Residual stress modification by post-weld treatment and its beneficial effect on fatigue strength of welded structures

Abstract

High tensile weld residual stress is one important factor contributing to fatigue crack development even under reversal or compressive cyclic loadings. A compressive stress induced by post-weld treatment is beneficial by eliminating the tensile residual stresses and generating compressive residual stresses, which improves fatigue strength of welded structures. A study is underway to characterize the magnitude and subsurface distribution of residual stresses produced by post-weld treatments, particularly by Ultrasonic Impact Treatment (UIT), and to establish the post-weld treatment effect on fatigue resistance. Two post-weld treatments, UIT and shot peening, are involved in the present study. Internal stresses were investigated on three base metal samples treated by UIT and shot peening and on one welded sample treated by UIT through neutron diffraction and X-ray diffraction techniques. This paper presents the experimental conditions and the results of these stress measurements. It is revealed that the peak compressive residual stress induced by both treatments exceeded the yield stress of the base material near the sample surface. The depth of compressive stress layer in base metal was 1.5–1.7 mm for UIT and ~0.8 mm for shot peening. The induced compressive residual stresses benefit to increase the threshold value of stress intensity factor range, Δ K th, for fatigue crack initiation and early propagation. The effect of residual stresses modification on fatigue strength is discussed based on fracture mechanics model and verified by fatigue test results of large-scale welded beams.