Loading variable effects on mode—I fatigue of wood–FRP composite bonded interface

In this paper, the effect of fatigue loading variables for waveform, load ratio and frequency is evaluated for red maple wood–phenolic fiber reinforced polymer (FRP) composite bonded interface. Experiments are performed with a contoured double cantilever beam (CDCB) specimen under load control, and the crack propagation rate is obtained by the compliance method. Three groups of tests are performed to study the effects of waveform, load ratio and frequency on crack propagation rate. In addition, the modified load shedding technique is used to explore the fatigue threshold and the load ratio effect on fatigue threshold. The tests are carried out under constant amplitude cyclic loading at room temperature and laboratory conditions. The results indicate that while waveform has negligible effect, both load ratio and frequency have significant effects on crack propagation rate of the bonded interfaces. A modified Paris Law model is proposed to include both load ratio and frequency effects. The dependence of the threshold strain energy release rate range, ΔGth, on load ratio is explained effectively by the crack closure concept.