Linear acoustical methods are mainly based on the measurements of the ultrasonic waves velocities and/or attenuation. Generally these methods are not capable to detect small changes due to damage especially when it is about characterising heterogeneous materials. To overcome this difficulty, other methods known to be « nonlinear » have been used and seem to be sensitive to small changes even for weak excitation levels [1,2]. In this study we have been interested by a nonlinear method based on the change of the resonance frequency of a given vibrating mode with respect to the excitation level. Indeed, when a material is damaged its modulus decreases creating thus a relationship between its vibratory behaviour and the used excitation level. This behaviour has been used to characterise a polymer based composite (Shield Moulding Compound SMC) and a civil engineering concrete.

For homogeneous materials, the ultrasonic immersion method, associated with a numerical optimization process mostly based on Newton's algorithm, allows the determination of elastic constants for various synthetic and natural composite materials. Nevertheless, a principal limitation of the existing optimization procedure occurs when the considered material is at the limit of the homogeneous hypothesis. Such is the case of the woven bidirectional SiC matrix and SiC fibre composite material. In this study, we have developed two numerical methods for the determination of the elastic constants of the 2D SiC/SiC composite material (2D SiC/SiC). The first one is based on Newton's algorithm: the elastic constants are obtained by minimizing the square deviation between experimental and calculated velocities. The second method is based on the Levenberg-Marquardt algorithm. We show that these algorithms give the same results in the case of homogeneous anisotropic composite materials. For the 2D SiC/SiC composite material, the two methods, using the same measured velocities, give different sets of elastic constants. We then note that the Levenberg-Marquardt algorithm enables a better convergence towards a global set of elastic constants in good agreement with the elastic properties, which can be measured using classical quasi-static methods.

The anisotropic elastic properties of two Aluminium Matrix Composites (AMC) were characterised using an ultrasonic method. The composite materials, fabricated by squeeze casting included aluminium matrix reinforced by alumina fibre and carbon fibre, respectively. The elastic constants were determined using an optimisation method associated to a classical ultrasonic immersion device. The methodology to recover the elastic constants consists in minimising the square deviation between calculated velocities and the experimental ones, measured under variable incidence from a computer controlled ultrasonic immersion device. The method of optimisation we use in this work is based on Levenberg–Marquard algorithm allowing a better convergence to the physical solution of the inverse problem. The elastic anisotropy of the two materials is discussed and the results are compared to experimental data taken from tensile tests and to values obtained by two models: the rule of mixture and a simplified Eshelby model.

For a quantitative study of acoustic emission signals or for the analysis of transient waveforms the transfer function of the sensors both in Rayleigh and longitudinal wave sound fields has to be known. To this end, the reciprocity calibration is applied to measure the response of acoustic emission sensors to the normal displacement velocity of Rayleigh and longitudinal waves. In this method neither a reference acoustic source nor a calibrated sensor is needed. The sensitivity is obtained through electrical measurements only, with three transducers alternately working as the sound source and receiver. Our contribution to this well-established calibration technique (the reciprocity method) comes from the use of spectral analysis techniques with short pulses, which enables the efficient determination of both the phase and the amplitude of the sensitivity in a broad frequency range. This study focuses on three points. First, we compare this method with laser interferometry calibration. Then we use the reciprocity technique to study the following points: the aperture effect and the fact that for quantitative acoustic emission analyses it is very important to take account of the material under test.

The classical NDE techniques of periodical maintenance are just now evolving towards the continuous health monitoring of materials and structures. The on line monitoring systems have to present a small size without damage initiation risk, good sensitivity and robustness. Taking account of the relation of this approach with the more general notion of smart materials, it seems useful to extend this passive concept to a more active one specially in the case of composite materials. Effectively, if sensors are embedded in a composite structure before curing in order to monitor and optimise the processing parameters, they are able, remaining in the structure, to assume the health monitoring in the second stage of the life. Moreover, in slightly damaged systems, it becomes reasonable to use the collected results for an on line ten tative prediction of the residual life. Finally in the last stage of the life, when the material is deeply damaged, it is sometimes possible to slow down, and very rarely, to heal the damage

