On the anomalous temperature dependency of fatigue crack growth of SS 316(N) weld

Fatigue crack growth behaviour of a nuclear grade SS 316(N) weld metal was examined in the Paris and threshold regimes in the as-welded condition, at 300, 573 and 823 K. The ΔKth values were 11.2, 7.5, and 9.5 MPa √m, respectively. These values were corrected for closure effects and the corresponding ΔKth,eff were found to be 7.7, 5.8 and 3.5 MPa √m, respectively. The anomalous behaviour, i.e., the high value of ΔKth at 823 K has been explained based on crack closure effect which is roughness induced at 300 K and oxide induced at 823 K, with both these insignificant at 573 K. The effect of temperature on crack growth mechanism and the associated closure mechanisms are discussed. The stress shielding at the crack tip due to closure is accounted for and the effective stress intensity factor experienced by the crack tip, ΔKeff,tip is determined. It is demonstrated that ΔKeff,tip qualifies as a more appropriate parameter as the driving force for the temperature-dependent crack growth in the near-threshold and Paris regimes.