Prediction of Inter-laminar Stresses in Composite Laminates using Mixed Finite Element Model

The introduction of composite materials has brought out a radical change in material applications in various fields. Its high strength to weight ratio, long fatigue life and other superior material properties have paved way for its use in highly weight sensitive applications such as aerospace & automotive structures. In all these applications, the level of precision and accuracy is very significant and hence a good understanding of the behavior of composite materials is essential. Delamination is one of the critical problems faced by composite laminates. It involves separation of composite laminae, especially at the free edge caused due to low strength along the ply interface and high local interlaminar stresses. High interlaminar shear stress on the free edge of angle-ply laminates can cause edge delamination, whereas the same effect will be produced by interlaminar normal stress in cross-ply laminates. Thus it is essential to accurately predict the interlaminar stresses to understand the failure mechanism involved in the delamination of laminated composites. In the present study, an 18-noded three-dimensional mixed finite element model is developed, having three transverse displacement and three interlaminar stresses as degrees of freedom per node, using minimum potential energy principle. The transverse stress quantities are invoked from the assumed displacement fields by using fundamental elasticity relations. This ensures the satisfaction of elasticity equations throughout an elastic continuum, which is lacking in numerical methods based on other mixed variational principles. The study would be extended for free and forced vibration analysis of the composite laminate.