g}_{3}\left( \mathbf{\xi }\right)   =\frac{\partial \left( \mathbf{\boldsymbol \varphi }\left( \xi _{1},\xi _{2}\right) +\zeta \lambda \mathbf{t}_{3}\right) }{\partial \zeta }=\lambda \mathbf{t}_{3} </math>
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| style="width: 5px;text-align: right;" | (16)
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This can be particularized for the points on the middle surface as
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{| style="text-align: left; margin:auto;" 
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| style="text-align: left;" | <math>\mathbf{a}_{\alpha } =\mathbf{g}_{\alpha }\left( \zeta=0\right) ={\boldsymbol \varphi  }_{^{\prime }\alpha }</math>
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| style="width: 5px;text-align: right;" | (17)
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{| style="text-align: left; margin:auto;" 
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| style="text-align: left;" | <math> \mathbf{a}_{3}   =\mathbf{g}_{3}\left( \zeta=0\right) =\lambda  \mathbf{t}_{3} </math>
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| style="width: 5px;text-align: right;" | (18)
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The covariant (first fundamental form) and contravariant metric tensors of the middle surface are
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>a_{\alpha \beta }=\mathbf{a}_{\alpha }\cdot \mathbf{a}_{\beta } </math>
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| style="width: 5px;text-align: right;" | (19)
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>a^{\alpha \beta }=\mathbf{a}^{\alpha }\cdot \mathbf{a}^{\beta }={\tilde{\boldsymbol \varphi }}_{^{\prime }\alpha }\cdot{\tilde{\boldsymbol \varphi }}_{^{\prime }\beta } </math>
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| style="width: 5px;text-align: right;" | (20)
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The Green-Lagrange strain vector of the middle surface points (membrane strains) is defined as
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>{\boldsymbol \varepsilon }_{m}=[\varepsilon _{m_{11}},\varepsilon _{m_{12}},\varepsilon _{m_{12}}]^{T} </math>
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| style="width: 5px;text-align: right;" | (21)
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with
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<span id="eq-22"></span>
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>\varepsilon _{m_{ij}}=\frac{1}{2}(a_{ij}-\delta _{ij}) </math>
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| style="width: 5px;text-align: right;" | (22)
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The curvatures (second fundamental form) of the middle surface are obtained by
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>\kappa _{\alpha \beta }=\frac{1}{2}\left( {\boldsymbol \varphi }_{^{\prime }\alpha } \cdot \mathbf{t}_{3^{\prime }\beta }+{\boldsymbol \varphi }_{^{\prime }\beta }\cdot  \mathbf{t}_{3^{\prime }\alpha }\right) =- \mathbf{t}_{3}\cdot{\boldsymbol \varphi }_{{\prime }\alpha \beta }\quad , \quad \alpha ,\beta=1,2 </math>
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| style="width: 5px;text-align: right;" | (23)
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The deformation gradient tensor is
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{| style="text-align: left; margin:auto;" 
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| style="text-align: center;" | <math>\mathbf{F=} [{x}_{{\prime }1},{x}_{{\prime }2},{x}_{{\prime }3}]=\left[ \begin{array}{ccc} {\boldsymbol \varphi }_{^{\prime }1}+\zeta \left( \lambda \mathbf{t}_{3}\right) _{^{\prime  }1} & {\boldsymbol \varphi }_{^{\prime }2}+\zeta \left( \lambda \mathbf{t}_{3}\right) _{^{\prime }2} & \lambda \mathbf{t}_{3} \end{array} \right] </math>
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| style="width: 5px;text-align: right;" | (24)
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The product <math display="inline">\mathbf{F}^{T}\mathbf{F=U}^{2}=\mathbf{C}</math> (where <math display="inline">\mathbf{U}</math> is the right stretch tensor, and <math display="inline">\mathbf{C}</math> the right Cauchy-Green deformation tensor) can be written as
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<span id="eq-25"></span>
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{| style="text-align: left; margin:auto;" 
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|-
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| style="text-align: center;" | <math>\mathbf{U}^{2}=\left[ \begin{array}{ccc} a_{11}+2\kappa _{11}\zeta \lambda & a_{12}+2\kappa _{12}\zeta \lambda & 0\\ a_{12}+2\kappa _{12}\zeta \lambda & a_{22}+2\kappa _{22}\zeta \lambda & 0\\ 0 & 0 & \lambda ^{2} \end{array} \right] </math>
354
|}
355
| style="width: 5px;text-align: right;" | (25)
356
|}
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In the derivation of expression ([[#eq-25|25]]) the derivatives of the thickness ratio <math display="inline">\lambda _{^{\prime }a}</math> and the terms associated to <math display="inline">\zeta ^{2}</math> have been neglected.
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Eq.([[#eq-25|25]]) shows that <math display="inline">\mathbf{U}^{2}</math> is not a unit tensor at the original configuration for curved surfaces (<math display="inline">\kappa _{ij}^{0}\neq{0}</math>). The changes of curvature of the middle surface are computed by
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
363
|-
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| 
365
{| style="text-align: left; margin:auto;" 
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|-
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| style="text-align: center;" | <math>\chi _{ij}=\kappa _{ij}-\kappa _{ij}^{0} </math>
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|}
369
| style="width: 5px;text-align: right;" | (26)
370
|}
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Note that <math display="inline">\delta \chi _{ij}=\delta \kappa _{ij}</math>.
373
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For computational convenience the following approximate expression (which is exact for initially flat surfaces) will be adopted
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
377
|-
378
| 
379
{| style="text-align: left; margin:auto;" 
380
|-
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| style="text-align: center;" | <math>\mathbf{U}^{2}=\left[ \begin{array}{ccc} a_{11}+2\chi _{11}\zeta \lambda & a_{12}+2\chi _{12}\zeta \lambda & 0\\ a_{12}+2\chi _{12}\zeta \lambda & a_{22}+2\chi _{22}\zeta \lambda & 0\\ 0 & 0 & \lambda ^{2} \end{array} \right] </math>
382
|}
383
| style="width: 5px;text-align: right;" | (27)
384
|}
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This expression is useful to compute different lagrangian strain measures. An advantage of these measures is that they are associated to material fibres, what makes it easy to take into account material anisotropy. It is also useful to compute the eigen decomposition of <math display="inline">\mathbf{U}</math> as
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
389
|-
390
| 
391
{| style="text-align: left; margin:auto;" 
392
|-
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| style="text-align: center;" | <math>\mathbf{U=}\sum _{\alpha=1}^{3}\lambda _{\alpha } \mathbf{r}_{\alpha } \otimes \mathbf{r}_{\alpha } </math>
394
|}
395
| style="width: 5px;text-align: right;" | (28)
396
|}
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where <math display="inline">\lambda _{\alpha }</math> and <math display="inline">\mathbf{r}_{\alpha }</math> are the eigenvalues and eigenvectors of <math display="inline">\mathbf{U}</math>.
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The resultant stresses  (axial forces and moments) are obtained by integrating across the original thickness the second Piola-Kirchhoff stress vector <math display="inline">{ \boldsymbol \sigma }</math> using the actual distance to the middle surface for  evaluating the bending moments, i.e.
401
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<span id="eq-29"></span>
403
{| class="formulaSCP" style="width: 100%; text-align: left;" 
404
|-
405
| 
406
{| style="text-align: left; margin:auto;" 
407
|-
408
| style="text-align: center;" | <math>{\boldsymbol \sigma }_{m}\equiv \lbrack N_{11},N_{22},N_{12}]^{T}=\int _{h^{0}}{\boldsymbol \sigma } d\zeta </math>
409
|}
410
| style="width: 5px;text-align: right;" | (29)
411
|}
412
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<span id="eq-30"></span>
414
{| class="formulaSCP" style="width: 100%; text-align: left;" 
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|-
416
| 
417
{| style="text-align: left; margin:auto;" 
418
|-
419
| style="text-align: center;" | <math>{\boldsymbol \sigma }_{b}\equiv \lbrack M_{11},M_{22},M_{12}]^{T}=\int _{h^{0}}{\boldsymbol \sigma  }\lambda \zeta  d\zeta </math>
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|}
421
| style="width: 5px;text-align: right;" | (30)
422
|}
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With these values the virtual work can be written as
425
426
<span id="eq-31"></span>
427
{| class="formulaSCP" style="width: 100%; text-align: left;" 
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|-
429
| 
430
{| style="text-align: left; margin:auto;" 
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|-
432
| style="text-align: center;" | <math>\int \int _{A^{0}}\left[ \delta{\boldsymbol \varepsilon }_{m}^{T}{\boldsymbol \sigma }_{m}+\delta{\boldsymbol \kappa  }^{T}{\boldsymbol \sigma }_{b}\right] dA=\int \int _{A^{0}}\delta \mathbf{u}^{T}\mathbf{t}dA </math>
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|}
434
| style="width: 5px;text-align: right;" | (31)
435
|}
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where <math display="inline">\delta \mathbf{u}</math> are virtual displacements, <math display="inline">\delta{\boldsymbol \varepsilon }_{m}</math> is the virtual Green-Lagrange membrane strain vector, <math display="inline">\delta{\boldsymbol \kappa }</math> are the virtual curvatures and <math display="inline">\mathbf{t}</math> are the surface loads. Other load types can be easily included into ([[#eq-31|31]]).
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===3.2 Constitutive models===
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In order to treat plasticity at finite strains an adequate stress-strain pair must be used. The Hencky measures will be adopted here. The (logarithmic) strains are defined as
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
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|-
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| 
446
{| style="text-align: left; margin:auto;" 
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|-
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| style="text-align: center;" | <math>\mathbf{E}_{\ln }\mathbf{=}\left[ \begin{array}{ccc} \varepsilon _{11} & \varepsilon _{21} & 0\\ \varepsilon _{12} & \varepsilon _{22} & 0\\ 0 & 0 & \varepsilon _{33} \end{array} \right] =\sum _{\alpha=1}^{3}\ln \left( \lambda _{\alpha }\right) \mathbf{r}_{\alpha }\otimes \mathbf{r}_{\alpha } </math>
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|}
450
| style="width: 5px;text-align: right;" | (32)
451
|}
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Two types of material models are considered here: an elastic-plastic material associated to thin rolled metal sheets and a hyper-elastic material for rubbers.
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In the case of metals, where the elastic strains are small, the use of a logarithmic strain measure reasonably allows to adopt an additive decomposition of elastic and plastic components as
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
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|-
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| 
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{| style="text-align: left; margin:auto;" 
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|-
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| style="text-align: center;" | <math>\mathbf{E}_{\ln }\mathbf{=E}_{\ln }^{e}+\mathbf{E}_{\ln }^{p} </math>
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|}
464
| style="width: 5px;text-align: right;" | (33)
465
|}
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A constant linear relationship between the (plane) Hencky stresses and the logarithmic elastic strains is  adopted giving
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
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|-
471
| 
472
{| style="text-align: left; margin:auto;" 
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|-
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| style="text-align: center;" | <math>\mathbf{T}=\mathbf{CE}_{\ln }^{e} </math>
475
|}
476
| style="width: 5px;text-align: right;" | (34)
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|}
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These constitutive equations are integrated using a standard return algorithm. The following Mises-Hill [30] yield function with non-linear isotropic hardening is chosen here
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
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|-
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| 
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{| style="text-align: left; margin:auto;" 
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|-
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| style="text-align: center;" | <math>\left( G+H\right) \;T_{11}^{2}+\left( F+H\right) \;T_{22}^{2} -2H\;T_{11}T_{22}+2N\;T_{12}^{2}=\sigma _0\left(e_{0}+e^{p}\right) ^{n} </math>
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|}
488
| style="width: 5px;text-align: right;" | (35)
489
|}
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where <math display="inline">F, G, H</math> and <math display="inline">N</math> define the non-isotropic shape of the yield surface and the parameters <math display="inline">\sigma _{0}</math>, <math display="inline">e_{0}</math> and <math display="inline">n</math> define its size as a function of the effective plastic strain <math display="inline">e^{p}</math>.
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The simple Mises-Hill yield function  allows, as a first approximation, to treat rolled thin metal sheets with planar and transversal anisotropy.
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For the case of rubbers, the Ogden [31] model extended to the compressible range is considered. The material behaviour is characterized by the strain energy density per unit undeformed volume defined as
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
498
|-
499
| 
500
{| style="text-align: left; margin:auto;" 
501
|-
502
| style="text-align: center;" | <math>\psi =\frac{K}{2}\left( \ln J\right) ^{2}+\sum _{p=1}^{N}\frac{\mu _{p}}{} {\alpha _{p}}\left[ J^{-\frac{\alpha _{p}}{3}}\left( \sum _{i=1}^{3}\lambda  _{i}^{\alpha _{p}-1}\right) -3\right] </math>
503
|}
504
| style="width: 5px;text-align: right;" | (36)
505
|}
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where <math display="inline">K</math> is the bulk modulus of the material, <math display="inline">J</math> is the determinant of <math display="inline">\mathbf{U}</math>, <math display="inline">N</math>, <math display="inline">\mu _{i}</math> and <math display="inline">\alpha _{i}</math> are material parameters, <math display="inline">\mu _{i}\,,\,\alpha _{i}</math> are real numbers such that <math display="inline">\mu _{i}\alpha _{i}>0</math> <math display="inline"> (\forall i=1,N)</math> and <math display="inline">N</math> is a positive integer.
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The stress measures associated to the principal logarithmic strains are denoted by <math display="inline">\beta _{i}</math>. They can be computed noting that
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
512
|-
513
| 
514
{| style="text-align: left; margin:auto;" 
515
|-
516
| style="text-align: center;" | <math>\beta _{i}=\frac{\partial \psi \left( \Lambda \right) }{\partial \left( \ln \lambda _{i}\right) }=K\left( \ln J\right) +\lambda _{i}\sum _{p=1}^{N} \mu _{p}J^{-\frac{\alpha _{p}}{3}}\left( \lambda _{i}^{\alpha _{p}-1}-\frac{1}{} {3}\frac{1}{\lambda _{i}}\sum _{j=1}^{3}\lambda _{j}^{\alpha _{p}}\right) </math>
517
|}
518
| style="width: 5px;text-align: right;" | (37)
519
|}
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521
we define now
522
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
524
|-
525
| 
526
{| style="text-align: left; margin:auto;" 
527
|-
528
| style="text-align: center;" | <math>a^{p}=\sum _{j=1}^{3}\lambda _{j}^{\alpha _{p}} </math>
529
|}
530
| style="width: 5px;text-align: right;" | (38)
531
|}
532
533
which gives
534
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
536
|-
537
| 
538
{| style="text-align: left; margin:auto;" 
539
|-
540
| style="text-align: center;" | <math>\beta _{i}=K\left( \ln J\right) +\sum _{p=1}^{N}\mu _{p}J^{-\frac{\alpha _{p} }{3}}\left( \lambda _{i}^{\alpha _{p}}-\frac{1}{3}a_{p}\right) </math>
541
|}
542
| style="width: 5px;text-align: right;" | (39)
543
|}
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The values of <math display="inline">\beta _{i}</math>, expressed in the principal strains directions, allow to evaluate the Hencky stresses in the convective coordinate system as
546
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
548
|-
549
| 
550
{| style="text-align: left; margin:auto;" 
551
|-
552
| style="text-align: center;" | <math>\mathbf{T}=\sum _{i=1}^{3}\beta _{i}\;\mathbf{r}_{i}\otimes \mathbf{r}_{i} </math>
553
|}
554
| style="width: 5px;text-align: right;" | (40)
555
|}
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The Hencky stress tensor <math display="inline">\mathbf{T}</math> can be easily particularized for the plane stress case.
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We define the rotated Hencky and second Piola-Kirchhoff stress tensors as
560
561
{| class="formulaSCP" style="width: 100%; text-align: left;" 
562
|-
563
| 
564
{| style="text-align: left; margin:auto;" 
565
|-
566
| style="text-align: left;" | <math>\mathbf{T}_{L}  </math>
567
| <math>  =\mathbf{R}_{L}^{T}\;\mathbf{T\;R}_{L}</math>
568
|}
569
| style="width: 5px;text-align: right;" | (41)
570
|-
571
| 
572
{| style="text-align: left; margin:auto;" 
573
|-
574
| style="text-align: left;" | <math> \mathbf{S}_{L}  </math>
575
| <math>  =\mathbf{R}_{L}^{T}\;\mathbf{S\;R}_{L} </math>
576
|}
577
| style="width: 5px;text-align: right;" | (42)
578
|}
579
580
581
where <math display="inline">\mathbf{R}_{L}</math> is the rotation tensor obtained from the eigenvectors of <math display="inline">\mathbf{U}</math> given by
582
583
{| class="formulaSCP" style="width: 100%; text-align: left;" 
584
|-
585
| 
586
{| style="text-align: left; margin:auto;" 
587
|-
588
| style="text-align: center;" | <math>\mathbf{R}_{L}=\left[ \begin{array}{ccc} \mathbf{r}_{1} & \mathbf{r}_{2} & \mathbf{r}_{3} \end{array} \right] </math>
589
|}
590
| style="width: 5px;text-align: right;" | (43)
591
|}
592
593
The relationship between the rotated Hencky and Piola-Kirchhoff stresses is
594
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
596
|-
597
| 
598
{| style="text-align: left; margin:auto;" 
599
|-
600
| style="text-align: right;" | <math>\left[ S_{L}\right] _{\alpha \alpha }  </math>
601
| <math>  =\frac{1}{\lambda _{\alpha }^{2} }\left[ T_{L}\right] _{\alpha \alpha }</math>
602
|-
603
| style="text-align: right;" | <math> \left[ S_{L}\right] _{\alpha \beta }  </math>
604
| <math>  =\frac{\ln \left( \lambda _{\alpha  }/\lambda _{\beta }\right) }{\frac{1}{2}\left( \lambda _{\alpha }^{2} -\lambda _{\beta }^{2}\right) }\left[ T_{L}\right] _{\alpha \beta } </math>
605
|}
606
| style="width: 5px;text-align: right;" | (44)
607
|}
608
609
The second Piola-Kirchhoff stress tensor can be computed by
610
611
{| class="formulaSCP" style="width: 100%; text-align: left;" 
612
|-
613
| 
614
{| style="text-align: left; margin:auto;" 
615
|-
616
| style="text-align: center;" | <math>\mathbf{S}=\mathbf{R}_{L}\;\mathbf{S}_{L}\mathbf{\;R}_{L}^{T} </math>
617
|}
618
| style="width: 5px;text-align: right;" | (45)
619
|}
620
621
The second Piola-Kirchhoff stress vector <math display="inline">{\boldsymbol \sigma }</math> of Eqs.([[#eq-29|29]]&#8211;[[#eq-30|30]]) can be readily extracted from the <math display="inline">\mathbf{S}</math> tensor.
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623
==4 TOTAL LAGRANGIAN FORMULATION OF THE BASIC SHELL TRIANGLE==
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625
===4.1 Definition of the element geometry and discretization of the displacement field===
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The rotation-free BST element has three nodes with three displacement degrees of freedom at each node. As before an element patch is defined by the central triangle  and the three adjacent elements (Figure [[#img-1|1]]). This patch helps to define the curvature field within the central triangle (the BST element) in terms of the displacement of the six patch nodes.
628
629
The node-ordering in the patch is the following (see Figure [[#img-1|1]])
630
631
* The nodes in the main element (M) are numbered locally as 1, 2 and 3. They are defined counter-clockwise around the positive normal.
632
633
* The sides in the main element are numbered locally as 1, 2, and 3. They are defined by the local node opposite to the side.
634
635
* The adjacent elements (which are part of the cell) are numbered with the number associated to the common side.
636
637
* The extra nodes of the cell are numbered locally as 4, 5 and 6, corresponding to nodes on adjacent elements opposite to sides 1, 2  and 3 respectively.
638
639
* The connectivities in the adjacent elements are defined beginning with the extra node as shown in Table 1.
