The Effect of Mean Stress on Fatigue Strength of Plain and Notched Stainless Steel Sheet in the Range From 10 to 107 Cycles

Abstract

Stainless steel sheet (18Cr-9Ni) was tested in fatigue under axial-load cycling in plain and notched conditions. Various stress ratios were used ranging from R = −1.0 to +0.91, and endurances (see Definitions) from 10 to 107 cycles were covered using testing frequencies of 5 to 15 cpm and 3000 cpm. The effect of mean stress on notch fatigue strength could not be predicted empirically solely from unnotched material data; at least one notched fatigue curve would be required. A fatigue strength reduction factor based on maximum stress for a particular mean stress and endurance provided the most reliable correlation between unnotched and notched data. Low-cycle and high-cycle fatigue curves matched up only to a limited extent at the overlap, but there was generally strength reduction at low frequency. Under certain conditions of mean stress and stress ratio a cyclic creep or ratchetting mechanism leading to ductile rupture was obtained at low endurances. Simple functions existed in the low-cycle region between stress range and plastic strain range and total energy and cycles to fracture, both of which were largely independent of stress ratio.