It is common thinking that shot peening is more efficient when applied to high strength materials than to lower strength materials including steel weldings. This is due to the fact that lower yield stress restricts the magnitude of residual stresses and makes them more prone to relax when cyclic loads are applied. This is due to local plasticity that has an important role in determining the fatigue and not only strength of notched components. Thus, the application of shot peening is not so usual in steels with the yield stress of up to 400 MPa.

Indeed, there is not so much research devoted to investigating and clarifying the role of shot peening in improving the mechanical behaviour of this kind of materials; anyway, the known results prove an increment of the fatigue limit at 2.000.000 of between 10% and 90%, with R ranging from 0 to 0.1 (R is the ratio between the minimum and the maximum stress of the fatigue cycle). Most of this data refers to welded connections (butt welds, fillet welds, bead-on-welds).

For shorter life, the experimental results show a limited or negligible effect of shot peening.

Bear in mind these considerations, the supposed reason for this limited benefit gotten from shot peening, is that the residual stress field induced by shot peening is not deep enough to remain stable during the fatigue tests.

Thus, it is natural to think of applying some other surface treatment to welded connections of low resistant steels: it is also natural to think of laser peening, an emerging treatment that has been proven to induce much deeper residual stress fields than shot peening in almost all the cases investigated and analysed. In fact, laser peening showed a deeper penetration of residual stresses and a strong increment of fatigue life and fatigue strength was observed in steels and light alloys. If the previous results can be summarized, the consequence is that for high strength materials laser peening seems to be more effective that shot peening, the high cost being the only obstacle to its diffusion.

However, recently, the Society of Automotive Engineers Fatigue Design and Evaluation (SAEFDE) Committee, proposed by SAE, developed research aimed at verifying the real benefits achievable by applying laser peening and comparing it with the improvement of the fatigue behaviour due to traditional shot peening.

Some of the results of this research were discussed at ICF 12 (12th International Conference on Fracture), Ottawa, July 2009. During that conference, some results of the research were presented by R.I. Stephens (University of Iowa). In his speech, Stephens described the tests done on a low carbon SAE 1010 cold rolled steel specimen (UTS = 390 MPa, Yield stress = 350 MPa, elongation = 42%). These latter were thin walled square tubes of 50.8 mm in width and height, 305 mm in length and with a wall thickness of 3.17 mm. A continuous seam weld ran the length of the tubes where the cold rolled tubes were joined together. All the tubular specimens received a transverse center weld bead on the surface adjacent to the seam weld.

Three groups of these specimens were formed (untreated, shot peened and laser peened) and fatigue tested. These latter tests consisted of constant amplitude tests and of variable amplitude tests. The fatigue cycle ratios R selected were R = 0.1 and R = 0.5.

Before the fatigue tests, residual stress measurements confirmed the ability of laser peening in causing a deeper residual stress field. 

The results of the constant amplitude fatigue tests show that only when R = 0.1 is laser peening effectively able to improve the fatigue limit and life more than traditional shot peening, while for R = 0.5 shot peening proved to cause a more significant effect of the fatigue strength of the welded specimens. But what is more surprising is that in the case of variable amplitude fatigue tests, laser peening gives a negligible effect in terms of life increment, much less marked than the one caused by shot peening. The authors of the research conclude that in light of the previous results the application of laser peening is not justified bearing in mind the limited improvement of the fatigue behavior got by using the laser peened specimens.

Generally speaking, maybe it is the case that further research should be supported to better understand the action of laser peening that, on the base of the exeprimental results, cannot be related only to the depth of the residual stress field.

Author: Mario Guagliano