VOL. 11 March ISSUE YEAR 2010
Shot Peening in the Automotive Industry
in Vol. 11 - March Issue - Year 2010
Springs Cannot Survive Without Shot Peening
I admit that the title of the present column is drawn to focus the attention of the potential readers and to stimulate them to read it.
Indeed, every mechanical component could be done without peening or other surface treatment, but the problem is that the final dimension would be too large for a convenient use in a mechanical system. This is even truer in the automotive industry, since performance, consumption reduction and lightness are the primary goals of the design process.
But for springs, peening becomes more and more important because of some peculiarity of this mechanical component, whose function is to absorb elastic energy and to smoothen the behaviour of the system under dynamic loads.
Let’s talk about helical springs, first of all. These components are made by using a steel wire with diameter equal to d, whose axis describes a helical trajectory around a cylinder of assigned diameter D. They are loaded by compressive forces, which induce a prevalently torsional stress state. To make the dimension of the spring compatible with the ones of a car, the diameter D is too small with respect to the wire diameter, d, to consider the spring as a usual beam: we cannot consider that inner fibres have the same length as outer ones. That is to say that normal stress formulas for beam analysis and calculation cannot be applied and that stress concentration arises at the inner part of the springs. So, with normal geometrical ratios, it is usual that, at the inner points of a helical spring, stresses are about 1.5-2 times the ones of linear beams subjected to the same internal forces. This makes helical springs particularly subjected to the problem of fatigue damage. Since it is well known that fatigue originates from local severe stress values and is promoted also by roughness, some surface treatment is usually performed to alleviate fatigue damage, and of these shot peening is the preferred one.
This is due to the possibility of peening the spring surface also at inner points which is the location of fatigue crack initiation: this is not possible with other mechanical treatments, since geometrical constraints prevent the correct performance.
However, even shot peening is not easy to apply. In fact, its execution must be carefully checked and it is necessary to be sure that the shot flow is able to impact the complete surface. It is sufficient that a small fraction of the inner surface is not plastically deformed by shot peening to make possible (and probable) the development of a fatigue crack from that point. In scientific literature some accidents have been reported about spring failures induced by incomplete execution of shot peening. In those cases the observation of the fracture surfaces clearly showed that shot was not able to reach that area and, consequently, to induce the compressive residual stresses able to prevent fatigue crack development. That is to say that particular care in quality control is unavoidable. Another way to improve the effectiveness of shot peening on helical springs is to perform it while the spring is mechanically loaded in compression. In this way larger compressive residual stresses can be introduced at the inner surface and higher fatigue strength and life achieved. Also in this case a strict plan for assessing the actual result of the treatment and a severe quality control plan is recommended. It would be useful also to include residual stress measurements at random points to assess that the residual stresses induced by peening are the expected ones.
Another kind of spring usually utilized in car suspension design is leaf springs. They are subjected to fatigue bending loads, of course. In this case performing shot peening is not particularly difficult, since leaf springs have straight geometry, but the optimal choice of shot peening parameters has to be carefully thought though, since leaf springs present also the problem of fretting damage, caused by a slight sliding between in-contact parts (the different leafs, if more than one, or the connections with the chassis.
The problem, in this case, is that there is not so much known data about the effectiveness of shot peening with to respect fretting damage, and it is not possible to find well established approaches able to help the engineers in choosing the correct treatment parameters. In these cases a good experimental program on these components could be developed, with the aim of finding the optimal peening parameters. Like in fatigue design, the experimental program should be based on tests performed on full scale springs, and not on small specimens, thus reproducing the real stress conditions of leaf springs.
Shot Peening in the Automotive Industry
by Mario Guagliano
Contributing Editor MFN and
Associate Professor of Technical University of Milan
20156 Milan, Italy
Author: Mario Guagliano