1983 Volume 32 Issue 355 Pages 400-405
Fatigue crack propagation under periodic overstressing i.e. intermittent application of a very small number of cycles of overstress during a cycling of very large number of understress below Kth, was studied with the normalized iron specimens and those stretched to 10% strain. Remarkable acceleration of crack propagation rate of about 200 times was observed in the former but it amounted about 100 times in the latter. Fracture surface observation revealed characteristic clam-shell markings corresponding to the varying stress. Etch pit study in the area where the markings are obvious suggested that the fracture surface formed by understressing has some bearing with crystallographic orientation. Direct observation of dislocation structure adjacent to the crack tip showed that acceleration of crack growth under the varying stressing is originated from cooperation of over- and understressing: Large crack tip opening displacement accompanied with formation of radial bands of dislocation structure emanating from crack tip on overstressing, and crack propagation along the band occurring on the way to recovery of the structure during understressing.
Some characteristics of acceleration under the periodic overstressing are discussed in the light of the new findings.