Simulation of Propagation Behavior of Microstructurally Small Fatigue Cracks
Yoshiaki Akiniwa, KeisukeTanaka, Hidehiko Kimura and Ken Kikuchi
Abstract:The propagation behavior of microstructurally small fatigue cracks was numerically simulated on the basis of the plasticity-induced crack closure model. By assuming that the crack growth rate was controlled by the crack tip openin displacement, ąCTOD, the simulation of the propagation of a crack nucleated in the weakest grain was conducted. The grain size, the critical value of microscopic stress intensity factor at grain boundary and the frictional stress of dislocation motion were given as random variables following two-parameter Weibull distributions. When the crack approached grains with higher frictional stresses, ąCTOD decreased, however the crack opening stress, Đop, increased. The grain boundary blocking and higher frictional stress act as a resistance of crack propagation. The scatters of ąCTOD and Đop diminished as the crack length becomes longer. When compared at the same stress intensity range, ąCTOD increased with stress ratio. On the other hand, the relation between ąCTOD and the effective stress intensity range was unique irrespective of the stress ratio. The crack propagation life was calculated as a function of ąCTOD. The life of the crack propagation and fatigue limit increased with decreasing grain size and with increasing critical value of microscopic stress intensity factor. The effect of the stress ratio on the fatigue limit was analyzed by the simulation. The fatigue limit as a function of the mean stress follows a modified Goodman relation. Key Words:Fatigue, Crack Closure, Crack Opening Displacement, Fatigue Limit, Mean Stress, Simulation, Microstructure, Grain Boundar