Correlation-Based Imaging Algorithm for Bolt Loosening Monitoring on Realistic Aerospace Structure

Abstract

In the present study, a correlation-based imaging technique called « Excitelet » is proposed to monitor bolt loosening on an aluminum lap-joint, representative of an aircraft component. The principle is based on guided wave generation and sensing using a compact micro-machined piezoceramic array and measurement of reflections induced by potential damage. The method uses a propagation model to correlate measured signals with a bank of signals and imaging is performed using a round-robin procedure (Full-Matrix Capture). This methods allows taking into account the transducer dynamics and finite dimensions, multi-modal and dispersive characteristics of the material and complex interaction between guided wave and damage. Experimental validation has been realized on an aluminum lap-joint instrumented with a compact linear piezoceramic array of 8 circular elements of 3 mm diameter each. The imaging algorithm is applied to detect the partial and complete loosening of one of the bolts. Imaging results obtained using both A0 and S0 modes at 300 kHz are presented for each loosening condition. It is demonstrated that variations in the bolt torque can be accurately detected, while the correlation level indicates the level of reflected energy, related to the loosening. An accuracy of approximately 5 mm on damage location estimation can be achieved, demonstrating the possibilities of the correlation-based imaging technique for damage monitoring of complex aerospace structures.