Propagation of Microscopic Fatigue Crack under Periodic Overstressing in Quenched and Tempered 0.45% C Steel

Abstract

Propagation of microscopic fatigue cracks by periodic overstressing was studied with quenched and tempered 0.45% C steel. Acceleration of crack propagation occured in 0.45% C steel as in the case of 0.15% C steel previously reported, when the crack length was more than 50μm. The lower limit of understress that caused such an acceleration was reduced steeply as the crack length became longer. The acceleration in 0.45% C steel was less than in 0.15% C steel in accordance with the previous result on macroscopic cracks in which the acceleration was less in materials of higher strength. In the cases of short cracks (below 200μm) and long cracks (above 200μm) loaded with low understress, the microscopic fracture surface consisted of small facets, which were supposed to be related to the crystal structure of steel, and cracks observed on the specimen surface propagated intermittently, suggesting that microstructure had a significant effect upon crack propagation under intermittent overstressing. In the case of long cracks loaded with high understress, the microscopic fracture surface consisted of large facets and cracks propagated steadily, indicating that microstructure had less effect. No appreciable difference was observed in crack opening behavior before and after overstressing, and the acceleration of crack propagation was not likely to be related to the crack closure phenomenon.