1993 Volume 42 Issue 472 Pages 22-28
The effect of mean stress on growing surface fatigue cracks in plates of annealed 0.15%C steel was studied under cyclic tension and compression loading and the crack tip opening loads at local area along the crack front were measured by an ultrasonic angle beam method (UAM). The crack was semielliptic and had a smaller aspect ratio under compressive mean stress than under tensile mean stress. The crack growth rate in the depth direction was higher than in the surface direction for the same ΔK-value, when the mean stress was tensile (R=0). When the mean stress was compresive (R=-2), the former was lower than the latter. For the same ΔKeff-value assessed on the crack tip opening load measured by UAM, not only the dependence of crack propagation rate on the mean stress but also the difference between the crack propagation rates in the surface and the depth directions almost disappeared. Prediction of aspect ratio variation of growing surface fatigue cracks in tension-compression loaded plates was made with the local ΔKeff-values and with the averaged ΔKeff-values obtained by a weighted averaging along the crack front. The averaged ΔKeff-value method overestimated the crack aspect ratio when the mean stress was tensile (R=0) and zero (R=-1). Estimation by the local ΔKeff-value method was fairly accurate irrespective of mean stress value.
