Scipedia: Documents published in 2020

Parametrized multifield variational principles in elasticity: II. Hybrid functionals and the Free Formulation

María Jesús Samper — Mon, 17 Aug 2020 10:57:55 +0200

A one-parameter family of d-generalized hybrid/mixed variational principles for linear elasticity is constructed following a domain subdivision. The family includes the d-generalized Hellinger-Reissner and potential energy principles as special cases. The parametrized principle is discretized by independently varied internal displacements, stresses and boundary displacements. The resulting finite element equations are studied following a physically motivated decomposition of the stress and internal displacement fields. The free formulation of Bergan and Nygård is shown to be a special case of this element type, and is obtained by assuming a constant internal stress field. The parameter appears as a scale factor of the higher-order stiffness.

The Extended Free Formulation of Finite Elements in Linear Elasticity

María Jesús Samper — Mon, 17 Aug 2020 10:43:27 +0200

The free formulation of Bergan and Nyga˚rd (1984) has been successfully used in the construction of high-performance finite elements for linear and nonlinear structural analysis. In its original form the formulation combines nonconforming internal displacement assumptions with a specialized version of the patch test. The original formulation is limited, however, by strict invertibility conditions linking the assumed displacement field to the nodal displacements. The present paper lifts those restrictions by recasting the free formulation within the framework of a mixed-hybrid functional that allows internal stresses, internal displacements, and boundary displacements to vary independently. This functional contains a free parameter and includes the potential energy and the Hellinger-Reissner principles as special cases. The parameter appears in the higher-order stiffness of the element equations

Towards parallel I/O in finite element simulations

María Jesús Samper — Mon, 17 Aug 2020 10:37:55 +0200

I/O issues in finite element analysis on parallel processors are addressed. Viable solutions for both local and shared memory multiprocessors are presented. The approach is simple but limited by currently available hardware and software systems. Implementation is carried out on a CRAY-2 system. Performance results are reported.

Electromagnetic axisymmetric finite elements based on a gauged four-potential variational principle

María Jesús Samper — Mon, 17 Aug 2020 10:25:47 +0200

Electromagnetic finite elements are derived based on a variational principle that uses the electromagnetic four-potential as a primary variable. The Lorentz gage normalization is incorporated as a constraint condition through a Lagrange multiplier field to construct elements suitable for downstream coupling with mechanical and thermal finite elements for the analysis of high-temperature superconductor devices with aerospace applications. The main advantages are: jump discontinuities on interfaces are naturally handled; no a priori approximations are invoked; and the number of degrees of freedom per node remains modest as the problem dimensionality increases. The new elements are tested on two magnetostatic axisymmetric problems. The results are in excellent agreement with analytical solutions and previous solutions for the 1D problem of a conducting infinite wire, in which case the multiplier field has no effect. For materials of widely different permeability, jump conditions are naturally accommodated by the present formulation.

The Effect of Wall Motions on the Governing Equations of Contained Fluids

María Jesús Samper — Mon, 17 Aug 2020 10:10:38 +0200

The equations of motion for an acoustic fluid enclosed in a moving or flexible container are studied. It is shown that the determination of the reference state must account for the surface-integrated effect of the wall motions. The governing equation of transient motions about this state in the displacement potential does not generally reduce to the classical wave equation unless special adjustments are made. The results are relevant to finite elements formulations based on the displacement potential.

Programming the isoparametric six-node triangle

María Jesús Samper — Mon, 17 Aug 2020 09:53:00 +0200

The formulation and programming of the six‐node, curved isoparametric triangle in plane stress is presented. This is intended to fill an apparent vacuum in the open finite element literature

A variational justification of the assumed natural strain formulation of finite elements—II. The C0 four-node plate element

María Jesús Samper — Mon, 17 Aug 2020 09:47:54 +0200

In Part II the four-node C0 plate bending element is used to explore some of the possibilities opened by the theory presented in Part I [C. Militello and C. A. Felippa, Comput. Struct.34, 431–438 (1990)]. This element is chosen because the version presented by Bathe and Dvorkin [Int. J. Numer. Meth. Engng21, 367–383 (1985)], MITC4, can be considered the simplest assumed natural strain element that allows several possibilities to be studied in a straightforward manner. Attention is focused on the governing functionals R and H presented in Part I, assuming independent strain fields only for the transverse shear strains. Besides MITC4, three formulations (two mixed and one hybrid) are considered that collectively represent a variational justification for the assumed strain technique. In addition, reduced and selective-integration elements are examined to compare their behavior with that of the present assumed strain elements.

Article Metrics Related Articles Cite Share Request Permissions Explore More Download PDF Detecting and Traversing Bifurcation Points in Nonlinear Structural Analysis

María Jesús Samper — Mon, 17 Aug 2020 09:43:18 +0200

We investigate procedures for detecting and traversing bifurcation points that occur in the geometrically nonlinear analysis of elastic structures. In particular, we study a family of test functions for sharp detection and identification of bifurcation points. To accomplish branch switching without recourse to artifical imperfections, the selection of appropriate predictors and correctors which exclude the known branch from the converged solution is considered. These techniques are tried and assesed on several three-dimensional truss structures that exhibit bifurcation behavior

The Individual Element Test Revisited

María Jesús Samper — Mon, 17 Aug 2020 09:23:38 +0200

The subject of the patch test for finite elements retains several unsettled aspects. In particular, the issue of one-element versus multielement tests needs clarification. Following a brief historical review, we present the individual element test (IET) of Bergan and Hanssen in an expanded context that encompasses several important classes of new elements. The relationship of the IET to the multielement forms A, B, and C of the patch test and to the single element test are clarified.

Probabilistic FEM for Nonlinear Concrete Structures. II: Applications

María Jesús Samper — Fri, 14 Aug 2020 14:31:12 +0200

This is the second part of an investigation on probabilistic finite element methods for nonlinear concrete structures. The study is concentrated on two reinforced concrete application examples including a simply supported beam and a portal frame. Nonlinearity in material and geometry, and randomness in loading, material, and geometry are considered. Extensive computations using a probabilistic finite element method inbedded in the computer code PFRAME developed at the University of Colorado, Boulder, are carried out to verify analytical results. The effect of Taylor series expansions about different values of various random variables on structural response is demonstrated in both application examples. Furthermore, using the simply supported beam example, it is shown that the probabilistic finite element method proposed is applicable to the assessment of structural safety of material and geometric nonlinear concrete structures. Further investigations are necessary in order to develop this probabilistic finite element approach into a mature structural safety assessment method.

Probabilistic FEM for Nonlinear Concrete Structures. I: Theory

María Jesús Samper — Fri, 14 Aug 2020 14:24:00 +0200

This is the first part of an investigation on probabilistic finite element methods for nonlinear concrete structures under random loads. The formulation of a method that accounts for both randomness and nonlinearity in material and geometryis proposed. The method is an extension of nonlinear finite element techniques, where; (1) The new quantities required at the element level are the derivatives of the external and internal forces with respect to the basic random variables; (2) the loads are applied in increments until collapse; and (3) the equilibrium at each load level is achieved by using a modified Newton iteration scheme. In contrast to currently available methods the proposed formulation allows for Taylor series expansion of the response about arbitrary reference values of the basic random variables. Furthermore, the collapse load of the system associated with a given combination of reference values of the random variables becomes a result of theanalysis. A computer code called PFRAME is developed for the application of the proposed method to concrete frame structures exhibiting geometric and material nonlinear behavior.

Mixed variational formulation of finite element analysis of acoustoelastic/slosh fluid-structure interaction

María Jesús Samper — Fri, 14 Aug 2020 13:38:22 +0200

A general three-field variational principle is obtained for the motion of an acoustic fluid enclosed in a rigid or flexible container by the method of canonical decomposition applied to a modified form of the wave equation in the displacement potential. The general principle is specialized to a mixed two-field principle that contains the fluid displacement potential and pressure as independent fields. This principle contains a free parameter alpha. Semidiscrete finite-element equations of motion based on this principle are displayed and applied to the transient response and free-vibrations of the coupled fluid-structure problem. It is shown that a particular setting of alpha yields a rich set of formulations that can be customized to fit physical and computational requirements. The variational principle is then extended to handle slosh motions in a uniform gravity field, and used to derive semidiscrete equations of motion that account for such effects.

A variational justification of the assumed natural strain formulation of finite elements—I. Variational principles

María Jesús Samper — Fri, 14 Aug 2020 13:31:02 +0200

The assumed natural strain (ANS) formulation of finite elements has undergone rapid development over the past five years. The key formulation step is the replacement, in the potential energy principle, of selected displacement-related strains by independently assumed strain fields in element natural coordinates. These strains are not generally derivable from displacements. This procedure was conceived as one of several competing methods with which to solve the element locking problem. Its most noteworthy feature is that, unlike many forms of reduced integration, it produces no rank deficiency; furthermore, it is easily extendible to geometrically non-linear problems. Many original formulations were not based on a variational principle. The objective of Part I is to study the ANS formulation from a variational standpoint. This study is based on two hybrid extensions of the Reissner-type functional that uses strains and displacements as independent fields. One of the forms is a genuine variational principle that contains an independent boundary traction field, whereas the other one represents a restricted variational principle. Two procedures for element-level elimination of the strain field are discussed, and one of them is shown to be equivalent to the inclusion of incompatible displacement modes. In Part II [C. Militello and C. A. Felippa, Comput. Struct.34, 439–444 (1990)], the four-node C0 plate bending quadrilateral element is used to illustrate applications of this theory.

Electromagnetic finite elements based on a four-potential variational principle

María Jesús Samper — Fri, 14 Aug 2020 13:20:43 +0200

We derive electromagnetic finite elements based on a variational principle that uses the electromagnetic four-potential as primary variable. This choice is used to construct elements suitable for downstream coupling with mechanical and thermal finite elements for the analysis of electromagnetic/mechanical systems that involve superconductors. The main advantages of the four-potential as a basis for finite element formulation are: the number of degrees of freedom per node remains modest as the problem dimensionality increases, jump discontinuities on interfaces are naturally accomodated, and statics as well as dynamics may be treated without any a priori approximations. The new elements are tested on an axisymmetric problem under steady-state forcing conditions. The results are in excellent agreement with analytical solutions.

The first ANDES elements: 9-dof plate bending triangles

María Jesús Samper — Fri, 14 Aug 2020 13:14:18 +0200

New elements are derived to validate and assess the assumed natural deviatoric strain (ANDES) formulation. This is a brand new variant of the assumed natural strain (ANS) formulation of finite elements, which has recently attracted attention as an effective method for constructing high-performance elements for linear and nonlinear analysis. The ANDES formulation is based on an extended parametrized variational principle developed in recent publications. The key concept is that only the deviatoric part of the strains is assumed over the element, whereas the mean strain part is discarded in favor of a constant stress assumption. Unlike conventional ANS elements, ANDES elements satisfy the individual element test (a stringent form of the patch test) a priori while retaining the favorable distortion-insensitivity properties of ANS elements. The first application of this new formulation is the development of several Kirchhoff plate bending triangular elements with the standard nine degrees of freedom. Linear curvature variations are sampled along the three sides with the corners as ‘gage reading’ points. These samples values are interpolated over the triangle using three schemes. Two schemes merge back to conventional ANS elements, one being identical to the discrete Kirchhoff triangle (DKT), whereas the third one products two new ANDES elements. Numerical experiments indicate that one of the ANDES element is relatively insensitive to distortion compared to previously derived high-performance plate-bending elements, while retaining accuracy for nondistorted elements.

