アルミニウム合金における変動荷重下の疲れき裂進展とき裂開閉口挙動 第ＩＩ領域におけるき裂進展速度曲線の折れ曲りと荷重変動の影響

Abstract

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, ..delta..K/sub eff/, exhibited a trilinear form in so-called Region II for both testing materials under constant amplitude loading. The relation between da/dn and ..delta..K/sub eff/ 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, K/sub op/, under variable loadings was found for both materials to be controlled by the maximum stress intensity range-pair, (..delta../sub rp/K)/sub max/, and its stress ratio, and further to coincide with the constant amplitude test results having the identical stress intensity range ..delta..K and stress ratio R. Fatigue crack growth under random loadings could be well predicted in terms of ..delta..K/sub eff/, using the modified relation between da/dn and ..delta..K/sub eff/ obtained from the repeated two-step loading tests accounting for crack growth characteristics depending on materials.