The Effect of Residual Stresses Induced by Shot‐peening on Fatigue Crack Propagation in Two High Strength Aluminium Alloys

Abstract

The crack initiation lives of peened specimens of aluminium alloys 7010 and 8090 are shorter than those of unpeened specimens. This is caused by the acceleration of crack initiation due to stress concentration in the rough peened surface, especially at fold-like defects. The crack growth rate in peened specimens is significantly reduced with increasing AK, i.e. with increasing crack length. At a crack length of approximately 0.3 mm this trend is reversed and the crack growth rate rapidly increases and attains the same level of crack growth rate as that in unpeened specimens. The point of smallest crack growth rate roughly corresponds to the point of maximum residual stress. The crack growth rate in a peened specimen has been modelled by assuming the effect of residual stress reduces to the equivalent stress ratio. The predicted results agree well with the experimental data.

3-D analysis of fatigue crack behaviour in a shot peened steam turbine blade material

“…Due to the more intense shot peening process, the T0 condition has a rougher surface than the T1 condition (see Table 1). Mutoh et al claimed that surface roughness produced by the shot peening process in an Al alloy contributed to the shorter crack initiation lives found when compared with the unpeened samples; the initial fatigue cracks were always sited at stress concentrations on the rough peened surface, especially the fold-like defects at a region of intense surface folding [3].…”

“…Interestingly, Mutoh et al found that the point at which the smallest crack growth rate is found roughly corresponded to the point of maximum compressive residual stress [3]; the lower value of the crack aspect ratio in shot peened samples is the result of the sub-surface compressive residual stress caused by shot peening [3]. The decrease in the crack aspect ratio in the peened specimen in the surface layer is caused by the fact that the source of the crack in the peened specimen is always sited at a region of intense surface folding and the initial crack has a larger crack length on the surface [3].…”

“…However, most of these studies which have focused on the effect of shot peening on fatigue crack initiation and propagation behaviour or the effect of stress relaxation, were generally confined to surface observations which provide no information about the three-dimensional (3D) shape of these cracks. Although Mutoh's research indicated that the fatigue crack morphologies varied significantly in shot peened and unpeened layers [3], there seems to be little detailed information available in the literature on how the 3D surface breaking fatigue crack morphologies are affected by the shot peened layers. Understanding this will provide important information on the evolution of the crack growth and will lead to more quantitative and micromechanically informed shot peening fatigue life assessments.…”

“…However, the growth of a small fatigue crack is a complex 3D process, especially for surface breaking or sub-surface initiating short cracks which typically have a more complex 3D crack shape than conventional through thickness long cracks. In this case the 3D interaction with other features such as local microstructure [2], residual stress [3], inclusions or second phases [4,5], can significantly affect initiation and propagation behaviour during fatigue since the stress (or strain) intensity distribution around the crack periphery changes. In addition simple measures of fatigue crack growth, da/dN, are based on underlying assumptions of a single crack with a crack driving force that can be described simply (e.g.…”

“…Extensive efforts have been made to study the influence of shot peening induced factors in the near-surface (surface roughness, compressive residual stress regions as well as the plastic strain layer) in both high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes in different material systems (such as steel [31,32], Al alloys [3,33] etc.). However, most of these studies which have focused on the effect of shot peening on fatigue crack initiation and propagation behaviour or the effect of stress relaxation, were generally confined to surface observations which provide no information about the three-dimensional (3D) shape of these cracks.…”