Materials (metals, polymers, composites…)

Whether it is for their qualification after manufacturing, the monitoring of possible aging or simply to constitute databases intended to feed calculation codes, the non-destructive evaluation of mechanical properties of materials represents both a scientific challenge and a real industrial need. Ultrasonic waves carry information required to achieve this objective because they apply mechanical and reversible loads to the propagation medium. Several types of waves may be used: bulk waves, guided waves, e.g. Lamb or Shear-Horizontal modes; but it is necessary to choose those which carry most information, which are the most suitable for the geometry of the medium, and also to properly choose the frequency domain according to the internal structure of the material. The experimental generation and detection of the selected waves then require dedicated equipment and specific processing of the measured signals. A numerical model faithfully simulating the experimental conditions is also necessary, as well as the resolution of the associated inverse problem, which will consist in the optimization of the material mechanical properties (input data in the propagation model) so that the calculated quantities match the measured ones (speeds, amplitudes, displacement fields, etc.). Various methods have been developed according to the type of materials, waves, surrounding medium (water, air), signal processing…

Related Publications:

[B. Hosten et al., JASA vol 82, 1987 / S. Baudoin et al., Ultrasonics vol 34, 1996 / C. Aristegui et al., JASA vol 101, 1997 / M. Castaings et al., NDT&E vol 33, 2000 / B. Hosten et al., QNDE vol 20, 2001].

Evaluation of the cohesive or interfacial mechanical properties of adhesively bonded assemblies

Research studies focus on NDT ultrasonic methods applied to adhesively bonded assemblies. The aim is to non-destructively characterize mechanical properties which are representative of the interfacial adhesion or bondline cohesion. The considered assemblies are made of aluminum substrates and epoxy adhesive. Depending on the geometry of the studied assemblies, two methods are used to obtain quantitative information on the adhesion level.

A first method is suitable for three-layered plate-like samples. It consists in analyzing the transmission of bulk ultrasonic plane waves through the assembly immersed in water. It has been shown that an inappropriate surface treatment of the substrates cause an apparent anisotropy of the measured elastic moduli of the joint. This anisotropy was quantified using two parameters whose values can reveal the quality of the interphases.

Stiffnesses kL and kT of the interfaces can be estimated. It was shown that their values are higher when the adhesion is nominal, and are strongly decreased when the adhesion is degraded. Measured shear strength for lap-joint samples bonded under the same conditions, decreased as the adhesion is supposed to be degraded, showing agreeing changes between mechanical tests and ultrasonic measurements.

The second ultrasonic method is more suitable for large lap joint samples. It is based on the Lamb or Shear Horizontal wave transmission coefficients measured from one substrate to the other, across the overlap zone. Those transmission coefficients are obtained thanks to home-made capacitive air coupled transducers (or to EMATs) which allow contactless generation and detection of Lamb waves (or SH waves) with a good signal to noise ratio.

Experimental measurements of the Lamb wave transmission coefficients were performed on two samples. One of them has interphases with nominal adhesion and the other has degraded interphases. The comparison between the measured results and numerical simulations obtained for different modes was used to determine the values of the interfacial stiffnesses for each sample. Again, it was observed that poor adhesion leads to low values of the interfacial stiffnesses of the interphases, that can be quantified using guided ultrasonic waves.  The obtained values of the stiffnesses were in good agreement with previous ones from the first ultrasonic method.

Related publications:

[Siryabe et al, Ultrasonics 79, 2016]

Topological Imaging

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

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.

Multi-Element Guided Waves (MEOG)

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].

Evaluation of material properties using sonothermography

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].

Synthetic focusing method for SHM

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].

3D Semi-analytical models of canonical geometries (multilayer plates and pipes)

Topological imaging of structures, like plane wings or pipes containing dangerous fluids, needs efficient computation of acoustic fields when these structures are healthy. By a semi-analytical approach, i.e. we know the exact solution in the perpendicular/radial direction after integral transformations with respect to time (Laplace transform) and to the other directions (Fourier transform/series), we are able to efficiently compute the 3D wave propagation in multilayer plates [Mora et al., Ultrasonics, 2016] or pipes [yet unpublished, PhD in progress], which can be immersed or embedded. The limitation is that the geometry must be invariant with respect to the latter other directions.

Numerical and experimental analysis nonlinear interaction between bulk waves and contact interfaces

The evaluation of damage at an early stage of fracture is relevant in many industrial applications such as aeronautics or nuclear plants. Ultrasonic methods based on linear wave scattering are ecient for detecting defects and for characterizing material elasticity, but are less sensitive to micro-cracks or closed cracks. In this context, nonlinear acoustics constitutes a good alternative for detection and evaluation of these defects, taking advantages of the nonlinear behavior of contact dynamics induced by an acoustic wave if its amplitude activates suciently nonlinear contact behavior. In order to exploit nonlinear effects (harmonic generation, DC-effect, nonlinear wave mixing…) it is essential to understand the complex interactions between waves and contact interfaces. To do that, the association of different models (analytical and numerical FE models) including specific algorithm for contact treatment and experiments are achieved. Interactions between bulk waves and contact interfaces [Meziane et. al. JASA 2011, Blanloeuil et. al., JASA, 2014], closed cracks [Blanloeuil et. al., Wave Motion, 2014] are intensively investigated in order to develop methodology based on nonlinear acoustics to quantitatively evaluate contact interface properties [Saidoun et. al., submitted in Wave Motion in 2019].