A modified loading method for separating the effect of residual stress on fatigue crack growth rate of welded joints

A novel loading method is proposed to remove the effect of residual stress on measured fatigue crack growth rate in welded joint, using which the influence of empirical crack growth rate correlation equations on the results can be avoided. The total stress intensity factor range and the effective stress ratios corresponding to the combined effect of applied and residual stress are kept constant during the fatigue crack growth test, by changing the applied maximum and minimum loads with the increase of crack length and changes in residual stress based on the superposition rule. The feasibility of the proposed method was validated by tests of compact tension specimens containing residual stresses introduced by four-point bending. The loading method was then applied to a test of welded compact tension specimen with coupled effect of residual stress and microstructure change on fatigue crack growth rate. The fluctuation in the separated crack growth rate reflects the effect of microstructure change on fatigue crack growth rate.