Fatigue crack growth at stress concentrations—the role of notch plasticity and crack closure

Abstract

Fatigue crack growth and closure behaviour in sharply notched specimens, made from AISI 316 stainless steel, BS 4360 grade 50B structural steel and a commercially pure aluminium, was monitored under constant amplitude cyclic loadings. The initial notch crack growth rates were in general high but decelerating. These observations are in contradiction with the predictions of linear elastic fracture mechanics. It is argued from experimental observations that plasticityinduced crack closure was mainly responsible for the initial deceleration of the notch crack. The effective stress intensity range, a parameter that has taken crack closure into account, was able to correlate most but not all of the notch crack growth data with the long crack data. When the applied loading is sufficiently high, a strain controlled condition exists just ahead of the notch tip. In this case, crack growth rates were better predicted by a shear decohesion model for high strain fatigue crack growth. Neither the nominal stress intensity range, a strain based intensity factor, a cyclic J integral nor a total shear parameter could bridge the gap between notch and long crack behaviour.