640
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642
<div class="center" style="font-size: 75%;">
643
'''Table 1'''. Element numbering and nodal connectivities of the four elements patch of Figure 1.</div>
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{| class="wikitable" style="text-align: left; margin: 1em auto;"
646
|- style="border-top: 2px solid;"
647
| '''Element''' 
648
| N1 
649
| N2 
650
| N3
651
|- style="border-top: 2px solid;"
652
|  '''M''' 
653
| 1 
654
| 2 
655
| 3
656
|- style="border-top: 2px solid;"
657
|  '''1''' 
658
| 4 
659
| 3 
660
| 2
661
|- style="border-top: 2px solid;"
662
|  '''2''' 
663
| 5 
664
| 1 
665
| 3
666
|- style="border-top: 2px solid;border-bottom: 2px solid;"
667
|  '''3''' 
668
| 6 
669
| 2 
670
| 1
671
672
|}
673
674
The following local cartesian coordinate system can be defined for the patch. In the main element the unit vector <math display="inline">\mathbf{t}_{1}</math>(associated to the local coordinate <math display="inline">\xi _{1}</math>) is directed along side 3 (from node 1 to node 2), <math display="inline">\mathbf{t}_{3}</math> (associated to the coordinate <math display="inline">\zeta </math>) is the unit normal to the plane, and finally <math display="inline">\mathbf{t}_{2}=\mathbf{t}_{3}\times \mathbf{t}_{1}</math> (associated to the coordinate <math display="inline">\xi _{2}</math>).
675
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The coordinates and the displacements are linearly interpolated within each three node triangle in the mesh in the standard manner, i.e.
677
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
679
|-
680
| 
681
{| style="text-align: left; margin:auto;" 
682
|-
683
| style="text-align: center;" | <math>{\boldsymbol \varphi } = \sum \limits _{i=1}^{3} L_{i}^e {\boldsymbol \varphi }_{i} = \sum \limits _{i=1}^{3} L_{i}^e ({\boldsymbol \varphi }^{0}_{i} + \mathbf{u}_{i}) </math>
684
|}
685
| style="width: 5px;text-align: right;" | (46)
686
|}
687
688
{| class="formulaSCP" style="width: 100%; text-align: left;" 
689
|-
690
| 
691
{| style="text-align: left; margin:auto;" 
692
|-
693
| style="text-align: center;" | <math>\mathbf{u}=\left\{ \begin{array}{c} u_{1}\\ u_{2}\\ u_{3} \end{array} \right\} =\sum \limits _{i=1}^{3}L_{i}^e\mathbf{u}_{i}\quad ,\quad \mathbf{u}_{i}=\left\{ \begin{array}{c} u_{1}\\ u_{2}\\ u_{3} \end{array} \right\} _{i} </math>
694
|}
695
| style="width: 5px;text-align: right;" | (47)
696
|}
697
698
In above <math display="inline">{\boldsymbol \varphi }_{i}</math> and <math display="inline">\mathbf{u}_{i}</math> contain respectively the three coordinates and the three displacements of node <math display="inline">i</math>.
699
700
===4.2 Computation of the membrane strains===
701
702
The Green-Lagrange membrane strains are expressed by substituting the linear displacement interpolation into Eq.([[#eq-22|22]]). This gives
703
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{| class="formulaSCP" style="width: 100%; text-align: left;" 
705
|-
706
| 
707
{| style="text-align: left; margin:auto;" 
708
|-
709
| style="text-align: center;" | <math>{\boldsymbol \varepsilon }_{m}=\frac{1}{2}\left[ \begin{array}{c}\boldsymbol \varphi _{^{\prime }1}\cdot \boldsymbol \varphi _{^{\prime  }1}-1 \\ \boldsymbol \varphi _{^{\prime }2}\cdot \boldsymbol \varphi _{^{\prime  }2}-1 \\ 2\boldsymbol \varphi _{^{\prime }1}\cdot \boldsymbol \varphi _{^{\prime }2} \end{array}\right] </math>
710
|}
711
| style="width: 5px;text-align: right;" | (48)
712
|}
713
714
The membrane strain field is constant within each triangle similarly as in the standard CST element. The variation of the membrane strains is simply obtained by
715
716
{| class="formulaSCP" style="width: 100%; text-align: left;" 
717
|-
718
| 
719
{| style="text-align: left; margin:auto;" 
720
|-
721
| style="text-align: center;" | <math>\delta{\boldsymbol \varepsilon }_{m}=\mathbf{B}_{m}\delta \mathbf{a}^{e} </math>
722
|}
723
| style="width: 5px;text-align: right;" | (49)
724
|}
725
726
with
727
728
<span id="eq-50"></span>
729
{| class="formulaSCP" style="width: 100%; text-align: left;" 
730
|-
731
| 
732
{| style="text-align: left; margin:auto;" 
733
|-
734
| style="text-align: center;" | <math>\mathbf{B}_{m}=[\mathbf{B}_{m_{1}},\mathbf{B}_{m_{2}},\mathbf{B}_{m_{3}} ]\quad ,\quad \mathbf{a}^{e}=\left\{ \begin{array}{c} \mathbf{u}_{1}\\ \mathbf{u}_{2}\\ \mathbf{u}_{3} \end{array} \right\} </math>
735
|}
736
| style="width: 5px;text-align: right;" | (50)
737
|}
738
739
and
740
741
<span id="eq-51"></span>
742
{| class="formulaSCP" style="width: 100%; text-align: left;" 
743
|-
744
| 
745
{| style="text-align: left; margin:auto;" 
746
|-
747
| style="text-align: center;" | <math>\begin{array}{c} \\ \mathbf{B}_{m_{i}}\\ 3\times{3} \end{array} =\left[ \begin{array}{c} L_{i,1}^M\boldsymbol \varphi _{^{\prime }1}^{T}\\ L_{i,2}^M\boldsymbol \varphi _{^{\prime }2}^{T}\\ L_{i,1}^M\boldsymbol \varphi _{^{\prime }2}^{T}+L_{i,2}^M\boldsymbol \varphi _{^{\prime }1}^{T} \end{array} \right]  </math>
748
|}
749
| style="width: 5px;text-align: right;" | (51)
750
|}
751
752
===4.3 Computation of bending strains (curvatures)===
753
754
We will assume the following constant curvature field within each element
755
756
{| class="formulaSCP" style="width: 100%; text-align: left;" 
757
|-
758
| 
759
{| style="text-align: left; margin:auto;" 
760
|-
761
| style="text-align: center;" | <math>\kappa _{\alpha \beta }=\hat{\kappa }_{\alpha \beta } </math>
762
|}
763
| style="width: 5px;text-align: right;" | (52)
764
|}
765
766
where <math display="inline">\hat{\kappa }_{\alpha \beta }</math> is the assumed constant curvature field defined by
767
768
<span id="eq-53"></span>
769
{| class="formulaSCP" style="width: 100%; text-align: left;" 
770
|-
771
| 
772
{| style="text-align: left; margin:auto;" 
773
|-
774
| style="text-align: center;" | <math>\hat{\kappa }_{\alpha \beta }=-\frac{1}{A_{M}^{0}}\int _{A_{M}^{0}}\mathbf{t}_{3}\cdot{\boldsymbol \varphi }_{^{\prime }\beta \alpha }\;dA^{0} </math>
775
|}
776
| style="width: 5px;text-align: right;" | (53)
777
|}
778
779
where <math display="inline">A_{M}^{0}</math> is the area (in the original configuration) of the central element in the patch.
780
781
Substituting Eq.(53) into (52) and integrating by parts the area integral gives the curvature vector within the element in terms of the following line integral
782
783
<span id="eq-54"></span>
784
{| class="formulaSCP" style="width: 100%; text-align: left;" 
785
|-
786
| 
787
{| style="text-align: left; margin:auto;" 
788
|-
789
| style="text-align: center;" | <math>{\boldsymbol \kappa }=\left\{ \begin{array}{c} \kappa _{11}\\ \kappa _{22}\\ 2\kappa _{12} \end{array} \right\} =\frac{1}{A_{M}^{0}}   {\displaystyle \oint _{\Gamma _{M}^{0}}}  \left[ \begin{array}{cc} -n_{1} & 0\\ 0 & -n_{2}\\ -n_{2} & -n_{1} \end{array} \right] \left[ \begin{array}{c} \mathbf{t}_{3}\cdot{\boldsymbol \varphi }_{^{\prime }1}\\ \mathbf{t}_{3}\cdot{\boldsymbol \varphi }_{^{\prime }2} \end{array} \right] d\Gamma </math>
790
|}
791
| style="width: 5px;text-align: right;" | (54)
792
|}
793
794
where <math display="inline">n_{i}</math> are the components (in the local system) of the normals to the element sides in the initial configuration <math display="inline">\Gamma _{M}^{0}</math>.
795
796
For the definition of the normal vector <math display="inline">\mathbf{t}_{3}</math>, the linear interpolation over the central element is used. In this case the tangent plane components are
797
798
{| class="formulaSCP" style="width: 100%; text-align: left;" 
799
|-
800
| 
801
{| style="text-align: left; margin:auto;" 
802
|-
803
| style="text-align: center;" | <math>{\boldsymbol \varphi }_{^{\prime }\alpha } = \sum _{i=1}^{3} L_{i,\alpha }^M {\boldsymbol \varphi }_{i} \quad ,\quad \alpha=1,2 </math>
804
|}
805
| style="width: 5px;text-align: right;" | (55)
806
|}
807
808
<span id="eq-56"></span>
809
{| class="formulaSCP" style="width: 100%; text-align: left;" 
810
|-
811
| 
812
{| style="text-align: left; margin:auto;" 
813
|-
814
| style="text-align: center;" | <math>\mathbf{t}_{3}=\frac{{\boldsymbol \varphi }_{\prime{1}}\times{\boldsymbol \varphi }_{\prime{2}}}{\left\vert {\boldsymbol \varphi }_{\prime{1}}\times{\boldsymbol \varphi }_{\prime{2}}\right\vert }=\lambda \;_{1}\times{\boldsymbol \varphi }_{2} </math>
815
|}
816
| style="width: 5px;text-align: right;" | (56)
817
|}
818
819
From these expressions it is also possible to compute in the original configuration the element area <math display="inline">A^{0}_{M}</math>, the outer normals <math display="inline">\left( n_{1} ,n_{2}\right) ^{i}</math> at each side and the side lengths <math display="inline">l_{i}^{M}</math>. Eq.([[#eq-56|56]]) also allows to evaluate the thickness ratio <math display="inline">\lambda </math> in the deformed configuration and the actual normal <math display="inline">\mathbf{t}_{3}</math>.
820
821
In order to compute the line integral of equation ([[#eq-54|54]]) the averaging procedure described in Section 2 is used. Hence along each side of the triangle the average value of <math display="inline">{\boldsymbol \varphi }_{^{\prime }\alpha }</math> between the main triangle and the adjacent one is taken leading to
822
823
{| class="formulaSCP" style="width: 100%; text-align: left;" 
824
|-
825
| 
826
{| style="text-align: left; margin:auto;" 
827
|-
828
| style="text-align: center;" | <math>{\boldsymbol \kappa }=\frac{1}{A^{0}_{M}}\sum _{I=1}^{3}\left[ \begin{array}{cc} -n_{1}^{i} & 0\\ 0 & n_{2}^{i}\\ n_{2}^{i} & -n_{1}^{i} \end{array} \right] \left[ \begin{array}{c} \mathbf{t}_{3}\cdot \frac{1}{2}\left( \mathbf{\boldsymbol \varphi }_{^{\prime }1} ^{M}+\mathbf{\boldsymbol \varphi }_{^{\prime }1}^{i}\right)\\ \mathbf{t}_{3}\cdot \frac{1}{2}\left( \mathbf{\boldsymbol \varphi }_{^{\prime }2} ^{M}+\mathbf{\boldsymbol \varphi }_{^{\prime }2}^{i}\right) \end{array} \right] </math>
829
|}
830
| style="width: 5px;text-align: right;" | (57)
831
|}
832
833
where the sum extends over the three elements adjacent to the central triangle <math display="inline">M</math>.
834
835
Noting that <math display="inline">\mathbf{t}_{3}\cdot \mathbf{\boldsymbol \varphi }_{^{\prime }\alpha }^{M}=0</math> in the main triangle it can be found <span id='citeF-24'></span>[[#cite-24|[24]]]
836
837
<span id="eq-58"></span>
838
{| class="formulaSCP" style="width: 100%; text-align: left;" 
839
|-
840
| 
841
{| style="text-align: left; margin:auto;" 
842
|-
843
| style="text-align: center;" | <math>{\boldsymbol \kappa }=\sum _{i=1}^{3}\left[ \begin{array}{cc} L_{i,1}^M & 0 \\         0 & L_{i,2}^M \\ L_{i,2}^M & L_{i,1}^M  \end{array} \right] \left[ \begin{array}{c} \mathbf{t}_{3}\cdot \mathbf{\boldsymbol \varphi }_{^{\prime }1}^{i}\\ \mathbf{t}_{3}\cdot \mathbf{\boldsymbol \varphi }_{^{\prime }2}^{i} \end{array} \right] </math>
844
|}
845
| style="width: 5px;text-align: right;" | (58)
846
|}
847
848
This can be seen as the projection of the local derivatives in the adjacent triangles <math display="inline">\mathbf{\boldsymbol \varphi }_{^{\prime }\alpha }^{i}</math> (where index <math display="inline">i</math> denotes values associated to the adjacent elements) over the normal to the main triangle <math display="inline">\mathbf{t}_{3}</math>. As the triangles have a common side, <math display="inline">\mathbf{t}_{3}\cdot \mathbf{\boldsymbol \varphi }_{^{\prime }s}^{i}=0</math>, where <math display="inline">\mathbf{\boldsymbol \varphi }_{^{\prime } s}^{i}</math> is the derivative along the side. Hence only the derivative along the side normal (<math display="inline">\mathbf{\boldsymbol \varphi }_{^{\prime }n}^{i}</math>) has non-zero component over <math display="inline">\mathbf{t}_{3}</math>. This gives
849
850
<span id="eq-59"></span>
851
{| class="formulaSCP" style="width: 100%; text-align: left;" 
852
|-
853
| 
854
{| style="text-align: left; margin:auto;" 
855
|-
856
| style="text-align: center;" | <math>\left[ \begin{array}{c} \mathbf{t}_{3}\cdot \mathbf{\boldsymbol \varphi }_{^{\prime }1}^{i}\\ \mathbf{t}_{3}\cdot \mathbf{\boldsymbol \varphi }_{^{\prime }2}^{i} \end{array} \right] =\left( \mathbf{t}_{3}\cdot \mathbf{\boldsymbol \varphi }_{^{\prime  }n}^{i}\right)\mathbf{n}^{i} </math>
857
|}
858
| style="width: 5px;text-align: right;" | (59)
859
|}
860
861
An alternative form to express the curvatures, which is useful when their variations are needed, is to define the vectors
862
863
<span id="eq-60"></span>
864
{| class="formulaSCP" style="width: 100%; text-align: left;" 
865
|-
866
| 
867
{| style="text-align: left; margin:auto;" 
868
|-
869
| style="text-align: center;" | <math>\mathbf{h}_{ij}=\sum _{k=1}^{3}\frac{1}{2}\left( L_{k,i}^{M}{\boldsymbol \varphi  }_{^{\prime }j}^{k}+L_{k,j}^{M}{\boldsymbol \varphi }_{\prime i}^{k}\right) </math>
870
|}
871
| style="width: 5px;text-align: right;" | (60)
872
|}
873
874
This gives
875
876
<span id="eq-61"></span>
877
{| class="formulaSCP" style="width: 100%; text-align: left;" 
878
|-
879
| 
880
{| style="text-align: left; margin:auto;" 
881
|-
882
| style="text-align: center;" | <math>\kappa _{ij}=\mathbf{h}_{ij}\cdot \mathbf{t}_{3}</math>
883
|}
884
| style="width: 5px;text-align: right;" | (61)
885
|}
886
887
The last expression allows to interpret the curvatures as the projections of the vectors <math display="inline">\mathbf{h}_{ij}</math> over the normal of the central element. The variation of the curvatures can be obtained as
888
889
<span id="eq-62"></span>
890
{| class="formulaSCP" style="width: 100%; text-align: left;" 
891
|-
892
| 
893
{| style="text-align: left; margin:auto;" 
894
|-
895
| style="text-align: center;" | <math>\delta{\boldsymbol \kappa }=\sum _{i=1}^{3}\left\{ \left[ \begin{array}{cc} L_{i,1}^{M} & 0\\ 0 & L_{i,2}^{M}\\ L_{i,2}^{M} & L_{i,1}^{M} \end{array} \right] \sum _{J=1}^{3}\left[ \begin{array}{c} L_{j,1}^{i}(\mathbf{t}_{3}\cdot \delta \mathbf{u}_{j}^{i})\\ N_{j,2}^{i}(\mathbf{t}_{3}\cdot \delta \mathbf{u}_{j}^{i}) \end{array} \right] -2\left[ \begin{array}{c} (L_{i,1}^{M}\rho _{11}^{1}+L_{i,2}^{M}\rho _{11}^{2})\\ (L_{i,1}^{M}\rho _{22}^{1}+L_{i,2}^{M}\rho _{22}^{2})\\ (L_{i,1}^{M}\rho _{12}^{1}+L_{i,2}^{M}\rho _{12}^{2}) \end{array} \right] (\mathbf{t}_{3}\cdot \delta \mathbf{u}_{i}^{M})\right\} </math>
896
|}
897
| style="width: 5px;text-align: right;" | (62)
898
|}
899
900
where the projections of the vectors <math display="inline">\mathbf{h}_{ij}</math> over the contravariant base vectors <math display="inline">\tilde{\boldsymbol \varphi }_{^{\prime }\alpha }</math> have been included
901
902
<span id="eq-63"></span>
903
{| class="formulaSCP" style="width: 100%; text-align: left;" 
904
|-
905
| 
906
{| style="text-align: left; margin:auto;" 
907
|-
908
| style="text-align: center;" | <math>\rho _{ij}^{\alpha }=\mathbf{h}_{ij}\cdot \tilde{\boldsymbol \varphi }_{^{\prime }\alpha } \quad ,\quad \alpha ,i,j=1,2</math>
909
|}
910
| style="width: 5px;text-align: right;" | (63)
911
|}
912
913
with
914
915
{| class="formulaSCP" style="width: 100%; text-align: left;" 
916
|-
917
| 
918
{| style="text-align: left; margin:auto;" 
919
|-
920
| style="text-align: right;" | <math>\mathbf{\tilde{\boldsymbol \varphi }}_{^{\prime }1}  </math>
921
| <math>  =\lambda \;\mathbf{\boldsymbol \varphi }_{^{\prime }2}\times \mathbf{t}_{3}</math>
922
|}
923
| style="width: 5px;text-align: right;" | (64)
924
|}
925
926
{| class="formulaSCP" style="width: 100%; text-align: left;" 
927
|-
928
| 
929
{| style="text-align: left; margin:auto;" 
930
|-
931
| style="text-align: right;" | <math> \mathbf{\tilde{\boldsymbol \varphi }}_{^{\prime }2}  </math>
932
| <math>  =-\lambda \;\mathbf{\boldsymbol \varphi  }_{^{\prime }1}\times \mathbf{t}_{3} </math>
933
|}
934
| style="width: 5px;text-align: right;" | (65)
935
|}
936
937
938
In above expressions superindexes in <math display="inline">L_{j}^k</math> and <math display="inline">\delta \mathbf{u}_{j}^k</math> refer to element numbers whereas subscripts denote node numbers. As usual the superindex <math display="inline">M</math> denotes values in the central triangle (Figure [[#img-1|1]]). Note that as expected the curvatures (and their variations) in the central element are a function of the nodal displacements of the six nodes in the four elements patch. Note also that
939
940
{| class="formulaSCP" style="width: 100%; text-align: left;" 
941
|-
942
| 
943
{| style="text-align: left; margin:auto;" 
944
|-
945
| style="text-align: center;" | <math>\lambda ={\frac{h}{h^{0}}}={\frac{A_{M}^{0}}{A_{M}}} </math>
946
|}
947
| style="width: 5px;text-align: right;" | (66)
948
|}
949
950
Details of the derivation of Eq.([[#eq-62|62]]) can be found in [27].
