Fatigue threshold and crack growth rate of adhesively bonded joints as a function of load/displacement ratio

The effect of the load and displacement ratios on the mixed-mode fatigue threshold and fatigue crack growth rate of a toughened epoxy adhesive was examined experimentally. The fatigue crack growth rates were presented as functions of various forms of the strain energy release rate, G. Calculating G using the magnitude of the load range, GΔP, is shown to reflect the range of crack-tip stresses during a loading cycle. This was found to eliminate the effect of load and displacement ratio on the fatigue threshold, Gth, in the low crack growth rate region, da/dN < 5 × 10−5 mm/cycle, where adhesive plasticity was relatively small. In contrast, fatigue crack growth at higher da/dN, where the crack-tip plastic zone is larger, can be better characterized using the range of G in a loading cycle; i.e. ΔG = Gmax − Gmin.