Variational formulation of high-performance finite elements: Parametrized variational principles

María Jesús Samper — Fri, 14 Aug 2020 12:56:03 +0200

High-performance (HP) elements are simple finite elements constructed to deliver engineering accuracy with coarse arbitrary grids. This paper is part of a series on the variational basis of HP elements, with emphasis on those constructed with the free formulation (FF) and assumed natural strain (ANS) methods. The present paper studies parametrized variational principles that provide a foundation for the FF and ANS methods, as well as for a combination of both methods.

Acoustic fluid volume modeling by the displacement potential formulation, with emphasis on the wedge element

María Jesús Samper — Fri, 14 Aug 2020 12:03:40 +0200

This paper studies three-dimensional finite elements for modeling acoustic fluid volumes using the displacement potential formulation. These elements find application in shock and vibration analysis of fluid-structure interaction problems such as submerged structures and flexible containers. The paper concentrates on the derivation and implementation of the six-node wedge element because of its unconventional nature. This element is validated on the Bleich-Sandler problem of fluid-structure transient analysis.

Membrane triangles with corner drilling freedoms—II. The ANDES element

María Jesús Samper — Fri, 14 Aug 2020 11:51:47 +0200

This is the second article in a three-part series on the construction of 3-node, 9-dof membrane elements with normal-to-its-plane rotational freedoms (the so-called drilling freedoms) using parametrized variational principles. In this part, one such element is derived within the context of the assumed natural deviatoric strain (ANDES) formulation. The higher-order strains are obtained by constructing three parallel-to-sides pure-bending modes from which natural strains are obtained at the corner points and interpolated over the element. To attain rank sufficiency, an additional higher-order “torsional” mode, corresponding to equal hierarchical rotations at each corner with all other motions precluded, is incorporated. The resulting formulation has five free parameters. When these parameters are optimized against pure bending by energy balance methods, the resulting element is found to coalesce with the optimal EFF element derived in Part I. Numerical integration as a strain filtering device is found to play a key role in this achievement.

Membrane triangles with corner drilling freedoms—I. The EFF element

María Jesús Samper — Fri, 14 Aug 2020 11:32:49 +0200

This paper is the first of a three-part series that studies the formulation of 3-node, 9-dof membrane elements with normal-to-element-plane rotations (the so-called drilling freedoms) within the context of parametrized variational principles. These principles supply a unified basis for several advanced element-construction techniques; in particular: the free formulation (FF), the extended free formulation (EFF) and the assumed natural deviatoric strain (ANDES) formulation. In Part I we construct an element of this type using the EFF. This derivation illustrates the basic steps in the application of that formulation to the construction of high-performance, rank-sufficient, nonconforming elements with corner rotations. The element is initially given the twelve degress of freedom of the linear strain triangle (LST), which allows the displacement expansion to be a complete quadratic in each component. The expansion basis contains the six linear basic functions and six energy-orthogonal quadratic higher-order functions. Three degrees of freedom, defined as the midpoint deviations from linearity along the triangle-median directions, are eliminated by kinematic constraints. The remaining hierarchical midpoint freedoms are transformed to corner rotations. The performance of the resulting element is evaluated in Part III.

Parametrized variational principles encompassing compressible and incompressible elasticity

María Jesús Samper — Fri, 14 Aug 2020 11:25:32 +0200

A parametrized five-field variational principle that can accommodate both compressible and incompressible hyperelasticity is presented. The primary variables are mean and deviatoric stresses, mean and deviatoric strains, and displacements. Through appropriate selection of parameters the functional of this general principle specializes to those previously presented by Atluri-Reissner, Herrmann and Franca.

Parametrized variational principles for micropolar elasticity

María Jesús Samper — Fri, 14 Aug 2020 11:16:14 +0200

A parametrized six-field variational principle for micropolar compressible linear elasticity is presented. The primary variables are symmetric and skew stresses, symmetric and skew strains, micropolar rotations and displacements. The governing functional is characterized by six free parameters. The connection between this formulation and the functionals with relaxed stress symmetry and independent rotations fields proposed by Reissner and Hughes-Brezzi for classical (non-polar) linear elasticity is examined. It is shown that the Hughes-Brezzi functionals are special cases of the parametrized functional but that the Reissner functionals are not. The former may be interpreted as a regularization (consistent stabilization) of the Reissner functionals that places them within the framework of micropolar elasticity. An eight-field parametrized principle that accounts for couple stresses is briefly described in the Appendix.

Membrane triangles with corner drilling freedoms— III. Implementation and performance evaluation

María Jesús Samper — Fri, 14 Aug 2020 11:10:03 +0200

This paper completes a three-part series on the formulation of 3-node, 9-dof membrane triangles with corner drilling freedoms based on parametrized variational principles. The first four sections cover element implementation details including determination of optimal parameters and treatment of distributed loads. Then three elements of this type, labeled ALL, FF and EFF-ANDES, are tested on standard plane stress problems. ALL represents numerically integrated versions of Allman's 1988 triangle; FF is based on the free formulation triangle presented by Bergan and Felippa in 1985; and EFF-ANDES represent two different formulations of the optimal triangle derived in Parts I and II. The numerical studies indicate that the ALL, FF and EFF-ANDES elements are comparable in accuracy for elements of unitary aspect ratios. The ALL elements are found to stiffen rapidly in inplane bending for high aspect ratios, whereas the FF and EFF elements maintain accuracy. The EFF and ANDES implementations have a moderate edge in formation speed over the FF.

Comparison of strain recovery techniques for the mixed iterative method

María Jesús Samper — Fri, 14 Aug 2020 10:50:53 +0200

Three strain recovery techniques are investigated in the framework of the mixed iterative method. The need for appropriate strain recovery for low-order elements is shown. The three techniques compared are: C-lumping, strain-smoothing and a new technique called C-splitting. It is shown that the use of C-lumping has negative effects on the convergence of the mixed iterative method. Both strain-smoothing and C-splitting deliver convergent results, but the new technique converges faster.

Three-dimensional non-linear Timoshenko beam based on the core-congruential formulation

María Jesús Samper — Fri, 14 Aug 2020 10:25:48 +0200

A three-dimensional, geometrically non-linear, two-node Timoshenko beam element based on the total Lagrangian description is derived. The element behaviour is assumed to be linear elastic, but no restrictions are placed on the magnitude of finite rotations. The resulting element has twelve degrees of freedom: six translational components and six rotational-vector components. The formulation uses the Green-Lagrange strains and second Piola-Kirchhoff stresses as energy-conjugate variables and accounts for bending-stretching and bending-torsional-coupling effects without special provisions. The core-congruential formulation (CCF) is used to derive the discrete equations in a staged manner. Core equations involving the internal force vector and tangent stiffness matrix are developed at the particle level. A sequence of matrix transformations carries these equations to beam cross-sections and finally to the element nodal degrees of freedom. The choice of finite rotation measure is made in the next-to-last transformation stage, and the choice of over-the-element interpolation in the last one. The tangent stiffness matrix is found to retain symmetry if the rotational vector is chosen to measure finite rotations. An extensive set of numerical examples are presented to test and validate the present element.

A survey of parametrized variational principles and applications to computational mechanics

María Jesús Samper — Fri, 14 Aug 2020 10:06:59 +0200

This survey paper describes recent developments in the area of parametrized variational principles (PVPs) and selected applications to finite-element computational mechanics. A PVP is a variational principle containing free parameters that have no effect on the Euler-Lagrange equations. The theory of single-field PVPs, based on gauge functions (also known as null Lagrangians) is a subset of the Inverse Problem of Variational Calculus that has limited value. On the other hand, multifield PVPs are more interesting from theoretical and practical standpoints. Following a tutorial introduction, the paper describes the recent construction of multifield PVPs in several areas of elasticity and electromagnetics. It then discusses three applications to finite-element computational mechanics: the derivation of high-performance finite elements, the development of element-level error indicators, and the construction of finite element templates. The paper concludes with an overview of open research areas.

A survey of the core-congruential formulation for geometrically nonlinear TL finite elements

María Jesús Samper — Fri, 14 Aug 2020 09:59:09 +0200

This article presents a survey of the Core-Congruential Formulation (CCF) for geometrically nonlinear mechanical finite elements based on the Total Lagrangian (TL) kinematic description. Although the key ideas behind the CCF can be traced back to Rajasekaran and Murray in 1973, it has not subsequently received serious attention. The CCF is distinguished by a two-phase development of the finite element stiffness equations. The initial phase develop equations for individual particles. These equations are expressed in terms of displacement gradients as degrees of freedom. The second phase involves congruential-type transformations that eventually binds the element particles of an individual element in terms of its node-displacement degrees of freedom. Two versions of the CCF, labeled Direct and Generalized, are distinguished. The Direct CCF (DCCF) is first described in general form and then applied to the derivation of geometrically nonlinear bar, and plane stress elements using the Green-Lagrange strain measure. The more complex Generalized CCF (GCCF) is described and applied to the derivation of 2D and 3D Timoshenko beam elements. Several advantages of the CCF, notably the physically clean separation of material and geometric stiffnesses, and its independence with respect to the ultimate choice of shape functions and element degrees of freedom, are noted. Application examples involving very large motions solved with the 3D beam element display the range of applicability of this formulation, which transcends the kinematic limitations commonly attributed to the TL description.

Superconducting axisymmetric finite elements based on a gauged potential variational principle—II. Solution and numerical results

María Jesús Samper — Fri, 14 Aug 2020 09:29:18 +0200

Part II of a two part paper discusses an incremental-iterative nonlinear solution technique for solving the nonlinear finite element equations of the superconducting state of a superconductor. The untreated equations are highly ill-conditioned and are impossible to solve within the typical 16-place double precision supplied by most computers. A combination of matrix scaling and mesh grading techniques is used to reduce the condition number of the tangent stiffness matrix and increase the accuracy of the current carrying boundary layer representation. Numerical results for a one-dimensional model of a time-independent superconductor treated by the Ginzburg-Landau model are presented and discussed. The computed solutions clearly display the Meissner effect of magnetic field expulsion from the central region of the superconductor. These results are compared to the physics of a low-viscosity fluid problem. From this analogy, a physical argument is advanced about the macroscopic behavior of superconductors.

Superconducting axisymmetric finite elements based on a gauged potential variational principle—I. Formulation

María Jesús Samper — Thu, 13 Aug 2020 14:43:09 +0200

The present work is part of a research program for the numerical simulation of electromagnetic (EM) fields within conventional Ginzburg-Landau (GL) superconductors. The final goal of this research is to formulate, develop and validate finite element (FE) models that can accurately capture electromagnetic, thermal and material phase changes in a superconductor. The formulations presented here are for a time-independent Ginzburg-Landau superconductor and are derived from a potential-based variational principle.

In Part I of this paper, we develop an appropriate variational formulation of time-independent superconductivity for the general three-dimensional case and specialize it to the one-dimensional case. Also developed are expressions for the material-dependent parameters α and β of GL theory and their dependence upon the temperature T. The one-dimensional formulation is then discretized for finite element purposes and the first variation of these equations is obtained. The resultant Euler equations contain nonlinear terms in the primary variables. To solve these equations, an incremental-iterative solution method is used. Expressions for the internal force vector, external force vector, loading vector and tangent stiffness matrix are therefore developed for use with the solution procedure.