In the paper industry, the need for on-line monitoring the mechanical properties of the paper during the manufacturing process is real, because of high production rates. To this end, a contactless method is considered where both the acoustic source and the sensor are laser based. The principle of laser generated ultrasounds (LUS) relies on the sharp increase of the temperature at the surface of a solid illuminated by a high power pulsed laser beam which leads to a transient mechanical stress, and therefore to the generation of elastic waves. Because this acoustic source is broadband in nature and non-contacting, it has been widely used since the early 1980's, in the Non Destructive Evaluation field. When used in conjunction with a laser beam detection system (interferometry), the presented method provides a mean to study the propagation of elastic waves without any mechanical contact, and is therefore well suited to our goals. This study focuses on the determination of the anisotropic elastic constants of paper, using this non-contact scheme. Although various type of elastic waves are generated with LUS. only the lowest antisymmetric Lamb wave mode is monitored, because of the small thickness of the samples, and the detection system characteristics. Experimented velocity dispersion curves for that mode are obtained in various directions, and the elastic constants are recovered by fitting the theoretical and experimental dispersion curves with a minimisation procedure. This non-contacting method is intended for being implemented on-line in the papennaking machine. The measured characteristics (elastic constants, wave speed...) may allow the real-time fe ed back control of the fabrication process to optimise the use of the raw materials or the paper quality.

In mechanics of material and material sciences, the early detection of damage, its non destructive characterisation and monitoring is of great importance. In this field, ultrasonic waves can he very helpful especially when nonlinear ultrasonics is considered. In this work, linear and non linear ultrasonics are used for damage characterisation. Concerning linear ultrasonics, we show that velocity and linear elastic constants are good damage indicators for composites materials. For these materials, ultrasonic velocity is measured in situ during a tensile test, at the same time acoustic emission is collected. Ultrasonic velocity decreases with damage. This decrease is very well correlated to the acoustic activity due to damage initiation and development. For other materials as metallic ones, linear ultrasonics is not sensitive enough for damage characterisation. In this case we show that nonlinear ultrasonics through the nonlinearity parameter s, is very sensitive to evaluate damage and to detect it very early. In this work we show that if linear ultrasonics through velocity measurements is a very good damage indicator especially for composite materials, nonlinear ultrasonics can he a very efficient tool for the early detection of damage and also for health monitoring. So it can greatly contribute to the understanding of damage initiation.

The DAMASCOS (Damage Assessment in Smart Composite Structures) project is a European Union funded programme of work bringing together a number of academic and industrial partners throughout Europe. The aim of DAMASCOS is to apply new ultrasonic detection and generation techniques integrated within the structure, together with advanced signal processing to realise damage assessment and ageing characterisation in composite structures. This paper describes the background, experimental findings and future applications of the technology as the project moves into its final phase.

When a piezoelectric element is inserted in a medium, the viscoelastic properties of this medium can be deduced from the measurement of the piezoelectric impedance of this insert by means of an optimisation technique. Previous laboratory developed studies have validated an original method enabling the in situ real time health monitoring of a composite structure. In order to extend such a method from laboratory conditions to the industrial situation, it is necessary to present different application fields of such an original method and to consider imperfections, as non correct positioning or bonding defect of the ceramic plate in the structure.

Applications of reinforced composites and heterogeneous solids are widespread, spanning technological areas of various aerospace and mechanical industries. A real challenge concerning these materials is their life time prediction when subjected to wide variety of environmental and mechanical loading conditions that can initiate damage and lead to failure. Indeed, damage at the smallest scales drives damage accumulation at larger length scales until some critical local damage state is attained that causes macroscopic failure. A key issue in predicting life time is to characterise distributed volumic and localised damage and to understand the mechanisms of its initiation, evolution and criticality and so, the identification of relevant precursors of failure. To answer to these questions, volumic and guided ultrasonic waves and acoustic emission are of particular interest. As a matter of fact volumic ultrasonic wave propagation is sensitive to homogeneously distributed microcracks and represents in that case a good damage indicator. Guided waves as Lamb waves especially when generated from inside the material using an inserted piezoelectric element offer a specific sensitivity to localised damage as cracks or delaminations. Besides, acoustic emission which corresponds to the energy released by the material during the damage processes is directly related to the damage mechanisms and so can give pertinent information about the damage initiation and development. In this paper, our aim is to show in the one hand the ability of volumic ultrasonic waves to characterise volumic damage of glass epoxy composites under hydrothermal ageing and also the ability of Lamb waves to detect and identify localised damage. In the other hand our purpose is to demonstrate the potentiality of acoustic emission in understanding the damage mechanisms that occurs during a tensile test of polymer fibre composites and to discriminate in real time the different types of damage occurring at the microscopic scale.

