Yavuz Kadioglu

Plastic Zones and Fatigue-Crack Closure Under Plane-Strain Double Slip

The results of a systematic investigation involving forward and reversed plastic zones for a growing fatigue crack under plane-strain double-slip conditions are presented. The study focuses on plastic-deformation fields outside the small-scale yielding regime. The size of the macroscopic forward plastic zone is found to be nearly proportional to the square of the applied-stress intensity over the critical resolved shear stress. The size of the forward plastic zone is also found to depend on the angles of the two microscopic slip directions with respect to the crack line. When the microscopic slip directions are kept symmetric about the crack-line normal, and the angle between them is varied, the forward plastic zone sizes hardly vary. However, when the angle between the slip lines is kept constant, and both planes are simultaneously rotated, the forward plastic zone sizes vary by a factor of three. The ratio of the reversed plastic zone size to the forward plastic zone size is also found to be dependent on the orientation of the microscopic slip planes. The ratio varies when the angle between the microscopic slip planes is changed, or when the orientations of both planes are rotated simultaneously. Stationary cracks are generally found to have larger reversed plastic zones than fatigue cracks, and the difference is attributed to crack closure.

FEM study of fatigue crack closure under double slip

Abstract An elastic—plastic finite element analysis of Mode I crack growth and plasticity induced crack closure is presented. The simulation, which models crack tip plasticity using two slip planes, predicts slip patterns experimentally observed in real crystals or grains. The model shows that plasticity induced crack closure will occur when the crack tip plastic zone is smaller than the grain size. Closure levels are found to vary as the crystallographic orientation of the model with respect to the crack line is changed. For some orientations the closure levels are found to be negligible, while in others the levels approach 0.35 of the maximum applied load. Closure levels also vary as the angle between the slip planes is varied. The values span a similar range. The amount of crack closure for any particular crystallographic orientation and/or slip plane configuration is found to be a balance between several variables. Primarily, the closure levels are dictated by the magnitude of the static crack opening displacement and amount of residual plasticity along the crack line.