951
952
Eq.([[#eq-62|62]]) can be rewritten in the form
953
954
{| class="formulaSCP" style="width: 100%; text-align: left;" 
955
|-
956
| 
957
{| style="text-align: left; margin:auto;" 
958
|-
959
| style="text-align: center;" | <math>\delta{\boldsymbol \kappa }=\mathbf{B}_{b}\delta \mathbf{a}^{p} </math>
960
|}
961
| style="width: 5px;text-align: right;" | (67)
962
|}
963
964
where
965
966
<span id="eq-68"></span>
967
{| class="formulaSCP" style="width: 100%; text-align: left;" 
968
|-
969
| 
970
{| style="text-align: left; margin:auto;" 
971
|-
972
| style="text-align: center;" | <math>\begin{array}{c} \\ \delta \mathbf{a}^{p}\\ 18\times{1} \end{array} =[\delta \mathbf{u}_{1}^{T},\delta \mathbf{u}_{2}^{T},\delta \mathbf{u}_{3} ^{T},\delta \mathbf{u}_{4}^{T},\delta \mathbf{u}_{5}^{T},\delta \mathbf{u}_{6}^{T}]^{T}</math>
973
|}
974
| style="width: 5px;text-align: right;" | (68)
975
|}
976
977
is the virtual displacement vector of the patch
978
979
<span id="eq-69"></span>
980
{| class="formulaSCP" style="width: 100%; text-align: left;" 
981
|-
982
| 
983
{| style="text-align: left; margin:auto;" 
984
|-
985
| style="text-align: center;" | <math>\mathbf{B}_{b}=[\mathbf{B}_{b1},\mathbf{B}_{b2}\cdots ,\mathbf{B}_{b6}]</math>
986
|}
987
| style="width: 5px;text-align: right;" | (69)
988
|}
989
990
is the curvature matrix relating the virtual curvatures within the central element and the 18 virtual displacements of the six nodes in the patch.
991
992
The form of matrix <math display="inline">\mathbf{B}_{b}</math> is given in the Appendix.
993
994
==5 ENHANCED BASIC SHELL TRIANGLE==
995
996
An enhanced version of the BST element (termed EBST) has been recently proposed by Flores and Oñate [27]. The main features of the element formulation are the following:
997
998
<ol>
999
1000
<li>The geometry of the patch formed by the central element and the three adjacent elements is ''quadratically interpolated'' from the position of the six nodes in the patch. </li>
1001
1002
<li>The membrane strains are assumed to vary ''linearly'' within the central triangle and are expressed in terms of the (continuous) values of the deformation gradient at the mid side points of the triangle. </li>
1003
1004
<li>The assumed ''constant curvature'' field within the central triangle is obtained by expression ([[#eq-54|54]]) using now twice the values of the (continuous) deformation gradient at the mid side points. </li>
1005
1006
</ol>
1007
1008
Details of the derivation of the EBST element are given below.
1009
1010
===5.1 Definition of the element geometry and computation of membrane strains===
1011
1012
As mentioned above a quadratic approximation of the geometry of the four elements patch is chosen using the position of the six nodes in the patch. It is useful to define the patch in the isoparametric space using the nodal positions given in the Table 2 (see also Figure 2).
1013
1014
1015
<div class="center" style="font-size: 75%;">
1016
'''Table 2'''. Isoparametric coordinates of the six nodes in the patch of Figure 2.</div>
1017
1018
{| class="wikitable" style="text-align: left; margin: 1em auto;"
1019
|- style="border-top: 2px solid;"
1020
|
1021
| 1 
1022
| 2 
1023
| 3 
1024
| 4 
1025
| 5 
1026
| 6
1027
|- style="border-top: 2px solid;"
1028
| <math display="inline">\xi </math> 
1029
| 0 
1030
| 1 
1031
| 0 
1032
| 1 
1033
| -1 
1034
| 1
1035
|- style="border-top: 2px solid;border-bottom: 2px solid;"
1036
| <math display="inline">\eta </math> 
1037
| 0 
1038
| 0 
1039
| 1 
1040
| 1 
1041
| 1 
1042
| -1
1043
1044
|}
1045
1046
The quadratic interpolation is defined by
1047
1048
<span id="eq-70"></span>
1049
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1050
|-
1051
| 
1052
{| style="text-align: left; margin:auto;" 
1053
|-
1054
| style="text-align: center;" | <math>{\boldsymbol \varphi }=\sum _{i=1}^{6}N_{i}{\boldsymbol \varphi }_{i}</math>
1055
|}
1056
| style="width: 5px;text-align: right;" | (70)
1057
|}
1058
1059
with (<math display="inline">\zeta=1-\xi-\eta</math>)
1060
1061
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1062
|-
1063
| 
1064
{| style="text-align: left; margin:auto;" 
1065
|-
1066
| style="text-align: center;" | <math>\begin{array}{ccc} N_{1}=\zeta{+\xi}\eta &  & N_{4}=\frac{\zeta }{2}\left( \zeta{-1}\right) \\ N_{2}=\xi{+\eta}\zeta &  & N_{5}=\frac{\xi }{2}\left( \xi{-1}\right) \\ N_{3}=\eta{+\zeta}\xi &  & N_{6}=\frac{\eta }{2}\left( \eta{-1}\right) \end{array} </math>
1067
|}
1068
| style="width: 5px;text-align: right;" | (71)
1069
|}
1070
1071
This interpolation allows to compute the displacement gradients at selected points in order to use an assumed strain approach. The computation of the gradients is performed at the mid side points of the central element of the patch denoted by <math display="inline">G_{1}</math>, <math display="inline">G_{2}</math> and <math display="inline">G_{3}</math> in Figure [[#img-2|2]]. This choice has the following advantages.
1072
1073
<div id='img-2'></div>
1074
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1075
|-
1076
|[[Image:draft_Samper_226033773-fig2.png|300px|Patch of elements in the isoparametric space.]]
1077
|- style="text-align: center; font-size: 75%;"
1078
| colspan="1" | '''Figure 2:''' Patch of elements in the isoparametric space.
1079
|}
1080
1081
* Gradients at the three mid side points depend only on the nodes belonging to the two elements adjacent to each side. This can be easily verified by sampling the derivatives of the shape functions at each mid-side point.
1082
1083
* When gradients are computed at the common mid-side point of two adjacent elements, the same values are obtained, as the coordinates of the same four points are used. This in practice means that the gradients at the mid-side points are independent of the element where they are computed. A side-oriented implementation of the finite element will therefore lead to a unique evaluation of the gradients per side.
1084
1085
The cartesian derivatives of the shape functions are computed at the original configuration by the standard expression
1086
1087
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1088
|-
1089
| 
1090
{| style="text-align: left; margin:auto;" 
1091
|-
1092
| style="text-align: center;" | <math>\left[ \begin{array}{c} N_{i,1}\\ N_{i,2} \end{array} \right] =\mathbf{J}^{-1}\left[ \begin{array}{c} N_{i,\xi } \\ N_{i,\eta } \end{array} \right] </math>
1093
|}
1094
| style="width: 5px;text-align: right;" | (72)
1095
|}
1096
1097
where the Jacobian matrix at the original configuration is
1098
1099
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1100
|-
1101
| 
1102
{| style="text-align: left; margin:auto;" 
1103
|-
1104
| style="text-align: center;" | <math>\mathbf{J=}\left[ \begin{array}{cc} \mathbf{\boldsymbol \varphi }_{^{\prime }\xi }^{0}\cdot \mathbf{t}_{1} & \mathbf{\boldsymbol \varphi  }_{^{\prime }\eta }^{0}\cdot \mathbf{t}_{1}\\ \mathbf{\boldsymbol \varphi }_{^{\prime }\xi }^{0}\cdot \mathbf{t}_{2} & \mathbf{\boldsymbol \varphi  }_{^{\prime }\eta }^{0}\cdot \mathbf{t}_{2} \end{array} \right] </math>
1105
|}
1106
| style="width: 5px;text-align: right;" | (73)
1107
|}
1108
1109
The deformation gradients on the middle surface, associated to an arbitrary spatial cartesian system and to the material cartesian system defined on the middle surface are related by
1110
1111
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1112
|-
1113
| 
1114
{| style="text-align: left; margin:auto;" 
1115
|-
1116
| style="text-align: center;" | <math>\left[ {\boldsymbol \varphi }_{^{\prime }1},\mathbf{\boldsymbol \varphi }_{^{\prime }2}\right] =\left[ \mathbf{\boldsymbol \varphi }_{^{\prime }\xi },\mathbf{\boldsymbol \varphi }_{^{\prime }\eta }\right]  \mathbf{J}^{-1} </math>
1117
|}
1118
| style="width: 5px;text-align: right;" | (74)
1119
|}
1120
1121
The Green-Lagrange membrane strains within the central triangle are now obtained using a linear assumed membrane strain field <math display="inline">\hat{\boldsymbol \varepsilon }_{m}</math>, i.e.
1122
1123
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1124
|-
1125
| 
1126
{| style="text-align: left; margin:auto;" 
1127
|-
1128
| style="text-align: center;" | <math>{\boldsymbol \varepsilon }_{m}=\hat{\boldsymbol \varepsilon }_{m} </math>
1129
|}
1130
| style="width: 5px;text-align: right;" | (75)
1131
|}
1132
1133
with
1134
1135
<span id="eq-76"></span>
1136
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1137
|-
1138
| 
1139
{| style="text-align: left; margin:auto;" 
1140
|-
1141
| style="text-align: center;" | <math>\hat{\boldsymbol \varepsilon }_{m}=(1-2\zeta ){\boldsymbol \varepsilon }_{m}^{1}+(1-2\xi ){\boldsymbol \varepsilon  }_{m}^{2}+(1-2\eta ){\boldsymbol \varepsilon }_{m}^{3}=\sum _{i=1}^{3}\bar{N}_{i} {\boldsymbol \varepsilon }_{m}^{i}</math>
1142
|}
1143
| style="width: 5px;text-align: right;" | (76)
1144
|}
1145
1146
where <math display="inline">{\boldsymbol \varepsilon }_{m}^{i}</math> are the membrane strains computed at the three mid side points <math display="inline">G_{i}</math> (<math display="inline">i=1,2,3</math>  see Figure [[#img-2|2]]). In Eq.([[#eq-76|76]]) <math display="inline">\bar{N}_{1}=(1-2\zeta )</math>, etc.
1147
1148
The gradient at each mid side point is computed from the quadratic interpolation ([[#eq-70|70]]):
1149
1150
<span id="eq-77"></span>
1151
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1152
|-
1153
| 
1154
{| style="text-align: left; margin:auto;" 
1155
|-
1156
| style="text-align: center;" | <math>\left( {\boldsymbol \varphi }_{^{\prime }\alpha }\right) _{G_{i}}={\boldsymbol \varphi }_{^{\prime  }\alpha }^{i}=\left[ \sum _{j=1}^{3}N_{j,\alpha }^{i}{\boldsymbol \varphi }_{j}\right] +N_{i+3,\alpha }^{i}{\boldsymbol \varphi }_{i+3}\quad ,\quad \alpha=1,2\quad ,\quad  i=1,2,3</math>
1157
|}
1158
| style="width: 5px;text-align: right;" | (77)
1159
|}
1160
1161
Substituting Eq.([[#eq-22|22]]) into ([[#eq-76|76]]) and using Eq.([[#eq-22|22]]) gives the membrane strain vector as
1162
1163
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1164
|-
1165
| 
1166
{| style="text-align: left; margin:auto;" 
1167
|-
1168
| style="text-align: center;" | <math>{\boldsymbol \varepsilon }_{m}=\sum _{i=1}^{3}\frac{1}{2}\bar{N}_{i}\left\{ \begin{array}{c} {\boldsymbol \varphi }_{^{\prime }1}^{i}\cdot \mathbf{\boldsymbol \varphi }_{^{\prime }1}^{i}-1\\ {\boldsymbol \varphi }_{^{\prime }2}^{i}\cdot \mathbf{\boldsymbol \varphi }_{^{\prime }2}^{i}-1\\ 2{\boldsymbol \varphi }_{^{\prime }1}^{i}\cdot \mathbf{\boldsymbol \varphi }_{^{\prime }2}^{i} \end{array} \right\} </math>
1169
|}
1170
| style="width: 5px;text-align: right;" | (78)
1171
|}
1172
1173
and the virtual membrane strains as
1174
1175
<span id="eq-79"></span>
1176
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1177
|-
1178
| 
1179
{| style="text-align: left; margin:auto;" 
1180
|-
1181
| style="text-align: center;" | <math>\delta{\boldsymbol \varepsilon }_{m}=\sum _{i=1}^{3}\bar{N}_{i}\left\{ \begin{array}{c} {\boldsymbol \varphi }_{^{\prime }1}^{i}\cdot \delta \mathbf{\boldsymbol \varphi }_{^{\prime }1}^{i}\\ {\boldsymbol \varphi }_{2}^{i}\cdot \delta \mathbf{\boldsymbol \varphi }_{^{\prime }2}^{i}\\ \delta{\boldsymbol \varphi }_{^{\prime }1}^{i}\cdot \mathbf{\boldsymbol \varphi }_{^{\prime }2} ^{i}+{\boldsymbol \varphi }_{^{\prime }1}^{i}\cdot \delta \mathbf{\boldsymbol \varphi }_{2}^{i} \end{array} \right\} </math>
1182
|}
1183
| style="width: 5px;text-align: right;" | (79)
1184
|}
1185
1186
We note that the gradient at each mid side point <math display="inline">G_{i}</math> depends only on the coordinates of the three nodes of the central triangle and on those of an additional node in the patch, associated to the side <math display="inline">i</math> where the gradient is computed.
1187
1188
Combining Eqs.([[#eq-79|79]]) and ([[#eq-77|77]]) gives
1189
1190
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1191
|-
1192
| 
1193
{| style="text-align: left; margin:auto;" 
1194
|-
1195
| style="text-align: center;" | <math>\delta{\boldsymbol \varepsilon }_{m}=\mathbf{B}_{m}\delta \mathbf{a}^{p} </math>
1196
|}
1197
| style="width: 5px;text-align: right;" | (80)
1198
|}
1199
1200
where <math display="inline">\delta \mathbf{a}^{p}</math> is the patch displacement vector (see Eq.([[#eq-68|68]])) and <math display="inline">\mathbf{B}_{m}</math> is the membrane strain matrix. An explicit form of this matrix is given in the Appendix.
1201
1202
Differently from the original BST element the membrane strains within the EBST element are now a function of the displacements of the six patch nodes.
1203
1204
===5.2 Computation of curvatures===
1205
1206
The constant curvature field assumed for the BST element is chosen again here. The numerical evaluation of the line  integral in Eq.([[#eq-54|54]]) results in a sum over the integration points at the element boundary which are, in fact, the same points used for evaluating the gradients when computing the membrane strains. As one integration point is used over each side, it is not necessary to distinguish between sides (<math display="inline">i</math>) and integration points (<math display="inline">G_{i}</math>). In this way the curvatures can be computed by
1207
1208
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1209
|-
1210
| 
1211
{| style="text-align: left; margin:auto;" 
1212
|-
1213
| style="text-align: center;" | <math>{\boldsymbol \kappa }=2\sum _{i=1}^{3}\left[ \begin{array}{cc} L_{i,1}^M & 0\\ 0         & L_{i,2}^M \\ L_{i,2}^M & L_{i,1}^M  \end{array} \right] \left[ \begin{array}{c} \mathbf{t}_{3}\cdot{\boldsymbol \varphi }_{^{\prime }1}^{i}\\ \mathbf{t}_{3}\cdot{\boldsymbol \varphi }_{^{\prime }2}^{i} \end{array} \right] </math>
1214
|}
1215
| style="width: 5px;text-align: right;" | (81)
1216
|}
1217
1218
In the standard BST element <span id='citeF-21'></span><span id='citeF-24'></span>[[#cite-21|[21,24]]] the gradient <math display="inline">\mathbf{\boldsymbol \varphi  }_{\prime \alpha }^{i}</math> is computed as the average of the linear approximations over the two adjacent elements (see Section 4.3). In the enhanced version, the gradient is evaluated at each side <math display="inline">G_{i}</math> from the quadratic interpolation
1219
1220
<span id="eq-82"></span>
1221
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1222
|-
1223
| 
1224
{| style="text-align: left; margin:auto;" 
1225
|-
1226
| style="text-align: center;" | <math>\left[ \begin{array}{c} {\boldsymbol \varphi }_{\prime{1}}^{i}\\ {\boldsymbol \varphi }_{\prime{2}}^{i} \end{array} \right] =\left[ \begin{array}{cccc} N_{1,1}^{i} & N_{2,1}^{i} & N_{3,1}^{i} & N_{i+3,1}^{i}\\ N_{1,2}^{i} & N_{2,2}^{i} & N_{3,2}^{i} & N_{i+3,2}^{i} \end{array} \right] \left[ \begin{array}{c} {\boldsymbol \varphi }_{1}\\ {\boldsymbol \varphi }_{2}\\ {\boldsymbol \varphi }_{3}\\ {\boldsymbol \varphi }_{i+3} \end{array} \right]  </math>
1227
|}
1228
| style="width: 5px;text-align: right;" | (82)
1229
|}
1230
1231
Note again than at each side the gradients depend only on the positions of the three nodes of the central triangle and of an extra node (<math display="inline">i+3</math>), associated precisely to the side (<math display="inline">G_{i}</math>) where the gradient is computed.
1232
1233
Direction '''t'''<math display="inline">_{3}</math> in Eq.([[#eq-82|82]]) can be seen as a reference direction. If a different direction than that given by Eq.([[#eq-56|56]]) is chosen, at an angle <math display="inline">\theta </math> with the former, this has an influence of order <math display="inline">\theta ^{2}</math> in the projection. This justifies Eq.([[#eq-56|56]]) for the definition of '''t'''<math display="inline">_{3}</math> as a function exclusively of the three nodes of the central triangle, instead of using the 6-node isoparametric interpolation.
1234
1235
The variation of the curvatures can be obtained as
1236
1237
<span id="eq-89"></span>
1238
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1239
|-
1240
| 
1241
{| style="text-align: left; margin:auto;" 
1242
|-
1243
| style="text-align: right;" | <math>\delta{\boldsymbol \kappa } </math>
1244
| <math>  =2\sum _{i=1}^{3}\left[ \begin{array}{cc} L_{i,1}^M & 0\\ 0         & L_{i,2}^M\\ L_{i,2}^M & L_{i,1}^M \end{array} \right] \left\{ \sum _{i=1}^{3}\left[ \begin{array}{c} N_{j,1}^{i}(\mathbf{t}_{3}\cdot \delta \mathbf{u}_{j})\\ N_{j,2}^{i}(\mathbf{t}_{3}\cdot \delta \mathbf{u}_{j}) \end{array} \right] +\left[ \begin{array}{c} N_{i+3,1}^{i}(\mathbf{t}_{3}\cdot \delta \mathbf{u}^{i+3})\\ N_{i+3,2}^{i}(\mathbf{t}_{3}\cdot \delta \mathbf{u}^{i+3}) \end{array} \right] \right\} -</math>
1245
|-
1246
| style="text-align: right;" | 
1247
| <math>  -\sum _{i=1}^{3}\left[ \begin{array}{c} (L_{i,1}^M\rho _{11}^{1}+L_{i,2}^M\rho _{11}^{2})\\ (L_{i,1}^M\rho _{22}^{1}+L_{i,2}^M\rho _{22}^{2})\\ (L_{i,1}^M\rho _{12}^{1}+L_{i,2}^M\rho _{12}^{2}) \end{array} \right] (\mathbf{t}_{3}\cdot \delta \mathbf{u}_{i})=\mathbf{B}_{b} \delta \mathbf{a}^{p}</math>
1248
|}
1249
| style="width: 5px;text-align: right;" | (83)
1250
|}
1251
1252
where the definitions ([[#eq-61|61]]) and ([[#eq-63|63]]) still hold but with the new definition of <math display="inline">\mathbf{h}_{ij}</math> given by <span id='citeF-27'></span>[[#cite-27|[27]]]
1253
1254
<span id="eq-90"></span>
1255
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1256
|-
1257
| 
1258
{| style="text-align: left; margin:auto;" 
1259
|-
1260
| style="text-align: center;" | <math>\mathbf{h}_{ij}=\sum _{k=1}^{3}\left( L_{k,i}^M{\boldsymbol \varphi }_{\prime j}^{k} +L_{k,j}^M{\boldsymbol \varphi }_{^{\prime }i}^{k}\right) </math>
1261
|}
1262
| style="width: 5px;text-align: right;" | (84)
1263
|}
1264
1265
In Eq.([[#eq-89|83]])
1266
1267
<span id="eq-91"></span>
1268
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1269
|-
1270
| 
1271
{| style="text-align: left; margin:auto;" 
1272
|-
1273
| style="text-align: center;" | <math>\mathbf{B}_{b}=[\mathbf{B}_{b_{1}},\mathbf{B}_{b_{2}},\cdots ,\mathbf{B}_{b_{6}}]</math>
1274
|}
1275
| style="width: 5px;text-align: right;" | (85)
1276
|}
1277
1278
The expression of the curvature matrix <math display="inline">\mathbf{B}_b</math> is given in the Appendix. Details of the derivation of Eq.([[#eq-89|89]]) can be found in [27].
