An analysis of the dependence of the material length parameter a0 on the stress ratio

Safe life approaches in notch fatigue of metallic components rely on the knowledge of the material response to fatigue stresses at notches, which is typically quantified by a materialdependent length parameter. As a general material behaviour, such material parameter depends on several factors and one of the most important of them is the mean stress. The present study is focused on the El Haddad-Smith-Topper material length parameter a0 for analysing under which circumstances and to which extent it may vary with the applied mean stress or stress ratio. The analysis starts by considering the broader problem of the mean stress influence on the fatigue limit of plain material, blunt and sharp notches and cracks. Eventually, an analysis of previous sets of experimental data on wrought steels led to simple relations for estimating the a0 parameter at different stress ratios on the basis of the sole Vickers Hardness