The emergence of new materials, geometries and SHM needs require imaging method able to deal with the complexity of these structures. Using a defect-free model of the complex structure, topological imaging allows locating defects when classical delay and sum methods tend to fail.

Guided waves in planar structures

Detecting defects in planar structures is a major industrial concern especially for aeronautics. Depending on the structure geometry and the inspection frequency, dispersion may be an issue. Taking into account the dispersion in the defect-free medium, defects are accurately localized [Rodriguez et al., Ultrasonics 54, 2014]. Anisotropy can also be taken into account [Rodriguez et al., EWSHM, 2014].

Reverberating media

Topological imaging is successfully applied to a reverberating plate made in a way that there is no direct wave path between the unique transducer and region of interest [Rodriguez et al., APL 105, 2014]. This method is actually applied to the control of hidden parts of railways.

Strongly heterogeneous media

The detection of buried defects in a periodic heterogeneous medium can also be achieved with topological imaging [H. Hafidi Alaoui et al., Ultrasonics 112, 2021]. It is a first step towards phononic crystal control.

This project led to the development of an efficient technique for the generation and detection of Lamb waves guided along large plate-like structures made from various types of materials (metal, polymer, fibre-reinforced composite, etc.). A multi-element matrix ultrasonic probe is driven using the well-known phased array principle, for launching and detecting pure Lamb modes in/from specific directions along the plate, which are arbitrary for isotropic materials and so far limited to principal directions (axis of symmetry) for anisotropic materials. The probe is gel-coupled to the tested specimen and allows quick inspection of large area from its fixed position, even of zones with limited access. The technique takes into account the frequency dispersive nature of guided modes, and is different than SHM-like (Structural Health Monitoring) inspection, since all transmitting or receiving elements are grouped together in a very localized area defined by the active surface of the probe, and not permanently attached to the tested structure. The use of a multi-element probe for long range Lamb waves-based inspection, is also distinctive from classical use of such probes, which consists of very local inspection of a material by steering the ultrasonic beam below and nearby the probe.

A prototype has been developed, and performed measurements showed efficient selectivity and directivity of the generated Lamb modes. Finally the detection and localisation of a through-thickness hole in a large aluminium plate, of a 20mm-in-diameter delamination-like defect in a carbon epoxy composite panel and of an impact damage on a stiffened composite curved structure have been successfully produced.

Related publications

[A. Leleux et al., J. of NDE vol 32, 2013 / P. Micheau et al., Contrôles Essais Mesures vol 46, 2014].

In the field of non-destructive evaluation and control (NDE/NDT) of materials and structures, there is no unique and satisfactory method for revealing any kind of defect. We proposed to combine ultrasounds and infrared thermography to characterize viscoelastic materials or defects. “Sonothermography” consists in applying vibrations or ultrasonic wave to a viscoelastic material.  Part of the mechanical energy is absorbed and is transformed into heat. If the level of energy is sufficient, the temperature field can be acquired with an infrared camera and be post-processed. The advantages of combining both method is to associate the generation of volumic sources induced by vibrations or waves and the ability of imaging of infrared cameras. Temperature field is sensitive both to mechanical and thermal properties of materials and structures. This technique has been applied to isotropic [Hosten et. al., JASA, 2008] and anisotropic materials [Hosten et al., NDT&E, 2012], evaluation of thermal properties [Kouadio et. al., QIRTJ, 2014], identification of thermal sources [GROZ et. al., Applied Science, 2020], and IR Chladni plate experiments [Rodriguez et. al, QIRTJ, 2018].

Structural health monitoring (SHM) using ultrasonic guided waves has proven to be attractive for the identification of damage in composite plate-like structures, due to its realization of both significant propagation distances and reasonable sensitivity to defects. However, topographical features such as bends, lap joints, and bonded stiffeners are often encountered in these structures, and they are susceptible to various types of defects as a consequence of stress concentration and cyclic loading during the service life. Therefore, the health condition of such features has to be assessed effectively to ensure the safe operation of the entire structure. This research work proposes a feature guided wave (FGW) based SHM strategy, in which proper FGWs are exploited as a screening tool to rapidly interrogate the representative stiffener-adhesive bond-composite skin assembly. An array of sensors permanently attached to the vicinity of the feature is used to capture scattered waves from the localized damage occurring in the bond line. This technique is combined with an imaging approach, and the damage reconstruction is achieved by the synthetic focusing algorithm using these scattered signals. The proposed SHM scheme is implemented in both the 3D finite element simulation and the experiment, and the results are in good agreement, demonstrating the feasibility of such SHM strategy [X. Yu et al., NDT&E vol 89, 2017 / X. Yu, Smart Mat. Struct. vol 27, 2018].