Welding is a popular joining method in the automotive and many other industrial sectors.

Due to the peculiarity of the process itself, welded details have a very peculiar fatigue behaviour. Firstly, we all know that welding induces a relevant residual stress field in the welding and in the zone around the welding. Unfortunately, the physics of the process generates tensile residual stress close to the welding; the worst part for fatigue strength and endurance. This is a critical factor since the tensile residual stresses strongly affect and reduce fatigue strength. In the case of welding, the tensile residual stresses are so relevant that the resulting fatigue strength is not affected by the mean stress of the fatigue cycle but only by its amplitude (at least if we consider steels).

But the fatigue behavior of weldings is particular also for other factors: apart from the notch effect induced by the macro-geometry, the intrinsic defects caused by the process make the fatigue crack initiation phase negligible. So, the fatigue strength of welded details in the as-welded state can be considered just in terms of fatigue crack propagation: this is the reason why the S-N (stress amplitude-number of cycles to failure) is quite different with respect to the base material and that the fatigue limit is rather less than the one of the base material, and the difference is so large that it cannot be justified just considering the notch effect.

In a nutshell, the fatigue behaviour of weldings is special and their fatigue strength is generally poor. If fatigue is critical for the design of a structural or a mechanical welded part, post-treatments of welding should be considered. 

Shot peening and allied processes are among the best candidates for reducing and annulling the critical factors of the fatigue behaviour of welded details.

Annealing, a common thermal post treatment, is able just to reduce the tensile residual stresses, but we know that shot peening induces compressive residual stresses in the surface layer of material also: this positively affects the fatigue behaviour of welding and strongly contributes to the improvement of the fatigue limit. But shot peening can also reduce the surface defect rate: thanks to the mechanical action of the impact of the shots against the welded surface shot peening that removes or reduces the presence of the defects. In fact, the plastic deformation induced by the shots impact can change the surface aspect in removing the defects or making them less dangerous regarding fatigue application. 

If applied in combination with some other treatment to modify the weld profile (like TIG-dressing, disc grinding or burr grinding) shot peening effectiveness can be increased, since it will act on a surface where defects and imperfections have already been removed and the weld profile has been geometrically improved. In these cases, it is possible to increase the fatigue limit in such a way that it is finally comparable with the one of the base material or even better.

However, these outstanding results can be achieved only after an accurate investigation on the case of interest. The choice of shot peening parameters should be chosen bearing in mind the material, the type of welded detail, the analysis of the stress state under the in-service load and considering that the characteristic behaviour of welded details does not permit consideration of the same approach used for other mechanical elements.

This is even more important in the automotive industry where the materials, fatigue cycles and safety potential issues need to adopt a “design by analysis” approach.

Contributing Editor MFN and 

Full Professor of Technical University of Milan

20156 Milan, Italy

E-mail: mario@mfn.li