FATIGUE THRESHOLD IN STEELS - MEAN STRESS AND MICROSTRUCTURE INFLUENCES

Tle 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, DELTAK(th). Essentially the extent of the R-ratio response on the DELTAK(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-DELTAK(th) trends exhibited poor commonality with the experimentally determined DELTAK(th) values. Marked complex yield strength sigma(y) effects on DELTAK(th) were observed at low R-ratio levels (R congruent-to 0), viz. (a) DELTAK(th) levels initially increased with sigma(y), (b) attained a maximum DELTAK(th) value of 9 MPa square-root m which remained constant with increasing sigma(y) level over the range 300-500 MPa and (c) above sigma(y) congruent-to 600 MPa DELTAK(th) decreased with sigma(y).