EFFECT OF LARGE PLASTIC STRAINS AND STRAIN GRADIENTS ON RESIDUAL STRESS RELAXATION IN SHOT PEENED IN100

摘要

Surface treatment processes, such as shot peening, are routinely used by several manufacturing industries (automotive, aerospace, medical, etc.) to impart compressive residual stresses onto the surface of metal components to increase damage tolerance and fatigue life. These compressive stresses are engineered into components to delay initiation of surface cracks and retard crack growth rate thereby improving fatigue life. In addition, the plastic deformation resulting from the shot peening process hardens the material increasing the yield strength and elastic stress range. For elastic material response, accurate fatigue life estimates that include credit for residual stresses are possible for complex geometries with complicated load histories. However, when a shot peened component is subjected to elevated temperatures or cyclic loading, thermal relaxation and cyclic plasticity, respectively, produce inelastic material response from which residual stresses may change continuously. When the material response is inelastic, taking full credit for compressive residual stresses in fatigue models would result in a nonconservative prediction. As a result, designers are reluctant to incorporate any compressive residual stresses into fatigue life predictions of turbine engine components, subject to elevated temperatures and inelastic loading conditions. This paper investigates relaxation of shot peened residual stresses in IN100 when subjected to applied stress profiles that produce large plastic strains and strain gradients in laboratory test samples. Model predictions supported by experiments results are presented to explain that redistribution of residual stress depth profiles are a function of geometry and applied stress/strain profile.