Effects of Compressive Residual Stress Near Precrack on Fatigue Crack Propagation Behavior Bent from a Slant Precrack (Based on Discontinuous Displacement Measured along Short Crack Immediately after bending)

Abstract

A testing for bent fatigue crack propagation under mixed-mode conditions was carried out using fatigue and annealed slant precracks with different slant angles, β, defined as the angle between loading and precrack directions in a rectangular plate. As a result, bent fatigue crack from the fatigue precrack with β=45 deg. propagated under mixed-mode conditions with mode II stress intensity factor (KII) est evaluated from the discontinuous displacement measured along the crack. On the other hand, bent fatigue crack from the fatigue precrack with β=60 deg. and from the annealed precracks with both slant angles of β=60 deg.and β=45deg, propagated under the mode I behavior. This was because the compressive residual stress near the fatigue precrack caused contact to each other between the upper and the lower surfaces of the precrack with the smaller slant angle β. The estimated fracture angles, θ est, calculated using (KI) est and (KII) est corresponded to the measured fracture angles, θ mes, even for the bent fatigue crack propagation under the mixed-mode condition containing the mode II component. Furthermore, the fatigue crack propagation rates indicated almost the same relationship for all the data, using the mixed-mode effective stress intensity factor (KM) est, calculated from the discontinuous displacement measured along the bent fatigue crack.