EFFECT OF TENSION HOLD IN CREEP-FATIGUE CRACK PROPAGATION IN NI-BASE SUPERALLOYS: TRANSITION FROM CRACK RETARDATION TO ACCELERATION

Effect of tension hold on crack propagation under subsequent fatigue loading during creep-fatigue crack propagation (CFCP) in single crystal (SC) and wrought Ni-base superalloys was investigated by conducting crack propagation tests with single tension hold applied during pure fatigue loading. Fatigue crack retardation occurred after the tension hold in the SC superalloy, whereas both retardation and acceleration occurred in the wrought superalloy depending on stress intensity factor, K. The retardation and acceleration were attributed to enhanced crack closure due to creep deformation and grain boundary (GB) embrittlement due to oxygen diffusion, respectively. Transition from the retardation to the acceleration was rationalized based on a comparison between sizes of residual compressive stress field and GB embrittlement area.
EXTENDED ABSTRACT