The effects of stress ratio on delamination onset behavior under cyclic loading was investigated in AS4/3502 [+/-252/90]s graphite/epoxy laminates. For the purpose of this study, delamination onset was defined as a delamination 5% of the laminate width. The resulting data showed definite changes in delamination behavior between stress ratios (R ratio) of 0.1 and 0.5.
For maximum amplitude stresses below about 80% of the static delamination stress, delamination onset occurred later at R = 0.5 than for R = 0.1. Also for the corresponding Gmax, delamination growth rates at delamination onset were slower for R = 0.5 than for R = 0.1. This was reflected in changes of power law exponent from 10.2 for R = 0.1 to 31.6 for R = 0.5 and the constant from 1.10 × 10-2 for R = 0.1 to 254 for R = 0.5.
Fractographic analysis suggested that the failure mode, which appeared to be predominantly Mode I, remained unchanged; however, the density of shallow hackles was greater for R = 0.1 than for R = 0.5. This presumably was due to a more rapid coalescence of microcracks which was consistent with the larger crack opening displacements for R = 0.1.