Fatigue damage detection in welded joints using a sideband peak intensity-based on nonlinear ultrasonic frequency mixing

Early detection of fatigue damage in welded joints is critical for ensuring structural safety and preventing catastrophic accidents. Nonlinear ultrasonic techniques have advanced substantially, enabling microscopic damage detection. However, there are still challenges in its early fatigue characterization, and its practical application in marine steel welded joints is rarely reported. This study proposes a method using nonlinear mixed frequency Rayleigh waves based on the sideband peak intensity (SPI) for quantitatively characterising and monitoring fatigue damage in welded marine steel joints. The impact of mixed-frequency orders on detection sensitivity for fatigue damage was also investigated. The SPI index provides a distinct signal of fatigue progression and serves as an early warning for potential fracture. Experimental results demonstrate that the SPI index steadily increases during fatigue accumulation and decreases sharply before fracture, with a 24% decline after 350,000 fatigue cycles. Furthermore, fifth-order mixing significantly improves the nonlinear parameter and correlates well with the SPI index, verifying its accuracy and reliability. Compared to the sideband peak counting index (SPC-I), the SPI index offers higher sensitivity, less threshold dependency, and stronger noise resistance. These results confirm the method’s effectiveness and practicality for early fatigue detection and structural health monitoring in marine engineering.