Abstract
A survey of the literature on the measurement of residual stress in metals and the effect of shot peening has been followed by a critical analysis of the layer removal method developed by Heyn, and Sachs and Espey, and later refined by Foppl. Some improvements on Foppl's method were made possible by the use of a computer for curve fitting. An analysis of the distribution of the radial residual stress in the plastic zone after shot peening has shown it to be sufficiently small to enable the stress system to be treated as a biaxial one without loss of accuracy. The optimum heat treatment conditions after peening, for maximum fatigue strength, were established, applied to shot peened cylindrical bars, and the distributions of hardness and residual stress in them determined. The core and surface hardnesses were then reproduced uniformly throughout torsional fatigue specimens and the effect of the different hardness levels on the fatigue strength evaluated. A third level of hardness was likewise investigated and the effect of shot peening on the fatigue strength of steels tempered to these three hardness levels was established. The effect of cyclical fatigue stressing upon the stress distribution due to shot peening, for levels of applied stress below and above the elastic limit, has shown that whereas the lower stresses had no effect, once the elastic limit of the matrix at the surface was exceeded, a considerable falling off of beneficial residual stress in the surface occurred, but it still maintained its original value 0.003 in below the surface. It has been shown that the point of maximum principal tensile stress in a shot peened and externally loaded torsion bar is about 0.C '5 in below the surface, and that as a result, there is no deleterious effect due to the presence of shot peening indentations which are stress raisers. Removal of such marks by polishing, therefore, in no way influenced fatigue performance. A study of the separate effects of residual stress and hardness increase due to cold work during shot peening has shown that the increase in fatigue strength is mainly and sometimes completely due to beneficial residual stress, although under certain conditions a small part of the benefit may be due to an increase in hardness.