Effect of matrix resin on delamination fatigue crack growth in CFRP laminates

Abstract The objective of this study is to examine the effects of matrix resin and fibers on the mechanisms and mechanics of delamination fatigue crack growth in unidirectional CFRP laminates. The near-threshold growth of delamination fatigue cracks was investigated with seven kinds of unidirectional laminates made from brittle epoxy matrix, toughened epoxy matrix and thermoplastic PEEK matrix. Tests were carried out under mode I loading at different stress ratios. The fatigue crack growth behavior of laminates tested here was mainly controlled by the toughness of the matrix resin. The crack growth rate under different stress ratios was a unique function of the equivalent stress intensity range proposed by the authors. The parameter, γ, to indicate the relative contribution of the maximum stress to the cyclic stress was determined from the analysis of the stress-ratio dependency. For the toughened matrix system, the crack growth rate was mainly controlled by the stress range; the γ value was small. On the other hand, for the case of brittle epoxy, the contribution of the maximum stress was large; the γ value was large. The resistance against fatigue crack growth increased with the increase of the static fracture toughness. However, the ratio of the fatigue threshold to the static fracture toughness decreased with the decrease of the γ value. Thus, the increase of the fracture toughness does not fully contribute to the fatigue crack growth resistance. The difference in growth behavior due to matrix resin and fibers was discussed on the bases of mechanism consideration and fractographic observation.