Analysis of load effect on nonlinear vibro-acoustic modulation used in on-line monitoring of fatigue cracks

Vibro-acoustic modulation is a convenient and effective nondestructive detecting approach. However, when using this technology to monitor structures under in-service conditions, conventional damage indices will not be able to effectively track the damage propagation due to the detrimental influence induced by service load. In this paper, an analysis model was approached based on plasticity induced crack closure stress analysis. This model reveals the linear dependence between the crack size and the changing rate of contact area with respect to external applied load. A load insensitive damage index (LIDI) was then developed aiming to track the crack growth under varying applied load. Results of on-line experiments confirm the load effect on the conventional damage index and prove that the LIDI is able to track the crack propagation under varying applied load. Furthermore, experimental results also demonstrate that the LIDI shows a higher sensitivity of small crack and also other improved tracking performance compared with the conventional one. We believe the LIDI holds a great potential in early damage inspection of in-service structures.