Effects of Mean Load on Creep and Fatigue Crack Growth at Elevated Temperature

The effects of mean stress on crack growth behavior have been studied by performing various load ratio experiments at elevated temperature. It was found that at low load ratios a transgranular crack growth mode dominates whereas at high load ratios an intergranular crack growth mode was observed. The crack closure phenomenon was observed for experiments in which the transgranular crack growth mode was significant. In such cases, the crack opening stress was found to be constant for Stage II crack growth for each individual experiment at fixed load ratio, but varied with load ratio. Based on this, it was found that the crack growth rate correlated well with the effective stress intensity factor range, ΔK_eff. At R = 0.6 and above, crack closure effects were not observed and the linear rate line integral parameter, C*, based on elastic-plastic fracture mechanics, was found to correlate better crack growth rate because time-dependent processes were significant in those experiments. In the regime where both fatigue and creep mechanisms might interact, the linear cumulative rule, which combines the two parameters ΔK_eff and C*, accounting for fatigue and creep effects, respectively, was found to overestimate crack growth rates.