The health of a structure depends on both the homogeneously distributed degradation of its mechanical properties during its life cycle and the presence of localised defects such as cracks or delaminations. The proposed non-destructive health monitoring method allows to recover both kinds of information using ultrasonic waves. To avoid traditional techniques limitations. such as coupling reproducibility for instance, we propose here to integrate a piezoelectric element into the plate like composite structure. The element dimensions are determined in order to uncouple the frequency ranges of the thickness and radial vibration modes. The thickness mode is used to monitor the homogeneous ageing of the structure through electrical impedance measurement. As for the radial vibrations, they are used to generate and detect Lamb waves, which have the advantage of propagating over long distances and offering specific sensitivity of various modes to different kinds of defects. The present work focuses on this last application and studies the ability of the proposed technique to detect and identify defects such as low speed impact-induced delaminations and cracks in composite plate-like structures.

The anisotropic elastic properties of an Al metal matrix composite unidirectionally reinforced with continuous α-alumina fibres fabricated, by a medium pressure infiltration technique, has been characterised using ultrasonic bulk and surface waves, tensile tests and microstructural analysis. The ultrasonic data are analysed under the continuum mechanics approximation. Elastic anisotropy is studied by varying the direction of propagation of ultrasonic waves. An optimisation process is used to recover all the volumic effective elastic moduli from bulk ultrasonic velocities. Two local (sub-surface) shear effective moduli are determined from the surface waves velocities. To discuss the elastic anisotropy of the material in relation with its microstructure and the manufacturing process, microstructural observations are correlated to the global and local ultrasonic evaluations and to the mechanical characterisation conducted in and out of symmetry axis.

Ultrasonic waves are commonly used for the characterization and the nondestructive testing of structural materials. The need of a coupling fluid weakens the reproducibility of traditional contact techniques and thus limits the monitoring of in-service structures. Our approach consists in integrating into the structure a piezoelectric element, which is designed for both monitoring the homogeneous mechanical properties and detecting localized defects. At first, from the electrical impedance of the embedded piezoelectric disc and using an appropriate modelling, it will be shown that it is possible to determine the viscoelastic properties of the medium surrounding the sensor. Such a method has proved its efficiency for the in-situ and real time monitoring of the life cycle of polymeric matrix composites, from the various polymerization stages to hydrolytic aging mechanisms. In addition the same piezoelectric sensor generates Lamb waves, which can explore large samples because of their weak attenuation. This notably accelerates the integrity control in composite plate-like structures. In this paper, guided waves propagation will be theoretically studied and illustrated through numerical calculations. Then, experimental results will demonstrate the ability of Lamb waves to detect real damages in carbon fibers reinforced composite plates.

La caractérisation des propriétés d’élasticité des matériaux anisotropes et l’évaluation de leur endommagement est réalisée à l’aide d’un dispositif ultrasonore à immersion. Les vitesses de propagation sont mesurées en incidence variable. Le problème inverse qui consiste à déterminer la matrice des constantes d’élasticité est résolu par optimisation en minimisant l’écart quadratique entre les vitesses ultrasonores théoriques et expérimentales. Ce processus est d’abord appliqué à un composite à matrice aluminium renforcée par des particules de carbure de silicium, qui est entièrement caractérisé. Ensuite, le comportement élastique d’un composite verre-époxy en vieillissement hygrothermique, à 70°C dans de l’eau distillée, est caractérisé par ultrasons. La dégradation qui en résulte est évaluée en terme de perte de rigidité et au travers d’une variable d’endommagement. Une observation au MEB permet de valider les mesures macroscopiques ultrasonores et l’anisotropie de l’endommagement.

