Critical crack size that causes retardation of short fatigue crack by single overload

摘要

The effect of track 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. In case retardation occurs, it is caused by the increase of crack closure stress as in a long crack. The occurrence of retardation, however, showed a complex dependency on crack length, yield strength of material and the value of baseline stress ratio R. At stress ratio R =-1, retardation did not occur for very short crack as short as 50 μm in the case of low yield strength material. This was because the increase of crack closure stress was prevented by the compressive stress component of baseline load. The plastically deformed layer near the crack tip was collapsed by the compressive stress of baseline load, which prevented the development of crack closure. In the case of high yield strength material, however, retardation occurred even in 50 μm crack. On the contrary at R = 0, retardation in the low yield strength material occurred even in 50 μm crack. The prevention of increase of crack closure did not occur in the case of R = 0. As a consequence, retardation basically occurs even in short crack as short as 50 μm unless the development of crack closure is impeded.