Fatigue Crack Growth Behavior of Unidirectional and Woven CFRP/Aluminum Adhesively Bonded Joints with a Thermosetting Film Epoxy Adhesive

Abstract

To investigate the effect of the direction of fiber orientation of the adherend CFRP on fatigue behavior of adhesively bonded CFRP/aluminum joints, fatigue crack growth tests were conducted on double cantilever beam (DCB) joints with unidirectional and woven CFRP and aluminum alloy adherends using a heat cured filmy type epoxy adhesive. Fatigue crack growth tests showed that fatigue threshold, Δ G_th of the woven CFRP/aluminum joint was slightly larger than that of the unidirectional CFRP/aluminum joint, and the slope of region II of the crack growth rate for the woven CFRP/aluminum joint was smaller than that of the unidirectional CFRP/aluminum joint. Moreover, it was also observed that cracks propagated to the aluminum interface for the unidirectional CFRP/aluminum joints and to the CFRP interface for the woven CFRP/aluminum joints. To investigate the cause of the difference in crack propagation direction between the two types of joints, crack propagation simulation was conducted using the finite element method for two types of joints, crack growth analysis also revealed that cracks of both the unidirectional and woven CFRP/aluminum joints propagated toward the CFRP interface. The direction of crack growth of the woven CFRP/aluminum join was consistent with the simulated result, whereas the direction of the unidirectional CFRP/aluminum joint differed from the simulated results. This cause was assumed to be due to that the maximum value of normal stress on the unidirectional CFRP/aluminum interface was larger than the maximum value on the woven CFRP/aluminum interface.