Abstract
The effect of periodic overstressing on fatigue crack propagation was studied with sharp notched specimens of low carbon steels (a rimmed steel and an Al-killed steel), a medium carbon steel, a high strength steel and an aluminum alloy. A very small number of cycles of overstress applied intermittently during a very large number of cycling of understress below the fatigue limit or threshold stress intensity Kth caused a remarkable acceleration of crack propagation (about or more than one hundred times) in all materials tested except the high strength steel. The fracture surface morphology in the cases with such remarkable acceleration was quite different from that of the cases without it, the characteristic feature being clam-shell markings with saw-tooth profile of hill-to-valley matching. This indicates the occurrence of crack propagation in zig-zag fashion in contrast to the straight propagation under steady cyclic stress conditions. The cracks propagated also under the understress below Kth. These results raise a question on the validity of Kth as a design criterion.
