A quantitative evaluation of fatigue crack shielding forces using photoelasticity

The mechanisms controlling the phenomenon of plasticity-induced shielding during fatigue are investigated and quantified by fitting a recently developed model to photoelastic data. The model derives from the Muskhelishvili approach and includes additional terms to describe the effect of plasticity on the elastic stress field around the crack tip. The photoelastic technique used a polycarbonate CT specimen containing a naturally propagating fatigue crack from which full-field data was obtained digitally using the phase-stepping method. The model was fitted to approximately 1000 data values for the isochromatic fringe order around the crack tip and generated values for the stress intensity factor and T-stress plus an interfacial shear stress intensity factor and a retardation intensity factor which together characterize the influence of plasticity on crack growth.