1279
1280
===5.3 The EBST1 element===
1281
1282
A simplified and yet very effective version of the EBST element can be obtained by using ''one point quadrature'' for the computation of all the element integrals. This element is termed EBST1. Note that this only affects the membrane stiffness matrices and it is equivalent to using a assumed constant membrane strain field defined by an average of the metric tensors computed at each side.
1283
1284
Numerical experiments have shown that both the EBST and the EBST1 elements are free of spurious energy modes.
1285
1286
==6 BOUNDARY CONDITIONS==
1287
1288
Elements at the domain boundary, where an adjacent element does not exist, deserve a special attention. The treatment of essential boundary conditions associated to translational constraints is straightforward, as they are the natural degrees of freedom of the element. The conditions associated to the normal vector are crucial in this formulation for bending. For clamped sides or symmetry planes, the normal vector <math display="inline">\mathbf{t}_{3}</math> must be kept fixed (clamped case), or constrained to move in the plane of symmetry (symmetry case). The former case can be seen as a special case of the latter, so we will consider symmetry planes only. This restriction can be imposed through the definition of the tangent plane at the boundary, including the normal to the plane of symmetry <math display="inline">\boldsymbol \varphi _{^{\prime }n}^{0}</math> that does not change during the process.
1289
1290
<div id='img-3'></div>
1291
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1292
|-
1293
|
1294
[[File:Draft_Samper_226033773_5749_Fig3.jpeg|600px|Local cartesian system for the treatment of symmetry boundary conditions]]
1295
|- style="text-align: center; font-size: 75%;"
1296
| colspan="1" | '''Figure 3:''' Local cartesian system for the treatment of symmetry boundary conditions
1297
|}
1298
1299
The tangent plane at the boundary (mid-side point) is expressed in terms of two orthogonal unit vectors referred to a local-to-the-boundary Cartesian system (see Figure [[#img-3|3]]) defined as
1300
1301
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1302
|-
1303
| 
1304
{| style="text-align: left; margin:auto;" 
1305
|-
1306
| style="text-align: center;" | <math>\left[\boldsymbol \varphi _{^{\prime }n}^{0},\;\bar{\boldsymbol \varphi }_{^{\prime }s}\right] </math>
1307
|}
1308
| style="width: 5px;text-align: right;" | (86)
1309
|}
1310
1311
where vector <math display="inline">\boldsymbol \varphi _{^{\prime }n}^{0}</math> is fixed during the process while direction <math display="inline">\bar{\boldsymbol \varphi }_{^{\prime }s}</math> emerges from the intersection of the symmetry plane with the plane defined by the central element (<math display="inline">M</math>). The plane (gradient) defined by the central element in the selected original convective Cartesian system (<math display="inline">\mathbf{t}_{1},\mathbf{t}_{2} </math>) is
1312
1313
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1314
|-
1315
| 
1316
{| style="text-align: left; margin:auto;" 
1317
|-
1318
| style="text-align: center;" | <math>\left[\boldsymbol \varphi _{^{\prime }1}^{M},\;\boldsymbol \varphi _{^{\prime  }2}^{M}\right] </math>
1319
|}
1320
| style="width: 5px;text-align: right;" | (87)
1321
|}
1322
1323
the intersection line (side <math display="inline">i</math>) of this plane with the plane of symmetry can be written in terms of the position of the nodes that define the side (<math display="inline">j </math> and <math display="inline">k</math>) and the original length of the side <math display="inline">l_{i}^{M}</math>, i.e.
1324
1325
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1326
|-
1327
| 
1328
{| style="text-align: left; margin:auto;" 
1329
|-
1330
| style="text-align: center;" | <math>\boldsymbol \varphi _{^{\prime }s}^{i}=\frac{1}{l_{i}^{M}}\left(\boldsymbol  \varphi _{k}-\boldsymbol \varphi _{j}\right) </math>
1331
|}
1332
| style="width: 5px;text-align: right;" | (88)
1333
|}
1334
1335
That together with the outer normal to the side <math display="inline">\mathbf{n}^{i} = \left[n_{1},n_{2}\right]^{T}=\left[\mathbf{n\cdot t}_{1},\mathbf{n\cdot t}_{2}\right]^{T}</math> (resolved in the selected original convective Cartesian system) leads to
1336
1337
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1338
|-
1339
| 
1340
{| style="text-align: left; margin:auto;" 
1341
|-
1342
| style="text-align: center;" | <math>\left[ \begin{array}{c}\boldsymbol \varphi _{^{\prime }1}^{iT} \\ \boldsymbol \varphi _{^{\prime }2}^{iT} \end{array}\right]=\left[ \begin{array}{cc}n_{1} & -n_{2} \\ n_{2} & n_{1} \end{array}\right]\left[ \begin{array}{c}\boldsymbol \varphi _{^{\prime }n}^{iT} \\ \boldsymbol \varphi _{^{\prime }s}^{iT} \end{array}\right] </math>
1343
|}
1344
| style="width: 5px;text-align: right;" | (89)
1345
|}
1346
1347
where, noting  that <math display="inline">\lambda </math> is the determinant of the gradient, the normal component of the gradient <math display="inline">\boldsymbol \varphi _{^{\prime }n}^{i}</math> can be approximated by
1348
1349
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1350
|-
1351
| 
1352
{| style="text-align: left; margin:auto;" 
1353
|-
1354
| style="text-align: center;" | <math>\boldsymbol \varphi _{^{\prime }n}^{i}=\frac{\boldsymbol \varphi _{^{\prime }n}^{0} }{\lambda |\boldsymbol \varphi _{^{\prime }s}^{i}|} </math>
1355
|}
1356
| style="width: 5px;text-align: right;" | (90)
1357
|}
1358
1359
In this way the contribution of the gradient at side <math display="inline">i</math> to vectors <math display="inline">\mathbf{ h}_{\alpha \beta }</math> (equations [[#eq-60|60]] and [[#eq-90|84]]) results in
1360
1361
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1362
|-
1363
| 
1364
{| style="text-align: left; margin:auto;" 
1365
|-
1366
| style="text-align: center;" | <math>\left[ \begin{array}{c}\mathbf{h}_{11}^{T} \\ \mathbf{h}_{22}^{T} \\ 2\mathbf{h}_{12}^{T} \end{array}\right]^{i}=2\left[ \begin{array}{cc}L_{i,1}^{M} & 0 \\ 0 & L_{i,2}^{M} \\ L_{i,2}^{M} & L_{i,1}^{M} \end{array}\right]\left[ \begin{array}{c}\boldsymbol \varphi _{^{\prime }1}^{iT} \\ \boldsymbol \varphi _{^{\prime }2}^{iT} \end{array}\right]=2\left[ \begin{array}{cc}L_{i,1}^{M} & 0 \\ 0 & L_{i,2}^{M} \\ L_{i,2}^{M} & L_{i,1}^{M} \end{array}\right]\left[ \begin{array}{cc}n_{1} & -n_{2} \\ n_{2} & n_{1} \end{array}\right]\left[ \begin{array}{c}\boldsymbol \varphi _{^{\prime }n}^{iT} \\ \boldsymbol \varphi _{^{\prime }s}^{iT} \end{array}\right] </math>
1367
|}
1368
| style="width: 5px;text-align: right;" | (91)
1369
|}
1370
1371
For the computation of the curvature variations, the contribution from the gradient at side <math display="inline">i</math> is now (see Ref. <span id='citeF-27'></span>[[#cite-27|[27]]])
1372
1373
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1374
|-
1375
| 
1376
{| style="text-align: left; margin:auto;" 
1377
|-
1378
| style="text-align: center;" | <math> \delta \left[ \begin{array}{c} \mathbf{h}_{11}^{T} \\ \mathbf{h}_{22}^{T} \\ 2\mathbf{h}_{12}^{T} \end{array} \right]^{i} =2\left[ \begin{array}{cc} L_{i,1}^{M} & 0 \\ 0 & L_{i,2}^{M} \\ L_{i,2}^{M} & L_{i,1}^{M} \end{array} \right]\left[ \begin{array}{cc} n_{1} & -n_{2} \\ n_{2} & n_{1} \end{array} \right]\left[ \begin{array}{c} \mathbf{0} \\ \frac{1}{L_{o}}\left[\delta \mathbf{u}_{k}-\delta \mathbf{u}_{j}\right]^{T} \end{array} \right]</math>
1379
|}
1380
| style="width: 5px;text-align: right;" |  (92a)
1381
|}
1382
1383
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1384
|-
1385
| 
1386
{| style="text-align: left; margin:auto;" 
1387
|-
1388
| style="text-align: center;" | <math> =\frac{2}{l_{i}^{M}}\left[ \begin{array}{c} -L_{i,1}^{M}n_{2} \\ L_{i,2}^{M}n_{1} \\ L_{i,1}^{M}n_{1}-L_{i,2}^{M}n_{2} \end{array} \right]\left[\delta \mathbf{u}_{k}-\delta \mathbf{u}_{j}\right]^{T}</math>
1389
|}
1390
| style="width: 5px;text-align: right;" |  (92b)
1391
|}
1392
1393
where the influence of variations in the length of vector <math display="inline">\boldsymbol \varphi  _{^{\prime }n}</math> has been neglected.
1394
1395
For a simple supported (hinged) side, the problem is not completely defined. The simplest choice is to neglect the contribution to the side rotations from the adjacent element missing in the patch in the evaluation of the curvatures via eq.([[#eq-54|54]]) <span id='citeF-29'></span>[[#cite-29|[21,24]]]. This is equivalent to assume that the gradient at the side is equal to the gradient in the central element, i.e.
1396
1397
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1398
|-
1399
| 
1400
{| style="text-align: left; margin:auto;" 
1401
|-
1402
| style="text-align: center;" | <math>\left[\boldsymbol \varphi _{^{\prime }1}^{i},\;\boldsymbol \varphi _{^{\prime }2}^{i} \right]=\left[\boldsymbol \varphi _{^{\prime }1}^{M},\;\boldsymbol \varphi  _{^{\prime }2}^{M}\right] </math>
1403
|}
1404
| style="width: 5px;text-align: right;" |  (93)
1405
|}
1406
1407
More precise changes can be however introduced to account for the different natural boundary conditions. One may assume that the curvature normal to the side is zero, and consider a contribution of the missing side to introduce this constraint. As the change of curvature parallel to the side is zero along the hinged side, both things lead to zero curvatures in both directions. Denoting the contribution to curvatures of the existing sides (<math display="inline">j </math> and <math display="inline">k</math>) by
1408
1409
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1410
|-
1411
| 
1412
{| style="text-align: left; margin:auto;" 
1413
|-
1414
| style="text-align: center;" | <math>\left[ \begin{array}{c}\kappa _{11} \\ \kappa _{22} \\ \kappa _{12} \end{array} \right]^{j-k} </math>
1415
|}
1416
|}
1417
1418
It can be easily shown that to set the normal curvature to zero the contribution of the simple supported side (<math display="inline">i</math>) should be
1419
1420
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1421
|-
1422
| 
1423
{| style="text-align: left; margin:auto;" 
1424
|-
1425
| style="text-align: center;" | <math>\left[ \begin{array}{c}\kappa _{11} \\ \kappa _{22} \\ \kappa _{12} \end{array} \right]^{i}=-\left[ \begin{array}{ccc}\left(n_{1}\right)^{4} & \left(n_{1}\right)^{2}\left(n_{2}\right)^{2} & \left(n_{1}\right)^{3}n_{2} \\ \left(n_{1}\right)^{2}\left(n_{2}\right)^{2} & \left(n_{2}\right)^{4} & n_{1}\left(n_{2}\right)^{3} \\ 2\left(n_{1}\right)^{3}n_{2} & 2n_{1}\left(n_{2}\right)^{3} & 2\left( n_{1}\right)^{2}\left(n_{2}\right)^{2} \end{array} \right]\left[ \begin{array}{c}\kappa _{11} \\ \kappa _{22} \\ \kappa _{12} \end{array} \right]^{j-k} </math>
1426
|}
1427
| style="width: 5px;text-align: right;" | (94)
1428
|}
1429
1430
For the case of a triangle with two sides associated to hinged sides, the normal curvatures to both sides must be zero. Denoting by <math display="inline">\mathbf{n}^{i}</math> and <math display="inline">\mathbf{n}^{j}</math> the normal to the sides, and by <math display="inline">\mathbf{m}^{i}</math> and <math display="inline"> \mathbf{m}^{j}</math> the dual base (associated to base <math display="inline">\mathbf{n}^{i}-</math> <math display="inline">\mathbf{ n}^{j}</math>), the contribution from the hinged sides (<math display="inline">i</math> and <math display="inline">j</math>) can be written as a function of the contribution of the only existing side (<math display="inline">k</math>):
1431
1432
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1433
|-
1434
| 
1435
{| style="text-align: left; margin:auto;" 
1436
|-
1437
| style="text-align: center;" | <math>\left[ \begin{array}{c}\kappa _{11} \\ \kappa _{22} \\ \kappa _{12} \end{array} \right]^{i-j}=-\left[ \begin{array}{c}m_{1}^{i}m_{1}^{j} \\ m_{2}^{i}m_{2}^{j} \\ m_{1}^{i}m_{2}^{j}+m_{2}^{i}m_{1}^{j} \end{array} \right]\left[ \begin{array}{ccc}2n_{1}^{i}n_{1}^{j} & 2n_{2}^{i}n_{2}^{j} & n_{1}^{i}n_{2}^{j}+n_{2}^{i}n_{1}^{j} \end{array} \right]\left[ \begin{array}{c}\kappa _{11} \\ \kappa _{22} \\ \kappa _{12} \end{array} \right]^{k} </math>
1438
|}
1439
| style="width: 5px;text-align: right;" | (95)
1440
|}
1441
1442
For a free edge the same approximation can be used but due to Poisson's effect this will lead to some error. The curvature variations of these contributions can be easily computed.
1443
1444
For the membrane formulation of element EBST, the gradient at the mid-side point of the boundary is assumed equal to the gradient of the main triangle.
1445
1446
==7 IMPLICIT SOLUTION SCHEME==
1447
1448
For a step <math display="inline">n</math> the configuration <math display="inline">\mathbf{\boldsymbol \varphi }^{n}</math> and the plastic strains <math display="inline">{\boldsymbol \varepsilon }_{p}^{n}</math> are known. The configuration <math display="inline">\mathbf{\boldsymbol \varphi }^{n}</math> is obtained by adding the total displacements to the original configuration <math display="inline"> \mathbf{\boldsymbol \varphi }^{n}=\mathbf{\boldsymbol \varphi }^{0} +\mathbf{u}^{n}</math>. The stresses are computed at each triangle using a single sampling (integration) point at the center and <math display="inline">N_{L}</math> integration points (layers) through the thickness. The plane stress state condition of the classical thin shell theory is assumed, so that for every layer three stress components are computed, (<math display="inline">\sigma _{11}</math>,<math display="inline">\sigma _{22}</math>, and <math display="inline">\sigma _{12}</math>) referred to the local cartesian system.
1449
1450
The computation of the incremental stresses is as follows:
1451
1452
<ol>
1453
1454
<li>Evaluate the incremental displacements: <math display="inline">\Delta \mathbf{u}^{n}=\mathbf{K}_{T}^{n}\mathbf{r}^{n}</math> where <math display="inline">\mathbf{K}_{T}</math> is the tangent stiffness matrix and '''r''' is the residual force vector  defined by for each element
1455
1456
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1457
|-
1458
| 
1459
{| style="text-align: left; margin:auto;" 
1460
|-
1461
| style="text-align: center;" | <math>\mathbf{r}^e_i =\int \int _A L_i {t}\, dA - \int \int _{A^\circ } ({B}_{m_i}^T {\boldsymbol \sigma }_m + {B}_{b_i}^T {\boldsymbol \sigma }_b)dA </math>
1462
|}
1463
| style="width: 5px;text-align: right;" | (96)
1464
|}</li>
1465
1466
The expression of the tangent stiffness matrix for the element is given below. Details of the derivation can be found in <span id='citeF-24'></span>[[#cite-24|[24]]],<span id='citeF-27'></span>[[#cite-27|[27]]].
1467
1468
<li>Generate the actual configuration <math display="inline">\mathbf{\boldsymbol \varphi }^{n+1} =\mathbf{\boldsymbol \varphi }^{n}+\Delta \mathbf{u}^{n}</math> </li>
1469
1470
<li>Compute the metric tensor <math display="inline">a_{\alpha \beta }^{n+1}\mathbf{ }</math>and the curvatures <math display="inline">\kappa _{\alpha \beta }^{n+1}</math> </li>
1471
1472
<li>Compute the total and elastic deformations at each layer <math display="inline">k</math> </li>
1473
1474
<span id="eq-96"></span>
1475
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1476
|-
1477
| 
1478
{| style="text-align: left; margin:auto;" 
1479
|-
1480
| style="text-align: right;" | <math>{\boldsymbol \varepsilon }_{k}^{n+1} </math>
1481
| <math>  ={\boldsymbol \varepsilon }_{m}^{n+1}+z_{k}{\boldsymbol \chi } ^{n+1}</math>
1482
|-
1483
| style="text-align: right;" | <math> \left[ {\boldsymbol \varepsilon }_{e}\right] _{k}^{n+1} </math>
1484
| <math>  ={\boldsymbol \varepsilon  }_{k}^{n+1}-\left[ {\boldsymbol \varepsilon }_{p}\right] _{k}^{n} </math>
1485
|}
1486
| style="width: 5px;text-align: right;vertical-align:center;" | (97)
1487
|}
1488
1489
<li>Compute the trial elastic stresses at each layer <math display="inline">k</math>
1490
1491
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1492
|-
1493
| 
1494
{| style="text-align: left; margin:auto;" 
1495
|-
1496
| style="text-align: center;" | <math>{\boldsymbol \sigma } _{k}^{n+1}=\mathbf{C}\left[ {\boldsymbol \varepsilon }_{e}\right] _{k}^{n+1} </math>
1497
|}
1498
| style="width: 5px;text-align: right;" | (98)
1499
|}</li>
1500
1501
<li>Check the plasticity condition and return to the plasticity surface. If necessary correct the plastic strains <math display="inline">\left[{\boldsymbol \varepsilon } _{p}\right] _{k}^{n+1}</math> at each layer (small strain plasticity) </li>
1502
1503
<li>Compute the generalized stresses
1504
1505
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1506
|-
1507
| 
1508
{| style="text-align: left; margin:auto;" 
1509
|-
1510
| style="text-align: right;" | <math>{\boldsymbol \sigma }^{n+1}_{m}  </math>
1511
| <math>  =\frac{h^{0}}{N_{L}}\sum _{k=1}^{N_{L}}\boldsymbol \sigma _{k} ^{n+1} w_{k}</math>
1512
|-
1513
| style="text-align: right;" | <math> {\boldsymbol \sigma }^{n+1}_{b}  </math>
1514
| <math>  =\frac{h^{0}}{N_{L}}\sum _{k=1}^{N_{L}}\boldsymbol \sigma _{k} ^{n+1}z_{k} w_{k} </math>
1515
|}
1516
| style="width: 5px;text-align: right;" | (99)
1517
|}</li>
1518
1519
Where <math display="inline"> w_{k}</math> is the weight of the through-the-thickness integration point. Recall that <math display="inline">z_{k}</math> is the current distance of the layer to the mid-surface and not the original distance. However, for small strain plasticity this distinction is not important.
