Fatigue damage is a common cause of failure in welded structures, and it is often difficult to detect it in the early stage. While ultrasonic-based methods can effectively monitor crack propagation, it remains a significant challenge to indicate the initiation of cracks. In this study, a novel method is proposed to monitor the diffuse ultrasonic field affected by ratcheting strain and microcracks formed in welded joints during fatigue degradation. The energy density in the diffuse ultrasonic signal is computed and correlated with different fatigue cycles, allowing for online monitoring of fatigue damage in welded joints. Six butt and cross-welded joints were studied under different fatigue conditions, and digital image correlation (DIC) technology was used for comparison throughout the fatigue tests. The results indicate that the correlation coefficient of the energy density in diffuse ultrasound exhibits a significant decreasing trend when crack initiation occurs, providing a unique signal to indicate crack initiation in welded joints. This signal may appear earlier than that from ratcheting strain monitored by DIC due to ultrasound's sensitivity to internal damages.
Keywords: Diffuse ultrasound; Early warning signal; Energy density; Fatigue damage monitoring; Welded joint.
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