Shot Peening in the Automotive Industry
in Vol. 9 - September Issue - Year 2008
Designing Shot Peening For Automotive Applications
Shot peening and the automotive industry have been related for a long time, maybe from when the automotive industry stopped being a sort of handicraft field of production and became a strictly organized industry, with plants where working people had a precise and unique role. That is to say after Henry Ford introduced assembly lines as an efficient way to produce cars for the masses.
It is not the case that in the same period, from the twenties and later, shot peening was developed independently in the two countries where the auto industry had the stronger development, USA and Germany. In Germany the work by Föppl and Thum was fundamental in recognizing the factors that make shot peening an efficient way to improve fatigue strength of mechanical components: residual stresses and surface work hardening. In the USA the pioneering work was made essentially by John Almen and Stephen Fuchs in the Labs of General Motors where they developed the first fatigue design criteria able to take into account shot peening. In particular, John Almen developed the well known “Almen Intensity” method to quantify the way to execute shot peening on mechanical components. This method is still used all over the world thanks to the fact that is really practical, cheap and can be easily introduced in quality control systems of companies.
Many years have passed from that time: today cars are full of electronic devices, are more and more comfortable, new materials have been introduced and there are a lot of plastics in every car. But when customers buy a car they look for performance, reduced weight and consumption. These factors lead designers and engineers to use materials more and more efficiently to design and produce components that are reliable, light and able to give ever better performance. This makes shot peening more interesting now than at the time of John Almen & Co.. Today shot peening is considered to be at the design stage, where structural elements are defined knowing that they will be peened before being mounted in the gearbox, or in the engine.
But, if the maximum result is the goal of shot peening application, this requires also a deeper knowledge of shot peening itself, since it makes necessary the development of a procedure able to lead engineers to design the treatment considering the operational conditions of the components (load, environment, temperature, number of fatigue cycle, load spectra, …..) and requires also the execution of preliminary experimental fatigue tests.
In other words the best application of shot peening should be based not only on previous experiences (that works but might be improved) but also on the analysis.
I know of a car manufacturer that used to peen the torsion bars utilized for rear suspensions with well determined peening parameters (that is to say with given Almen Intensity, coverage and shot type) and considered these parameters as fixed (probably they did not remind the origin of that choice, but I do not know). Indeed the parameters gave good results without any specific problem. But the manufacturer started to sell its vehicles in South and Central America and some cases of fatigue fracture of the torsion bar occured. Oh my God, this is not a good thing, why did it happen? This was the reaction of engineers and technicians. The first idea was that shot peening was badly executed (not correct coverage, maybe damaged shots that made surface defects on the bars, ….) but the analysis that was executed did not reveal any of this: shot peening was correctly done.
So it was necessary to look more in depth and a series of experimental tests aimed to assess the operational fatigue condition justified what happened. Torsion bars were instrumented with strain gauges and were put on the road for thousands and thousands of kilometres on typical South American roads and a lot of data were recorded and elaborated: the results was the definition of the typical load spectrum for that application, that is to say that the typical number of repetition of fatigue cycles with given amplitude and mean stress was determined. Unfortunately this latter was very different from the one of European roads: there were more severe fatigue cycles (due to the conditions of the road) able to strongly modify the residual stress field induced by shot peening.
It was necessary to re-design shot peening, choosing parameters able to give acceptable results even for the new operational conditions. In the end the problem was solved and no other fatigue fracture took place in those countries.
What is the final thought? Shot peening is ever more important in an automotive market that is ever more competitive. If we want to fully appreciate its beneficial effect on performances of mechanical parts it is better to combine it with adequate analysis to be able to relate the choice of shot peening parameters with the real operational conditions of the parts. That is to say that “Designing by Analysis” can lead to much better performances. It costs, but the results are sure.
Shot Peening in the Automotive Industry
by Mario Guagliano
Contributing Editor MFN and
Associate Professor of Technical University of Milan
20156 Milan, Italy