Effect of shot peening residual stresses on fatigue strength and crack propagation threshold: experience on helicopter components made in Al 7475 alloy

Shot peening is a mechanical process which cold works the surface of a structural part by means of a propelled stream of spherical shots. It is used to improve the fatigue properties of the part by introducing on the surface and in a small layer underneath beneficial compressive stresses which retards or sometimes prevents fatigue cracking. In the presence of shot peening a surface flaw is partially or totally embedded in the compressive stress field. When considering the traditional LEFM methodology, this compressive stress field is the main source of the closure of the crack, which therefore grows at a highly reduced rate. The same closure effect is also present when considering the threshold stresses for non propagation of cracks emanating from the original flaw, so that the detrimental effect due to the presence of the flaw is reduced. The present paper shows the effect of shot peening for a typical aluminum alloy (Al7475-T7351) on the fatigue limit and on the threshold to propagation of a semicircular surface flaw 0.36 mm deep, for two different peening intensities (A6 and A12) and for three different stress ratios (-1, 0.1, 0.5) of the external load. The results have been obtained experimentally and are part of a wider program whose aim is the optimization of the shot peening parameters versus fatigue strength and threshold to crack propagation of small defects. According to the current activity the beneficial effect of the shot peening reduces with increasing stress ratio. Furthermore, while the fatigue limit of pristine material is not affected by the Almen intensity, the threshold stress to propagation of surface cracks increases with increasing peening intensity.