OS11W0026 Modeling of crack initiation and simulation of low cycle fatigue damage in biaxial stress states

Abstract

In the present work, crack initiation in biaxial fatigue was first modeled as the slip band formation in an individual grain constituting a polycrystalline metal. In modeled grains, normal directions of slip planes and slip directions on respective planes were independently given at random. The stress states in individual grains were determined in the following two ways. In the first model, it was assumed that every grain is subjected to the same stress state as that in the bulk material. In the second model, stress variations were supposed to exist in distinct grains, and the stresses in each grain were also randomly given. As a crack initiation criterion, a slip-band crack was presumed to be initiated along the given slip direction on the specified slip plane when the resolved shear stress calculated in the slip direction exceeded a critical shear stress. The crack initiation life was evaluated using a dislocation pile-up model, in which the calculated resolved shear stress was incorporated. Simulations on crack initiation were carried out for three values of biaxiality; i.e. axial, combined axial-torsional and torsional modes. Space-time information on initiated cracks was able to be obtained by simulation using the aforementioned model. Simulated directions of initiated cracks under biaxial modes were compared with experimental results which had been observed in fatigue tests using tubular specimens of pure copper under axial, combined axial-torsional and torsional loading modes. As a whole trend, the estimation based on the proposed model showed a good agreement with the experimental observation.