1520
1521
This computation of stresses is adequate for an implicit scheme independent of the step size and it is exact for an elastic problem.
1522
1523
<li>Compute the residual force vector. The contribution for the <math display="inline">M</math>th element is given by
1524
1525
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1526
|-
1527
| 
1528
{| style="text-align: left; margin:auto;" 
1529
|-
1530
| style="text-align: center;" | <math>(\mathbf{r}^{M})^{n+1}=-A_{M}^{0}\left[ \begin{array}{cc} \mathbf{B}_{m}^{T} & \mathbf{B}_{b}^{T} \end{array} \right] ^{n+1}\left[ \begin{array}{c} \boldsymbol \sigma _{m}\\ \boldsymbol \sigma _{b} \end{array} \right] ^{n+1} </math>
1531
|}
1532
| style="width: 5px;text-align: right;" | (100)
1533
|}</li>
1534
1535
</ol>
1536
1537
===7.1 Tangent stiffness matrix===
1538
1539
As usual the tangent stiffness matrix is split into material and geometric components. The material tangent stiffness matrix is simply computed from the integral
1540
1541
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1542
|-
1543
| 
1544
{| style="text-align: left; margin:auto;" 
1545
|-
1546
| style="text-align: center;" | <math>\mathbf{K}^{M}=\int \int _{A^{M}}\mathbf{B}^{T}\mathbf{C}\mathbf{B}dA </math>
1547
|}
1548
| style="width: 5px;text-align: right;" | (101)
1549
|}
1550
1551
where <math display="inline">\mathbf{B}=\mathbf{B}_{m}+\mathbf{B}_{b}</math> includes:<br/>
1552
1553
*   a membrane contribution <math display="inline">\mathbf{B}_{m}</math> given by Eq.([[#eq-51|51]]) or Eq.(80).
1554
1555
*   a bending contribution <math display="inline">\mathbf{B}_{b}</math> given by Eq.([[#eq-69|69]]) or Eq.([[#eq-91|85]])  which is constant over the element.
1556
1557
<br/>
1558
1559
A three point quadrature is used for integrating the stiffness terms <math display="inline">\mathbf{B}_{m}^{T}\mathbf{C}\mathbf{B}_{m}</math> (recall that for the EBST element the membrane strains vary linearly within the element) whereas one point quadrature is chosen for the rest of the terms in <math display="inline">\mathbf{K}^{M}</math>.
1560
1561
===7.2 Geometric tangent stiffness matrix===
1562
1563
The geometric stiffness is written as
1564
1565
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1566
|-
1567
| 
1568
{| style="text-align: left; margin:auto;" 
1569
|-
1570
| style="text-align: center;" | <math>\mathbf{K}^{G}=\mathbf{K}_{m}^{G}+\mathbf{K}_{b}^{G} </math>
1571
|}
1572
| style="width: 5px;text-align: right;" | (102)
1573
|}
1574
1575
where subscripts <math display="inline">m</math> and <math display="inline">b</math> denote as usual membrane and bending contributions. For the BST element the membrane part is the same than for the standard constant strain triangle, leading to
1576
1577
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1578
|-
1579
| 
1580
{| style="text-align: left; margin:auto;" 
1581
|-
1582
| style="text-align: right;" | <math>\delta \mathbf{u}^{T}\mathbf{K}_{m}^{G}\mathbf{\;}\Delta \mathbf{u} </math>
1583
| <math> =A^{M}\sum _{i=1}^{3}\sum _{j=1}^{3}\left\{ \delta \mathbf{u}_{i}\;\left[ \begin{array}{cc} L_{i,1}^{M} & L_{i,2}^{M} \end{array} \right] \left[ \begin{array}{cc} N_{11} & N_{12}\\ N_{21} & N_{22} \end{array} \right] \left[ \begin{array}{c} L_{j,1}^{M}\\ L_{j,2}^{M} \end{array} \right] \Delta \mathbf{u}_{j}\right\} </math>
1584
|-
1585
| style="text-align: right;" | 
1586
| 
1587
|}
1588
| style="width: 5px;text-align: right;" | (103)
1589
|}
1590
1591
While for the EBST element the membrane part is computed as the sum of the contributions of the three sides, i.e.
1592
1593
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1594
|-
1595
| 
1596
{| style="text-align: left; margin:auto;" 
1597
|-
1598
| style="text-align: right;" | <math>\delta \mathbf{u}^{T}\mathbf{K}_{m}^{G}\mathbf{\;}\Delta \mathbf{u} </math>
1599
| <math> =\frac{A^{M}}{3}\sum _{k=1}^{3}\sum _{i=1}^{6}\sum _{j=1}^{6}\left\{ \delta \mathbf{u}_{i}\;\left[ \begin{array}{cc} N_{i,1}^{k} & N_{i,2}^{k} \end{array} \right] \left[ \begin{array}{cc} N_{11}^{k} & N_{12}^{k}\\ N_{21}^{k} & N_{22}^{k} \end{array} \right] \left[ \begin{array}{c} N_{j,1}^{k}\\ N_{j,2}^{k} \end{array} \right] \Delta \mathbf{u}_{j}\right\} </math>
1600
|-
1601
| style="text-align: right;" | 
1602
| 
1603
|}
1604
| style="width: 5px;text-align: right;" | (104)
1605
|}
1606
1607
where <math display="inline">N_{ij}={\sigma _{m}}_{ij}</math> are the axial forces defined in Eq.(29).
1608
1609
The geometric stiffness associated to bending moments is much more involved and can be found in  [27]. Numerical experiments have shown that the bending part of the geometric stiffness is not so important and can be disregarded in the iterative process.
1610
1611
Again three and one point quadratures are used for computing the membrane and bending contributions to the geometric stiffness matrix. We note that for elastic-plastic problems a uniform one point quadrature has been chosen for integration of both the membrane and bending stiffness matrices.
1612
1613
==8 EXPLICIT SOLUTION SCHEME==
1614
1615
For simulations including large non-linearities, such as frictional contact conditions on complex geometries or large instabilities in membranes, convergence is difficult to achieve with implicit schemes. In those cases an explicit solution algorithm is typically most advantageous. This scheme provides the solution for dynamic problems and also for static problems if an adequate damping is chosen.
1616
1617
The dynamic equations of motion to solve are of the form
1618
1619
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1620
|-
1621
| 
1622
{| style="text-align: left; margin:auto;" 
1623
|-
1624
| style="text-align: center;" | <math>\mathbf{r}(\mathbf{u}) + \mathbf{C} \dot{\mathbf{u}} + \mathbf{M} \ddot{\mathbf{u}} = 0 </math>
1625
|}
1626
| style="width: 5px;text-align: right;" | (105)
1627
|}
1628
1629
where <math display="inline">\mathbf{M}</math> is the mass matrix, <math display="inline">\mathbf{C}</math> is the damping matrix and the dot means the time derivative. The solution is performed using the ''central difference method''. To make the method competitive a diagonal (lumped) <math display="inline">\mathbf{M}</math> matrix is typically used and <math display="inline">\mathbf{C}</math> is taken proportional to <math display="inline">\mathbf{M}</math>. As usual, mass lumping is performed by assigning, one third of the triangular element mass to each node in the central element.
1630
1631
The explicit solution scheme can be summarized as follows. At each time step <math display="inline">n</math> where displacements have been computed:
1632
1633
<ol>
1634
1635
<li>Compute the internal forces <math display="inline">\mathbf{r}^{n}</math>. This simply follows the same steps (2-8) described for the implicit scheme in the previous section. </li>
1636
1637
<li>Compute the accelerations at time <math display="inline">t_{n}</math>
1638
1639
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1640
|-
1641
| 
1642
{| style="text-align: left; margin:auto;" 
1643
|-
1644
| style="text-align: center;" | <math>\ddot{\mathbf{u}}^{n} = {M}_d^{-1} [ \mathbf{r}^{n} - \mathbf{C} \dot{\mathbf{u}}^{n-1/2} ]  </math>
1645
|}
1646
|}</li>
1647
1648
where <math display="inline">{M}_d</math> is the diagonal (lumped) mass matrix.
1649
1650
<li>Compute the velocities at time <math display="inline">t_{n+1/2}</math>
1651
1652
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1653
|-
1654
| 
1655
{| style="text-align: left; margin:auto;" 
1656
|-
1657
| style="text-align: center;" | <math>\dot{\mathbf{u}}^{n+1/2} = \dot{\mathbf{u}}^{n-1/2} \ddot{\mathbf{u}}^{n} \delta t  </math>
1658
|}
1659
|}</li>
1660
1661
<li>Compute the displacements at  time <math display="inline">t_{n+1}</math>
1662
1663
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1664
|-
1665
| 
1666
{| style="text-align: left; margin:auto;" 
1667
|-
1668
| style="text-align: center;" | <math>\mathbf{u}^{n+1} = \mathbf{u}^{n} +\dot{\mathbf{u}}^{n+1/2} \delta t  </math>
1669
|}
1670
|}</li>
1671
<li>Update the shell geometry </li>
1672
<li>Check frictional contact conditions </li>
1673
</ol>
1674
1675
1676
Further details of the implementation of the standard BST element within an explicit solution scheme can be found in [26].
1677
1678
==9 EXAMPLES==
1679
1680
In this section several examples are presented to show the good performance of the rotation-free shell elements (BST, EBST and EBST1). The first five static examples are solved using an implicit code. The rest of the examples are computed using the explicit dynamic scheme. For the explicit scheme the  EBST element is always integrated using one integration point per element (EBST1 version) although not indicated.
1681
1682
===9.1 Patch tests===
1683
1684
The three elements considered (BST, EBST and EBST1) satisfy the membrane patch test defined in Figure [[#img-4|4]]. A uniform axial tensile stress is obtained in all cases.
1685
1686
<div id='img-4'></div>
1687
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1688
|-
1689
|
1690
[[File:Draft_Samper_226033773_1361_Fig4.jpeg|300px|Patch test for uniform tensile stress]]
1691
|- style="text-align: center; font-size: 75%;"
1692
| colspan="1" | '''Figure 4:''' Patch test for uniform tensile stress
1693
|}
1694
1695
<div id='img-5'></div>
1696
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1697
|-
1698
|
1699
[[File:Draft_Samper_226033773_8502_Fig5.jpeg|400px|Patch test for uniform torsion]]
1700
|- style="text-align: center; font-size: 75%;"
1701
| colspan="1" | '''Figure 5:''' Patch test for uniform torsion
1702
|}
1703
1704
The element bending formulation does not allow to apply external bending moments (there are not rotational DOFs). Hence it is not possible to analyse a patch of elements under loads leading to a uniform bending moment. A uniform torsion can be considered if a point load is applied at the corner of a rectangular plate with two consecutive free sides and two simple supported sides. Figure [[#img-5|5]] shows three patches leading to correct results both in displacements and stresses. All three patches are structured meshes. When the central node in the third patch is shifted from the center, the results obtained with the EBST and EBST1 elements are not correct. This however does not seems to preclude the excellent performance of these elements, as proved in the rest of the examples analyzed. On the other hand, the BST element  gives correct results in all torsion patch tests if natural boundary conditions are imposed in the formulation. If this is not the case, incorrect results are obtained even with structured meshes.
1705
1706
===9.2 Cook's membrane problem===
1707
1708
This example is used to assess the membrane performance of the EBST and EBST1 elements and to compare it with the standard linear triangle (constant strain) and the quadratic triangle (linear strain). This example involves important shear energy and was proposed to assess the distortion capability of elements. Figure [[#img-6|6]].a shows the geometry and the applied load. Figure [[#img-6|6]].b plots the vertical displacement of the upper vertex as a function of the number of nodes in the mesh. Results obtained with other isoparametric elements have also been  plotted for comparison. They include the constant strain triangle (CST), the bilinear quadrilateral (QUAD4) and the linear strain triangle (LST). Note that as this is a pure  membrane problem  the BST and the CST elements give identical results.
1709
1710
<div id='img-6'></div>
1711
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1712
|-
1713
|[[Image:draft_Samper_226033773-fig6a.png|300px|]]
1714
|
1715
[[File:Draft_Samper_226033773_1597_Fig6b.jpg|300px|Cook membrane problem (a) Geometry (b) Results]]
1716
|-
1717
|style="text-align: center; font-size: 75%;padding:10px;"|(a)
1718
|style="text-align: center; font-size: 75%;padding:10px;"|(b)
1719
|- style="text-align: center; font-size: 75%;"
1720
| colspan="2" | '''Figure 6:''' Cook membrane problem (a) Geometry (b) Results
1721
|}
1722
1723
From the plot shown it can be seen that the enhanced element with three integration points (EBST) gives values slightly better that the constant strain triangle for the most coarse mesh (only two elements). However, when the mesh is refined, a performance similar to the linear strain triangle is obtained that is dramatically superior that the former. On the other hand, if a one point quadrature is used (EBST1) the convergence in the reported displacement is notably better that for the rest of the elements.
1724
1725
===9.3 Cylindrical roof===
1726
1727
In this example an effective membrane interpolation is of primary importance. The geometry is a cylindrical roof supported by a rigid diaphragm at both ends and it is loaded by a uniform dead weight (see Figure [[#img-7|7]].a.). Only one quarter of the structure is modelled due to symmetry conditions. Unstructured and structured meshes are considered. In the latter case two orientations are possible (Figure [[#img-7|7]] shows orientation B).
1728
1729
Tables [[#table-3|3]], [[#table-4|4]] and [[#table-5|5]] present the normalized vertical displacements at the crown (point A) and at the midpoint of the free side (point B) for the two orientations of the structured meshes and for the non-structured mesh. Values used for normalization are <math display="inline">u_{A}=0.5407</math> y <math display="inline">u_{B}=-3.610</math> that are quoted in reference [32].
1730
1731
<div id='img-7'></div>
1732
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1733
|-
1734
|
1735
[[File:Draft_Samper_226033773_7308_Fig7a.jpeg|400px|]]
1736
|[[Image:draft_Samper_226033773-fig7b.png|400px|]]
1737
|- style="text-align: center; font-size: 75%;"
1738
| colspan="2" | '''Figure 7:''' Cylindrical roof under dead weight. <math>E=3 \times 10^{6}</math>, <math>\nu=0.0</math>, Thickness =3.0, shell weight =0.625 per unit area.
1739
|}
1740
1741
1742
<div class="center" style="font-size: 75%;">
1743
'''Table 3'''. Cylindrical roof under dead weight. Normalized vertical displacements for mesh orientation A</div>
1744
1745
{| class="wikitable" style="text-align: left; margin: 1em auto;"
1746
|- 
1747
| 
1748
| colspan='3' style="text-align: center;" | Point-A
1749
| colspan='3' style="text-align: center;" | Point-B
1750
|- 
1751
|  NDOFs 
1752
| EBST 
1753
| EBST1 
1754
| BST 
1755
| CBST 
1756
| EBST1 
1757
| BST
1758
|-
1759
| 16 
1760
| 0.65724 
1761
| 0.91855 
1762
| 0.74161 
1763
| 0.40950 
1764
| 0.70100 
1765
| 1.35230
1766
|-
1767
| 56 
1768
| 0.53790 
1769
| 1.03331 
1770
| 0.74006 
1771
| 0.54859 
1772
| 1.00759 
1773
| 0.75590
1774
|-
1775
| 208 
1776
| 0.89588 
1777
| 1.04374 
1778
| 0.88491 
1779
| 0.91612 
1780
| 1.02155 
1781
| 0.88269
1782
|-
1783
| 800 
1784
| 0.99658 
1785
| 1.01391 
1786
| 0.96521 
1787
| 0.99263 
1788
| 1.00607 
1789
| 0.96393
1790
|- 
1791
| 3136 
1792
| 1.00142 
1793
| 1.00385 
1794
| 0.99105 
1795
| 0.99881 
1796
| 1.00102 
1797
| 0.98992
1798
1799
|}
1800
1801
1802
<div class="center" style="font-size: 75%;">
1803
'''Table 4'''. Cylindrical roof under dead weight. Normalized vertical displacements for mesh orientation B</div>
1804
1805
{| class="wikitable" style="text-align: left; margin: 1em auto;"
1806
|- 
1807
| 
1808
| colspan='3' style="text-align: center;" | Point-A
1809
| colspan='3' style="text-align: center;" | Point-B
1810
|- 
1811
|  NDOFs 
1812
| EBST 
1813
| EBST1 
1814
| BST 
1815
| CBST 
1816
| EBST1 
1817
| BST
1818
|-
1819
| 16 
1820
| 0.26029 
1821
| 0.83917 
1822
| 0.40416 
1823
| 0.52601 
1824
| 0.86133 
1825
| 0.89778
1826
|-
1827
| 56 
1828
| 0.81274 
1829
| 1.10368 
1830
| 0.61642 
1831
| 0.67898 
1832
| 0.93931 
1833
| 0.68238
1834
|-
1835
| 208 
1836
| 0.97651 
1837
| 1.04256 
1838
| 0.85010 
1839
| 0.93704 
1840
| 1.00255 
1841
| 0.86366
1842
|-
1843
| 800 
1844
| 1.00085 
1845
| 1.01195 
1846
| 0.95626 
1847
| 0.99194 
1848
| 1.00211 
1849
| 0.95864
1850
|- 
1851
| 3136 
1852
| 1.00129 
1853
| 1.00337 
1854
| 0.98879 
1855
| 0.99828 
1856
| 1.00017 
1857
| 0.98848
1858
1859
|}
1860
1861
1862
<div class="center" style="font-size: 75%;">
1863
'''Table 5'''. Cylindrical roof under dead weight. Normalized vertical displacements for non-structured mesh</div>
1864
1865
{| class="wikitable" style="text-align: left; margin: 1em auto;"
1866
|- 
1867
| 
1868
| colspan='3' style="text-align: center;" | Point-A
1869
| colspan='3' style="text-align: center;" | Point-B
1870
|- 
1871
|  NDOFs 
1872
| EBST 
1873
| EBST1 
1874
| BST 
1875
| EBST 
1876
| EBST1 
1877
| BST
1878
|-
1879
| 851 
1880
| 0.97546 
1881
| 0.8581 
1882
| 0.97598 
1883
| 0.97662 
1884
| 1.0027 
1885
| 0.97194
1886
|-
1887
| 3311 
1888
| 0.98729 
1889
| 0.9682 
1890
| 0.98968 
1891
| 0.98476 
1892
| 1.0083 
1893
| 0.98598
1894
|- 
1895
| 13536 
1896
| 0.99582 
1897
| 0.9992 
1898
| 1.00057 
1899
| 0.99316 
1900
| 0.9973 
1901
| 0.99596
1902
1903
|}
1904
1905
Plots in Figure [[#img-7|7]].b show the normalized displacement of point-B for structured meshes as a function of the number of degrees of freedom for each case studied. An excellent convergence for the EBST element can be seen. The version with only one integration point (EBST1) presents a behavior a little more flexible and converges from above for structured meshes. Table [[#table-5|5]] shows that both the EBST and the EBST1 elements have an excellent behavior for non structured meshes.
1906
1907
===9.4 Open semi-spherical dome with point loads===
1908
1909
The main problem of finite elements with initially curved geometry is the so called membrane locking. The EBST element  has a quadratic interpolation of the geometry, then it may suffer from this problem. To assess this we resort to an example of inextensional bending. This is an hemispherical shell of radius <math display="inline">r=10</math> and thickness <math display="inline">h=0.04</math> with an 18<math display="inline">^{o}</math> hole in the pole and free at all boundaries, subjected to two inward and two outward forces 90<math display="inline">^{o}</math> apart. Material properties are <math display="inline">E=6.825\times{10}^{7}</math> and <math display="inline">\nu=0.3</math>. Figure [[#img-8|8]].a shows the discretized geometry (only one quarter of the geometry is considered due to symmetry).
