This paper presents the investigation of nonlinear scattering features of guided waves from fatigue cracks. The fatigue cracks nucleated from a rivet hole are studied as the representative case. A small-size numerical model based on the Local Interaction Simulation Approach (LISA) is introduced, which enables the efficient analysis of the Contact Acoustic Nonlinearity (CAN) of guided waves. Fatigue tests on a thin aluminum plate with a rivet hole is conducted to induce cracks in the specimen. An active sensor array surrounding the crack zone is implemented to generate and receive ultrasonic guided waves in various directions. Several distinctive aspects of the nonlinear scattering phenomenon are discussed: (1) the directivity and mode conversion features, which addresses the scattering direction dependence of fundamental and superharmonic wave mode components; (2) the amplitude effect, which stems from the rough crack surface condition with initial openings and closures; (3) the nonlinear resonance phenomenon, which maximizes the nonlinear response during the wave crack interactions at certain excitation frequency ranges. All these features may provide insights and guidelines for nonlinear guided wave based Structural Health Monitoring (SHM) system design. The numerical studies are compared with experimental data. The paper finishes with discussion, concluding remarks, and suggestions for future work.

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Yanfeng Shen, Junzhen Wang, and Wu Xu, "Nonlinear scattering features of guided waves from fatigue cracks ," Proc. SPIE 10600, Health Monitoring of Structural and Biological Systems XII, 106000A (Presented at SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring: March 05, 2018; Published: 27 March 2018); https://doi.org/10.1117/12.2296413.