50 Year classic reprint: An appreciation of R. Courant's ‘variational methods for the solution of problems of equilibrium and vibrations,’ 1943

María Jesús Samper — Thu, 13 Aug 2020 14:33:16 +0200

Fifty years elapsed since the appearance of the title paper, in Bull. Am. Math. Soc., 49, 1-23 (1943), and twenty six since my original encounter with it. I first read it in 1967 during an internal workshop on the origins of the finite element method (FEM), held at UC Berkeley, where I was a Post-Doctoral Research Associate. I must admit at not having been overly impressed - the pride of having just finished a thesis in refined elements coupled with the arrogance of youth to dismiss what to me seemed like tricial hand calculations for a simple torsion problem. Why, a 1967 CDC 6600 computer wrap those up in milliseconds...

From the individual element test to finite element templates: Evolution of the patch test

María Jesús Samper — Thu, 13 Aug 2020 13:59:21 +0200

This paper starts a sequence of three articles that follow an unconventional approach in finite element research. The ultimate objective is to construct high-performance elements and element-level error estimators for those elements. The approach takes off from our previous work in high-performance elements and culminates with the development of finite element templates. The present paper concentrates on the patch test and evolved versions of the test that have played a key role in this research. Following a brief review of the historical roots, we present the Individual Element Test (IET) of Bergan and Hanssen in an expanded context that encompasses several important classes of new elements. The relationship of the IET to the multielement forms A, B and C of the patch test and to the single-element test are investigated. An important consequence of the IET application is that the element stiffness equations decompose naturally into basic and higher-order parts. The application of this decomposition to the “sanitization” of the non-convergent BCIZ element is described and verified with numerical experiments. Two sequel papers in preparation are subtitled ‘the algebraic approach’ and ‘element-level error estimation’. These apply the fundamental decomposition to the derivation of templates for specific mechanical elements and to the construction of element-level error estimators, respectively.

Parametric unification of matrix structural analysis: classical formulation and d-connected mixed elements

María Jesús Samper — Thu, 13 Aug 2020 13:52:38 +0200

Concepts and techniques from the field of Parametrized Variational Principles (PVPs) are extended to Matrix Structural Analysis (MSA). Free parameters are used as weighting factors of governing discrete equations. Combining this idea with matrix manipulation techniques yields a continuous spectrum of supermatrix equations. Setting parameters to numerical values provides specific solution methods, some of which are well known whereas others are not. The approach is applied to the classical MSA of truss and framework structures as well as to displacement-connected FE models generated by a parametrized mixed functional. The main advantage of this “top down” derivation of solution schemes is the unification of seemingly disjoint methods for instructional and classification purposes. In addition, the question of duality between range-space and null-space representations is clarified.

Recent developments in parametrized variational principles for mechanics

María Jesús Samper — Thu, 13 Aug 2020 13:44:12 +0200

This paper gives an introduction to the formulation of parametrized variational principles (PVPs) in mechanics. This is complemented by more advanced material describing selected recent developments in hybrid and nonlinear variational principles. A PVP is a variational principle containing free parameters that have no effect on the Euler-Lagrange equations and natural boundary conditions. The theory of single-field PVPs, based on gauge functions, is a subset of the Inverse Problem of Variational Calculus that has limited value. On the other hand, multified PVPs are more interesting from both theoretical and practical standpoints. The two-dimensional Poisson equation is used to present, in a tutorial fashion, the formulation of parametrized mixed functionals. This treatment is then extended to internal interfaces, which are useful in treatment of discontinuities, subdomain linkage and construction of parametrized hybrid functionals. This is followed by a similar but more compact treatment of three-dimensional classical elasticity, and a parametrization of nonlinear hyperelasticity.

Parametrized variational principles in dynamics applied to the optimization of dynamic models of plates

María Jesús Samper — Thu, 13 Aug 2020 13:37:31 +0200

We investigate the use of Parametrized Variational Principles (PVP) in linear structural dynamics. Our main objective is to assess whether the free parameters can be used to enhance the accuracy of dynamic models on a fixed mesh. Consistent, boundary-consistent and lumped mass matrices are defined within the framework of the PVP. The accuracy provided by three different mass matrices in the computation of plate frequencies is numerically studied. A method to update the free parameters on an element by element basis to improve the dynamic model is presented. Numerical experiments that characterize that improvement for the vibration and transient-response analysis of plates are presented. These experiments suggest that such update is primarily beneficial for modes in the intermediate frequency range.

Everyone may benefit from subsidising entry to risky occupations

María Jesús Samper — Thu, 13 Aug 2020 13:32:24 +0200

This paper shows that in the presence of costly state verification, directly or indirectly subsidising entry to risky occupations may benefit everyone. The result holds even in the presence of private insurance. Indeed, it may be desirable to prohibit private insurance in favour of subsidies to hazardous activities. These findings do not depend on the government having an advantage over the private sector in observing outcomes. The explanation is that through its influence on equilibrium price, feasible fiscal policy can shift the return distribution so as to create collective insurance more cheaply than is possible through private contracting with its requirement of costly auditing. Amongst applications is a case for a loss-making state bank offering high interest-rate loans.

An algebraically partitioned FETI method for parallel structural analysis: performance evaluation

María Jesús Samper — Thu, 13 Aug 2020 13:26:55 +0200

This paper presents the algorithmic performance of an algebraically partitioned Finite Element Tearing and Interconnection (FETI) method presented in a companion paper. A simple structural assembly topology is employed to illustrate the implementation steps in a Matlab software environment. Numerical results indicate that the method is scalable, provided the iterative solution preconditioner employs the reduced interface Dirichlet preconditioner. A limited comparison of the present method with the differentially partitioned FETI method with corner modes is also offered. Based on this comparison and a reasonable extrapolation, we conclude the present algebraically partitioned FETI method possesses a similar iteration convergence property of the differentially partitioned FETI method with corner modes

An algebraically partitioned FETI method for parallel structural analysis: algorithm description

María Jesús Samper — Thu, 13 Aug 2020 13:21:05 +0200

An algebraically partitioned FETI method for the solution of structural engineering problems on parallel computers is presented. The present algorithm consists of three attributes: an explicit generation of the orthogonal null‐space matrix associated with the interface nodal forces, the floating subdomain rigid‐body modes computed from the subdomain static equilibrium equation of the classical force method and the identification of redundant interface force constraint operator that emanates when the interface force computations are localized. Comparisons of the present method with the previously developed differentially partitioned FETI method are offered in terms of the saddle‐point formulations at the end of the paper. A companion paper reports implementation details and numerical performance of the proposed algorithm.

A direct flexibility method

María Jesús Samper — Thu, 13 Aug 2020 12:56:07 +0200

We present a Direct Flexibility Method (DFM) for the solution of finite element equations. This method is based on a decomposition of the finite element model into substructures, which may reduce to individual elements. Substructures are preprocessed by the Direct Stiffness Method (DSM) to generate free-free flexibility matrices for floating substructures. The interface problem is solved for the interface forces and the solution recovered over substructure interiors. The DFM shares with the DSM the advantages of being automatic, maintaining locality and sparseness, efficiently handling continuum elements, and requiring only the availability of element stiffness libraries. The new method appears to be advantageous for specific applications. These include: massively parallel processing, inverse problems, treatment of rigid members and inclusions, and use of under-integrated elements without spurious-mode stabilization.

A Variational Framework for Solution Method Developments in Structural Mechanics

María Jesús Samper — Thu, 13 Aug 2020 12:49:38 +0200

We present a variational framework for the development of partitioned solution algorithms in structural mechanics. This framework is obtained by decomposing the discrete virtual work of an assembled structure into that of partitioned substructures in terms of partitioned substructural deformations, substructural rigid-body displacements and interface forces on substructural partition boundaries. New aspects of the formulation are: the explicit use of substructural rigid-body mode amplitudes as independent variables and direct construction of rank-sufficient interface compatibility conditions. The resulting discrete variational functional is shown to be variationally complete, thus yielding a full-rank solution matrix. Four specializations of the present framework are discussed. Two of them have been successfully applied to parallel solution methods and to system identification. The potential of the two untested specializations is briefly discussed.

ETA Expressions for the High Frequency Impedance of a Uniformly Pulsating Submerged Torus

María Jesús Samper — Thu, 13 Aug 2020 12:46:21 +0200

The present abstract was motivated by the search for clean yet nontrivial benchmark problems for wave propagation and scattering in an infinite acoustic medium. The submerged sphere, cylinder and disk illustrate important aspects of scattering and radiation while admitting complete analytical solutions. These three classical configurations are thus indispensable part of any set of benchmark problems.

The construction of free–free flexibility matrices as generalized stiffness inverses

María Jesús Samper — Thu, 13 Aug 2020 12:03:51 +0200

We present generalizations of the classical structural flexibility matrix. Direct or indirect computation of flexibilities as ‘influence coefficients’ has traditionally required pre-removal of rigid body modes by imposing appropriate support conditions. Here the flexibility of an individual element or substructure is directly obtained as a particular generalized inverse of the free–free stiffness matrix. This entity is called a free–free flexibility matrix. It preserves exactly the rigid body modes. The definition is element independent. It only involves access to the stiffness generated by a standard finite element program as well as a separate geometric construction of the rigid body modes. With this information, the computation of the free–free flexibility can be done by solving linear equations and does not require the solution of an eigenvalue problem or performing a singular value decomposition. Flexibility expressions for symmetric and unsymmetric free–free stiffnesses are studied. For the unsymmetric case two flexibilities, one preserving the Penrose conditions and the other the spectral properties, are examined. The two versions coalesce for symmetric matrices

Construction of optimal 3-node plate bending triangles by templates

María Jesús Samper — Thu, 13 Aug 2020 11:47:19 +0200

A finite element template is a parametrized algebraic form that reduces to specific finite elements by setting numerical values to the free parameters. The present study concerns Kirchhoff Plate-Bending Triangles (KPT) with 3 nodes and 9 degrees of freedom. A 37-parameter template is constructed using the Assumed Natural Deviatoric Strain (ANDES). Specialization of this template includes well known elements such as DKT and HCT. The question addressed here is: can these parameters be selected to produce high performance elements? The study is carried out by staged application of constraints on the free parameters. The first stage produces element families satisfying invariance and aspect ratio insensitivity conditions. Application of energy balance constraints produces specific elements. The performance of such elements in benchmark tests is presently under study.

Recent Developments In Basic Finite Element Technologies

María Jesús Samper — Thu, 13 Aug 2020 11:41:48 +0200

High performance (HP) elements are simple finite elements constructed to deliver engineering accuracy with arbitrary coarse meshes. This chapter reviews original developments based on parametrized variational principles, which provide a common foundation for various approaches to HP element construction. Templates are parametrized algebraic forms of finite element stiffness equations, which hold future promise in the unification of finite element methods and connection to arbitrary grid finite differences. Essential to this unification work is the availability of powerful computer algebra systems. The application of templates to the construction of an optimal 3-node plate bending element is outlined in the chapter.

