アルミニウム合金の変動応力下の応力腐食割れにおけるき裂進展

Abstract

The effect of interaction between corrosion fatigue and stress corrosion cracking on the crack propagation behaviour of 7278-T6 aluminum alloy has been studied under zero-to-tension low cycle fatigue tests with a hold time in salt water (3.0% NaCl).The results obtained are summarized as follows:(1) From the low-cycle corrosion fatigue tests with trapezoidal stress waves, the crack growth rate was found to increase with increasing stress hold time and was approximately proportional to the cyclic Loading frequency fα. The crack growth rate during the stress hold time was accelerated by the stress cycling as compared with that of stress corrosion cracking.(2) The tests of alternate loading of stress cycling and holding at a constant stress were conducted and the crack growth rate was determined. The crack growth rate calculated in terms of that with trapezoidal stress waves was larger than that of trapezoidal stress waves itself in the region of short holding time. On the other hand, it decreased with increasing stress hold time and became smaller than that of trapezoidal stress waves in longer holding time because of crack tip blunting.(3) The crack propagation rate during the stress hold time was approximately proportional to th-β, where th is the stress hold time, β≈0.5 for trapezoidal stress waves and 1.4 for alternate loading. It was suggested that β is affected by the sensitivity to environmental attack at crack tip and slip dissolution.(4) Environmental enhancement for crack growth rate was found in the low stress intensity range, and it increased as the testing frequency was lowered in the case of trapezoidal stress waves. For the alternate loading, a critical frequency was found at which the maximum environmental attack occured.