Crack Closure Behavior in Short Carbon Fiber Reinforced Plastics

Abstract

Using the compact tension (CT) specimen made of the short carbon fiber reinforced plastic (sCFRP), the fatigue crack propagation and crack closure behavior was observed in this study. The fatigue crack propagation rate, da/dN, was lower for the specimen loaded to the molding direction than that loaded to the transverse direction in the relationship between da/dN and stress intensity factor range, ΔK. For both MD specimen and TD specimen, da/dN is higher in the specimen with high stress ratio, R(=P_min/P_max), under the same maximum load, P_max. The displacement on the edge of the notch was measured by a clip-gage at the same frequency of fatigue test. Then, crack opening load, P_op, and closure load P_cl were measured from the load-displacement diagrams by unloading elastic compliance method. P_cl was higher in MD specimen than in TD specimen. The effective fractions, U=(P_max-P_op)/(P_max-P_min) or U=(P_max-P_cl)/(P_max-P_min), were calculated and the effective stress intensity factor range, ΔK_eff(=UΔK), and ΔK_eff/E, in which E is Young's modulus, were evaluated. The relationship between da/dN- ΔK_eff/E evaluated from P_cl was relatively identical for the different specimens with the various R.