Abstract
The effects of stress ratio and a water environment on the near-threshold growth of delamination fatigue cracks were investigated with unidirectional laminates made from ICI APC-2 thermoplastic prepregs (AS4/PEEK). Tests were carried out under mode I opening loading by using double cantilever beam specimens. The fatigue crack growth rate near the threshold region was mainly controlled by the stress intensity range rather than the maximum energy release rate and the energy release rate range. The fatigue crack growth resistance of AS4/PEEK laminates was much higher than that of conventional CF/epoxy laminates even near the threshold region. However, the increase in fatigue resistance was smaller than that in fracture toughness. Tests in water did not show large detrimental effects. The crack propagation rate in water was expressed as two power functions of the energy release rate range. The exponent of the power function in the low propagation rate region was smaller than that in the high propagation rate region, and the growth threshold was not observed. The mechanism of the existence of the two power functions was discussed by conducting tests in water at lower frequency of stress cycling. Mesoscopic mechanism of delamination fatigue crack growth in air and in water was discussed on the basis of the fractographic observation.
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