VOL. 12 September ISSUE YEAR 2011
Shot Peening in the Automotive Industry
in Vol. 12 - September Issue - Year 2011
Shot Peening For Very High Cycle Fatigue: A Stimulating Challenge
Fatigue damage is one of the most frequent causes of failure in mechanical structures and components. As almost all engineers and technicians know, the first cases of fatigue failure date to the XIX century and from that time, an increasing number of studies and researches have been dedicated to fully understand this phenomenon and to develop design approaches able to avoid fatigue failure or, in most recent times, to develop design criteria allowing to substitute a fatigued part before its failure, once a load spectrum is assigned (i.e.: fail-safe, safe life and damage tolerant approaches, used in aeronautic systems).
A great effort was also dedicated to develop experimental devices able to make the fatigue tests faster and cheaper to determine the properties of materials and components under cyclic loads. This provides the basic data for the evaluation of the fatigue strength of a component.
Anyway, due to technical and economic reasons, the fatigue properties of materials and components are often determined with tests with a maximum number of cycles equal to 10 million cycles. And if a specimen does not break after this number of cycles, the test is interrupted and it is considered that it will not break for an infinite number of cycles. The concept of fatigue limit is based on this assumption.
But many times, machine elements and structural parts are subjected, during their life span, to a larger number of cycles, some with orders of magnitude higher than 107. This is valid also for automotive parts, motor components and it is true that they can fail even for an ultra-high number of cycles (108-1010). In 1999, Bathias stated that there is no infinite life for metallic materials and some years later, Cetin Morris Sonsino concluded that a real fatigue limit can only be achieved in absence of microstructural inhomogeneities or when there is not interference with a corrosive medium.
This justifies the great effort dedicated in the last years to investigate the fatigue behaviour of materials in the field of the so-called very high-cycle fatigue (VHCF). These investigations include the need to develop reliable experimental devices to obtain data in a reasonable test time, that is to say, that high-frequency (ultrasonic) machines were developed to make the time needed for a VHCF test sustainable. The results obtained until now show that, most of the time, when failure happens in the VHCF regime, the crack initiation point is no longer on the surface but in correspondence to some internal defect, usually a non-metallic inclusion.
After this result, the natural question that all people interested in surface modification techniques will ask themselves if it is still useful to perform some surface treatment to improve the behaviour of the treated parts in the VHCF field. If the attention is focussed on shot peening, some data were recently obtained and published and seem to be somewhat contradictory. Ochi and co-workers found that at 109 cycles, shot-peened ADI (bainitic nodular graphite iron) specimens have lower fatigue strength than non-peened specimens. But they were not able to find an explanation.
In another investigation, a different Japanese group found that for a number of cycles equal to 108, ultrasonic shot peening improves the behaviour of austenitic AISI 304 steel, both at room temperature and at 300°C. And there is another study showing that shot peening improves the fatigue-corrosion strength in the VHCF field. It is also interesting to note that a Chinese research group has performed fatigue tests on welded details, finding that shot peening is able to improve the fatigue behaviour even for very long durations, that is to say, up to 109 cycles. In other investigations, it is remarked that residual stresses induced by shot peening do not vary in the VHCF while in other papers the results are opposite. And I could mention other results referring to the VHCF strength of materials peened with parameters able to induce microstructural modification and grain size refinement (if the reader is interested references to the papers cited in this column can be provided).
But almost all the studies in this field that I know do not adequately consider one important factor for understanding if shot peening can be useful to improve the fatigue strength of materials and components in the VHCF regime: the notch effect and the related stress/strain gradient - and this is very important for many automotive parts. Indeed, it is true that shot peening is more effective when applied to notched elements and it is expected that even for the VHCF it should be the same, but there are many aspects that have to be investigated and clarified before we are able to fine-tune the correct peening parameters for this kind of applications and for the materials subjected to VHCF failure. It could be a stimulating challenge for all these interesting subjects.
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