14th International Conference on Shot Peening

A SED-based approach to predict the fatigue strength of notched and plain shot-peened parts

M. Benedettia, M. Bandini, S. Raghavendraa, V. Fontanaria, F. Bertoc

University of Trento, Dept. of Industrial Engineering, Trento, Italy – matteo.benedetti@unitn.it

b PeenService srl, Bologna, Italy – m.bandini@peenservice.it

c Norwegian University of Science and Technology, Dep. of Mechanical and Industrial Engineering, Trondheim, Norway – filippo.berto@ntnu.no

Introduction

Shot peening is a widely used surface treatment known to greatly improve the fatigue strength of machine elements, especially in the presence of stress raisers. Its beneficial effect is mainly related to the introduction of compressive residual stresses (RS) into the surface layer.

A reliable fatigue calculation method of shot-peened parts necessitates the incorporation of residual stresses into a suitable fatigue criterion. In stress-based approaches, RS are treated as mean stresses superimposed to the external stress field. Sometimes, the notch effect is accounted for according to the Theory of Critical Distances. In fracture mechanics approaches, RS are encapsulated into the definition of a closure stress intensity factor that reduces the effective stress intensity range available for the fatigue crack propagation.

In recent years, the concept of strain energy density (SED) emerged as a very promising theoretical framework for accurate fatigue calculations. However, to the authors’ best knowledge, none attempted so far to incorporate shot-peening RS into the definition of SED.

Objectives

The starting basis is the novel SED-based fatigue criterion recently devised by the author to take into account the mean stress effect [1]. In this paper, in contrast to most of the previous literature, residual stresses/strains are thought as a stress/strain field independent of the external one.

When tensile strains fluctuate in the presence of compressive residual strains, a negative work is done and this reduces the SED associated to the external stresses, thus accounting for the beneficial effect of compressive RS.