Fatigue strength of blast cleaned and stress relief annealed butt joints made of structural steel S355J2+N for offshore wind support structures

•Blast cleaning on welded butt joints leads to increase in fatigue strength.•Quantification of cold work hardening and compressive residual stresses.•Stress relief annealing decreases residual stresses and cold work hardness.•Stress relief annealing is not suitable to simulate residual stress reduction.•Numerical validation with structural hot spot stress and effective notch stress. The effects of blast cleaning (BC) and stress relief annealing (SRA) on butt joints were investigated. BC leads to a significant increase in fatigue strength due to insertion of compressive residual stresses and cold work hardening at the surface. Additional SRA leads to a decrease of fatigue strength down to the as-welded state. It is shown that the SRA not only leads to a decrease of compressive residual stresses but also to a decrease of cold work hardness at the surface. This shows that SRA is not an appropriate method to show the effect of the reduction of residual stresses in service of mechanical post weld treated weld seams since surface hardness is decreased as well. SRA of as-welded specimens did not lead to an increase in fatigue strength. It is shown that compressive residual stresses and surface hardness are stable under high tensile loads and high cycle numbers. Fatigue tests were accompanied by strain gauge measurements to identify the structural hot-spot stresses in correlation to axial and angular misalignments. A parametric FE model was used to calculate the stress concentration factors and compared to the measured stresses. Angular misalignment shows good correlation with stress increase compared to nominal stresses.