The propagation of defects under fatigue loading

Fatigue crack propagation has been studied in QlN, which is a weldable low alloy steel. Fatigue tests were conducted on specimens in the tempered martensitic condition, and the propagation of through-thickness and semi-elliptical cracks was studied in air, vacuum, and in saltwater. Load-shedding fatigue tests were carried out in air on through-thickness cracks for stress ratios of 0.2, 0.35, 0.5 and 0.7. Crack closure measurements were made during these tests using Back Face Strain (BFS) gauges and a clip gauge. These showed that the effects of stress ratio can be explained largely by crack closure concepts. Similarly, load-shedding and load-increasing fatigue tests were carried out in air on specimens containing single semi-elliptical cracks for stress ratios of 0.2, 0.35, 0.5 and 0.7. Crack closure measurements were also made using BFS gauges, clip-gauge, and near-tip gauges. Effects of stress ratio on crack growth-rates and crack shape development were rationalised using crack closure arguments, and differences between the propagation at the surface and depth positons were explained by considering the transition from plane stress conditions at the surface, to plane strain conditions at the depth. Investigations of the interaction and coalescence of coplanar semi-elliptical cracks were also carried out. Stress intensity factors and crack growth predictions were obtained for these cracks using finite element techniques, and the effects of stress ratio and initial separation on the interaction and coalescence of coplanar semi-elliptical cracks were studied using load-shedding and load-increasing fatigue tests. These showed that coplanar semi-elliptical cracks grow almost independently before coalescence which occurs rapidly to evolve single `thumbnail' cracks. The studies also revealed the changes in fracture mode associated with the interaction and coalescence stages. The propagation of non-coplanar semi-elliptical cracks was studied using specimens containing a sequence of parallel collinear and parallel offset semi-elliptical cracks with different initial separations. These tests were carried out at a stress ratio of 0.5. Parallel collinear cracks were found to be very sensitive to crack-tip shielding, and the largest cracks tended to grow independently through the specimens whilst the smaller cracks were retarded until they became non-propagating as a result of crack-tip shielding. Offset non-coplanar cracks also grew almost independently until their adjacent crack tips overlapped. The overlapping crack tips then grew towards the planes of the adjacent cracks, although they never actually came into contact with the latter before propagating through the breadth of the specimens. The final section of the project involved fatigue tests on through-thickness and single semi-elliptical cracks in saltwater and in vacuum. These were carried out at stress ratios of 0.2 and 0.7. The effects of environment were explained by considering the irreversibility of slip at the crack-tip due to the chemisorption of water/water vapour on the freshly exposed crack faces, and also by crack closure concepts.