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Health Monitoring of Aerospace Bolted Lap Joints Using Nonlinear Ultrasonic Spectroscopy: Theory and Experiments



This paper presents theoretical and experimental study of nonlinear ultrasonic spectroscopy method for health monitoring of aerospace bolted lap joints. In this study, the interrogating waves generated by a transmitter piezoelectric wafer active sensor (T-PWAS) propagate along the structure, interact with the lap joint contact surfaces, carry the information of bolt preload and are picked up by a receiver PWAS (RPWAS). The contact acoustic nonlinearity (CAN) introduced by the interaction between guided waves and contact surfaces serves as an index for the assessment of bolt tight/loose status. Contact finite element models (FEM) are built to simulate the contact behavior of the lap joint surfaces under ultrasonic wave cyclic loading. The relationship between bolt preload and the CAN is investigated. Experiments on an aerospace bolted lap joint are carried out to verify the FEM predictions. The nonlinearity from the electronic equipment (function generator, amplifier, etc.) is addressed. Scanning Doppler laser vibrometer is used to visualize the wave propagation and interaction with the lap joint, and the results are compared with FEM simulation. Nonlinear effects such as higher harmonics are observed from the FEM predictions and the experimental results. The paper finishes with conclusions and suggestions for future work.

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