FATIGUE GROWTH ANALYSIS OF INTERACTING AND COALESCING SURFACE-DEFECTS

Fatigue crack-growth is simulated for multiple surface cracks in a pla te subjected to different combinations of tension and bending loads. T he numerical technique employed is based on the step-by-step integrati on of a Paris' type of fatigue crack-growth law at a set of points of crack front, which enables the crack shape change during propagation t o be traced. The stress intensity factors along the crack front are es timated by the three-dimensional finite element method. The technique has also accommodated an automatic procedure for the regeneration of f inite element models as the crack grows, so that the prediction of com plex crack shape change undergoing particularly during the crack coale scence is made possible and easy. The fatigue growth behaviours at thr ee typical growth stages, i.e. pre-coalescence, coalescence and post-c oalescence, are analysed for two multiple crack configurations. The pr edicted results are also compared with those obtained by the ASME XI c ode and a 'no interaction and immediate transition (NIIT)' simplified method. It is shown that each individual crack before they touch propa gates almost independently and the interaction between them is general ly limited despite being dependent on actual crack configurations. The analyses of fatigue lives show that both the ASME XI code and the NII T method give more conservative results than the present simulation te chnique, however, the NIIT method is relatively less conservative than the ASME XI code which is excessively conservative for the crack conf iguration examined.