OVERLOAD RETARDATION OF FATIGUE CRACK-GROWTH IN A 9-PERCENT-CR 1-PERCENT-MO STEEL AT ELEVATED-TEMPERATURES

This paper describes an investigation into the effects of a single-peak tensile overload on fatigue crack propagation in a 9% Cr 1% Mo steel. Overloads were applied during cycling at a constant stress intensity range (DELTAK), and any consequent transients in growth rate were recorded. The severity of retardation rises as the magnitude of the applied overload is increased. The effect of temperature is complex, but a 525-degrees-C retardation is significantly less marked than at 25 or 225-degrees-C. Signs of crack face contact are seen on post-overload fracture surfaces, but there is little crack branching. The dominant cause of overload retardation in this steel appears to be plasticity-induced closure. At 525-degrees-C. post-overload dwell periods significantly reduce the severity of retardations. This is not observed it lower temperatures, indicating that the residual clamping stresses that lead to closure are gradually believed at 525-degrees-C.