Effect of Load Ratio on Fatigue Crack Growth Behavior of SS316LN using Two-Parameter (ΔK and Kmax) Model

Abstract

AbstractFatigue crack growth (FCG) behavior of SS316L (N) with 0.12 wt.% nitrogen has been studied with different load ratios at 300 K and 823 K in Paris and the threshold region. It is observed that, with increasing load ratio, the crack growth rate (da/dN) increases, and the threshold value (ΔKth) decreases. The variation in the ΔKth value with load ratio is generally attributed to the crack closure effects. Nevertheless, uncertainty exists in the consistent measurement of crack closure quantification. The present study aimed to develop a master curve using the two parametric concepts without invoking the crack closure approach. Attempts have been made to develop an FCG master curve using two-parameter models of Walker (ΔKw) and Kujawski (K*). The Kujawski (K*) model is found to merge the FCG data into a single master curve for the entire range of load ratios compared to the Walker model. Thus, the Kujawski (K*) model was found to be appropriate for life prediction analysis. However, two adjustable fitting parameters are required to get a single curve in this study since the two kinds of crack growth mechanisms are operative over the tested load ratio range. As revealed by the fractographic examination, the crack growth occurred by a faceted mode at lower load ratios for the tested temperature (300 K and 823 K) and the sustained load fracture is also observed at the higher (R = 0.7) load ratio, which comes through the Kmax.