Estimation of residual fatigue life of high-strength steel welded joints based on diffuse ultrasound

Fatigue significantly affects the durability and capacity of high-strength steel welded joints (WJs) in engineering. Despite the capability of ultrasonic-based methods to issue early warnings for fatigue crack initiation and effectively monitor crack expansion, estimating residual life before surface cracks poses a considerable challenge. This study introduces a novel method that relies on the correlation coefficient of the ultrasonic energy density, alpha, to estimate the residual life of WJs before surface cracks become evident. The fatigue damage of 20 butt- and cross-WJs of Q690qENH and Q960E, subjected to different fatigue stresses, was monitored using alpha. The results revealed a cubic polynomial relationship between alpha and the number of fatigue cycles when a sample experienced early fatigue damage. Consequently, we introduced an alpha-N early fatigue damage monitoring curve. The zero value of its second derivative functioned as an early warning indicator, and the corresponding life of the sample was recorded as zeta. Finally, a warning of 20%-50% of the fatigue life for WJs made of two high-strength steels was attainable. A linear relationship between zeta and fatigue stress, denoted as the zeta-S curve, facilitated residual life estimation before surface cracks developed. The blind test produced an approximately +/- 10% error with 4 samples.