Benefits of surface finish and shot peening
In cyclic loading conditions failure by fatigue can be prevented / delayed by employing surface finish techniques like shot/laser penning.
Fatigue in metals refers to the development of cracking in components as a result of cyclically applied loading, often over a long period. Most frequently this fatigue crack initiation is aided by surface stress concentrating features which can significantly reduce a components fatigue life.
Since fatigue cracks frequently originate from the surface of a component as a result of applied cyclic stresses, it is important that the combination of such cyclic stresses and stress raising features are well below the fatigue limit for the component, or else fatigue cracks will indeed initiate and subsequently propagate. It is important when considered with the applied cyclic stress that surface stress intensity K (combined effect of tensile stress and flaw parameters) is below this fatigue threshold limit, ∆Kth. Such fatigue threshold values are available for specific material/environment combinations or in specific/critical applications can be measured experimentally to facilitate appropriate fatigue design. As the magnitude of the cyclic stress intensity is dependent on the net locally applied tensile stress, the superposition of a compressive stress can afford significant improvement of fatigue life. Such compressive stresses can be induced by both chemical and mechanical methods such as rolling and shot peening. Both shot and laser peening induce compressive residual stresses into the surface of the component (where applied stresses are often highest) to a depth of up to 1mm in shot peening and 2mm in shock peening.
In a recent investigation of the fatigue performance of a titanium alloy, the as-fabricated surface roughness had an RA of the order of 12 microns, from which fatigue cracking could readily initiate and propagate, under typical stresses. By polishing the surface to a fine mirror finish (RA less than 0.5 micron) there was a three to four fold increase in fatigue life. Once polished and shot peened, the fatigue life increased by a further two to three orders of magnitude. Laser peening gives even better results. Although significant improvement of fatigue life can be achieved sub surface crack initiation can occur and hence the depth of peening stress is important.
If fatigue is a problem in sophisticated components (or cannot be solved using more standard approaches), peening is a very viable methodology for improving fatigue life and should not be discounted.
Published in Technical Tips by Origen Engineering Solutions on 1 August 2015