A localized version of the method of Lagrange multipliers and its applications

María Jesús Samper — Thu, 13 Aug 2020 11:34:42 +0200

This paper describes a novel version of the method of Lagrange multipliers for an improved modeling of multi-point constraints that emanate from contact-impact problems, partitioned structural analysis using parallel computers, and structural inverse problems. It is shown that the classical method of Lagrange multipliers can lead to a non-unique set of constraint conditions for the modeling of interfaces involving more than two or multi-point substructural interface nodes. The proposed version of the method of Lagrange multipliers leads not only to unique construction of constraints but also encounters no singularity in modeling an arbitrary number of multi-point constraints. An important utilization of the present method is in the regularized modeling of interfaces whose rigidities are radically different from one to another. The present approach is demonstrated via several examples for its simplicity in modeling constraints, ease of implementation and computational advantages

A variational principle for the formulation of partitioned structural systems

María Jesús Samper — Thu, 13 Aug 2020 11:11:35 +0200

A continuum‐based variational principle is presented for the formulation of the discrete governing equations of partitioned structural systems. This application includes coupled substructures as well as subdomains obtained by mesh decomposition. The present variational principle is derived by a series of modifications of a hybrid functional originally proposed by Atluri for finite element development. The interface is treated by a displacement frame and a localized version of the method of Lagrange multipliers. Interior displacements are decomposed into rigid‐body and deformational components to handle floating subdomains. Both static and dynamic versions are considered. An important application of the present principle is the treatment of nonmatching meshes that arise from various sources such as separate discretization of substructures, independent mesh refinement, and global–local analysis. The present principle is compared with that of a globalized version of the multiplier method.

On the Original Publication of the General Canonical Functional of Linear Elasticity

María Jesús Samper — Thu, 13 Aug 2020 10:40:58 +0200

The general canonical functional of linear elastostatics is associated with the names of Hu and Washizu, who published it independently in 1955. This note discusses how that functional, in a generalized four-field form, had been derived by B. M. Fraeijs de Veubeke in a 1951 technical report. This report presents five of the seven canonical functionals of elasticity. In addition to the general functional, it exhibits what is likely the first derivation of the strain-displacement dual of the Hellinger-Reissner functional. The tour of five variational principles takes only a relatively small portion of the report: 8 pages out of 56. The bulk is devoted to the use of energy methods for analysis of wing structures. The title, technology focus, and limited dissemination may account for the subsequent neglect of this original contribution to variational mechanics.

Customizing high performance elements by fourier methods

María Jesús Samper — Thu, 13 Aug 2020 10:24:44 +0200

This paper illustrates, through a worked out example, the application of finite element templates to construct high performance bending elements for vibration and buckling problems. The example focuses on the improvement of the mass and geometric stiffness matrices of plane beams. Similar methods can be used for Kirchhoff plate bending elements, but this material is omitted because of space constraints. The process interweaves classical tools: Fourier analysis and orthogonal polynomials, with newer ones: finite element templates and computer algebra systems. Templates are parametrized algebraic forms that uniquely characterize an element population by a "genetic signature" defined by the set of free parameters. Specific elements are obtained by assigning numeric values to the parameters. This freedom of choice can be used to design custom elements. For the beam example, Legendre polynomials are used to construct templates for the material stiffness, geometric stiffness and mass matrices. Fourier analysis carried out through symbolic computation searches for template signatures of mass and stiffness that deliver matrices with desirable attributes for specific target situations. Three objectives are noted: high accuracy for vibration and buckling analysis, wide separation of acoustic and optical branches, and low sensitivity to mesh distortion and boundary conditions. This paper examines in some detail only the first objective

Interfacing nonmatching FEM meshes: the zero moment rule

María Jesús Samper — Thu, 13 Aug 2020 10:14:27 +0200

This is a tutorial paper that outlines, through a benchmark example, an efiective method for coupling nonmatched flnite element meshes. Coupling is done through a displacement frame interposed between the interface meshes. That frame is treated with a FEM discretization and \glued" to the meshes through localized Lagrange multipliers collocated at mesh-interface nodes. The approach can be used to couple meshes of arbitrary geometry, discretization type (e.g., FEM and BEM) and even meshes of difierent physics (e.g., structure and ∞uids). The example, however, focuses on a simplifled 2D case that can be explained within space constraints. The requirement for preservation of constant stress states leads to an easily visualized condition, called the zero-moment rule or ZMR, that can be used to locate frame nodes on frame geometry. The ZMR provides all possible consistent frame-node conflgurations. Generalizations of the 2D ZMR rule to 3D problems involve partitioning of the frame and are not reported here. Thus the ZMR is necessary but not su-cient. The constant stress consistency condition may be interpreted, according to the reader's taste, as either an interface patch test, a vanishing of the flrst variation of the interface potential under admissible kinematic modes, or a requirement for energy conservation across the interface.

Special issue: Advances in computational methods for fluid-structure interaction and coupled problems - Preface

María Jesús Samper — Thu, 13 Aug 2020 09:31:39 +0200

The computer based simulation of Fluid-Structure Interaction (FSI) problems has historical and practical importance in computational mechanics. FSI was one of the first systematically developed areas in the wider framework of what is now called computational multiphysics. Its development propelled original advances in modeling, solution algorithms and computer implementation of coupled models coming from two different disciplines: solid and fluid mechanics. Despite that relatively early start, the practical importance of computational FSI has remained undiminished. Among its important scientific and technological applications we can cite the following.

Customizing the mass and geometric stiffness of plane beam elements by Fourier methods

María Jesús Samper — Wed, 12 Aug 2020 14:05:39 +0200

Teaches by example the application of finite element templates in constructing high performance elements. The example discusses the improvement of the mass and geometric stiffness matrices of a Bernoulli-Euler plane beam. This process interweaves classical techniques (Fourier analysis and weighted orthogonal polynomials) with newer tools (finite element templates and computer algebra systems). Templates are parameterized algebraic forms that uniquely characterize an element population by a "genetic signature" defined by the set of free parameters. Specific elements are obtained by assigning numeric values to the parameters. This freedom of choice can be used to design "custom" elements. For this example weighted orthogonal polynomials are used to construct templates for the beam material stiffness, geometric stiffness and mass matrices. Fourier analysis carried out through symbolic computation searches for template signatures of mass and geometric stiffness that deliver matrices with desirable properties when used in conjunction with the well-known Hermitian beam material stiffness. For mass-stiffness combinations, three objectives are noted: high accuracy for vibration analysis, wide separation of acoustic and optical branches, and low sensitivity to mesh distortion and boundary conditions. Only the first objective is examined in detail.

Partitioned formulation of internal fluid–structure interaction problems by localized Lagrange multipliers

María Jesús Samper — Wed, 12 Aug 2020 14:00:52 +0200

A partitioned formulation of compressible internal fluid–structure interaction problems is presented by employing a displacement model for both the fluid and structure. Partitioning is effected by a localized version of the method of Lagrange multipliers, which assigns two independent sets of Lagrange multipliers to the structural and fluid interfaces. Two major features of the present formulation include: an interface compliance normalization that helps capture the predominant physics of interaction phenomena when the interfaces are characterized by two radically different rigidities, and a novel transformation of the displacement model into a fluid-pressure model that is suitable for both transient and vibration analyses. The present formulation first solves for the interface Lagrange multipliers, which are subsequently used to solve for the structural displacements and the fluid displacement or pressure by employing two independent analysis modules.

Partitioned analysis of coupled system

María Jesús Samper — Wed, 12 Aug 2020 13:47:57 +0200

This is a tutorial article that reviews the use of partitioned analysis procedures for the analysis of coupled dynamical systems. Attention is focused on the computational simulation of systems in which a structure is a major component. Important applications in that class are provided by thermomechanics, fluid–structure interaction and control–structure interaction. In the partitioned solution approach, systems are spatially decomposed into partitions. This decomposition is driven by physical or computational considerations. The solution is separately advanced in time over each partition. Interaction effects are accounted for by transmission and synchronization of coupled state variables. Recent developments in the use of this approach for multilevel decomposition aimed at massively parallel computation are discussed.

A historical outline of matrix structural analysis: a play in three acts

María Jesús Samper — Wed, 12 Aug 2020 13:42:29 +0200

The evolution of matrix structural analysis (MSA) from 1930 through 1970 is outlined. Highlighted are major contributions by Collar and Duncan, Argyris, and Turner, which shaped this evolution. To enliven the narrative the outline is configured as a three-act play. Act I describes the pre-WWII formative period. Act II spans a period of confusion during which matrix methods assumed bewildering complexity in response to conflicting demands and restrictions. Act III outlines the cleanup and consolidation driven by the appearance of the direct stiffness method, through which MSA completed morphing into the present implementation of the finite element method (FEM). No attempt is made at chronicling the more complex history of FEM itself.

Fitting Strains and Displacements by Minimizing Dislocation Energy

María Jesús Samper — Wed, 12 Aug 2020 13:32:31 +0200

We present a procedure for matching a displacement field to a given strain field, or vice- versa, over an arbitrary domain, which can be a finite element. The fitting criterion used is minimization of a dislocation energy functional. The strain field, whether given or fitted, need not be compatible. The method has four immediate applications: (i) finite element stiffness formulation based on fitting assumed-natural-strain (ANS) fields to node displacements; (ii) pointwise recovery of an internal displacement field in ANS elements as required for consistent mass, body load or geometric stiffness computations; (iii) recovery of smoothed strains from node displacements for stress post-processing, and (iv) system identification and damage detection from experimental data. The article focuses on application (i) for the strain fitting (SF) problem and (ii) for the displacement fitting (DF) problem. The separation of mean and deviatoric strains is emphasized whenever it is found convenient to simplify calculations.

The construction of free–free flexibility matrices for multilevel structural analysis

María Jesús Samper — Wed, 12 Aug 2020 13:26:29 +0200

This article considers a generalization of the classical structural flexibility matrix. It expands on previous papers by taking a deeper look at computational considerations at the substructure level. Direct or indirect computation of flexibilities as “influence coefficients” has traditionally required pre-removal of rigid body modes by imposing appropriate support conditions, mimicking experimental arrangements. With the method presented here the flexibility of an individual element or substructure is directly obtained as a particular generalized inverse of the free–free stiffness matrix. This generalized inverse preserves the stiffness spectrum. The definition is element independent and only involves access to the stiffness generated by a standard finite element program and the separate construction of an orthonormal rigid-body mode basis. The free–free flexibility has proven useful in special application areas of finite element structural analysis, notably massively parallel processing, model reduction and damage localization. It can be computed by solving sets of linear equations and does not require processing an eigenproblem or performing a singular value decomposition. If substructures contain thousands of d.o.f., exploitation of the stiffness sparseness is important. For that case this paper presents a computation procedure based on an exact penalty method, and a projected rank-regularized inverse stiffness with diagonal entries inserted by the sparse factorization process. These entries can be physically interpreted as penalty springs. This procedure takes advantage of the stiffness sparseness while forming the full free–free flexibility, or a boundary subset, and is backed by an in-depth null space analysis for robustness.

A simple algorithm for localized construction of non‐matching structural interfaces

María Jesús Samper — Wed, 12 Aug 2020 13:21:21 +0200

A simple and effective algorithm for the modular construction of non-matched interfaces is presented for the partitioned solution of large-scale structural problems. The formulation is based on a recently developed four-field variational principle, which introduces a connection frame between the interfaced partitions. A key result of the present study is a frame nodal placement criterion that uniquely determines the frame discretization into piecewise linear elements so that the interface patch test condition is satisfied a priori. The method is demonstrated with several 2D and 3D example problems

A contact formulation based on localized Lagrange multipliers: Formulation and application to two-dimensional problems

María Jesús Samper — Wed, 12 Aug 2020 13:00:35 +0200

The non-penetration condition in contact problems is traditionally based on the classical Lagrange multiplier method. This method makes extensive use of modelling details of the contacting bodies for contact enforcement as the contact surface meshes are in general non-matching. To deal with this problem we introduce a novel element in the Lagrange multiplier approach of contact modelling, namely, a contact frame placed in between contacting bodies. It acts as a medium through which contact forces are transferred without violating equilibrium in the contact domain for discrete contact models. Only nodal information of the contacting bodies is required which makes the proposed contact enforcement generic. The contact frame has its own independent freedoms, which allows the formulation to pass contact patch tests by design.

