in Vol. 14  November Issue  Year 2013

The Importance Of The Stress Gradient
Everyone who deals with shot peening knows that if you apply this treatment to a smooth (e.g. cylindrical) part subjected to a uniaxial cyclically variable tensile load, the effect will be very limited, if any. Indeed the analysis of the fracture surfaces of samples broken this way will show that the crack initiation point is probably shifted from the free surface to a subsurface site, most times in correspondence with some internal nonmetallic inclusion. If the same cylindrical part is loaded by a system of forces able to result in cyclic bending, the result, after the application of shot peening, will be more marked: the expected improvement of the fatigue limit with respect of the untreated parts should be about 1020% and the crack initiation point will probably remain on the free surface. Finally, if a notched part with a small fillet radius (or holes or grooves), as in most automotive machine elements usually considered, the result, after the application of shot peening, will be much more remarkable: the fatigue limit will be much higher than the one of the unpeened parts and it is not rare to improve it by more than 50%. But it is true that the final results can be highly dependent upon the shotpeening parameters and that these latter results are strongly influenced by the geometry of the part of interest. That is to say, that the same shotpeening parameters applied to different parts made of the same material can give very different results. This is due to the different stress trend from the surface to the core. In particular, the geometry of geometrical details strongly influences the stress gradient. And the stress gradient is a very important factor in fatigue strength of the notched element and it must be considered for assessing the real effect of notches on the fatigue behaviour. Also, the choice of shotpeening parameters should consider the geometry and the stress gradient. In fact the effect of shot peening is to introduce a compressive residual stress field in the surface layer of material together with surface hardening in about the same layer. The residual stress field presents a compressive maximum at a distance from the surface and then decreases and changes sign. Both the depth of the compressive maximum and the changing of sign are related to the choice of the peening parameters; furthermore, the beneficial effect of shot peening on notched specimens is related to its ability to stop the propagation of microcracks (thanks to the residual stress and the surface work hardening and grain distortion); in fact, in this case, the crack is growing in an applied stress field that is rapidly decreasing (due to the strong gradient), while the compressive residual stress field initially increases. Bearing these facts in mind, it is understandable that the choice of the peening parameters should be chosen (not ignoring the actual stress trend) in such a way as to have the best "synergy" between the applied and the residual stress trend and to induce the right depth of the surface work hardening. This means that the optimization of shot peening of conrods, crankshafts, springs, etc., should be customized by considering not only the different materials but the real stress field caused by the applied loads as well. That is, that fine calculations and finite element simulations could help the right choice of the shotpeening parameters. On the other hand, the development and the refinement of fatigue design approaches able to consider the shotpeening effect would help a more accurate evaluation of the expected improvement when shot peening is applied.

