Investigation of Critical Crack Size that Causes Retardation of Fatigue Crack Propagation by Single Overload

Abstract

The effect of crack length on the retardation of fatigue crack propagation caused by single overload was investigated using carbon steels S25C and S45C. The retardation and crack arrest occurred even in short cracks ranging from 50 to several hundred microns. Retardation was caused by the increase of crack closure stress caused by overload. The occurrence of retardation, however, showed a complex dependency on crack length, yield strength of material and stress ratio of baseline stress R. At R = -1, retardation did not occur for very small crack as small as 50μm deep in the case of S25C whose yield strength is relatively low. This was because the increase of crack closure stress was prevented by the compressive stress component of baseline load. The plastically deformed layer of crack flank was collapsed by the compressive stress component of baseline load, which prevented the development of crack closure. Retardation occurred even in 50μm deep crack at R = -1 in the case of S45C which has higher yield strength. On the contrary at R = 0, retardation occurred even in small crack as small as 50μm. Prevention of the development of crack closure did not occur in the case of R = 0.