Fatigue crack growth behavior of continuous fiber reinforced composite materials was studied under variable stress conditions. Two kinds of laminates with different ductility were employed: a hybrid laminate of glass fiber-reinforced epoxy and 7075-T6 aluminum alloy (GLARE1) and a plain-weave roving fabric laminate of glass fiber-reinforced epoxy (GFRP). Fatigue tests of center-notched (GLARE1) and double-edge-notched (GFRP) plate specimens were performed for the cases of stress-increase and stress-decrease, under repeated tension-tension and alternating tension-compression loading conditions. For GLARE1, acceleration and retardation of crack growth were observed under repeated tension-tension loading condition, while they were not under alternating tension-compression loading condition. The fatigue crack of this material propagated only in the aluminum layer, and its growth rate was not adequately correlated with the stress intensity factor range ΔK. It was found that the crack growth rate was successfully correlated with an effective stress intensity factor range ΔKeff defined taking into account both the crack-closure and the fiber-bridging in the crack wake so that ΔKeff=ΔKbr-ΔKcl. The acceleration and retardation were explained using this parameter. For GFRP, on the other hand, no appreciable effect of the stress change on fatigue crack growth was detected. This was attributed to the brittleness of this laminate. The crack growth rate, in this case, was correlated well with the stress intensity factor range.
