1989 Volume 38 Issue 428 Pages 520-526
Microscopic fatigue crack propagation studies were carried out under periodic overstressing with different number of overstress cycles n2 using normalized 0.15% C and quenched and tempered 0.45% C steels. With decreasing n2, the acceleration of crack propagation became higher and for cracks longer than 50μm the lower limit of the understress that caused the acceleration decreased considerably. When n2 was very small (2 cycles), the acceleration in 0.15% C steel was higher than that in 0.45% C steel in agreement with the previous results on macroscopic cracks in which the acceleration was higher in materials of lower strength. Microscopic fracture surfaces in short cracks (less than 200μm) consisted of small facets which were supposed to be related to crystal structures and were mostly tilted with respect to the direction perpendicular to the tensile axis. Cracks observed on specimen surfaces sometimes stopped at grain boundaries and propagated intermittently. These observations suggest that microstructure (grain boundary and crystallographic orientation) has a significant effect on propagation of short cracks. As for long cracks, microscopic fracture surfaces consisted of large facets which were mostly perpendicular to the tensile axis and cracks propagated steadily, indicating that microstructure has less effect. Hill-to-valley matching of striation-like patterns was observed on pairs of fracture surfaces under overstressing, indicating that the cracks propagated in zigzag manner. Under steady stressing, matching of striation-like patterns was not observed. These suggest that the acceleration under overstressing is related to the zigzag crack propagation.
