Effect of residual stress on crack-tip for fatigue crack propagation

Abstract

Fatigue tests were performed in cyclic loading at 10Hz and stress ratios, R of 0.1 and 0.2 at room temperature. The type 304 stainless steel specimens are prepared into two different thickness (3mm and 25mm). The fatigue crack growth rate of 25mm thick specimen is faster than that of 3mm thick specimen, and this result is attributed to the difference of plasticity in the crack tip region from the elastic-plastic fracture mechanical analysis. The residual stress fields are induced by the surface hardening treatments by induction heating at temperatures of ~600℃, 750℃ and 900℃, and the induced residual stresses are verified by the X-ray measurements of the induction heating treated specimen at 750℃. It is observed that the tensile residual stress is formed at heat affected zone and, behind this zone, the compressive residual stress is formed with maximum value of ~200 MPa. The results of fatigue tests showed the acceleration of the growth rate in tensile region, and the retardation in compressive region. The results of constant stress intensity tests illustrated that the variations of the crack growth rate are related with the change of crack closure level. A simple model for fatigue crack propagation of materials is derived with energy balance approach on the basis of the Dugdale model, and this model is verified through the experiments of two kinds of materials (type 304 stainless steel and Inconel 718). The energy balance model demonstrated that the variations of material``s tearing modulus results in the change of fatigue crack propagation.