Fatigue threshold in steels—Mean stress and microstructure influences

Abstract

The purpose of the present paper was (i) to closely investigate the effect of R-ratio on a large variety of microstructures commonly found in low C steels and (ii) to assess the predictability of a selection of approaches or models which deal specifically with R-ratio effects on threshold fatigue stress intensity range values, ΔK th . Essentially the extent of the R-ratio response on the ΔK th was found to be significantly microstructure dependent but the trends were by no means simple. Generally the predictions of the various models dealing with R-ratio-ΔK th trends exhibited poor commonality with the experimentally determined ΔK th values. Marked complex yield strength σ y effects on ΔK th were observed at low R-ratio levels ( R ⋍ 0 ), viz. (a) ΔK th levels initially increased with σ y , (b) attained a maximum ΔK th value of 9 MPa✓m which remained constant with increasing σ y level over the range 300–500 MPa and (c) above σ y ⋍ 600 MPa ΔK th decreased with σ y .