Cure path dependency of static and dynamic mode II interlaminar fracture toughness of interlayer toughened composite laminates

Abstract Faster cure cycles are desirable to achieve higher production rates for thermoset prepreg composites. This work studies the effect of cure cycle on the static and dynamic Mode II interlaminar fracture toughness ( G I I c ) of an interlayer toughened thermoset prepreg composite, Toray T800SC/3900-2 B. Unidirectional laminates with different interlayer microstructures were manufactured to the same degree of cure using cure cycles with varying heating rates (0.1–7.0 °C/min) and cure temperatures (120–200 °C). Initiation and propagation G I I c were measured for a range of loading rates (0.0083–2871 mm/s) using non-precracked (NPC) and precracked (PC) end-notched flexure (ENF) tests, respectively. Results show that initiation toughness is not sensitive to curing conditions, but static propagation toughness is. Furthermore, the cure dependence of propagation G I I c decreases with increasing loading rate. This was found to be due to the delamination migration produced when loaded at different rates. Specifically, it was found that the interlayer microstructure and loading rate both influence the formation of interlaminar microcracks and the subsequent rate at which the crack migrates from the toughened interlayer to the more brittle intralayer. In laminates made with high heating rates or high cure temperatures, the crack mainly remains in the toughened interlayer during Mode II loading regardless of loading rate, and there is no noticeable reduction of propagation G I I c . This study suggests that it may be possible to develop faster cure cycles than the one currently recommended for T800SC/3900-2 B laminates, yet retain the same interlaminar fracture toughness.