Fatigue crack growth rate in ferrite-martensite dual-phase steel

Abstract

An empirical study is made on the fatigue crack growth rate in ferrite-martensite dual-phase (FMDP) steel. Particular attention is given to the effect of ferrite content in the range of 24.2% to 41.5% where good fatigue resistance was found at 33.8%. Variations in ferrite content did not affect the crack growth rate da/dN when plotted against the effective stress intensity factor range AK~I ~ which was assumed to follow a linear relation with the crack tip stress intensity factor range AK. A high AK~f~. corresponds to uniformly distributed small size ferrite and martensite. No other appreciable correlation could be related to the microstructure morphology of the FMDP steel. The closure stress intensity factor K d, however, is affected by the ferrite content with K,q/Km~,× reaching a maximum value of 0.7. In general, crack growth followed the interphase between the martensite and ferrite.Dividing the fatigue crack growth process into Stage I and It where the former would be highly sensitive to changes in AK and the latter would increase with AK depending on the R = Crmi n/Om~x ratio. The same data when correlated with the strain energy density factor range AS showed negligible dependence on mean stress or R ratio for Stage I crack growth. A parameter a involving the ratio of ultimate stress to yield stress, percent reduction of area and R is introduced for Stage II crack growth so that the da/dN data for different R would collapse onto a single curve with a narrow scatter band when ploned against aAS.