1910
1911
<div id='img-8'></div>
1912
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1913
|-
1914
|
1915
[[File:Draft_Samper_226033773_7798_Fig8a.jpeg|400px|]]
1916
|[[Image:draft_Samper_226033773-fig8b.png|400px|]]
1917
|-
1918
|style="text-align: center; font-size: 75%;"|(a)
1919
|style="text-align: center; font-size: 75%;"|(b)
1920
|- style="text-align: center; font-size: 75%;"
1921
| colspan="2" | '''Figure 8:''' Pinched hemispherical shell with a hole, (a) geometry, (b) normalized displacement
1922
|}
1923
1924
In Figure [[#img-8|8]].b the displacements of the points under the loads have been plotted versus the number of nodes used in the discretization. Due to the orientation of the meshes chosen, the displacement of the point under the inward load is not the same as the displacement under the outward load, so in the figure an average (the absolute values) has been used. Results obtained with other elements have been included for comparison: two membrane locking free elements, namely the original linear BST element and a transverse shear-deformable quadrilateral (QUAD) [33]; a transverse shear deformable quadratic triangle (TRIA) [2] that is vulnerable to locking and an assumed strain quadratic triangle (TRIC) [3] that does not exhibit membrane locking.
1925
1926
From the plotted results it can be seen that the EBST element presents slight membrane locking in bending dominated problems with initially curved geometries. This locking is much less severe than in a standard quadratic triangle. Membrane locking disappears when only one integration point is used (EBST1 element).
1927
1928
===9.5 Inflation of a sphere===
1929
1930
The example is the inflation of a spherical shell under internal pressure. An incompressible Mooney-Rivlin constitutive material has been considered. The Ogden parameters are <math display="inline">N=2</math>, <math display="inline">\alpha _{1}=2</math>, <math display="inline">\mu _{1}=40</math>, <math display="inline">\alpha _{2}=-2</math>, <math display="inline">\mu _{2}=-20</math>. Due to the simple geometry an analytical solution exists [34] (with <math display="inline">\gamma =R/R^{0}</math>):
1931
1932
{| class="formulaSCP" style="width: 100%; text-align: left;" 
1933
|-
1934
| 
1935
{| style="text-align: left; margin:auto;" 
1936
|-
1937
| style="text-align: center;" | <math> p=\frac{h^{0}}{R^{0}\gamma ^{2}}\frac{dW}{d\gamma }=\frac{8h^{0} }{R^{0}\gamma ^{2}} \left( \gamma ^{6}-1\right) \left( \mu _{1}-\mu _{2}\gamma ^{2}\right) </math>
1938
|}
1939
|}
1940
1941
In this numerical simulation the same geometric and material parameters used in Ref. <span id='citeF-23'></span>[[#cite-23|[23]]] have been adopted: <math display="inline">R^{0}=1</math> and <math display="inline">h^{0}=0.02</math>. The three meshes of EBST1 element considered to evaluate convergence are shown in Figure [[#img-9|9]].a. The value of the actual radius as a function of the internal pressure is plotted in Figure [[#img-9|9]].b for the different meshes and is also compared with the analytical solution. It can be seen that with a few degrees of freedom it is possible to obtain an excellent agreement for the range of strains considered. The final value corresponds to a  ratio of <math display="inline">h/R=0.00024</math>.
1942
1943
<div id='img-9'></div>
1944
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1945
|-
1946
|[[File:Draft_Samper_226033773_5233_Fig9.jpeg|500px|]]
1947
|-
1948
| style="text-align: center; font-size: 75%;|(a)
1949
|-
1950
|[[File:Draft_Samper_226033773_9697_Fig9b.jpeg|300px|]]
1951
|-
1952
| style="text-align: center; font-size: 75%;|(b)
1953
|- style="text-align: center; font-size: 75%;padding-top:10px;"
1954
| '''Figure 9:''' Inflation of sphere of Mooney-Rivlin material. (a) Meshes of EBST1 elements used in the analysis (b) Change of radius as a function of the internal pressure.
1955
|}
1956
1957
===9.6 Clamped spherical dome under impulse pressure loading===
1958
1959
The geometry of the dome and the material properties chosen are shown in Figure [[#img-10|10]]. A uniform pressure load of 600 psi is applied to the upper surface of the dome. The different meshes used in the analysis are shown in Figure [[#img-11|11]]. One fourth of the dome is considered only due to symmetry. Two different analyses under elastic and elastic-plastic conditions were carried out. The number of thickness layers in eq.([[#eq-96|96]]) is four. Numerical experiments show that this suffice to provide an accurate solution for large elastic-plastic problems [26]. Results are obtained using the explicit scheme.
1960
1961
<div id='img-10'></div>
1962
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1963
|-
1964
|[[Image:draft_Samper_226033773-fig10.png|400px|Spherical dome under impulse pressure. Geometry and material]]
1965
|- style="text-align: center; font-size: 75%;"
1966
| colspan="1" | '''Figure 10:''' Spherical dome under impulse pressure. Geometry and material
1967
|}
1968
1969
<div id='img-11'></div>
1970
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1971
|-
1972
|
1973
[[File:Draft_Samper_226033773_1877_Fig11.jpeg|600px|Spherical dome under impulse pressure. Meshes used in the analysis. Mesh-1 with 338 elements, Mesh-2 with 1250 elements, and Mesh-3 with 2888 elements]]
1974
|- style="text-align: center; font-size: 75%;"
1975
| colspan="1" | '''Figure 11:''' Spherical dome under impulse pressure. Meshes used in the analysis. Mesh-1 with 338 elements, Mesh-2 with 1250 elements, and Mesh-3 with 2888 elements
1976
|}
1977
1978
1979
Figure [[#img-12|12]] shows results for the time history of the central deflection using different meshes and ''elastic material properties'' for both  BST and EBST1 elements. Results are almost identical for mesh-2 and mesh-3, showing the excellent convergence properties. The coarsest mesh shows some differences between both elements, but for the finer meshes the results are almost identical. Figure [[#img-13|13]] shows similar results but now for an ''elastic-plastic material''. The excellent convergence  of the BST and EBST elements is again noticeable.
1980
1981
1982
<div id='img-12'></div>
1983
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1984
|-
1985
|[[Image:draft_Samper_226033773-fig12.png|400px|Spherical dome under impulse pressure. History of central deflection for elastic material]]
1986
|- style="text-align: center; font-size: 75%;"
1987
| colspan="1" | '''Figure 12:''' Spherical dome under impulse pressure. History of central deflection for elastic material
1988
|}
1989
1990
<div id='img-13'></div>
1991
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
1992
|-
1993
|[[Image:draft_Samper_226033773-fig13.png|400px|Spherical dome under impulse pressure. History of central deflection for elastic-plastic material]]
1994
|- style="text-align: center; font-size: 75%;"
1995
| colspan="1" | '''Figure 13:''' Spherical dome under impulse pressure. History of central deflection for elastic-plastic material
1996
|}
1997
1998
1999
Results obtained with the present elements compare very well with published results using fine meshes. See for example ABAQUS Explicit example problems manual <span id='citeF-35'></span>[[#cite-35|[35]]] and WHAMS-3D manual [36], showing plots comparing results using different shell elements.
2000
2001
A summary of results for the central deflection at significant times is given in Tables [[#table-6|6]] and [[#table-7|7]]. Further details on the solution of this problem with the standard  BST element can be found in [26].
2002
2003
<div class="center" style="font-size: 75%;">
2004
'''Table 6'''. Spherical dome. Elastic material. Comparison of the central deflection values at the mid point obtained with the BST and EBST1  elements for different meshes</div>
2005
2006
{| class="wikitable" style="text-align: right; margin: 1em auto;"
2007
|- style="border-top: 2px solid;"
2008
| style="text-align: left;border-left: 2px solid;border-right: 2px solid;" |  Element/mesh 
2009
| style="border-left: 2px solid;border-right: 2px solid;" | <math>t = 0.2 ms</math>
2010
| style="border-left: 2px solid;border-right: 2px solid;" | <math>t = 0.4 ms</math>
2011
| style="border-left: 2px solid;border-right: 2px solid;" | <math>t = 0.6 ms</math>
2012
| style="border-left: 2px solid;border-right: 2px solid;" | <math>t = 0.8 ms</math>
2013
|- style="border-top: 2px solid;"
2014
| style="text-align: left;border-left: 2px solid;border-right: 2px solid;" |   BST Coarse  
2015
| style="border-left: 2px solid;border-right: 2px solid;" | -0.05155 
2016
| style="border-left: 2px solid;border-right: 2px solid;" | -0.09130 
2017
| style="border-left: 2px solid;border-right: 2px solid;" | 0.04414 
2018
| style="border-left: 2px solid;border-right: 2px solid;" | -0.08945 
2019
|-
2020
| style="text-align: left;border-left: 2px solid;border-right: 2px solid;" | BST Medium  
2021
| style="border-left: 2px solid;border-right: 2px solid;" | -0.04542 
2022
| style="border-left: 2px solid;border-right: 2px solid;" | -0.09177 
2023
| style="border-left: 2px solid;border-right: 2px solid;" | 0.03863 
2024
| style="border-left: 2px solid;border-right: 2px solid;" | -0.08052 
2025
|-
2026
| style="text-align: left;border-left: 2px solid;border-right: 2px solid;" | BST Fine    
2027
| style="border-left: 2px solid;border-right: 2px solid;" | -0.04460 
2028
| style="border-left: 2px solid;border-right: 2px solid;" | -0.09022 
2029
| style="border-left: 2px solid;border-right: 2px solid;" | 0.03514 
2030
| style="border-left: 2px solid;border-right: 2px solid;" | -0.08132 
2031
|- style="border-top: 2px solid;"
2032
| style="text-align: left;border-left: 2px solid;border-right: 2px solid;" |  EBST1 Coarse  
2033
| style="border-left: 2px solid;border-right: 2px solid;" | -0.05088 
2034
| style="border-left: 2px solid;border-right: 2px solid;" | -0.08929 
2035
| style="border-left: 2px solid;border-right: 2px solid;" | 0.04348 
2036
| style="border-left: 2px solid;border-right: 2px solid;" | -0.08708 
2037
|-
2038
| style="text-align: left;border-left: 2px solid;border-right: 2px solid;" | EBST1 Medium  
2039
| style="border-left: 2px solid;border-right: 2px solid;" | -0.04527 
2040
| style="border-left: 2px solid;border-right: 2px solid;" | -0.09134 
2041
| style="border-left: 2px solid;border-right: 2px solid;" | 0.03865 
2042
| style="border-left: 2px solid;border-right: 2px solid;" | -0.07979 
2043
|- style="border-bottom: 2px solid;"
2044
| style="text-align: left;border-left: 2px solid;border-right: 2px solid;" | EBST1 Fine    
2045
| style="border-left: 2px solid;border-right: 2px solid;" | -0.04453 
2046
| style="border-left: 2px solid;border-right: 2px solid;" | -0.09004 
2047
| style="border-left: 2px solid;border-right: 2px solid;" | 0.03510 
2048
| style="border-left: 2px solid;border-right: 2px solid;" | -0.08099 
2049
2050
|}
2051
2052
<div class="center" style="font-size: 75%;">
2053
'''Table 7'''. Spherical dome. Elastic-plastic material. Comparison of the central deflection values at the mid point obtained with the BST and EBST1  elements for different meshes</div>
2054
2055
{| class="wikitable" style="text-align: left; margin: 1em auto;"
2056
|- style="border-top: 2px solid;"
2057
| Element/mesh 
2058
| <math>t = 0.2 ms</math>
2059
| <math>t = 0.4 ms</math>
2060
| <math>t = 0.6 ms</math>
2061
| <math>t = 0.8 ms</math>
2062
|- style="border-top: 2px solid;"
2063
|   BST Coarse  
2064
| -0.05888 
2065
| -0.05869 
2066
| -0.02938 
2067
| -0.06521 
2068
|-
2069
| BST Medium  
2070
| -0.05376 
2071
| -0.06000 
2072
| -0.02564 
2073
| -0.06098 
2074
|-
2075
| BST Fine    
2076
| -0.05312 
2077
| -0.05993 
2078
| -0.02464 
2079
| -0.06105 
2080
|- style="border-top: 2px solid;"
2081
|  EBST1 Coarse  
2082
| -0.05827 
2083
| -0.05478 
2084
| -0.02792 
2085
| -0.06187 
2086
|-
2087
| EBST1 Medium  
2088
| -0.05374 
2089
| -0.05884 
2090
| -0.02543 
2091
| -0.06080 
2092
|- style="border-bottom: 2px solid;"
2093
| EBST1 Fine    
2094
| -0.05317 
2095
| -0.05935 
2096
| -0.02458 
2097
| -0.06085 
2098
2099
|}
2100
2101
===9.7 Cylindrical panel under impulse loading===
2102
2103
The geometry of the cylinder and the material properties are shown in Figure [[#img-14|14]]. A prescribed initial normal velocity of <math display="inline">v_{o}=-5650</math> in/sec is applied to the points in the region shown modelling the effect of the detonation of an explosive layer. The panel is assumed clamped along all the boundary. One half of the cylinder is discretized only due to symmetry conditions. Three different meshes of <math display="inline">6\times{12}</math>, <math display="inline">12\times{32}</math> and <math display="inline">18\times{48}</math>  BST elements were used for the analysis. The deformed configurations for <math display="inline">time =1 msec</math> are shown for the three meshes in Figure [[#img-15|15]].
2104
2105
<div id='img-14'></div>
2106
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2107
|-
2108
|[[Image:draft_Samper_226033773-fig14.png|400px|Cylindrical panel under impulse loading. Geometry and material properties]]
2109
|- style="text-align: center; font-size: 75%;"
2110
| colspan="1" | '''Figure 14:''' Cylindrical panel under impulse loading. Geometry and material properties
2111
|}
2112
2113
<div id='img-15'></div>
2114
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2115
|-
2116
|[[Image:draft_Samper_226033773-fig15.png|600px|Impulsively loaded cylindrical panel. Deformed meshes for time =1 msec]]
2117
|- style="text-align: center; font-size: 75%;"
2118
| colspan="1" | '''Figure 15:''' Impulsively loaded cylindrical panel. Deformed meshes for <math>time =1 msec</math>
2119
|}
2120
2121
2122
The analysis was performed assuming an elastic-perfect plastic material behaviour (<math display="inline">\sigma _y = k</math> <math display="inline">k'=0</math>). A study of the convergence of the solution with the number of thickness layers showed again that four layers suffice to capture accurately the non linear material response [26].
2123
2124
A comparison of the results obtained with both elements using the coarse mesh and the finer mesh is shown in Figure [[#img-16|16]] where experimental results reported in <span id='citeF-37'></span>[[#cite-37|[37]]] have also been plotted for comparison purposes. Good agreement between the numerical and experimental results is obtained. Figure [[#img-16|16]] show the time evolution of the vertical displacement of two reference points along the center line located at <math display="inline">y=6.28</math>in and <math display="inline">y=9.42</math>in, respectively. For the finer mesh results between both elements are almost identical. For the coarse mesh it can been seen again that the element BST is more flexible than element EBST1.
2125
2126
2127
<div id='img-16'></div>
2128
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2129
|-
2130
|[[Image:draft_Samper_226033773-fig16.png|400px|Cylindrical panel under impulse loading. Time evolution of the displacement of two points along the crown line. Comparison of results obtained with BST and EBST1 elements (mesh 1: 6×12 elements and mesh 3: 18×48 elements) and experimental values ]]
2131
|- style="text-align: center; font-size: 75%;"
2132
| colspan="1" | '''Figure 16:''' Cylindrical panel under impulse loading. Time evolution of the displacement of two points along the crown line. Comparison of results obtained with BST and EBST1 elements (mesh 1: <math>6\times{12}</math> elements and mesh 3: <math>18\times{48}</math> elements) and experimental values 
2133
|}
2134
2135
2136
The numerical values of the vertical displacement at the two reference points obtained with the BST and EBST1  elements after a time of 0.4 ms using the <math display="inline">16\times{32}</math> mesh are compared in Table [[#table-8|8]]  with a numerical solution obtained by Stolarski ''et al.'' [38] using a curved triangular shell element and the <math display="inline">16\times{32}</math> mesh. Experimental results reported in [37] are also given for comparison. It is interesting to note the reasonable agreement of the results for <math display="inline">y=6.28</math>in. and the discrepancy of present and other published numerical solutions with the experimental value for <math display="inline">y=9.42</math>in.
2137
2138
2139
<div class="center" style="font-size: 75%;">
2140
'''Table 8'''. Cylindrical panel under impulse load. Comparison of vertical displacement values of two central points for <math>t=0.4</math> ms</div>
2141
2142
{| class="wikitable" style="text-align: left; margin: 1em auto;"
2143
|- style="border-top: 2px solid;"
2144
| 
2145
| colspan='2' style="text-align: left;" | Vertical displacement (in.)
2146
|- 
2147
|  element/mesh                
2148
| <math>y=6.28</math>in 
2149
| <math>y=9.42</math>in 
2150
|- 
2151
|  BST  (<math display="inline"> 6\times 12</math> el.)    
2152
| -1.310     
2153
| -0.679      
2154
|-
2155
| BST  (<math display="inline">18\times 48</math> el.)    
2156
| -1.181     
2157
| -0.587      
2158
|-
2159
| EBST1 (<math display="inline"> 6\times 12</math> el.)    
2160
| -1.147     
2161
| -0.575      
2162
|-
2163
| EBST1 (<math display="inline">18\times 48</math> el.)    
2164
| -1.171     
2165
| -0.584      
2166
|-
2167
| Stolarski ''et al.'' [38] 
2168
| -1.183     
2169
| -0.530      
2170
|- 
2171
| Experimental [37] 
2172
| -1.280     
2173
| -0.700      
2174
2175
|}
2176
2177
2178
The deformed shapes of the transverse section for <math display="inline">y=6.28</math>in. and the longitudinal section for <math display="inline">x=0</math> obtained with the both elements for the coarse and the fine meshes after 1ms. are compared with the experimental results in Figures [[#img-17|17]] and [[#img-18|18]].  Excellent agreement is observed for the fine mesh for both elements.
2179
2180
2181
<div id='img-17'></div>
2182
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2183
|-
2184
|[[Image:draft_Samper_226033773-fig17.png|400px|Cylindrical panel under impulse loading. Final deformation (t=1 msec) of the panel at the cross section y=6.28 in Comparison with experimental values. ]]
2185
|- style="text-align: center; font-size: 75%;"
2186
| colspan="1" | '''Figure 17:''' Cylindrical panel under impulse loading. Final deformation (<math>t=1 msec</math>) of the panel at the cross section <math>y=6.28 in</math> Comparison with experimental values. 
2187
|}
2188
2189
<div id='img-18'></div>
2190
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2191
|-
2192
|[[Image:draft_Samper_226033773-fig18.png|550px|Cylindrical panel under impulse loading. Final deformation (t=1 msec) of the panel at the crown line (x=0.00 in). Comparison with experimental values. ]]
2193
|- style="text-align: center; font-size: 75%;"
2194
| colspan="1" | '''Figure 18:''' Cylindrical panel under impulse loading. Final deformation (<math>t=1 msec</math>) of the panel at the crown line (<math>x=0.00 in</math>). Comparison with experimental values. 
2195
|}
2196
2197
===9.8 Airbag Membranes===
2198
2199
'''Inflation/deflation of a circular airbag'''
2200
2201
This example has been taken from Ref.[23] where it is shown that the final configuration is mesh dependent due to the strong instabilities leading to a non-uniqueness of the solution. In [23]  it is also discussed the important regularizing properties of the bending energy, that when disregarded leads to massive wrinkling in the compressed zones.
