Experimental investigation on monitoring the evolution of fatigue microcracks in marine high-strength steel by nonlinear ultrasonic technique

Due to harsh service conditions, fatigue damage is the main reason for numerous catastrophic accidents of ships and offshore constructions. This paper uses a nonlinear ultrasonic technique to monitor the evolution of fatigue microcracks in marine steel AH36. An experimental system was established and nonlinear ultrasonic inspections were conducted on the specimens with different fatigue degrees. The microstructural changes induced by fatigue loading were also studied by microscopic observation. The experimental results demonstrate that the normalised nonlinear parameters show a closed relation to the evolution of fatigue microcracks in steel AH36. Meanwhile, microscopic observation indicates that the microcrack generation mode and grain boundary cracking are the two mechanisms for microcrack generation. Both the generation and growth of the microcracks contribute to the rise of the nonlinear effect. Consequently, the nonlinear ultrasonic techniques provide a reliable and effective method for monitoring the evolution of fatigue microcracks in marine high-strength steel.