Mixed-frequency nonlinear numerical simulation of fatigue microcracks on air-frame surfaces by focused ultrasonic waves

This study addresses the challenge of detecting surface fatigue micro-cracks in aircraft structures subjected to complex loading conditions. A numerical model incorporating a fatigue micro-crack interface force model is developed to simulate the mixing non-linearity of focused surface acoustic waves (FSAWs) interacting with micro-cracks. Simulations reveal that FSAWs, unlike linear surface waves, concentrate ultrasonic energy at the focal point, leading to a significant enhancement of the mixing non-linearity at the micro-crack site and improve the detection sensitivity and localization. The mixing non-linearity increases with crack length within the focal zone but remains relatively constant beyond it. These findings offer a new numerical simulation approach for air-frame structural health monitoring and micro-damage identification.