Determination of a local stress concentration induced by machining and its effect on fatigue life of an aluminum alloy

Abstract

The influence of machined surface roughness on the fatigue life of 7010 aluminium alloy has been investigated. Four-point bending specimen have been machined according to various machining conditions and tested in fatigue. It is shown that roughness has a predominant influence on the fatigue life. However, the geometric roughness parameter Ra is not able to fully describe the difference in fatigue strength between all the samples. In order to model this effect, an approach based on the finite element analysis of measured surface topography is proposed. Surface grooves due to machining are supposed to generate local stress concentrations. Surface topography is then characterized from a mechanical point of view without the use of geometrical parameters: local stress concentration factor Kt is calculated by finite element analysis from surface measurements. This so-calculated stress concentration factor is integrated in two different modelling to predict limited fatigue lives and fatigue limit. The whole approach (measurement of surface topography, determination of Kt, fatigue life prediction) provides a reliable mean to predict fatigue life when changing machining parameters and processes in an industrial frame without time-consuming and expensive tests.