Fretting is a kind of surface deterioration that arises when relative motion of microscopic amplitude occurs between two metal surfaces. The microscopic sliding surfaces rub: the consequence is the formation of fine abrasive oxides, which contribute to the scoring of the surfaces.
Fretting generates one or more forms of damage, such as fretting corrosion, fretting wear and/or fretting fatigue. Fretting corrosion and fretting wear are both corrosion damage and wear damage respectively, that occur as a direct result of fretting. The aspect of the fretted surfaces includes discoloration, pitting, oxide-layer build-up, seizing and loss of fit, are all characteristics of these forms of damage. Fretting fatigue is any fatigue damage that initiates from fretting. The most notable characteristic of this form of damage is a reduction of fatigue strength of the parts.
Fretting-fatigue damage is initiated during the first few load cycles. Therefore, such damage can be prevented or reduced by restricting slip between contacting surfaces and creating and/or maintaining and effective lubricating third body between the surfaces before the first loading cycle.
As the reader can imagine, fretting is a very dangerous phenomenon, since it can be the ultimate cause of unexpected failures, especially in all couplings where there should be no relative motion between the coupled parts but there is for some not-well-chosen design data. For instance, in pressed fits, there shouldn’t be any relative motion between the mating parts, but, if the geometry of the hub and the shaft is not well chosen, the distribution of the contact pressure is not uniform and some relative motion between the two pressed parts can take place.
Shot peening has proven to be successful in retarding fretting and eventual fretting fatigue by increasing the surface hardening through cold working of the metal and generating residual compressive stresses at the fretting surfaces. However, there is another effect induced by shot peeling that is beneficial as regards fretting, being the minute pockets that are produced at the surface through shot dimples that act as oil reservoirs, thus resulting in longer lubricant retention.
In the automotive field, fretting is a well-known damage and can take place in many contacting parts. In particular, fretting is a particularly urgent problem in connecting rods. Indeed, these machine elements are shot peened in many points and not only against fretting. Sometimes, the thread roots are peened, which can impart to a cut thread almost the same fatigue strength as a rolled thread. Connecting rod bolts are also shot peened for axial fatigue in the shank to head fillet and for fretting fatigue in the shank itself. Some large con rods are also peened in the oil holes and in the fillets of the bolts. About fretting, fretting fatigue is prevented by peening the serrations between the rod and the cap, the bearing surfaces, and the boltholes.
Nevertheless, if it is true that there is lot of data and research concerning the effect of shot peening with respect of fatigue, not so many studies have been developed (or published) about its effect on fretting. Hence, it cannot be affirmed whether shot peening effect on fretting is the same as its effects against fatigue failure. In fact, the approach to select the correct peening parameters is often based on empirical considerations.
Indeed, fretting is by itself a complex phenomenon, not really completely understood. In addition, this reflects the difficulty in developing analytical design approaches to avoid or to limit it.
The difficulty increases when shot peening and its effects on fretting are considered. The increased fretting resistance of shot-peened parts is mainly related to residual stress state, but it is difficult to find well accepted damage parameters able to indicate the effect of the residual stresses on the development of fretting and able to indicate to engineers the correct peening parameters to be used to avoid fretting under assigned in-service conditions.
But the main still unknown factor is the effect of surface roughness induced by shot peening. In fact, even if it is clear that the dimples caused by the impact of the shots are beneficial for fretting, since they improve the lubrication conditions, the relation between the parameters that defines roughness (Ra, Rt, …) and the fretting development is not well known. This could be a good topic for future work, and to increase the application of shot peening.

Shot Peening in the Automotive Industry
by Mario Guagliano
Contributing Editor MFN and
Associate Professor of Technical University of Milan
20156 Milan, Italy
E-mail: mario@mfn.li

Author: Mario Guagliano