A study of optimal membrane triangles with drilling freedoms

María Jesús Samper — Wed, 12 Aug 2020 12:55:50 +0200

This article compares derivation methods for constructing optimal membrane triangles with corner drilling freedoms. The term “optimal” is used in the sense of exact inplane pure-bending response of rectangular mesh units of arbitrary aspect ratio. Following a comparative summary of element formulation approaches, the construction of an optimal three-node triangle using the ANDES formulation is presented. The construction is based upon techniques developed by 1991 in student term projects, but taking advantage of the more general framework of templates developed since. The optimal element that fits the ANDES template is shown to be unique if energy orthogonality constraints are enforced. Two other formulations are examined and compared with the optimal model. Retrofitting the conventional linear strain triangle element by midpoint-migrating and congruential transformations is shown unable to produce an optimal element, while rank deficiency is inevitable. Use of the quadratic strain field of the 1988 Allman triangle, or linear filtered versions thereof, is also unable to reproduce the optimal element. Moreover these elements exhibit serious aspect ratio lock. These predictions are verified on benchmark examples.

A distortion-insensitive four-noded membrane quadrilateral that passes the patch test

María Jesús Samper — Wed, 12 Aug 2020 12:52:28 +0200

This chapter discusses the possibility to develop a quadrilateral called Unidirectional Bending-Optimal Quadrilateral Panel (UBOQP) that passes the patch test for any geometry while being bending exact along one direction. This model is directly constructed using the framework of finite element templates by energy fitting arguments. UBOQP is not obtainable by any conventional finite element method (FEM) formulation, which illustrates the customization power of templates. Numerical experiments in a well-known mesh distortion test verify the exact response to bending for any material law, aspect ratio, and skew angles. The four-noded membrane (plane stress) flat quadrilateral is one of the most interesting models from the standpoint of its impact on both theory and practice of the FEM. The rectangular specialization is one of the two multidimensional finite element stiffnesses that is explicitly derived by Turner. It is widely believed that a four-noded flat quadrilateral membrane element that passes the patch test must be sensitive to the mesh distortion response to inplane bending actions.

A compendium of FEM integration formulas for symbolic work

María Jesús Samper — Wed, 12 Aug 2020 11:31:58 +0200

This paper presents a set of Mathematica modules that organizes numerical integration rules considered useful for finite element work. Seven regions are considered: line segments, triangles, quadrilaterals, tetrahedral, wedges, pyramids and hexahedra. Information can be returned in floating-point (numerical) form, or in exact symbolic form. The latter is useful for computer-algebra aided FEM work that carries along symbolic variables. A few quadrature rules were extracted from sources in the FEM and computational mathematics literature, and placed in symbolic form using Mathematica to generate own code. A larger class of formulas, previously known only numerically, were directly obtained through symbolic computations. Some unpublished non-product rules for pyramid regions were found and included in the collection. For certain regions: quadrilaterals, wedges and hexahedra, only product rules were included to economize programming. The collection embodies most FEM-useful formulas of low and moderate order for the seven regions noted above. Some gaps as regard region geometries and omission of non-product rules are noted in the conclusions. The collection may be used “as is” in support of symbolic FEM work thus avoiding contamination with floating arithmetic that precludes simplification. It can also be used as generator for low-level floating-point code modules in Fortran or C. Floating point accuracy can be selected arbitrarily. No similar modular collection applicable to a range of FEM work, whether symbolic or numeric, has been published before

A unified formulation of small-strain corotational finite elements: I. Theory

María Jesús Samper — Wed, 12 Aug 2020 11:21:59 +0200

This paper presents a unified theoretical framework for the corotational (CR) formulation of finite elements in geometrically nonlinear structural analysis. The key assumptions behind CR are: (i) strains from a corotated configuration are small while (ii) the magnitude of rotations from a base configuration is not restricted. Following a historical outline the basic steps of the element independent CR formulation are presented. The element internal force and consistent tangent stiffness matrix are derived by taking variations of the internal energy with respect to nodal freedoms. It is shown that this framework permits the derivation of a set of CR variants through selective simplifications. This set includes some previously used by other investigators. The different variants are compared with respect to a set of desirable qualities, including self-equilibrium in the deformed configuration, tangent stiffness consistency, invariance, symmetrizability, and element independence. We discuss the main benefits of the CR formulation as well as its modeling limitations.

Partitioned formulation of frictional contact problems using localized Lagrange multipliers

María Jesús Samper — Wed, 12 Aug 2020 11:16:06 +0200

Interface treatment methods for the contact problem between non-matching meshes have traditionally been based on a direct coupling of the contacting solids employing a master–slave strategy or classical Lagrange multipliers. These methods tend to generate strongly coupled systems that is dependent on the discretization characteristics on each side of the contact zone. In this work a displacement frame is intercalated between the interface meshes. The frame is then discretized so that the discrete frame nodes are connected to the contacting substructures using the localized Lagrange multipliers collocated at the interface nodes. The resulting methodology alleviates the need for master–slave book-keeping and provides a partitioned formulation which preserves software modularity, facilitates non-matching mesh treatment and passes the contact patch test. Frictional contact problems are used to demonstrate the salient features of the proposed method.

Supernatural QUAD4: A template formulation

María Jesús Samper — Wed, 12 Aug 2020 11:12:03 +0200

A universal template for a 4-node quadrilateral in plane stress is constructed using a combination of old and new techniques. The use of natural quantities, strongly advocated by J.H. Argyris since his Continua and Discontinua 1965 exposition, is further expanded. The qualifier ‘supernatural’ means that all governing equations: kinematic, constitutive and equilibrium, are expressed in both Cartesian and natural forms. These two sets are used to build different components of the template. With timely help from a computer algebra system, a template that include all possible quadrilateral elements that pass the Individual Element Test of Bergan and Hanssen emerges. It yields an infinite number of hitherto undiscovered instances that may be customized to fit special needs. A striking example is the construction of a two-trapezoid macroelement that is bending exact about one direction, for any amount of distortion. This concludes a five decade search that begins with the formulation of the wing-cover rectangular panel in Argyris’ 1954 seminal serial on Energy Methods and Structural Analysis.

Construction of Customized Mass-Stiffness Pairs Using Templates

María Jesús Samper — Wed, 12 Aug 2020 10:43:21 +0200

This paper is a tutorial exposition of the template approach to the construction of customized mass-stiffness pairs for selected applications in structural dynamics. The exposition focuses on adjusting the mass matrix while a separately provided stiffness matrix is kept fixed. Two well known kinetic-energy discretization methods described in finite-element method (FEM) textbooks since the mid-1960s lead to diagonally lumped and consistent mass matrices, respectively. These two models are sufficient to cover many engineering applications. Occasionally, however, they fall short. The gap can be filled with a more general approach that relies on the use of templates. These are algebraic forms that carry free parameters. This approach is discussed in this paper using one-dimensional structural elements as examples. Templates have the virtue of producing a set of mass matrices that satisfy certain a priori constraint conditions such as symmetry, nonnegativity, invariance, and momentum conservation. In particular, the diagonally lumped and consistent versions can be obtained as instances. Thus those standard models are not excluded. Availability of free parameters, however, allows the mass matrix to be customized to special needs, such as high precision vibration frequencies or minimally dispersive wave propagation. An attractive feature of templates for FEM programming is that only one element implementation as module with free parameters is needed, and need not be recoded when the application problem class changes.

FEM and BEM coupling in elastostatics using localized Lagrange multipliers

María Jesús Samper — Wed, 12 Aug 2020 10:04:06 +0200

The equations produced by the finite and boundary element methods in structural mechanics are expressed in different variables and cannot be linked without modifications. Conventional coupling methods of these numerical techniques have traditionally been based on the use of classical Lagrange multipliers making a direct connection of the two solids through their common interfaces using matching meshes and altering the formulation of one of the methods to make it compatible with the other. In this work, a discrete surface called frame is interposed between the connected subdomains to approximate their common interface displacements, it is treated using a finite element discretization and connected to each substructure using localized Lagrange multipliers collocated at the interface nodes. This methodology facilitates the connection of non-matching finite and boundary element meshes avoiding modifications to the numerical methods used and providing a partitioned formulation, which preserves software modularity.

Treatment of acoustic fluid–structure interaction by localized Lagrange multipliers and comparison to alternative interface-coupling methods

María Jesús Samper — Wed, 12 Aug 2020 09:47:29 +0200

This paper is a sequel on the topic of localized Lagrange multipliers (LLM) for applications of fluid–structure interaction (FSI) between finite-element models of an acoustic fluid and an elastic structure. The prequel paper formulated the spatial-discretization methods, the LLM interface treatment, the time-marching partitioned analysis procedures, and the application to 1D benchmark problems. Here, we expand on formulation aspects required for successful application to more realistic 2D and 3D problems. Additional topics include duality relations at the fluid–structure interface, partitioned vibration analysis, reduced-order modeling, handling of curved interface surfaces, and comparison of LLM with other coupling methods. Emphasis is given to non-matching fluid–structure meshes. We present benchmark examples that illustrate the benefits and drawbacks of competing interface treatments. Realistic application problems involving the seismic response of two existing dams are considered. These include 2D modal analyses of the Koyna gravity dam, transient-response analyses of that dam with and without reduced-order modeling, incorporation of nonlinear cavitation effects, and the 3D transient-response analysis of the Morrow Point arch dam

A formulation based on localized Lagrange multipliers for BEM–FEM coupling in contact problems

María Jesús Samper — Wed, 12 Aug 2020 09:25:43 +0200

This paper presents a unified formulation for the combination of the finite element method (FEM) and the boundary element method (BEM) in 3D frictional contact problems that is based on the use of localized Lagrange multipliers (LLMs). Resolution methods for the contact problem between non-matching meshes have traditionally been based on a direct coupling of the contacting solids using classical Lagrange multipliers. These methods tend to generate strongly coupled systems that require a deep knowledge of the discretization characteristics on each side of the contact zone complicating the process of mixing different numerical techniques. In this work a displacement contact frame is inserted between the FE and BE interface meshes, discretized and finally connected to the contacting substructures using LLMs collocated at the msh-interface nodes. This methodology will provide a partitioned formulation which preserves software modularity and facilitates the connection of non-matching FE and BE meshes.

Treatment of acoustic fluid–structure interaction by localized Lagrange multipliers: Formulation

María Jesús Samper — Tue, 11 Aug 2020 13:46:24 +0200

A new concept is presented for modeling the dynamic interaction between an acoustic fluid and an elastic structure. The coupling of this multiphysics system is done by inserting a kinematic interface frame between the fluid and the structure, and using node-collocated Lagrange multipliers to connect the frame to each subsystem. The time-domain response analysis is performed by a partitioned analysis procedure. The main advantages of this localized Lagrange multiplier (LLM) primal-dual coupling method are: complete localization of the structure and fluid subsystems, elimination of the conventional predictor in the partitioned time integration method, and the ability to accommodate non-matching meshes. The standard Newmark time integrator is used on both the fluid and structure models. It is shown that if the integrator is A-stable and second-order accurate for a monolithic treatment, it retains those properties for both Mortar and LLM partitioned solution procedures. Infinite and finite piston problems are used to explain and verify the methodology. A sequel paper under preparation presents and discusses a set of benchmark and application examples that involve the response of existing dams to seismic excitation.

