In a recent conference held in Rotterdam about fatigue in aerospace structures (ICAF), one of the main discussed topics was the use of cold working processes to increase the fatigue strength of components and parts of airplanes. Among these ones a great emphasis  was devoted to shot peening and to processes related to shot peening. Besides this, other cold working treatments were presented and developed: for example, press fits, laser peening and other less known and more particular treatments. 
Everyone remained surprised at the great increment of fatigue life that can be obtained by introducing residual stresses with some mechanical work hardening process. A great part of the research presented showed the results of fatigue tests in terms of endurance, while no data about the increment of the fatigue limit was displayed. This is not surprising since aerospace structures are designed following the damage tolerant methodology. This implies that critical elements are periodically inspected, visually or by some ND technique, to assess presence of growing fatigue cracks and to evaluate if their presence can be tolerated until the next inspection.
We have also to add that shot peening is also applied to used components, to create a favourable residual stress field where fatigue damage is developing.
The importance of surface treatments is fundamental to diminish the frequency of the inspections,  that is to say in delaying fatigue crack growth. This is an important objective, since it allows for considerable savings. This is a key factor that drives manufacturers and the related companies to start and develop new research programs, aimed at obtaining structures that are ever more safe, light, reliable and economic. All these themes are not completely solved and they are subject to interest from researchers, engineers and manufacturers.
In the automotive field, the approach is completely different: crankshafts, gears, springs, valves, connecting rods, welds and many other machine elements are designed to be mounted once in their lifetime and to be not inspected by any periodical check. Most of the mechanical parts of a car are designed for an infinite life; that is to say not to break over the assumed in-service life of the car. In other cases mechanical parts are to be substituted when they are worn. This means that shot peening is executed not for increasing the fatigue life but for improving the fatigue limit of a mechanical element.
In these cases, the beneficial effects induced are related both to the residual stress field and to the surface work hardening and it is not so immediate to distinguish the parts due to the two factors.
Even though shot peening was introduced many years ago in the automotive industry in the design phase, there is not so much research about it. The parameters are generally the same as twenty years ago. In other words my feeling is that in many cases shot peening is considered as a mature technology, whose optimal parameters have already been found. But, if I look around me, I can see that it is not so. There is a lot of research about shot peening and the possibility to relate the treatment parameters to the fatigue strength of components and until now I have not seen any definitive solution or conclusion. For example, a current field of study is the possibility to obtain nanostructured layers of materials by means of air blast shot peening. These results can be obtained by increasing the energy of impact of the shots and the time of exposure to the shot flow. It is a promising field of research. Some papers have been published and showed the possibility of obtaining better results with respect to traditional peening. Another interesting topic is the application of the finite element method for predicting the effect of shot peening on a structural part. But, if I look at the companies that supported this research, only in a few cases do I find automotive industries or companies related to the latter.
I could cite other research topics that could be of interest to automotive manufacturers but the note should be the same: there is not so much attention to shot peening with respect to the possibility of obtaining better results with this treatment.
What is the conclusion? How does aerospace fit in with automotive?
The treatment is the same but the approach is different. In aerospace there is much interest in optimizing shot peening and in looking for better results, in the automotive industry it is common to consider shot peening as a mature treatment. In practical terms, it means that a great number of research proposals are coming out from aerospace promoters while it is not so in the automotive field (I am referring mainly to UE research projects). But are we really sure that the automotive industry does not need research into shot peening?

Author: Mario Guagliano