1991 Volume 40 Issue 454 Pages 921-926
The effect of mean stress on growing surface fatigue cracks in bending plates of annealed 0.20% C steel was studied under constant amplitude stressing and periodic overstressing. In constant amplitude stressing tests, the crack was semi-elliptic and had a smaller aspect ratio under compressive mean stress than under tensile mean stress. The crack propagation rate in depth direction was almost equal to that in surface direction for the same ΔK value when mean stress was tensile (R=0). When mean stress was compressive (R=-2), the former was lower than the latter. In periodic overstressing tests, the crack propagation rate became remarkably accelerated (several hundred times) as mean stress changing from tension to compression. Large acceleration (more than one hundred times) also occurred even in the case of tensile mean stress of overstress provided that it was large enough (R=0). The fracture surface morphology in most cases except small acceleration showed that the crack propagated in zigzag manner in the same way as the cases with large acceleration in through-thickness cracks reported previously.