This project is concerned with the fundamental study of an innovative damage detection and imaging method for structural monitoring. The principle is based on the use of ambient acoustic and vibrational noise as a substitute to incident ultrasound waves generated in conventional non-destructive testing methods.

The applicability of uncontrolled noise for defect detection in plate-like structures has been validated in previous works. Though very encouraging academic results have been obtained in terms of defect detection and localisation, the use of linear defect responses makes hazardous an application in actual conditions, where environmentally-related changes in recorded signals would constitute a major problem. Therefore, we propose in this project to take benefit of contact acoustic nonlinearity (CAN) phenomena in damages, in order to develop more robust passive monitoring techniques. Indeed, a damaged area subject to static or low-frequency (LF) mechanical loading often exhibits nonlinear behaviour that can induce two kinds of characteristic signatures in high-frequency (HF) ultrasound signals: (1) acoustic emission and harmonic generation through solid-solid contact phenomena; (2) pump-probe effects entailing modulations of HF waves incident on the damage.

Exploitation of these effects using ambient LF vibrations such as generated in regular service will be the key aspect of this project. Through original processing of HF waves naturally produced or modulated by the CAN under LF loading, we expect to achieve robust detection and, possibly, localisation and characterisation of critical structural damages. Indeed, since they will rely on repetitive probing of the same damages in different LF loading states, rather than on hypothetical baseline (“damage-free”) signals, this kind of techniques should be much more immune to environmental conditions than linear techniques.

Moreover, the envisaged techniques do not depend on artificially-driven acoustic sources. Therefore, they are expected to be applicable with lower resources (power-consumption, reduced hardware) than classical emission-reception techniques. One of the main applications envisaged is structural health monitoring (SHM) of structures whose usage conditions make them subject to ambient acoustic waves and vibrations. Aeronautical structures are obviously a notorious example. In this context, the principles developed are thought to be a promising solution to deploy on-board and on-service structural monitoring systems that would virtually not impact the aircraft power consumption. This would be a significant gain over classical, more power-consuming and heavier, active monitoring systems.