Competing failure mechanisms and fretting-sensitive shot peening optimization for a prestressed leaf spring

Abstract

To improve the fatigue resistance of high-stress leaf springs, prestressed shot peening is widely used. However, conventional process optimization based solely on Almen saturation and residual stress magnitude is insufficient to explain premature failures in clamp/contact-sensitive regions. In this study, three Almen C intensity levels (0.27C, 0.31C, and 0.33C) were applied under identical prestress conditions. Through-depth residual stress and full-width at half-maximum (FWHM) were characterized at two representative locations: 90 mm (clamp-sensitive) and 450 mm (bending-dominated). Finite element (FE) modeling with a service-equivalent M16 U-bolt clamping preload was used to identify fretting-sensitive zones, while scanning electron microscopy (SEM) and metallographic analyses were used to clarify the associated micro-failure mechanisms. The results indicate that increasing the peening intensity deepened the compressive residual-stress layer but progressively degraded surface integrity. The 0.31C condition provided the best balance, giving the highest mean fatigue life and fractures confined to the 450 mm region, where fine dimples were consistent with stable cyclic bending. By contrast, 0.33C over-peening shifted the dominant fracture location toward the 90 mm region. Microstructural evidence suggested that severe surface defects, together with local fretting wear, accelerated crack initiation, as indicated by friction-induced wear scars, extrusion folds, and larger microstructural features in the rapid-fracture zone. These findings suggest that fatigue performance is governed by a competition between beneficial subsurface strengthening and detrimental contact effects, supporting the use of a moderate peening intensity and targeted fretting control rather than uniform maximization of peening intensity.