Plastic buckling and collapse of thin shell structures, using layered plastic modeling and co-rotational ANDES finite elements

María Jesús Samper — Tue, 11 Aug 2020 13:38:31 +0200

This study reveals an analysis of plastic buckling and collapse of thin shell structures. For this purpose, the co-rotational and layered plastic model as well as ANDES (Assumed Natural Deviatoric Strain) finite element formulations are used. The co-rotational kinematics formulation splits the translational and rotational deformations in a small deformation analysis. The ANDES finite element is modified to elastoplastic ANDES finite element by the introduction of the von Mises yield criterion elastoplastic formulation on its original deformation model. In order to accommodate the plasticity formulation, the Gauss point layered integration is inserted through of thickness of the element to produce the internal force vector and material stiffness matrix. Special effort is devoted to maintain the consistency of the internal forces and tangent stiffness as well as to enhance the robustness of element level computations. The arc-length method is used to follow the postbuckling equilibrium path. Results are presented for several benchmark elastoplastic shell problems available in the present literature, which are generally in agreement with the present work

The d'Alembert–Lagrange principal equations and applications to floating flexible systems

María Jesús Samper — Tue, 11 Aug 2020 13:26:13 +0200

This paper addresses the dynamics and quasi‐statics of floating flexible structures as well as extensions to unconstrained substructures and partitions of coupled mechanical systems. The principal solution is defined as the state of self‐equilibrated forces obtained as the particular solution of the rigid motion and interface equilibrium equations. This solution is independent of the stress–strain constitutive properties as well as of the compatibility equations. For statically determinate systems, the principal solution is the final force solution. For statically indeterminate systems, the correction due to flexibility and compatibility is orthogonal to the principal solution. The formulation is done in the context of d'Alembert's principle, which supplies the d 'Alembert–Lagrange principal equations for floating bodies. These are obtained by summation of virtually working forces and moments acting on the floating systems. Applications of this approach are demonstrated on a set of dynamic and quasi‐static example problems of increasing generality. Linkage to variational principles with an interface potential is eventually discussed as providing the theoretical foundation for handling interacting semi‐discrete subsystems linked by node‐collocated Lagrange multipliers.

Partitioned formulation of internal and gravity waves interacting with flexible structures

María Jesús Samper — Tue, 11 Aug 2020 12:47:25 +0200

This paper presents a partitioned modeling of internal and gravity fluid waves that interact with flexible structures. The governing interaction model consists of three completely partitioned entities: fluid model, structural model, and interface model that acts as an internal constraint on the fluid–structure interface boundary. Thus, the proposed partitioned multi-physics modeling can employ two completely modular fluid and structure software modules plus an interface solver, hence amenable to partitioned solution algorithms. The interface discretization can exploit the nonmatching interface algorithm previously developed via the method of localized Lagrange multipliers. Also noted is that the present fluid model can make use of widely available finite element software for standard Poisson-type problems.

Partitioned vibration analysis of internal fluid-structure interaction problems

María Jesús Samper — Tue, 11 Aug 2020 11:44:36 +0200

A partitioned, continuum‐based, internal fluid–structure interaction (FSI) formulation is developed for modeling combined sloshing, acoustic waves, and the presence of an initial pressurized state. The present formulation and its computer implementation use the method of localized Lagrange multipliers to treat both matching and non‐matching interfaces. It is shown that, with the context of continuum Lagrangian kinematics, the fluid sloshing and acoustic stiffness terms originate from an initial pressure term akin to that responsible for geometric stiffness effects in solid mechanics. The present formulation is applicable to both linearized vibration analysis and nonlinear FSI transient analysis provided that a convected kinematics is adopted for updating the mesh geometry in a finite element discretization. Numerical examples illustrate the capability of the present procedure for solving coupled vibration and nonlinear sloshing problems.

Reusing linear finite elements in material and geometrically nonlinear analysis—Application to plane stress problems

María Jesús Samper — Tue, 11 Aug 2020 11:18:29 +0200

This paper describes a computational approach suitable for combined material and geometrically nonlinear analysis by the Finite Element Method. Its main advantage is reuse: once a finite element has been developed with good performance in linear analysis, extension to material and geometrically nonlinear problems is simplified. Extension to geometrically nonlinear problems is enabled by a corotational kinematic description, and that to material nonlinear problems by an optimization-based solution algorithm. The approach thus comprises three ingredients—the development of a high performance linear finite element (for example, using the ANDES concept), a corotational kinematic description, and an optimization algorithm. The main constraint in the application of the corotational formulation is restriction to small deformational displacements. The paper illustrates the realization of the three ingredients on plane stress problems that exhibit elasto-plastic material behavior. Numerical examples are presented to illustrate the effectiveness of the approach. Comparison is made with respect to solutions provided by the commercial nonlinear code ABAQUS as reference.

A solid‐shell corotational element based on ANDES, ANS and EAS for geometrically nonlinear structural analysis

María Jesús Samper — Tue, 11 Aug 2020 11:06:20 +0200

This paper describes an eight‐node, assumed strain, solid‐shell, corotational element for geometrically nonlinear structural analysis. The locally linear kinematics of the element is separated into in‐plane (which is further decoupled into membrane and bending), thickness and transverse shear components. This separation allows using any type of membrane quadrilateral formulation for the in‐plane response. Assumed strain fields for the three components are constructed using different approaches. The Assumed Natural Deviatoric Strain approach is used for the in‐plane response, whereas the Assumed Natural Strain approach is used for the thickness and transverse shear components. A strain enhancement based on Enhanced Assumed Strain concepts is also used for the thickness component. The resulting element passes well‐known shell element patch tests and exhibits good performance in a number of challenging benchmark tests. The formulation is extended to the geometric nonlinear regime using an element‐independent corotational approach. Some key properties of the corotational kinematic description are discussed. The element is tested in several well‐known shell benchmarks and compared with other thin‐shell and solid‐shell elements available in the literature, as well as with commercial nonlinear FEM codes.

Mass Matrix Templates: General Description and 1D Examples

María Jesús Samper — Tue, 11 Aug 2020 11:00:54 +0200

This article is a tutorial exposition of the template approach to the construction of customized mass-stiffness pairs for selected applications in structural dynamics. The main focus is on adjusting the mass matrix. Two well known discretization methods, described in FEM textbooks since the late 1960s, lead to diagonally lumped and consistent mass matrices, respectively. Those models are sufficient to cover many engineering applications but for some problems they fall short. The gap can be filled with a more general approach that relies on the use of templates. These are algebraic forms that carry free parameters. Templates have the virtue of producing a set of mass matrices that satisfy certain a priori constraint conditions such as symmetry, nonnegativity, invariance and momentum conservation. In particular, the diagonally lumped and consistent versions can be obtained as instances. Availability of free parameters, however, allows the mass matrix to be customized to special needs, such as high precision vibration frequencies or minimally dispersive wave propagation. An attractive feature of templates for FEM programming is that only one element implementation as module with free parameters is needed, and need not be recoded when the application problem class changes. The paper provides a general overview of the topic, and illustrates it with one-dimensional structural elements: bars and beams.

Inverse mass matrix via the method of localized lagrange multipliers

María Jesús Samper — Tue, 11 Aug 2020 10:36:45 +0200

An efficient method for generating the mass matrix inverse of structural dynamic problems is presented, which can be tailored to improve the accuracy of target frequency ranges and/or wave contents. The present method bypasses the use of biorthogonal construction of a kernel inverse mass matrix that requires special procedures for boundary conditions and free edges or surfaces and constructs the free‐free inverse mass matrix using the standard FEM procedure. The various boundary conditions are realized by the the method of localized Lagrange multipliers. In particular, the present paper constructs the kernel inverse matrix by using the standard FEM elemental mass matrices. It is shown that the accuracy of the present inverse mass matrix is almost identical to that of a conventional consistent mass matrix or a combination of lumped and consistent mass matrices. Numerical experiments with the proposed inverse mass matrix are conducted to validate its effectiveness when applied to vibration analysis of bars, beams, and plain stress problems.

A Numerical Model for the Prediction of Microstructure Distribution across the Thickness of Quenched Steel Plates

María Jesús Samper — Tue, 11 Aug 2020 09:43:17 +0200

In this work we apply a numerical inverse analysis procedure based on the ICME framework to ensure a required microstructure and combination of properties in the quenched plate. The microstructure is decided first, and the cooling profile needed to obtain such microstructure is then calculated using the CALPHAD approach. Subsequently, an inverse analysis of the heat transfer problem provides the description of the convection mechanism in quenching that results in the demanded cooling profile. An additional constraint is set on the through-thickness thermal gradient to achieve a homogeneous microstructure. Finally, the resultant microstructure is computed. By means of the proposed model we are able to retrieve the necessary quenching process parameters and to quantify the influence of these parameters on the temperature and microstructure distribution within the plate after the quenching.

Computation of energy release rate for interfacial crack in orthotropic bimaterials

Salah Bouziane — Sun, 09 Aug 2020 10:44:03 +0200

The interfacial crack in bimaterials is a very interesting problem for composite materials and which has received particular attention from several researchers. In this study, we will propose a numerical modeling of the interfacial crack between two orthotropic materials using a special mixed finite element. For the calculation of the energy release rate, a technique, based on the association of the present mixed finite element with the virtual crack extension method, was used. The numerical model proposed, in this work, was used to study a problem of interfacial crack in bimaterials. Two cases were treated: isotropic and orthotropic bimaterials. The results obtained, using the present element, were compared with the values of the analytical solution and other numerical models found in the literature.

Tiempos de pandemia

Rafael Jesus Gasperi Romero — Fri, 07 Aug 2020 20:02:02 +0200

Estamos viviendo tiempo de pandemia de un virus tipo coronavirus el COVID 19, causante de una enfermedad que si bien el 80% de la población cursa como una gripe cualquiera con catarro, dolor de garganta, malestar general, tos, fiebre y se mejora rápidamente, en un 20% de la población este puede causar más daño prolongándose y profundizándose sus síntomas causando daños respiratorios, cardiovasculares y deterioro del estado general ameritando la hospitalización, tratamiento médico específico, algunos necesitaran apoyo ventilatorio en una UCI y un 2% de estos pacientes morirán . Es por ello que debemos estar conscientes de la situación que enfrentamos; una pandemia, un caso inédito en la historia reciente de la humanidad con repercusiones globales no solo desde el punto de vista Sanitario sino social y económico.

Dr. Oswaldo Valdivia: docente e investigador icono de la Universidad Centroccidental Lisandro Alvarado.

Rafael Jesus Gasperi Romero — Fri, 07 Aug 2020 19:54:03 +0200

El Doctor Oswaldo Valdivia nació en 1937 en Arequipa, Perú. En 1958 inició sus estudios en la carrera de medicina, en el primer año nació el amor por la neuroanatomía y en el cuarto año inicio su labor como docente. Se graduó como médico en 1964 y de anatomopatólogo en 1966. Contrajo matrimonio en 1962 con Zulema Valdivia con quien tuvo 3 hijos, en 1968 fue becado para entrenamiento de microscopia electrónica en la universidad de Wisconsin. En 1973 decidió emigrar hacia Venezuela debido a la situación del Perú y por una oferta laboral de parte de la Universidad Centroccidental Lisandro Alvarado (UCLA), donde se desenvolvió como profesor de anatomía microscópica a dedicación exclusiva, ampliadas contribuciones como la incorporación de la microscopia electrónica en la región centroccidental del país en 1980. Realizó diversos trabajos de investigación, principalmente sobre leishmaniasis, fue profesor jubilado activo de la UCLA hasta sus últimos días. En su ejercicio profesional dio grandes aportes a la salud pública, participando en jornadas de pesquisa de patológica cervical y formando profesionales de la salud. Fallece el 18 de noviembre de 2019 en Perú, su tierra natal.