2202
2203
The airbag geometry is initially circular with an undeformed radius of <math display="inline">0.35</math>.  The constitutive material is a linear isotropic elastic one with modulus of elasticity <math display="inline">E=6\times 10^{7}</math>Pa, Poisson's ratio <math display="inline">\nu =0.3</math> and density <math display="inline">\rho = 2000</math>kg/m<math display="inline">^3</math>.  Arbitrarily only one quarter of the geometry has been modelled.  Only the normal displacement to the original plane is constrained along the boundaries.  The thickness considered is <math display="inline">h=0.0004</math>m and the inflation pressure is <math display="inline">5000</math>Pa. Pressure is linearly increased from 0 to the final value in <math display="inline">t=0.15</math> sec.
2204
2205
Figure 19 shows the final deformed configurations for a mesh with 10201 nodes and 20000 EBST1 elements.  The figure on the left (a) corresponds to an analysis including full bending effects and the right figure (b) is a pure membrane analysis.
2206
2207
We note that when the bending energy is included a more regular final pattern is obtained.  Also the final pattern is rather independent of the discretization (note that the solution is non unique due to the strong instabilities), and a massive wrinkling appears in the center of the modelled region.  On the other hand, the pure membrane solution shows a wrinkling pattern where the width of the wrinkle is the length of the element.
2208
2209
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2210
|-
2211
|
2212
[[File:Draft_Samper_226033773_2615_Fig19.jpeg|600px|Inflation of a circular airbag. Deformed configurations for final pressure. (a) bending formulation; (b) membrane formulation.]]
2213
|- style="text-align: center; font-size: 75%;"
2214
| colspan="1" | '''Figure 19:''' Inflation of a circular airbag. Deformed configurations for final pressure. (a) bending formulation; (b) membrane formulation.
2215
|}
2216
2217
Figure 20 shows the results obtained for the de-inflation process.  Note that the spherical membrane falls down due to the self weight.  The final configuration is of course non-unique.
2218
2219
<div id='img-20'></div>
2220
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2221
|-
2222
|
2223
[[File:Draft_Samper_226033773_9407_Fig20.jpeg|600px|Inflation and deflation of a circular air-bag.]]
2224
|- style="text-align: center; font-size: 75%;"
2225
| colspan="1" | '''Figure 20:''' Inflation and deflation of a circular air-bag.
2226
|}
2227
2228
2229
The next problem is the study of the inflating and de-inflating of a tube with a semi-spherical end cap.  The tube diameter is <math display="inline">D=1</math>, its total length is <math display="inline">L=5</math>m and the thickness <math display="inline">h=5\times 10^{-3}</math>m.  The material has the following properties <math display="inline">E=4\times 10^{8}</math>Pa, <math display="inline">\nu =0.35 </math>, <math display="inline">\rho =5\times 10^{4}</math>kg/m<math display="inline">^3</math>.  The tube is inflated fast until a pressure of <math display="inline">10^4</math> and then is de-inflated under self weight.  The analysis is performed with a mesh of 4176 EBST1 elements and 2163 nodes modelling a quarter of the geometry.  The evolution of the tube walls during the de-inflating process can be seen in Figure 21.  Note that the central part collapses as expected, while the semi-spherical cap remains unaltered.
2230
2231
2232
<div id='img-21'></div>
2233
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2234
|-
2235
|
2236
[[File:Draft_Samper_226033773_9850_Fig21.jpeg|600px|Inflation and deflation of a closed  tube. L=5, D=1, h=5×10⁻³.]]
2237
|- style="text-align: center; font-size: 75%;"
2238
| colspan="1" | '''Figure 21:''' Inflation and deflation of a closed  tube. <math>L=5</math>, <math>D=1</math>, <math>h=5\times 10^{-3}</math>.
2239
|}
2240
2241
2242
The same analysis is repeated for a longer and thinner tube (<math display="inline">L=6</math>m and <math display="inline">h=3\times 10^{-3}</math>m).  The same material than in the previous case was chosen with a higher density (<math display="inline">\rho =7.5\times 10^{4}</math>kg/m<math display="inline">^3</math>).  The evolution of the tube walls is shown in Figure 22.  Note that the central part collapses again but in a less smoother manner due to the smaller thickness.
2243
2244
2245
<div id='img-22'></div>
2246
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2247
|-
2248
|
2249
[[File:Draft_Samper_226033773_8861_Fig22.jpeg|600px|Inflation and deflation of a closed  tube. L=6, D=1, h=3×10⁻³.]]
2250
|- style="text-align: center; font-size: 75%;"
2251
| colspan="1" | '''Figure 22:''' Inflation and deflation of a closed  tube. <math>L=6</math>, <math>D=1</math>, <math>h=3\times 10^{-3}</math>.
2252
|}
2253
2254
2255
The last example of this kind is the inflation of a square airbag supporting a spherical object.  This example resembles a problem studied (numerically and experimentally) in Ref.[39], where fluid-structure interaction is the main subject.  Here the fluid is not modelled, and a uniform pressure is applied over all the internal surfaces.  The lower surface part of the airbag is limited by a rigid plane and on the upper part a spherical dummy object is set to study the interaction between the airbag and the object.
2256
2257
The airbag geometry is initially square with an undeformed side length of 0.643m.  The constitutive material chosen is a linear isotropic elastic one with <math display="inline">E=5.88\times 10^8</math>Pa, <math display="inline">\nu =0.4</math> and a density of <math display="inline">\rho = 1000</math> kg/m<math display="inline">^3</math>.  Only one quarter of the geometry has been modelled due to symmetry.  The thickness <math display="inline">h=0.00075</math>m and the inflation pressure is 250000Pa.  Pressure is linearly incremented from 0 to the final value in <math display="inline">t=0.15</math>sec.  The spherical object has a radius <math display="inline">r=0.08</math>m and a mass of 4.8kg (one quarter), and is subjected to gravity load during all the process.
2258
2259
The mesh includes 8192 EBST1 elements and 4225 nodes on each surface of the airbag.  Figure 23 shows the deformed configurations for three different times.  The sequence on the left of the figure corresponds to an analysis including full bending effects and the sequence on the right is the result of a pure membrane analysis.  A standard penalty formulation is used for frictionless contact.
2260
2261
2262
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2263
|-
2264
|
2265
[[File:Draft_Samper_226033773_6495_fig23.jpg|600px|Inflation of a square airbag against an spherical object. Deformed configurations for different times. Left figure: results obtained with the full bending formulation. Right figure: results obtained with a pure membrane solution.]]
2266
|- style="text-align: center; font-size: 75%;"
2267
| colspan="1" | '''Figure 23:''' Inflation of a square airbag against an spherical object. Deformed configurations for different times. Left figure: results obtained with the full bending formulation. Right figure: results obtained with a pure membrane solution.
2268
|}
2269
2270
===9.9 S-rail sheet stamping===
2271
2272
The final problem corresponds to one of the sheet stamping benchmark tests proposed in NUMISHEET'96 <span id='citeF-40'></span>[[#cite-40|[40]]].  The analysis comprises two parts, namely, stamping of a S-rail sheet component and springback computations once the stamping tools are removed.  Figure [[#img-24|24]] shows the deformed sheet after springback.
2273
2274
<div id='img-24'></div>
2275
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2276
|-
2277
|[[Image:draft_Samper_226033773-fig_n1.png|600px|Stamping of a S-rail. Final deformation of the sheet after springback obtained in the simulation. The triangular mesh of the deformed sheet is also shown]]
2278
|- style="text-align: center; font-size: 75%;"
2279
| colspan="1" | '''Figure 24:''' Stamping of a S-rail. Final deformation of the sheet after springback obtained in the simulation. The triangular mesh of the deformed sheet is also shown
2280
|}
2281
2282
The detailed geometry and material data can be found in the proceedings of the conference <span id='citeF-40'></span>[[#cite-40|[40]]] or in the web <span id='citeF-41'></span>[[#cite-41|[41]]]. The mesh used for the sheet has 6000 three  node triangular elements and 3111 points (Figure 24). The tools are treated as rigid bodies. The meshes used for the sheet and the tools are those provided by the  benchmark organizers. The material considered here is a mild steel (IF) with Young Modulus <math display="inline">E=2.06 GPa</math> and Poisson ratio <math display="inline">\nu=0.3</math>. Mises yield criterion was used for plasticity behaviour with non-linear isotropic hardening defined by <math display="inline">k(e^p) = 545(0.13+e^p)^{0.267} [MPa]</math>. A uniform friction of 0.15 was used for all the tools. A low (10kN) blank holder force was considered in this simulation.
2283
2284
Figure [[#img-25|25]] compares the punch force during the stamping stage obtained with both BST and EBST1 elements for the simulation and experimental values. Also for reference the average values of the simulations presented in the conference are included. Explicit and implicit simulations are considered as different curves. There is a remarkable coincidence between the experimental values and the results obtained with BST and EBST1 elements.
2285
2286
<div id='img-25'></div>
2287
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2288
|-
2289
|[[Image:draft_Samper_226033773-fig_n2.png|500px|Stamping of a S-rail. Punch force versus punch travel. Average of explicit and implicit results reported at the benchmark are also shown. ]]
2290
|- style="text-align: center; font-size: 75%;"
2291
| colspan="1" | '''Figure 25:''' Stamping of a S-rail. Punch force versus punch travel. Average of explicit and implicit results reported at the benchmark are also shown. 
2292
|}
2293
2294
Figure [[#img-26|26]] plots the <math display="inline">Z</math> coordinate along line B"&#8211;G" after springback stage. The top surface of the sheet does not remain plane due to some instabilities for the low blank holder force used. Results obtained with the simulations compare very well with the experimental results.
2295
2296
<div id='img-26'></div>
2297
{| style="text-align: center; border: 1px solid #BBB; margin: 1em auto; width: 100%;max-width: 100%;"
2298
|-
2299
|[[Image:draft_Samper_226033773-fig_n3.png|500px|Stamping of a S-rail. Z-coordinate along line B''&#8211;-G'' after springback. Average of explicit and implicit results reported at the benchmark are also shown. ]]
2300
|- style="text-align: center; font-size: 75%;"
2301
| colspan="1" | '''Figure 26:''' Stamping of a S-rail. Z-coordinate along line B''&#8211;-G'' after springback. Average of explicit and implicit results reported at the benchmark are also shown. 
2302
|}
2303
2304
==10 CONCLUDING REMARKS==
2305
2306
We have presented in the paper two alternative formulations for the rotation-free basic shell triangle (BST) using an assumed strain approach.  The simplest element of the family is based on an assumed constant curvature field expressed in terms of the nodal deflections of a patch of four elements and a constant membrane field computed from the standard linear interpolation of the displacements within each triangle. An enhanced version of the element is obtained by using a quadratic interpolation of the geometry in terms of the six patch nodes.  This allows to compute an assumed linear membrane strain field which improves the in-plane behaviour of the original element.  A simple and economic version of the element using a single integration point has been presented.  The efficiency of the different rotation-free shell triangles has been demonstrated in many examples of application including linear and non linear analysis of shells under static and dynamic loads, the inflation and de-inflation of membranes and a sheet stamping problem.
2307
2308
The enhanced rotation-free basic shell triangle element with a single integration point (the EBST1 element) has proven to be an excellent candidate for solving practical engineering shell and membrane problems involving complex geometry, dynamics, material non linearity and frictional contact conditions.
2309
2310
==ACKNOWLEDGEMENTS==
2311
2312
The problems analyzed with the explicit formulation were solved with the computer code STAMPACK <span id='citeF-42'></span>[[#cite-42|[42]]] where the rotation-free elements here presented have been implemented.  The support of the company QUANTECH (www.quantech.es) providing the code STAMPACK is gratefully acknowledged.
2313
2314
==REFERENCES==
2315
2316
<div id="cite-1"></div>
2317
[[#citeF-1|[1]]] E. Oñate. A review of some finite element families for thick and thin plate and shell analysis. Publication CIMNE N.53, May 1994.
2318
2319
<div id="cite-2"></div>
2320
[2] F.G. Flores, E. Oñate and F. Zárate. New assumed strain triangles for non-linear shell analysis. ''Computational Mechanics'', 17, 107&#8211;114, 1995.
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2323
[3] J.H. Argyris, M. Papadrakakis, C. Aportolopoulun and S. Koutsourelakis. The TRIC element. Theoretical and numerical investigation. ''Comput. Meth. Appl. Mech. Engrg.'', 182, 217&#8211;245, 2000.
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2326
[4] O.C. Zienkiewicz and R.L. Taylor. ''The finite element method. Solid Mechanics''. Vol II, Butterworth-Heinemann, 2000.
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<div id="cite-5"></div>
2329
[5]  H. Stolarski, T. Belytschko and S.-H. Lee. A review of shell finite elements and corotational theories. ''Computational Mechanics Advances'', Vol. 2 N.2, North-Holland, 1995.
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[[#citeF-6|[6]]]  E. Ramm and W.A. Wall. Shells in advanced computational environment. In ''V World Congress on Computational Mechanics'', J. Eberhardsteiner, H. Mang and F. Rammerstorfer (Eds.), Vienna, Austria, July 7&#8211;12, 2002. http://wccm.tuwien.ac.at.
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2335
[[#citeF-7|[7]]]  D. Bushnell and B.O. Almroth, “Finite difference energy method for non linear shell analysis”, ''J. Computers and Structures'', Vol. 1, 361, 1971.
2336
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<div id="cite-8"></div>
2338
[8] S.P. Timoshenko. ''Theory of Plates and Shells'', McGraw Hill, New York, 1971.
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2341
[[#citeF-9|[9]]] A.C. Ugural. ''Stresses in  Plates and Shells'', McGraw Hill, New York, 1981.
2342
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2344
[10] R.A. Nay and S. Utku. An alternative to the finite element method. ''Variational Methods Eng.'', Vol. 1, 1972.
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2347
[11] J.K. Hampshire, B.H.V. Topping and H.C.  Chan. Three node triangular elements with one degree of freedom per node. ''Engng. Comput''. Vol. 9, pp. 49&#8211;62, 1992.
2348
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2350
[12] R. Phaal and C.R. Calladine. A simple class of finite elements for plate and shell problems. I: Elements for beams and thin plates. ''Int. J. Num. Meth. Engng.'', Vol. 35, pp. 955&#8211;977, 1992.
2351
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2353
[13] R. Phaal and C.R.  Calladine. A simple class of finite elements for plate and shell problems. II: An element for thin shells with only translational degrees of freedom. ''Int. J. Num. Meth. Engng.'', Vol. 35,  pp. 979&#8211;996, 1992.
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<div id="cite-14"></div>
2356
[14] E. Oñate and Cervera M. Derivation of thin plate bending elements with one degree of freedom per node. ''Engineering Computations'', Vol. 10, pp 553&#8211;561, 1993.
2357
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<div id="cite-15"></div>
2359
[15] M. Brunet and F. Sabourin. Prediction of necking and wrinkles with a simplified shell element in sheet forming. ''Int. Conf. of Metal Forming Simulation in Industry'', Vol. II, pp. 27&#8211;48, B. Kröplin (Ed.), 1994.
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<div id="cite-16"></div>
2362
[16] G. Rio, B. Tathi and H. Laurent. A new efficient finite element model of shell with only three degrees of freedom per node. Applications to industrial deep drawing test. in ''Recent Developments in Sheet Metal Forming Technology'', Ed. M.J.M. Barata Marques, 18th IDDRG Biennial Congress, Lisbon, 1994.
2363
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<div id="cite-17"></div>
2365
[17] G. Rio, B. Tathi and  H. Laurent. A new efficient finite element model of shell with only three degrees of freedom per node. Applications to industrial deep drawing test. in ''Recent Developments in Sheet Metal Forming Technology'', Ed. M.J.M. Barata Marques, 18th IDDRG Biennial Congress, Lisbon, 1994.
2366
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<div id="cite-18"></div>
2368
[18]  J. Rojek and E. Oñate. Sheet springback analysis using a simple shell triangle with translational degrees of freedom only. ''Int. J. of Forming Processes'', Vol. 1, No. 3, 275&#8211;296, 1998.
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2371
[19]  J. Rojek, E. Oñate and E. Postek. Application of explicit FE codes to simulation of sheet and bulk forming processes. ''J. of Materials Processing Technology'', Vols. 80-81, 620&#8211;627, 1998.
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2374
[20]  J. Jovicevic and E. Oñate. ''Analysis of beams and shells using a rotation-free finite element-finite volume formulation'', Monograph 43, CIMNE, Barcelona, 1999.
2375
2376
<div id="cite-21"></div>
2377
[[#citeF-21|[21]]] E. Oñate and F. Zárate. Rotation-free plate and shell triangles. ''Int. J. Num. Meth. Engng.'', 47, pp. 557&#8211;603, 2000.
2378
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2380
[22] F. Cirak and M. Ortiz. Subdivision surfaces: A new paradigm for thin-shell finite element analysis. ''Int. J. Num. Meths in Engng'',  Vol. 47, 2000, 2039-2072.
2381
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2383
[[#citeF-23|[23]]] F. Cirak and M. Ortiz. Fully <math display="inline">C^{1}</math>-conforming subdivision elements for finite deformations thin-shell analysis. ''Int. J. Num. Meths in Engng'', vol. 51, 2001, 813-833.
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[[#citeF-24|[24]]] F.G. Flores and E. Oñate. A basic thin shell triangle with only translational DOFs for large strain plasticity. ''Int. J. Num. Meths in Engng'', Vol. 51, pp 57-83, 2001.
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[25] G. Engel, K. Garikipati, T.J.R. Hughes, M.G. Larson, L. Mazzei and R.L. Taylor. Continuous/discontinuous finite element approximation of fourth-order elliptic problems in structural and continuum mechanics with applications to thin beams and plates, and strain gradient elasticity. ''Comput. Methods Appl. Mech. Engrg.'', Vol. 191, 3669&#8211;3750, 2002.
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[26]  E. Oñate, P. Cendoya and J. Miquel. Non linear explicit dynamic analysis of shells using the BST rotation-free triangle. ''Engineering Computations'', 19 (6), 662&#8211;706, 2002.
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2395
[[#citeF-27|[27]]] F.G. Flores and E. Oñate. Improvements in the membrane behaviour of the three node rotation-free BST shell triangle  using an assumed strain approach. ''Computer Methods in Applied Mechanics and Engineering'', in press, 2003.
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[28]  O.C. Zienkiewicz and E. Oñate. Finite Elements vs. Finite Volumes. Is there really a choice?. ''Nonlinear Computational Mechanics''. State of the Art. (Eds. P. Wriggers and R. Wagner). Springer Verlag, Heidelberg, 1991.
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[[#citeF-29|[29]]] E. Oñate, M.  Cervera  and O.C. Zienkiewicz. A finite volume format for structural mechanics. ''Int. J. Num. Meth. Engng.'', 37, pp. 181&#8211;201, 1994.
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2404
[30] R. Hill. A Theory of the Yielding and Plastic Flow of Anisotropic Metals. ''Proc. Royal Society London'', Vol. A193, pp. 281, 1948.
2405
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<div id="cite-31"></div>
2407
[31] R.W. Ogden. Large deformation isotropic elasticity: on the correlation of theory and experiments for incompressible rubberlike solids. ''Proceedings of the Royal Society of London'', Vol. A326, pp. 565&#8211;584, 1972.
2408
2409
<div id="cite-32"></div>
2410
[32] H.C. Huang, ''Static and Dynamic Analysis of Plates and Shells'', page 40, Springer-Verlag, Berlin, 1989.
2411
2412
<div id="cite-33"></div>
2413
[33] E.N. Dvorkin and K.J. Bathe. A continuum mechanics based four node shell element for general non-linear analysis. ''Eng. Comp.'', 1, 77&#8211;88, 1984.
2414
2415
<div id="cite-34"></div>
2416
[34] A. Needleman. Inflation of spherical rubber ballons. ''Int. J. of Solids and Structures'', 13, 409&#8211;421, 1977.
2417
2418
<div id="cite-35"></div>
2419
[[#citeF-35|[35]]]  Hibbit, Karlson and Sorensen Inc. ABAQUS, version 5.8, Pawtucket, USA, 1998.
2420
2421
<div id="cite-36"></div>
2422
[36] WHAMS-3D. An explicit 3D finite element program. KBS2  Inc., Willow Springs, Illinois 60480, USA.
