On Microcracking in Composite Laminates under Thermal and Mechanical Loading

Composite laminates used in many applications are exposed to both thermal and mechanical loads. These loads can cause cracks in the matrix, changing the laminates’ properties. An analytical methodology is developed to predict microcrack density in a general laminate exposed to an arbitrary thermomechanical load history. The analysis uses a shear lag stress solution in conjunction with an energy-based cracking criterion. It is found that crack densities depend on the stress in the cracking ply group, regardless of the source of the stress. Crack densities are a nonlinear function of stress, however, so if both thermal and mechanical loads are present, the cracking response is coupled. Experimental investigation is used to verify the analysis. Correlation between analysis and experiment is generally excellent. The method successfully predicts the initiation of cracking and crack accumulation under both thermal and mechanical loads, in most ply groups in a variety of laminates, using a single set of ply material properties. The cases where the analysis and experiment do not agree shed some light on the limits of current modelling techniques. Given an understanding of these limits, the analysis appears to be a useful preliminary design tool.