Comparison of FEA and LPT Based Failure Analysis of P75 Graphite/934 Epoxy Composites

A comparison between a micro-mechanical finite element model (FEM) and the macro-mechanical laminated plate theory (LPT) has been undertaken to investigate the failure of an 8-ply [0/0/90/90]s P75 Graphite/934 Epoxy composite subjected to mechanical and thermal loads. This study deals with the initiation of failure, ultimate failure, and resulting stresses at each failure, as well as the effect of thermal residual stresses on the failure of the composite material. Thermal residual stresses develop as a result of the difference in the coefficients of thermal expansion of the fiber and the matrix, and the operating (75°F) and stress-free or cure temperature (350°F). Relationships between the boundary condition of the FEM and LPT are also explored. The ANSYS finite element program has been used in this research. The initiation of failure is based on the ultimate radial stress criterion developed by Bowles and Griffin. For LPT analysis, the first-ply failure and consequent failures are derived using the Tsai-Wu failure theory. The analyses indicate intermediate degradation of the laminate as the applied stress/strain is increased. This information is used to predict the loss in overall stiffness as a function of loading.