La agroalimentación y la salud pública en Venezuela. Una visión desde el pensamiento complejo.

Rafael Jesus Gasperi Romero — Fri, 07 Aug 2020 19:30:02 +0200

El presente ensayo está fundamentado en el desarrollo de un cuerpo de argumentaciones de la realidad en la agroalimentación y la salud pública. Se sustenta en el modelo agroalimentario de dominación hegemónica, justificando así la importancia de la transformación del desarrollo económico productivo, sus relaciones con la salud pública, desde la complejidad de la realidad las condiciones de vida y la reproducción social; es por ello que se plantea como propósito el preámbulo de una teoría argumentada que oriente el desarrollo agroalimentario en la esfera de la producción, distribución, consumo, acceso a las oportunidades de alimentación en Venezuela y su relación con el mundo en el contexto de la preservación ambiental y la vida desde el paradigma de la complejidad,

Medicina del estilo de vida. Una alternativa ante el avance de las enfermedades no transmisibles.

Rafael Jesus Gasperi Romero — Fri, 07 Aug 2020 19:15:03 +0200

La medicina del estilo de vida (MdelEV) ha ganado relevancia en la prevención, el tratamiento y la reversión de la mayoría de las enfermedades crónicas, abordando directamente sus causas subyacentes. Con base en lo anterior, este artículo tuvo como objetivo realizar una revisión descriptiva de los elementos fundamentales que caracterizan a la Medicina del Estilo de Vida, sus avances en la investigación clínica en las áreas de nutrición, ejercicio y reducción del estrés, así como la tendencia creciente de convertirla en un componente integral de los planes de estudio de las escuelas de medicina. Se realizó una búsqueda exhaustiva en diferentes fuentes documentales, utilizando las categorías Medicina de Estilo de Vida, Principios básicos de Medicina de Estilo de Vida, Medicina Convencional vs. Medicina de Estilo de Vida, estudios clínicos en MdelEV, así como medicina de estilo de vida como subespecialidad médica. Se revisaron aproximadamente 60 artículos científicos abstractos. Mediante el uso de criterios de inclusión, se seleccionaron 27 artículos y se sometieron a una lectura integral y analítica. En el desarrollo y discusión del tema se presentan los resultados de la revisión, destacando el uso del modelo de determinantes sociales de la salud propuesto por la OMS, hallazgos de investigaciones experimentales sobre la reversión de los factores de riesgo en las ETS y avances en la formación. profesional. En la discusión combinan los resultados de todos los artículos seleccionados con una posición crítica. hallazgos de investigaciones experimentales sobre la reversión de los factores de riesgo en las ETS y avances en la formación profesional. En la discusión combinan los resultados de todos los artículos seleccionados con una posición crítica. hallazgos de investigaciones experimentales sobre la reversión de los factores de riesgo en las ETS y avances en la formación profesional. En la discusión combinan los resultados de todos los artículos seleccionados con una posición crítica.

Zoonosis como problema de salud pública desde una visión integral

Rafael Jesus Gasperi Romero — Fri, 07 Aug 2020 01:59:02 +0200

Las zoonosis como resultado de un proceso en el que intervienen diversos factores y están relacionados con variables epidemiológicas, económicas y sociales, se desarrolló este artículo de revisión, con el objetivo de reflexionar sobre la contextualización de los programas de control de las zoonosis como un problema de salud pública. Para la argumentación reflexiva de este documento, se realizó una revisión documental, con extracción, recopilación de información y análisis de los fundamentos teóricos y los resultados presentados en las investigaciones consultadas, visualizando los criterios descritos por diferentes autores, que pueden ser de gran utilidad. contribución debido a la poca información sobre enfermedades transmisibles de animales a humanos y las acciones del INSAI, como el órgano rector para abordar los problemas zoosanitarios y fitosanitarios,Destacando como conclusión la necesidad de capacitación del personal, participó en forma activa e interdisciplinaria los profesionales de la salud humana y animal.

Satisfacción en la prestación de servicios de salud en adultos mayores Colombia 2018

Rafael Jesus Gasperi Romero — Fri, 07 Aug 2020 01:49:02 +0200

El termino envejecimiento activo es relativamente novedoso y muy poco conocido a pesar de que en los últimos tiempos se ha hablado mucho de esto. El presente trabajo tiene como objetivo revisar la literatura frente a la atención en salud del adulto mayor. El método que se utilizó fue una revisión narrativa a través de una exploración de artículos científicos en las bases de datos: Science direct, Pubmed y Scielo. En los resultados se encontró que en varios artículos los adultos mayores no se encontraban satisfechos con la atención que recibían en salud. Como conclusiones según lo referido por las personas mayores, hay una brecha grande entre lo que se dice en las políticas públicas de envejecimiento y lo que realmente se está haciendo en la prestación de servicios. En cuanto a la percepción que tienen los adultos mayores de su salud en Antioquia, hay un porcentaje muy alto que cree que la atención en salud no es la adecuada.

Enteroparasitosis en escolares: importancia de los parásitos asociados

Rafael Jesus Gasperi Romero — Fri, 07 Aug 2020 01:18:02 +0200

La presencia simultánea de varios parásitos puede tener importancia clínica-epidemiológica y también podría determinar la conducta terapéutica a seguir en la persona poliparasitada. Se realizó un estudio retrospectivo de tipo correlacional, partiendo de niños en edad escolar poliparasitados, se determinó el índice de afinidad de Fager para establecer las asociaciones entre los principales enteroparásitos identificados y determinar su importancia. Entre los años 2007 y 2016 fueron evaluados 11 escuelas (10 en el municipio Heres y 1 en el municipio Caroní) y 1277 niños entre 5 y 15 años de edad. La distribución de los niños evaluados fue homogénea con relación al género ya que se estudiaron 48,0% (n = 613) del género femenino y 52% (n = 664) del masculino. De las 1277 muestras analizadas, 811 muestras formas evolutivas de enteroparásitos, lo que indica un 63,7% de frecuencia. En orden los más comunes fueron los cromistas mostraron porBlastocystis spp. (49,3%), seguido de los protozoarios Endolimax nana (14,5%) y Entamoeba coli (11,7%), y en último lugar, los helmintos, donde el mayor frecuencia fue Ascaris lumbricoides (5,5% ) Los escolares resultaron poliparasitados en 43.2% de los casos, siendo el parásito más específicamente asociado Blastocystis spp. con 90% de los casos, seguido de E. nana (43,1%) y E. coli (35,4%). Los índices de afinidad (I _AB ) y las pruebas de "t" efectuadas, señalan asociación entre Blastocystis spp. y los protozoarios G. intestinalis , E. nana y E. coliy entre los geohelmintos Trichuris trichiura y A. lumbricoides . En conclusión, las asociaciones que expresaron significación estadística de acuerdo al índice de afinidad de Fager y al estudio estadístico fueron: entre el cromista Blastocystis spp. y los protozoarios G. intestinalis , E. nana y E. coli y entre los helmintos Ascaris lumbricoides-Trichuris trichiura .

ARTERIAL HYPERTENSION AND SOCIAL LONELINESS IN AN OLDER POPULATION FROM 55 YEARS AND OLDER. BARQUISIMETO, VENEZUELA

Rafael Jesus Gasperi Romero — Fri, 07 Aug 2020 00:57:02 +0200

Social loneliness has been shown to be closely related to the health status of the population. The present research assessed the prevalence of arterial hypertension and its association with the perception of social loneliness in residents over 55 years of age in the Antonio Carrillo community. In this regard, an analytical prevalence study, 92 people were studied as population, the previous diagnosis of arterial hypertension was asked, blood pressure was measured, and the data collection tool ESTE II was subsequently applied. It was found that, in the population over 55 years of age, according to the general characteristics of the population, the female sex predominated (63%); in addition, the majority of the habitants were between the ages of 55 and 64 (52.2%); according to marital status, married residents prevailed (41.3%); with regard to education, the majority of the population underwent incomplete primary education (24%) and in regards of the inactivity by the unemployment situation (76.1%). 65.2 % of the population was found to be hypertensive, and in terms of the perception of social loneliness, 68.5% of the population was found to have the same conditions. As in fact, when the association between blood pressure and the perception of social loneliness was made, it was specified that 68.3% of the habitants with perception of social loneliness were hypertensive (p>0.05).

Prueba tbala

Héctor Tabares-Ospina — Thu, 06 Aug 2020 20:58:03 +0200

Predecir alteraciones clínico–metabólicas mediante el índice circunferencia abdominal-talla depende del sitio de medición

Rafael Jesus Gasperi Romero — Thu, 06 Aug 2020 01:03:02 +0200

El objetivo fue determinar la capacidad del índice circunferencia abdominal – talla para predecir dos o más alteraciones clínico-metabólicas, según sitio de medición de circunferencia abdominal. Se hizo un estudio observacional analítico transversal. Se estudiaron 800 adolescentes de ambos sexos (405 hombres), entre 15 y 19 años de edad, inscritos en 31 instituciones de educación media – diversificada y educación universitaria del Municipio Iribarren – Lara - Venezuela. Los jóvenes firmaron consentimiento informado. Se evaluó nivel socioeconómico, hábitos alimentarios, actividad física, talla y circunferencia abdominal en tres sitios anatómicos: encima de crestas ilíacas, punto medio y debajo del reborde costal. Se calculó la índice circunferencia abdominal – talla para cada sitio. Se evaluó presión arterial, perfil lipídico y glucemia. Se realizaron análisis estadísticos bivariados y regresión logística. Como resultado se logró modelar la probabilidad de dos o más alteraciones clínico – metabólicas como una función del índice circunferencia abdominal – talla, tanto debajo del reborde costal como en crestas ilíacas, sin embargo, el grado de asociación varió según el sitio anatómico, siendo mayor la probabilidad de dos o más alteraciones clínico – metabólicas en el primero. Tal probabilidad fue directamente proporcional al índice circunferencia abdominal – talla y mayor en hombres. Se concluye que la grasa abdominal superior predijo mejor dos o más alteraciones clínico – metabólicas en comparación con la grasa inferior del abdomen. Con esto podría unificarse el sitio anatómico de medición de la circunferencia abdominal, inmediatamente por debajo del reborde costal, para identificar adolescentes con mayor probabilidad de presentar dichas alteraciones, como factores de riesgo cardiovascular.

