The Effect of Stress Ratio on Growth Behaviour of Small Fatigue Cracks in an Aluminum Alloy 7075-t6 (with Special Interest in Stage I Crack Growth)

Abstract

The growth behaviour of small fatigue cracks was investigated on an aluminum alloy 7075-T6 at the stress ratios R of 0, -1 and -2. The effect of stress ratio was discussed on the basis of the detailed observation with special interest in the stage I region of small crack growth. Cracks initiated at R=-1 and -Z, grew to a certain depth by the stage I growth mechanism, and the stage II crack growth followed. The stage I to stage II transition occurred under a constant maximum stress intensity factor which was approximately consistent with the effective threshold stress intensity range, ΔKeff, th, for large cracks. At R=0, on the other hand, the stage I crack growth was not observed because of crack initiation at inclusions. At all the stress ratios, small cracks grew more rapidly than large cracks in the same nominal stress intensity range, and grew below the threshold stress intensity range, ΔKth, for large cracks. Particularly, the stage I cracks showed much higher growth rate and grew below ΔKeff, th for large cracks. It is suggested that the stage II crack growth rate can be characterized in terms of the effective stress intensity range, while a micromechanics approach is successful to evaluate the stage I crack growth rate. © 1989, The Society of Materials Science, Japan. All rights reserved.