Effects of tensile and compressive residual stresses on behavior of fatigue crack growth and crack closure are investigated for plate specimens of SM 50A (JIS) steel welded by electron beam.
A simple method for measuring residual stresses in a plate is proposed first. In this method the residual stresses are evaluated using fracture mechanics approach, that is, the strains and displacements measured at a point on the edge of a plate as a crack is introduced and extended from the edge are used to deduce the state of stresses that existed in the uncracked plate. Through finite element analyses and experiments this method is shown to be valid and effective for measuring the 2-dimensional residual stress distribution of a welded plate.
Experiments of fatigue crack growth are performed for various values of R. It is found that in most cases the effective range of stress intensity factor, ΔKeff or ΔK combined with the effective stress ratio, R' can successfully describe the fatigue crack propagation in the residual stress field. However, when the fatigue crack, which starts initially at compressive residual stress field, propagates near the region of transition to tensile residual stress field, the crack shows a quite different behavior. It appears that the crack tip undergoes a partial opening in the transition region. It is found that the behavior in the region is well described in terms of Kpart. eff based on the concept of the partial opening.
