抄録
Fatigue crack growth tests were carried out on two aluminum alloys, ZK141-T7 and A7075-T6, under constant amplitude, repeated two-step and random loadings. Crack length and crack closure were measured by using the minicomputer-aided unloading elastic compliance method.
The crack growth rate curve represented in terms of effective stress intensity range, ΔKeff, exhibited a trilinear form in so-called Region II for both testing materials under constant amplitude loading. The relation between da/dn and ΔKeff of ZK141-T7 under repeated two-step loadings was found different from the constant amplitude results due to load interaction, whereas the relation of A7075-T6 was hardly affected by load variation.
The crack opening stress intensity, Kop, under variable loadings was found for both materials to be controlled by the maximum stress intensity range-pair, (ΔrpK)max, and its stress ratio, and further to coincide with the constant amplitude test results having the identical stress intensity range ΔK and stress ratio R.
Fatigue crack growth under random loadings could be well predicted in terms of ΔKeff, using the modified relation between da/dn and ΔKeff obtained from the repeated two-step loading tests accounting for crack growth characteristics depending on materials.
