Abstract

Abstract

The numerical simulation of complex failure modes of composite materials, such as delamination, can be computationally very demanding, as it requires special elements and/or numerical strategies to characterize damage onset and propagation. This work presents several formulations developed to optimize the computational performance of an explicit finite element code designed specifically for the simulation of large scale composite structures. The composite mechanical performance is obtained with the matrix-reinforced mixing theory, a simplified version of the serial/parallel mixing theory that does not require an iterative procedure or the calculation of the tangent stiffness matrix. The number of elements required to perform the simulation is reduced by stacking several layers inside a single finite element. This work also proposes a modification of the isotropic damage law, capable of taking into account the residual strength provided by friction in type II fracture modes. The ability of these formulations to successfully predict the mechanical performance of composite materials is assessed with the ply drop-off test. In this test a laminate with a change of thickness in its mid-span is loaded until it breaks due to a delamination process. The formulation proposed obtains a very accurate prediction of the experimental response of the test, as it provides a very good characterization of the initial laminate stiffness, the delamination onset, and its propagation along the specimen. 

Abstract

Based on the classical laminated plate theory, a variational approach for the study of the statical and dynamical behaviour of arbitrary quadrilateral anisotropic plates with various boundary conditions is developed. The analytical formulation uses the Ritz method in conjunction with natural coordinates to express the geometry of general plates in a simple form. The deflection of the plate is approximated by a set of beam characteristic orthogonal polynomials generated using the Gram–Schmidt procedure. The algorithm developed is quite general and can be used to study fibre reinforced composite laminates with symmetric lay-ups, which may have general anisotropy and any combinations of clamped, simply supported and free edge support conditions. Various numerical applications are presented and some results are compared with existing values in the literature to demonstrate the accuracy and flexibility of the present method. New results were also determined for plates with different geometrical shapes, combinations of boundary conditions, several stacking sequences and various angles of fibre orientation.

Abstract

The scale effect in Composite Materials is known as the delay in the appearance of damage in the weakest laminas of a laminate (typically 90 degrees laminas). The availability of ultra-thin plies, of up to 20 microns of thickness, has attracted the attention on this effect, as the use of these plies can imply the delay of the onset of damage at a laminate in a significant manner. The frame of the present work is a micromechanical study of the different phases of damage assumed to lead to the failure of the weakest lamina. Thus, it is assumed that the damage starts by isolated debondings between fibres and matrix, then continuing by the abandon of the interphase, the debonding crack kinking into the matrix, and finally the damage progressing either through the matrix itself or along the interfaces between fibres and matrix, to generate a transversal crack in the 90 degrees lamina. The study is carried out using a multi-scale model based on the Boundary Element Method, with control cells in which the damage at micromechanical level previously described is allowed. Concepts derived from Fracture Mechanics and Interfacial Fracture Mechanics are applied in the analysis. The conclusions obtained with this study are compared with those that clearly manifest the scale effect based on a global energy balance from the pristine state to the final situation with the 90 degrees lamina having a complete transverse crack.