2423
2424
<div id="cite-37"></div>
2425
[[#citeF-37|[37]]]  H.A. Balmer and E.A. Witmer. Theoretical experimental correlation of large dynamic and permanent deformation of impulsively loaded simple structures. ''Air force flight Dynamic Lab. Rep. FDQ-TDR-64-108'', Wright-Patterson AFB, Ohio, USA, 1964.
2426
2427
<div id="cite-38"></div>
2428
[38]  H. Stolarski, T. Belytschko and N. Carpenter. A simple triangular curved shell element. ''Eng. Comput.'', Vol. 1, 210&#8211;218, 1984.
2429
2430
<div id="cite-39"></div>
2431
[39]  P.O. Marklund and L. Nilsson. Simulation of airbag inflation processes using a coupled fluid structure approach. ''Computational Mechanics'', 29, 289&#8211;297, 2002.
2432
2433
<div id="cite-40"></div>
2434
[[#citeF-40|[40]]] NUMISHEET'96, ''Third International Conference and Workshop on Numerical Simulation of 3D Sheet Forming Processes, NUMISHEET'96'', E.H. Lee, G.L. Kinzel and R.H. Wagoner (Eds.), Dearbon-Michigan, USA, 1996.
2435
2436
<div id="cite-41"></div>
2437
[[#citeF-41|[41]]]  <code>http://rclsgi.eng.ohio-state.edu/%Elee-j-k/numisheet96/</code>
2438
2439
<div id="cite-42"></div>
2440
[[#citeF-42|[42]]] STAMPACK. ''A General Finite Element System for Sheet Stamping and Forming Problems'', Quantech ATZ, Barcelona, Spain, 2003 (www.quantech.es).
2441
2442
==APPENDIX==
2443
2444
==A.1 Curvature matrix for the BST element==
2445
2446
{| class="formulaSCP" style="width: 100%; text-align: left;" 
2447
|-
2448
| 
2449
{| style="text-align: left; margin:auto;" 
2450
|-
2451
| style="text-align: center;" | <math>\delta{\boldsymbol \kappa }=\mathbf{B}_{b} \times \mathbf{t}_3 \delta \mathbf{a}^{p}  </math>
2452
|}
2453
|}
2454
2455
with
2456
2457
{| class="formulaSCP" style="width: 100%; text-align: left;" 
2458
|-
2459
| 
2460
{| style="text-align: left; margin:auto;" 
2461
|-
2462
| style="text-align: center;" | <math>\begin{array}{c} \\ \delta \mathbf{a}^{p}\\ 18\times{1} \end{array} =[\delta \mathbf{u}_{1}^{T},\delta \mathbf{u}_{2}^{T},\delta \mathbf{u}_{3} ^{T},\delta \mathbf{u}_{4}^{T},\delta \mathbf{u}_{5}^{T},\delta \mathbf{u}_{6}^{T}]^{T}  </math>
2463
|}
2464
|}
2465
2466
and 
2467
2468
<math>\mathbf{B}_{b}^{T}=</math>
2469
2470
2471
{| class="wikitable" style="text-align: center; margin: 1em auto;"
2472
|- style="border-top: 2px solid;"
2473
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">+L^{M}_{2,1} L^{2}_{2,1}    +L^{M}_{3,1} L^{3}_{3,1} </math> 
2474
| style="border-left: 2px solid;border-right: 2px solid;" | <math>+L^{M}_{2,2} L^{2}_{2,2}    +L^{M}_{3,2} L^{3}_{3,2} </math>
2475
| style="border-left: 2px solid;border-right: 2px solid;" | <math>+L^{M}_{2,2} L^{2}_{2,1} +L^{M}_{2,1} L^{2}_{2,2}    +L^{M}_{3,2} L^{3}_{3,1} +L^{M}_{3,1} L^{3}_{3,2} </math>
2476
|- style="border-top: 2px solid;"
2477
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> L^{M}_{1,1} L^{1}_{3,1}    +L^{M}_{3,1} L^{3}_{2,1} </math> 
2478
| style="border-left: 2px solid;border-right: 2px solid;" | <math> L^{M}_{1,2} L^{1}_{3,2}    +L^{M}_{3,2} L^{3}_{2,2} </math>
2479
| style="border-left: 2px solid;border-right: 2px solid;" | <math> L^{M}_{1,2} L^{1}_{3,1} +L^{M}_{1,1} L^{1}_{3,2}    +L^{M}_{3,2} L^{3}_{2,1} +L^{M}_{3,1} L^{3}_{2,2} </math>
2480
|- style="border-top: 2px solid;"
2481
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> L^{M}_{1,1} L^{1}_{2,1}    +L^{M}_{2,1} L^{2}_{3,1} </math> 
2482
| style="border-left: 2px solid;border-right: 2px solid;" | <math> L^{M}_{1,2} L^{1}_{2,2}    +L^{M}_{2,2} L^{2}_{3,2} </math>
2483
| style="border-left: 2px solid;border-right: 2px solid;" | <math> L^{M}_{1,2} L^{1}_{2,1} +L^{M}_{1,1} L^{1}_{j,3}    +L^{M}_{2,2} L^{2}_{3,1} +L^{M}_{2,1} L^{2}_{3,2} </math>
2484
|- style="border-top: 2px solid;"
2485
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L^{M}_{1,1} L^{1}_{1,1} </math> 
2486
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L^{M}_{1,2} L^{1}_{1,2} </math>
2487
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L^{M}_{1,2} L^{1}_{1,1} +L^{M}_{1,1} L^{1}_{1,3} </math>
2488
|- style="border-top: 2px solid;"
2489
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L^{M}_{2,1} L^{2}_{1,1} </math> 
2490
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L^{M}_{2,2} L^{2}_{1,2} </math>
2491
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L^{M}_{2,2} L^{2}_{1,1} +L^{M}_{2,1} L^{2}_{1,3} </math>
2492
|- style="border-top: 2px solid;border-bottom: 2px solid;"
2493
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L^{M}_{3,1} L^{3}_{1,1} </math> 
2494
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L^{M}_{3,2} L^{3}_{1,2} </math>
2495
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L^{M}_{3,2} L^{3}_{1,1} +L^{M}_{3,1} L^{3}_{1,3} </math>
2496
2497
|}
2498
2499
<math display="inline">-2</math> 
2500
{| class="wikitable" style="text-align: center; margin: 1em auto;"
2501
|- style="border-top: 2px solid;"
2502
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> L^{M}_{1,1}\rho _{11}^{1}+L^{M}_{1,2}\rho _{11}^{2} </math> 
2503
| style="border-left: 2px solid;border-right: 2px solid;" | <math> L^{M}_{1,1}\rho _{22}^{1}+L^{M}_{i,2}\rho _{22}^{2} </math>
2504
| style="border-left: 2px solid;border-right: 2px solid;" | <math> L^{M}_{1,1}\rho _{12}^{1}+L^{M}_{1,2}\rho _{12}^{2} </math>
2505
|- style="border-top: 2px solid;"
2506
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> L^{M}_{2,1}\rho _{11}^{1}+L^{M}_{2,2}\rho _{11}^{2} </math> 
2507
| style="border-left: 2px solid;border-right: 2px solid;" | <math> L^{M}_{2,1}\rho _{22}^{1}+L^{M}_{2,2}\rho _{22}^{2} </math>
2508
| style="border-left: 2px solid;border-right: 2px solid;" | <math> L^{M}_{2,1}\rho _{12}^{1}+L^{M}_{2,2}\rho _{12}^{2} </math>
2509
|- style="border-top: 2px solid;"
2510
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> L^{M}_{3,1}\rho _{11}^{1}+L^{M}_{3,2}\rho _{11}^{2} </math> 
2511
| style="border-left: 2px solid;border-right: 2px solid;" | <math> L^{M}_{3,1}\rho _{22}^{1}+L^{M}_{3,2}\rho _{22}^{2} </math>
2512
| style="border-left: 2px solid;border-right: 2px solid;" | <math> L^{M}_{3,1}\rho _{12}^{1}+L^{M}_{3,2}\rho _{12}^{2} </math>
2513
|- style="border-top: 2px solid;"
2514
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">0</math> 
2515
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2516
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2517
|- style="border-top: 2px solid;"
2518
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">0</math> 
2519
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2520
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2521
|- style="border-top: 2px solid;border-bottom: 2px solid;"
2522
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">0</math> 
2523
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2524
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2525
2526
|}
2527
2528
<br/><br/>
2529
2530
==A.2 Membrane strain matrix and curvature matrix for the EBST element==
2531
2532
===A.2.1 Membrane strain matrix===
2533
2534
{| class="formulaSCP" style="width: 100%; text-align: left;" 
2535
|-
2536
| 
2537
{| style="text-align: left; margin:auto;" 
2538
|-
2539
| style="text-align: center;" | <math>\delta {\boldsymbol \varepsilon }_m ={B}_m \delta {a}^p  </math>
2540
|}
2541
|}
2542
2543
<math>\mathbf{B}_{m}^{T}=\frac{1}{3}</math>
2544
2545
2546
{| class="wikitable" style="text-align: center; margin: 1em auto;"
2547
|- style="border-top: 2px solid;"
2548
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{1}_{1,1}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }1}   + N^{2}_{1,1}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }1}   + N^{3}_{1,1}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }1} </math> 
2549
| style="border-left: 2px solid;border-right: 2px solid;" | <math> N^{1}_{1,2}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }2}   + N^{2}_{1,2}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }2}   + N^{3}_{1,2}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }2} </math>
2550
|- style="border-top: 2px solid;"
2551
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{1}_{2,1}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }1}   + N^{2}_{2,1}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }1}   + N^{3}_{2,1}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }1} </math> 
2552
| style="border-left: 2px solid;border-right: 2px solid;" | <math> N^{1}_{2,2}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }2}   + N^{2}_{2,2}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }2}   + N^{3}_{2,2}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }2} </math>
2553
|- style="border-top: 2px solid;"
2554
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{1}_{3,1}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }1}   + N^{2}_{3,1}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }1}   + N^{3}_{3,1}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }1} </math> 
2555
| style="border-left: 2px solid;border-right: 2px solid;" | <math> N^{1}_{3,2}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }2}   + N^{2}_{3,2}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }2}   + N^{3}_{3,2}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }2} </math>
2556
|- style="border-top: 2px solid;"
2557
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{1}_{4,1}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }1} </math> 
2558
| style="border-left: 2px solid;border-right: 2px solid;" | <math> N^{1}_{4,2}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }2} </math>
2559
|- style="border-top: 2px solid;"
2560
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{2}_{5,1}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }1} </math> 
2561
| style="border-left: 2px solid;border-right: 2px solid;" | <math> N^{2}_{5,2}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }2} </math>
2562
|- style="border-top: 2px solid;border-bottom: 2px solid;"
2563
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{3}_{6,1}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }1} </math> 
2564
| style="border-left: 2px solid;border-right: 2px solid;" | <math> N^{3}_{6,2}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }2} </math>
2565
2566
|}
2567
2568
2569
{| class="wikitable" style="text-align: center; margin: 1em auto;"
2570
|- style="border-top: 2px solid;"
2571
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{1}_{1,2}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }1}    +N^{1}_{1,1}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }2}    +N^{2}_{1,2}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }1}    +N^{2}_{1,1}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }2}    +N^{3}_{1,2}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }1}    +N^{3}_{1,1}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }2} </math> 
2572
|- style="border-top: 2px solid;"
2573
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{1}_{2,2}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }1}    +N^{1}_{2,1}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }2}   + N^{2}_{2,2}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }1}    +N^{2}_{2,1}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }2}   + N^{3}_{2,2}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }1}    +N^{3}_{2,1}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }2} </math> 
2574
|- style="border-top: 2px solid;"
2575
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{1}_{3,2}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }1}    +N^{1}_{3,1}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }2}    +N^{2}_{3,2}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }1}    +N^{2}_{3,1}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }2}    +N^{3}_{3,2}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }1}    +N^{3}_{3,1}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }2} </math> 
2576
|- style="border-top: 2px solid;"
2577
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{1}_{4,2}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }1}    +N^{1}_{4,1}\mathbf{\boldsymbol \varphi }^{1}_{^{\prime }2} </math> 
2578
|- style="border-top: 2px solid;"
2579
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{2}_{5,2}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }1}    +N^{2}_{5,1}\mathbf{\boldsymbol \varphi }^{2}_{^{\prime }2} </math> 
2580
|- style="border-top: 2px solid;border-bottom: 2px solid;"
2581
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline"> N^{3}_{6,2}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }1}    +N^{3}_{6,1}\mathbf{\boldsymbol \varphi }^{3}_{^{\prime }2} </math> 
2582
2583
|}
2584
2585
===A.2.2 Curvature matrix===
2586
2587
{| class="formulaSCP" style="width: 100%; text-align: left;" 
2588
|-
2589
| 
2590
{| style="text-align: left; margin:auto;" 
2591
|-
2592
| style="text-align: center;" | <math>\delta {\boldsymbol \kappa } ={B}_b \times \mathbf{t}_3 \delta {a}^p  </math>
2593
|}
2594
|}
2595
2596
<math>\mathbf{B}_{b}^{T}=2</math>
2597
2598
2599
{| class="wikitable" style="text-align: center; margin: 1em auto;"
2600
|- style="border-top: 2px solid;"
2601
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{1,1}\left(N_{1,1}\right)_{G_{1}}   +L_{2,1}\left(N_{1,1}\right)_{G_{2}}   +L_{3,1}\left(N_{1,1}\right)_{G_{3}}</math> 
2602
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L_{1,2}\left(N_{1,2}\right)_{G_{1}}   +L_{2,2}\left(N_{1,2}\right)_{G_{2}}   +L_{3,2}\left(N_{1,2}\right)_{G_{3}}</math>
2603
|- style="border-top: 2px solid;"
2604
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{1,1}\left(N_{2,1}\right)_{G_{1}}   +L_{2,1}\left(N_{2,1}\right)_{G_{2}}   +L_{3,1}\left(N_{2,1}\right)_{G_{3}}</math> 
2605
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L_{1,2}\left(N_{2,2}\right)_{G_{1}}   +L_{2,2}\left(N_{2,2}\right)_{G_{2}}   +L_{3,2}\left(N_{2,2}\right)_{G_{3}}</math>
2606
|- style="border-top: 2px solid;"
2607
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{1,1}\left(N_{3,1}\right)_{G_{1}}   +L_{2,1}\left(N_{3,1}\right)_{G_{2}}   +L_{3,1}\left(N_{3,1}\right)_{G_{3}}</math> 
2608
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L_{1,2}\left(N_{3,2}\right)_{G_{1}}   +L_{2,2}\left(N_{3,2}\right)_{G_{2}}   +L_{3,2}\left(N_{3,2}\right)_{G_{3}}</math>
2609
|- style="border-top: 2px solid;"
2610
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{1,1}\left(N_{4,1}\right)_{G_{1}}</math> 
2611
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L_{1,2}\left(N_{4,2}\right)_{G_{1}}</math>
2612
|- style="border-top: 2px solid;"
2613
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{2,1}\left(N_{5,1}\right)_{G_{2}}</math> 
2614
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L_{2,2}\left(N_{5,2}\right)_{G_{2}}</math>
2615
|- style="border-top: 2px solid;border-bottom: 2px solid;"
2616
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{3,1}\left(N_{6,1}\right)_{G_{3}}</math> 
2617
| style="border-left: 2px solid;border-right: 2px solid;" | <math>L_{3,2}\left(N_{6,2}\right)_{G_{3}}</math>
2618
2619
|}
2620
2621
2622
{| class="wikitable" style="text-align: center; margin: 1em auto;"
2623
|- style="border-top: 2px solid;"
2624
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{1,2}\left(N_{1,1}\right)_{G_{1}}+L_{1,1}\left(N_{1,2}\right)_{G_{1}}   +L_{2,2}\left(N_{1,1}\right)_{G_{2}}+L_{2,1}\left(N_{1,2}\right)_{G_{2}}   +L_{3,2}\left(N_{1,1}\right)_{G_{3}}+L_{3,1}\left(N_{1,2}\right)_{G_{3}}</math> 
2625
|- style="border-top: 2px solid;"
2626
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{1,2}\left(N_{2,1}\right)_{G_{1}}+L_{1,1}\left(N_{2,2}\right)_{G_{1}}   +L_{2,2}\left(N_{2,1}\right)_{G_{2}}+L_{2,1}\left(N_{2,2}\right)_{G_{2}}   +L_{3,2}\left(N_{2,1}\right)_{G_{3}}+L_{3,1}\left(N_{2,2}\right)_{G_{3}}</math> 
2627
|- style="border-top: 2px solid;"
2628
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{1,2}\left(N_{3,1}\right)_{G_{1}}+L_{1,1}\left(N_{j,3}\right)_{G_{1}}   +L_{2,2}\left(N_{3,1}\right)_{G_{2}}+L_{2,1}\left(N_{j,3}\right)_{G_{2}}   +L_{3,2}\left(N_{3,1}\right)_{G_{3}}+L_{3,1}\left(N_{j,3}\right)_{G_{3}}</math> 
2629
|- style="border-top: 2px solid;"
2630
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{1,2}\left(N_{4,1}\right)_{G_{1}}+L_{1,1}\left(N_{4,3}\right)_{G_{1}}</math> 
2631
|- style="border-top: 2px solid;"
2632
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{2,2}\left(N_{5,1}\right)_{G_{2}}+L_{2,1}\left(N_{5,3}\right)_{G_{2}}</math> 
2633
|- style="border-top: 2px solid;border-bottom: 2px solid;"
2634
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">L_{3,2}\left(N_{6,1}\right)_{G_{3}}+L_{3,1}\left(N_{6,3}\right)_{G_{6}}</math> 
2635
2636
|}
2637
2638
<math>-2</math>
2639
2640
2641
{| class="wikitable" style="text-align: center; margin: 1em auto;"
2642
|- style="border-top: 2px solid;"
2643
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">\left(L_{1,1}\rho _{11}^{1}+L_{1,2}\rho _{11}^{2}\right)</math> 
2644
| style="border-left: 2px solid;border-right: 2px solid;" | <math>\left(L_{1,1}\rho _{22}^{1}+L_{i,2}\rho _{22}^{2}\right)</math>
2645
| style="border-left: 2px solid;border-right: 2px solid;" | <math>\left(L_{1,1}\rho _{12}^{1}+L_{1,2}\rho _{12}^{2}\right)</math>
2646
|- style="border-top: 2px solid;"
2647
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">\left(L_{2,1}\rho _{11}^{1}+L_{2,2}\rho _{11}^{2}\right)</math> 
2648
| style="border-left: 2px solid;border-right: 2px solid;" | <math>\left(L_{2,1}\rho _{22}^{1}+L_{2,2}\rho _{22}^{2}\right)</math>
2649
| style="border-left: 2px solid;border-right: 2px solid;" | <math>\left(L_{2,1}\rho _{12}^{1}+L_{2,2}\rho _{12}^{2}\right)</math>
2650
|- style="border-top: 2px solid;"
2651
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">\left(L_{3,1}\rho _{11}^{1}+L_{3,2}\rho _{11}^{2}\right)</math> 
2652
| style="border-left: 2px solid;border-right: 2px solid;" | <math>\left(L_{3,1}\rho _{22}^{1}+L_{3,2}\rho _{22}^{2}\right)</math>
2653
| style="border-left: 2px solid;border-right: 2px solid;" | <math>\left(L_{3,1}\rho _{12}^{1}+L_{3,2}\rho _{12}^{2}\right)</math>
2654
|- style="border-top: 2px solid;"
2655
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">0</math> 
2656
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2657
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2658
|- style="border-top: 2px solid;"
2659
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">0</math> 
2660
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2661
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2662
|- style="border-top: 2px solid;border-bottom: 2px solid;"
2663
| style="border-left: 2px solid;border-right: 2px solid;" | <math display="inline">0</math> 
2664
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2665
| style="border-left: 2px solid;border-right: 2px solid;" | <math>0</math>
2666
2667
|}
2668
2669
2670
In this last expression <math display="inline">L_{i,j} =L_{i,j}^{M}</math>
2671

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Published on 01/01/2005

DOI: 10.1016/j.cma.2004.07.039
Licence: CC BY-NC-SA license

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