Evolution of residual stress at a fatigue crack tip and its influence on crack tip shielding and plasticity

The evolution of residual stresses (RS) at a crack tip and associated changes in crack shielding effects during fatigue crack growth (FCG) has a first order effect on the fatigue life of welded joints. This paper reports the results of fatigue crack growth rate (FCGR) tests conducted on as-welded and post-weld heat-treated (PWHT) specimens. By combining digital image correlation (DIC) techniques with the CJP model of crack tip stress and displacement fields, the stress intensity factor KCJP was obtained. Additionally, the da/dN-Delta KCJP curve was derived from the displacement field. Subsequently, the variation in KCJP-max and crack opening displacement delta were compared for as-welded and PWHT specimens. The evolution of RS and crack opening force Fop at different crack lengths were investigated. Results indicated that during FCG, the RS at the crack tip showed a trend towards a progressive acceleration in its decrease, until it was completely relaxed when a/W was approximately 0.56. Due to the crack tip shielding effects contribution an associated slight reduction in KCJP-max was observed in the initial stages of crack growth. Finally, the impact of the RS changes on crack tip shielding effects and the plastic zone size at the crack tip during the FCG process was systematically analysed. An improved model for assessing the plastic zone size at the crack tip, which considers the evolution of RS and crack tip shielding effects, was established and validated.