Thermally induced damage in composite laminates: Predictive methodology and experimental investigation

A general analysis method is presented to predict matrix cracks in a composite laminate, together with the resulting degradations of laminate properties, as functions of temperature or thermal cycles. All plies except the surface plies are considered. A shear-lag solution for the stresses in the vicinity of cracks and a fracture mechanics crack formation criterion are used to predict cracks. Damage is modeled incrementally, which allows the inclusion of the effects of temperature-dependent material properties and softening of the laminate due to previous cracking. The analysis is incorporated into an easy-to-use computer program. Experimentally, crack densities are measured in a variety of laminates exposed to decreasing temperatures. Crack densities are measured at the edges of specimens by microscopic inspection, and throughout the specimen volumes by X-radiography and sanding down of the edges. In specimens with thick ply groups (several plies of the same angle stacked together), cracks behave 'classically', running the width of the specimens. In specimens with single ply groups, smaller, discontinuous cracks developed. Correlation between the analytical results and the edge crack densities were reasonable for those specimens which behaved classically. Crack densities measured at specimen edges do not agree with internal crack densities (or analyses) in specimens with non-classical cracks. A free-edge stress analysis clarifies the reasons for these discrepancies. (C) 1996 Elsevier Science Limited