The effect of mean stresses on short fatigue crack growth behaviour.

This thesis investigates the effects of mean stresses on the fatigue behaviour of short cracks and related mechanisms. An experimental programme was carried out to investigate the effect of mean stresses on short fatigue crack growth behaviour and fatigue life in air. The material used was a 1.99% NiCrMo steel. Specimens with a shallow hour-glass profile were tested under torsional and uniaxial loading with various alternating and mean stress levels. Crack propagation was monitored by means of a surface replication technique applied periodically during the tests. Several results have been obtained. Satisfactory predictions of the MSC growth rate and high cycle fatigue lifetime data from the following four types of tests have been achieved on the basis of this model: (a) fully reversed torsion tests (R = -1); (b) mean torsional shear stress tests; ( c) fully reversed uniaxial loading (R = -1); (d) mean stress uniaxial tests (tensile or compressive mean stress loading).