Fretting-Fatigue Analysis of Shot-Peened Al 7075-T651 Test Specimens

Vicente Martín,Jesús Vázquez,Jaime Domínguez,Carlos Navarro

doi:10.3390/met9050586

Abstract

Shot peening is a mechanical treatment that induces several changes in the material: surface roughness, increased hardness close to the surface, and, the most important, compressive residual stresses. This paper analyzes the effect of this treatment on alloy Al 7075-T651 in the case of fretting fatigue with cylindrical contact through the results of 114 fretting fatigue tests. There are three independent loads applied in this type of test: a constant normal load N, pressing the contact pad against the specimen; a cyclic bulk stress σ in the specimen; and a cyclic tangential load Q through the contact. Four specimens at each of 23 different combinations of these three parameters were tested—two specimens without any treatment and two treated with shot peening. The fatigue lives, contact surface, fracture surface, and residual stresses and hardness were studied. Improvement in fatigue life ranged from 3 to 22, depending on fatigue life. The relaxation of residual-stress distribution related to the number of applied cycles was also measured. Finally, another group of specimens treated with shot peening was polished and tested, obtaining similar lives as in the tests with specimens that were shot-peened but not polished.

Highlights

Fretting is a phenomenon produced when two elements in contact under pressure are subjected to very small amplitude relative displacements
The normal contact pressure and the shear tractions produced by the interaction between the contact surfaces usually produce a high-stress field close to the contact that cyclically varies with the relative displacements
The objective of this paper is to analyze the effect of shot peening in general, as well as for each one of the main transformations produced by the treatment, such as residual stress and its relaxation, hardening, or roughness on the fretting-fatigue strength of Al 7075-T651

Summary

Introduction

Fretting is a phenomenon produced when two elements in contact under pressure are subjected to very small amplitude relative displacements. The normal contact pressure and the shear tractions produced by the interaction between the contact surfaces usually produce a high-stress field close to the contact that cyclically varies with the relative displacements. These varying stresses initiate cracks that grow through the zone where stresses are high enough. If the global stress field, far from the contact, is small, the initiated cracks stop growing when they reach a low-stress zone In this case, the effect of fretting is only wear and the phenomenon is called fretting wear. If the global stress field is high enough, the initiated cracks continue growing until final failure In this case, the phenomenon is called fretting fatigue. There are many examples of mechanical elements prone to fretting-fatigue failure, such as bolted joints, rotor-blade dovetail connections, metal cables, or shrink-fitted couplings [1,2]

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