Life Prediction for Composite Material Structures

Abstract

Abstract Recently developed high performance composite materials, such as ceramic or metal matrix composites, exhibit a wide variety of mechanical and thermal effects. A simultaneous development has occurred in manufacturing, where two and three dimensional weaves are now common. These new manufacturing techniques have increased the complications already inherent in predicting the life of advanced composite material components. As a result a general life prediction model for composite material is being developed. A finite element analysis, using a nonlinear finite element code, forms the backbone of the system. A viscoplastic model, applicable to generally orthotropic materials, is included in the code. The finite element analysis simulates the loading history and response for at least one cycle, and possibly several cycles. These cycles do not need to be successive. The results of the finite element analysis are written to a post process file for use in the life model. The viscoplastic model includes parameters that can vary with environmental degradation. For example, SiC fibers may oxidize when present in air at high temperatures. The properties will then degrade with time. This degradation is included through the use of a chemical kinetics and diffusion model. Diffusion, in the case of SiC fibers, is primarily due to oxygen along the fiber matrix interface, but does occur through the fibers and the matrix. Hence the diffusion is orthotropic for a single layer of uniaxial material. The diffusion is modeled using a thermal finite element analysis. The chemical kinetics is included as a heat source. The results of the analysis are written to a post process file for use in the viscoplastic model and the life model. The life model can be either probabilistic or deterministic. The life probabilistic model is based on weakest link theory and includes time dependence. The probabilistic model can be run in a mode that is equivalent to existing damage models.