Effect of stress ratio on near-threshold propagation of delimination fatigue cracks in unidirectional CFRP

The effect of the stress ratio on near-threshold growth of delamination fatigue cracks was investigated with unidirectional laminates made from Ciba Geigy 914C prepegs (T300/914) and from Toray P305 prepegs (T300/#2500). Tests of delamination fatigue crack propagation were carried out under mode I opening loading by using double cantilever beam specimens. The normalized gradient of energy release range was controlled in load-shedding tests. In the region of crack growth rates above about 5 × 10^?10 m/cycle, the growth rate was expressed as a power function of fracture mechanics parameters. Below this region, there existed a growth threshold. The influence of the stress ratio became smaller when the rate was correlated to the energy release rate range than when the rate was correlated to the stress intensity range or the maximum energy release rate. A controlling fracture mechanics parameter is discussed on the basis of fractographic observation and mechanism consideration. A new phenomenological law of fatigue crack propagation is derived.