Mode I notch fatigue crack growth behaviour under constant amplitude loading and due to the application of a single tensile overload

Low carbon steel plates having different single edge U-shaped notches with different geometries were tested near their fatigue limits at different stress ratios and constant amplitude axial stresses. The present work demonstrates experimentally obtained early mode I fatigue crack growth rates from such notches. The experimental fatigue crack growth behaviour clearly showed a minimum rate when the propagating crack reached a length approximately equal to the extent of the notch cyclic plastic zone. Further experiments investigated the effect on the notch fatigue crack growth behaviour of a single tensile overload applied within constant amplitude base load cycles. The tested parameters included the notch geometry, the maximum applied stress and the stress ratio of the base load, the overload ratio and the location of the crack tip at which the overload was applied. In case of an overload cycle applied after having an initiated fatigue crack propagating from the root of a notch, the effects of both overload and stress ratios on the proceeding fatigue crack growth rates were similar to those commonly observed in un-notched plates. When the overload cycle was applied before fatigue testing, more resistance to fatigue crack initiation and propagation was observed. In tests with an overload cycle applied after having a short fatigue crack with its tip located outside the notch root cyclic plastic zone, two minimum rates were observed. One minimum rate was observed if the overload cycle was applied when the initiated crack tip was propagating inside the notch root cyclic plastic zone.