THESIS : Pierre GOISLOT

Rapporteurs:

• Mrs Odile ABRAHAM - Université Gustave Eiffel
• Mr Marc REBILLAT - Arts et Métiers

Examiners:

• Mr Koen VAN DEN ABEELE - KU Leuven
• Mr Régis MARCHIANO - Sorbonne University

Thesis co-supervisor:

• Mrs Guillemette RIBAY

Thesis co-director:

• Mr Emmanuel MOULIN - INSA Hauts-de-France
• Mrs Lynda CHEHAMI - Université Polytechnique Hauts-de-France

The aeronautics industry is making a transition from traditional materials to composite materials, which are lighter for equivalent mechanical strength, enabling a significant reduction in fuel consumption. However, these materials are subject to complex damage, in particular as a result of impacts generating defects known as BVID (Barely Visible Impact Damage), which are often invisible on the surface and constitute a major challenge. Caused by light impacts (hail, debris or tool falls during maintenance), they can evolve into critical states following repeated mechanical stress. Safety is a constant preoccupation in the aeronautics industry, and this is particularly true of structural integrity monitoring. Traditional non-destructive testing (NDT) techniques rely on costly, scheduled spot inspections, which limit the frequency of inspections. Structural Health Monitoring (SHM) aims to overcome these limitations by providing continuous monitoring of the structure using on-board sensors. Among existing SHM methods, those using guided elastic waves are naturally sensitive to these defects. However, traditional approaches based on a reference state are poorly suited to structures evolving in harsh environments. The aim of this work is to develop a method for detecting and localizing BVIDs without a reference state, via a pump-probe method based on the principle of vibro-acoustic modulation, where a low-frequency vibration
(pump) modifies the reference state. frequency vibration (pump) modifies the contact state of the defect, which is then detected by an ultrasonic wave (probe). As the signatures obtained with this family of methods are of a lower order of magnitude than those obtained with a reference state, a coherent imaging method was first developed, for highly anisotropic structures. It takes account of phase-deviation-inducing focusing effects, and was able to locate a BVID with reference state in the presence of very high noise. An artificial contact defect was then designed and modeled, for the implementation of reference-state-free imaging methods. It was shown that this defect imposes a variable force on the vibrating plate, generating a variable contact interface. The modulation of a guided elastic wave across this defect was observed by piezoelectric sensors and confirmed by ultrasound field observation. Several strategies were explored, ranging from incoherent approaches, which do not require phase information from the ultrasonic signatures, to coherent approaches, which are more efficient but also more constraining. The case of a monochromatic pump was studied, followed by that of a random pump using a method based on singular-value decomposition of differential matrices; a second matrix approach, based on decomposition into independent components, enabled the detection of weak modulations embedded in the noise. Finally, the methods developed have been applied to a real case on a BVID. The multimodal nature of guided elastic waves and the presence of modulations competing with those of the fault make interpretation of the results tricky. The modulations observed on the A0 and S0 modes were studied, as well as the modulation of conversions between these modes using multimodal imaging algorithms. An alternative approach, based on the frequency mixing of two ultrasonic waves, revealed a non-linear interaction in the BVID zone via a measurement of the ultrasonic field, and provides avenues for future improvements to the method.