Effect of process zone constraint on the fracture toughness of a laminated graphite/epoxy composite

Abstract

Results are presented from an experimental study investigating the effects of process zone constraint on the fracture toughness of fiber reinforced laminated composites. Three specimen types were each tested to fracture in mode I, mode II and mixed mode (Gn/G=0.4). The specimen types consisted of a unidirectional laminate with high resin content, a unidirectional laminate with low resin content, and a high resin content laminate with a stacking sequence designed to constrain the size of the process zone (referred to here as a laminate). Comparison of the unidirectional laminate test results show that there is a correlation between resin content, process zone constraint, and fracture toughness. Laminates with higher resin content exhibited higher fracture toughness than low resin content laminates, which may be attributed to less process zone constraint in the high resin content laminates. Fracture toughnesses for higher resin content laminates were also equal to or larger than those from than the constrained laminates. This may also be attributed to less process zone constraint in the unidirectional high resin content laminates. It is therefore recommended that constrained specimens be used for all subsequent fracture toughness tests to provide the most conservative value of G c for all mode ratios. Additionally, for fracture critical structure, it is shown how test results can be used to achieve an optimum design.