Crack Jumping in Fabric Composite Fracture Testing

Abstract

Interlaminar fracture is a failure mode that fiber-reinforced polymer composites (FRPC) are commonly susceptible to during loading. The strain energy release rate associated with delaminations can be the limiting factor in a laminate’s design. Standard test methods have been developed to measure the critical strain energy release rates using precracked coupons, such as the double cantilever beam (DCB). However, since the adherends in these coupons are laminates themselves, often the crack can initiate a secondary crack within one of the adherends and propagate along a secondary interface as well as the primary, precracked, interface. Deconvoluting the effects of the two cracks, a bridged ply, and multiple crack tips can turn a relatively simple test in determining a material property into a very complicated structural problem. In most cases, it is best to scrap the data collected after the crack has jumped interfaces and start with a fresh specimen. For fabric composites, this phenomenon can be quite common due to the variation in bond line thickness between plies resulting from the architecture of the fabric itself (tow size, weave architecture) as well as manufacturing flaws (voids, foreign object debris). This study aims to use the crack jumping phenomenon to learn more about the characteristics of the process zone through the insertion of designed flaws as well as design a method for evaluating the fracture properties of a toughened film adhesive in a co-cured context.