Accidentes de trabajo y discapacidad en policías Municipales de Iribarren del Estado Lara

Rafael Jesus Gasperi Romero — Thu, 06 Aug 2020 00:55:01 +0200

La presente investigación tuvo como objetivo determinar las características de los accidentes de trabajo y discapacidad en los policías Municipales de Iribarren del Estado Lara. El estudio estuvo enmarcado en una investigación de campo descriptiva, transversal. Se seleccionó una población objetivo de 297 trabajadores de dicho cuerpo de seguridad. Se aplicó un cuestionario previamente validado mediante el juicio de tres expertos, utilizando la técnica de la entrevista, con el fin de determinar los aspectos sociodemográficos y laborales de los policías, las características epidemiológicas y laborales de los accidentes de trabajo, las características clínicas y laborales de los accidentes de trabajo y la discapacidad según aspectos clínicos- legales. Los resultados más significativos fueron: 37,71% de los policías tenían edades comprendidas 33-37 años, 88,55 % eran del sexo masculino, su grado de instrucción era secundaria en 50.84%, %, el estado civil es casado 54,88%, tenían turno mixto el 84,85%, el rango operativo 94,61 %, el 41,08 % de los policías eran oficiales agregados, en cuanto a la antigüedad laboral es de 6 a 10 años en un 51,85 %. La frecuencia de accidente fue 59.60%, de los instrumentos causantes del accidente fueron armas de fuego 57,85%, la actividad realizada al momento del accidente fue el patrullaje en 22,60%, el tipo de accidente fue por el hecho en 51,98%, el 73,45% de los policías no notifico el accidente, el sitio anatómico más afectado fueron los miembros superiores 27,68%, la severidad de la lesión fue grave en 42,37%, el tipo de discapacidad más común fue la parcial permanente en 38,42%. Este estudio es un aporte para la Policía Municipal y para organizaciones similares, pues sus resultados proporcionan datos estadísticos que pudieran servir para la realización de programas preventivos que permitan buscar soluciones al problema de los accidentes de trabajo y la discapacidad.

Los trabajos de fans como crítica

Belén García de Pablos — Tue, 28 Jul 2020 12:00:03 +0200

Habitualmente se piensa que los fans y los trabajos hechos por fans se basan únicamente en la adoración de éstos por los textos de origen, pero esto no es cierto: los fans son muchas veces los críticos más feroces ya que, al conocerlos tan bien y formar parte de una comunidad en torno al texto, son capaces de encontrar recursos (literarios, cinematográficos, etc.) recurrentes que muchas veces contienen elementos machistas, racistas o similares, que los fans pasan a criticar; en otros casos es el mismo texto el que les permite hacer una crítica de la sociedad; y en otros, los fans se sienten traicionados por el autor del texto base y pasan a criticar la actitud de éste. Estas críticas pueden realizarse de muy distintas maneras: ya sea por medio de textos escritos en las que se analiza el texto base, por medio de relatos, películas, cómics, vídeos u otros en los que se expone la crítica. Son en los trabajos creativos (relatos/películas/vídeos) realizados por fans en los que nos centraremos, ya que son los que se alejan más de la crítica tradicional y, aunque formalmente algunos de ellos puedan estar más cerca de la parodia o la sátira, el fondo normalmente se aleja de ellas tanto por la relación de los fans con el texto, como por el contenido de las mismas.

Proper Use of Technical Standards in Offshore Petroleum Industry

Dejan Brkić — Sat, 25 Jul 2020 13:04:02 +0200

Ships for drilling need to operate in the territorial waters of many different countries which can have different technical standards and procedures. For example, the European Union and European Economic Area EU/EEA product safety directives exclude from their scope drilling ships and related equipment onboard. On the other hand, the EU/EEA offshore safety directive requires the application of all the best technical standards that are used worldwide in the oil and gas industry. Consequently, it is not easy to select the most appropriate technical standards that increase the overall level of safety and environmental protection whilst avoiding the costs of additional certifications. We will show how some technical standards and procedures, which are recognized worldwide by the petroleum industry, can be accepted by various standardization bodies, and how they can fulfil the essential health and safety requirements of certain directives. Emphasis will be placed on the prevention of fire and explosion, on the safe use of equipment under pressure, and on the protection of personnel who work with machinery. Additionally considered is how the proper use of adequate procedures available at the time would have prevented three large scale offshore petroleum accidents: the Macondo Deepwater Horizon in the Gulf of Mexico in 2010; the Montara in the Timor Sea in 2009; the Piper Alpha in the North Sea in 1988.

PLAN DE NEGOCIOS

Annia Quintero — Wed, 22 Jul 2020 22:12:02 +0200

Occicorps Cosmetics, C.A. es una empresa conformada por la Corporación Occipital (constituido por Adriana Castellanos, Annia Quintero, Thalía Samarjian, Miguel Tovar y Glen Zea), y surge como una iniciativa privada de cinco jóvenes farmacéuticos que desean atender las necesidades de negocio de empresas pequeñas a medianas que requieran fabricar productos cosméticos y de cuidado personal, pero que no cuentan con el conocimiento, instrumentos, o las instalaciones para concretar sus proyectos.

Occicorps Cosmetics C.A. presenta un plan de negocios como una empresa dedicada al diseño, producción y almacenamiento de productos cosméticos líquidos y semisólidos de pequeña a mediana escala, contribuyendo al desarrollo de otros emprendedores. Diseñamos y producimos cosméticos de alta calidad.

Este plan de negocios tiene como propósito dar a conocer el objetivo general y los objetivos específicos de la empresa, aunado a nuestra razón de ser como la misión, visión y los valores que rigen a la compañía. También exponemos el abanico de servicios que estamos en la posibilidad de ofertar y estrategia de marketing. A partir de esta información, se evaluará la rentabilidad y viabilidad de la empresa.

Project B2 - Evaluation of Work Zone Mobility by Utilizing Naturalistic Driving Study Data

Scipedia content — Tue, 21 Jul 2020 11:46:19 +0200

The purpose of this research is to evaluate work zone mobility using SHRP2 NDS naturalistic driving data. In this study, forward, rear-view video, and time series traces for traversed work zones are collected at 0.1 second intervals. Using the forward and rear-ward videos work zone configurations such as traffic control devices, area types, presence of dynamic message signs (DMS), intelligent transportation systems technologies (ITS), as well as the presence of workers and equipment are identified. These videos are also used for estimating traffic density by observing the number of vehicles surrounding the participant, and average traffic flow by observed speed and traffic density. The capacities of different work zone sections and configurations are estimated based on developed speed-flow relationships and this capacity is used to verify and calibrate the work zone capacity method defined by the newest addition of Highway Capacity Manual (HCM). The probability of breakdown is then estimated for each level of flow rates and volume to capacity ratios can be used to verify the results of HCM.

CLOUD TECHNOLOGY: A CATALYST FOR EFFECTIVE ENTREPRENEURSHIP EDUCATION IN NIGERIA

UYAI EMMANUEL AKPANOBONG — Mon, 20 Jul 2020 21:40:02 +0200

Information technology plays a major role in organising and operating a business by any entrepreneur. Entrepreneurship involves the ability to be creative, innovative and taking risk as well as the capability to plan and manage projects to achieve the set objectives. Entrepreneurship education contributes to economic growth in developed nations. The financial crisis and the economic recession being experienced now in the country has really affected the entrepreneurs who operate Small and Medium Scale Enterprises (SMEs) in Nigeria due to inability to explore modern technology in their businesses. Therefore, there is need for cloud technology to be adopted as a powerful catalyst that would enhance job creation through entrepreneurship education. The potential for cloud computing to boost entrepreneurs as a platform for entrepreneurship education is significant. This technology can be adapted to support entrepreneurship education as it can be scaled over time to offer the required resources. Research has shown that cloud computing has played a catalytic role in boosting education in the world. Thus, the paper examined the potential benefits and issues involved in adopting cloud computing to support entrepreneurship education. It was concluded that the adoption of cloud computing would help to generate thousands of new jobs in Nigeria. It was also recommended among others that the National Information Technology Development Agency should organise workshops, seminars and training for existing entrepreneurs on the benefits of using cloud technology. Cloud technology should also be introduced as one of the courses in business education and allied fields.

Machine learning techniques to predict the compressive strength of concrete

Priscila F. S. Silva — Wed, 15 Jul 2020 02:51:13 +0200

Conventional concrete is the most common material used in civil construction, and its behavior is highly nonlinear, mainly because of its heterogeneous characteristics. Compressive strength is one of the most critical parameters when designing concrete structures, and it is widely used by engineers. This parameter is usually determined through expensive laboratory tests, causing a loss of resources, materials, and time. However, artificial intelligence and its numerous applications are examples of new technologies that have been used successfully in scientific applications. Artificial neural network (ANN) and support vector machine (SVM) models are generally used to resolve engineering problems. In this work, three models are designed, implemented, and tested to determine the compressive strength of concrete: random forest, SVM, and ANNs. Pre-processing data, statistical methods, and data visualization techniques are also employed to gain a better understanding of the database. Finally, the results obtained show high efficiency and are compared with other works, which also captured the compressive strength of the concrete.

The solution of non‐linear hyperbolic equation systems by the finite element method

María Jesús Samper — Thu, 09 Jul 2020 17:07:56 +0200

The difficulties experienced in the treatment of hyperbolic systems of equations by the finite element method (or other) spatial discretization procedures are well known. In this paper a temporal discretization precedes the spatial one which in principle is considered along the characteristics to achieve a self adjoint form. By a suitable expansion, the original co‐ordinates are preserved and combined with the use of a standard Galerkin process to achieve an accurate discretization. It is shown that the process is equivalent to the Taylor‐Galerkin methods of Donea.17

Several examples illustrate the accuracy and efficiency attainable in such problems as transport, shallow water equations, transonic flow etc.

Accelerated ‘relaxation’ or direct solution? Future prospects for fem

María Jesús Samper — Thu, 09 Jul 2020 16:39:18 +0200

‘Dynamic’ or ‘viscous’ relaxation procedures have not gained much popularity in finite element analysis in which the direct (Gaussian elimination) solution dominates. Reasons for this are various—the most important being the rather slow convergence generally achieved for such procedures. However, it is possible to accelerate this quite dramatically and a method of doing so is shown in this paper. With the use of such acceleration and the inherent advantages of greatly reduced storage requirements and simplicity of programming, relaxation procedures promise an exciting possibility for the solution of large two‐ and three‐dimensional problems in both linear and nonlinear ranges.

Recent developments in FEM-CFD

María Jesús Samper — Thu, 09 Jul 2020 16:34:44 +0200

The current status of CFD with regard to unstructured grids employing finite element methods and Eulerian frames is reviewed. Algorithms suitable for the computation of large three-dimensional problems involving flow past arbitrary geometries are developed. Adaptive mesh refinement strategy is reviewed, and domain splitting or local time-stepping are briefly addressed. The development of search algorithms of optimal order, variable time-stepping Jacobi smoothers for elliptic problems, and transport concepts for hyperbolics to help achieve good performance for unstructured multigrid processes is discussed. As examples, transient supersonic flow in a channel, regular shock reflection of a wall, viscous flow past a protruberance, potential flow past a cylinder, and Burgers equation are considered.

A simple extension to multidimensional problems of the artificial viscosity due to Lapidus

María Jesús Samper — Thu, 09 Jul 2020 16:31:03 +0200

An artificial viscosity suitable for use in the simulation of multidimensional high-speed compressible flows by the finite element method is proposed. The form adopted is a consistent extension of the method due to Lapidus.

An adaptive finite element procedure for compressible high speed flows

María Jesús Samper — Thu, 09 Jul 2020 16:27:54 +0200

A two-step explicit finite element procedure for obtaining time-accurate solutions to the compressible Euler equations is described. The method is coupled with an adaptive mesh refinement process which enables steady state solutions of improved quality to be obtained.

Effective Programming of Finte Element Methods for Computational Fluid Dynamics on Supercomputers

María Jesús Samper — Thu, 09 Jul 2020 16:15:54 +0200

The effective programming of Finite Element Methods for CFD on vector-machines is discussed. It is shown, that for unstructured grids the performance observed on this class of machine depends heavily of the availability of hardware Gather/Scatter. Timings obtained for a 3-D Euler code are presented for the CYBER-205, CRAY-XMP11 and CRAY-XMP48.