Characterisation of fatigue crack growth and related damage mechanisms in FRP–metal hybrid laminates

Fatigue crack growth and related damage mechanisms were investigated experimentally in a hybrid laminate consisting of carbon fibre-reinforced epoxy and an aluminium alloy. During the fatigue cycling, the strain within a defined area in the vicinity of the crack was measured directly using embedded fibre-optic Bragg grating sensors. The progress of delamination damage was monitored by means of an in situ ultrasonic C-scanning technique. A simplified empirical model has been applied to determine an effective stress intensity factor applied to the crack tip in the alloy material. Fatigue crack growth rate in the hybrid laminate, characterised using the effective stress intensity factor, was demonstrated to be in reasonable agreement with the data for the monolithic aluminium alloy, showing that the methods used to partition the load were reliable. The paper thus demonstrates a viable technology for in situ strain measurements, offers a new insight into the damage mechanisms that prevail in hybrid laminates, and validates simplified procedures for estimating crack growth.