Fracture mechanism transitions in laminate composites

Abstract

Possible effects of using standard metallurgical fracture toughness specimens to measure the fracture toughness KIC of laminates and high-volume-fraction composites have been analysed. It has been shown that fracture can take place either through the laminates or by delamination. The method of load application causes tensile stresses perpendicular to the laminates and shear stresses parallel to the laminates, either of which can cause pre-existing flaws in the laminate interface to propagate or the bond strength to be exceeded. Expressions have been determined that permit prediction of the mechanism of crack extension, provided that certain geometric and materials parameters are known. They indicate that delamination is more likely for small specimens, short cracks, high transverse fracture toughness, and low interfacial toughnesses and strengths. Thus, geometric factors can play a more important role in fracture of composites than in isotropic materials, and care must be exercised when assessing the results of toughness tests.