Periodical maintenance NDE-based inspections are today of a general acceptance for almost all complex technological structures. Nevertheless, the idea that the integrated and continuous sensing techniques can optimize the operating conditions is now in progress. Different aspects of this evolution towards health monitoring are discussed in relation with the smart materials concept through a non-exhaustive review of some realizations or experiences essentially focused on the field of sensitive materials. Piezoelectric implant technique and in situ electric resistance measurements on carbon epoxy composites are presented with more details. Finally, by extension, it seems useful to introduce the adaptive materials-based new concept of 'health saving'.

Electricité de France has started a study in collaboration with the Metallurgical and Materials Physics Study Group (GEMPPM) of INSA-Lyon, to evaluate the effect of metallurgical structures of austenitic stainless steel welds on wave propagation for application to ultrasonic nondestructive testing. Experimentally, the anisotropic and heterogeneous characteristics of austenitic welds together with a coarse-grained structure (elongated and oriented grains) lead to the following phenomena: scattering, attenuation, skewing, splitting, and divergence of the ultrasonic beam. To study and predict these phenomena, simulation studies are most helpful. The theory of wave propagation into anisotropic and homogeneous media already allows the prediction of beam skewing and divergence effects. When considering the more complicated case of heterogeneous anisotropic structures, simulation studies require realistic descriptions of the various kind of weld structures which can be encountered. The present paper discusses the methods and their results developed to describe the welds by revealing homogeneous domains at the ultrasonic scale, and to characterize in each domain the elastic properties affecting wave propagation (crystallographic orientations, elasticity tensor).

This study concerns the characterisation of the elastic properties of a long-fibre-reinforced ceramic-matrix composite. Seven of the nine independent elastic constants of a woven 2.5 D carbon-fibre reinforced SiC ceramic matrix have been measured by an ultrasonic technique associated with a numerical optimisation process. The elastic moduli are recovered by minimising the square deviation between measured and theoretical velocities. The ultrasonic measurements are discussed with the approximation of continuum mechanics, in regard to the wavelength and the size of the microstructural details of the material. For comparison, an estimate of the elastic moduli in all directions is performed with an Eshelby-based model, assuming the composite as a two-dimensional tow-reinforced matrix containing voids. The effective moduli for the tow composite structure are estimated from a first homogenisation step. Volume fractions of tow and matrix cracks slightly opened are taken into account in the microstructural description of the composite from experimental data on mean crack orientations, quantities and shapes. The specific effect of neglecting waviness is estimated from finite-element calculations. In the limit of the uncertainties on the phase moduli, the estimates of elastic moduli are in agreement with the available measured ones. As relevant, estimates are thus provided for the missing measurements.

Ultrasonics is a suitable technique for nondestructive evaluation of structural materials degradation. Although the most common objective of ultrasonic testing is the detection and location of overt flaws, ultrasonics can be used to identify microstructural factors that alter strength and performance. However, while single large defects can be individually detected and characterized, widely dispersed discontinuities are impossible to resolve and only their global effects on bulk properties can be observed and measured. The aim of this paper is, using an immersion technique, to characterize damage progression of a glass fiber epoxy matrix composite during hygrothermal aging by means of attenuation frequency-dependence measured in normal incidence and longitudinal and transversal ultrasonic velocities measured as function of the propagation direction. The anisotropic elastic constants of the material are recovered from the ultrasonic velocities using an optimization process. Effects of hygrothermal aging on the attenuation dispersion, longitudinal and transversal ultrasonic velocities angular dependence and on the elastic properties are discussed. Anisotropic damage is assessed in terms of loss of stiffness. We show that, these macroscopic effects of damage result from located fiber/matrix interface decohesion. A microscopic investigation conducted using scanning electronic microscopy (SEM) on the damaged sample confirms this assumption and shows an important debonding at the fiber-matrix interface mostly located at the pole of the fibers in the thickness direction.

Previous laboratory-developed studies have validated an original method enabling the in situ real-time health monitoring of a composite structure by means of an inserted piezoelectric sensor. In order to extend such a method from laboratory conditions to the industrial situation, it is necessary to take the two kinds of defects linked to the non-correct positioning and to the imperfect bonding of the piezoelectric plate in the structure to be characterized into account. We have proposed a specific criterion to detect and evaluate these types of imperfection by using analytical relations linking acoustical parameters extracted by the numerical method. The applications have concerned the in situ monitoring of the curing and the local damage assessment of a polymer-based composite.