Daniel L. Balageas

Structural health monitoring system based on diffracted Lamb wave analysis by multiresolution processing

A health monitoring system is presented composed of integrated disc-shaped, 100 µm thick and 5 mm diameter piezoelectric transducers (PZTs) working sequentially as Lamb wave emitters and receivers. The diagnostic is based on the analysis of Lamb wave signals recorded before and after damage. In the composite, delaminations are discontinuities producing mode conversion processes generating various outgoing modes. The multiresolution processing allows the isolation of various propagation modes and their extraction in order to measure, for various propagation paths, the time delay between the arrivals of the main burst and of a specific outgoing mode. This process permits, with good accuracy, the localization of damage and the estimation of its extent. The robustness and portability of this technique is demonstrated by the fact that, after validation in our laboratory, it was successfully applied to data coming from an experiment conducted in another laboratory using its own acousto-ultrasonic health monitoring hardware system.

A Hybrid Electromagnetic Acousto-Ultrasonic Method for SHM of Carbon/Epoxy Structures

A model of electromagnetic behavior of composite materials like carbon/epoxy or glass/epoxy structures has been developed. Based on this model, an electromagnetic method allowing to evaluate the electric conductivity and the electric polarization of this type of material by measurement of magnetic and electric components of an incident electromagnetic wave crossing through the material has been also developed. A local measurement of magnetic and electric field allows to detect the presence and the extent of a damage if it induces a local variation of electric conductivity and/or electric polarization. This method presents a great sensitivity to detect damages inducing local variations of electric characteristics such as burning and liquid ingress. However, the sensitivity to mechanical damages such as delaminations is clearly lower, contrary to acousto-ultrasonic methods. Based on the complementarity of these two techniques, a new concept is proposed, combining them in an unique Structural Health Monitoring System (SHMS). The new SHMS which consists of a network of electromagnetic and ceramic piezo-electric sensors, is presented. The main results performed with this dual method applied to a carbon/epoxy structure including various defects (impact delaminations and local burning) are presented and discussed. Various images obtained by data reduction are presented.

Structural health monitoring R&D at the “European Research Establishments in Aeronautics” (EREA)

A description of the compared activities in structural health monitoring of the seven main research establishments in aeronautics and aerospace sciences in Europe, grouped in the EREA association, is made, with emphasis on the more recent and interesting results obtained in this field.

Application of a two-layer semi-analytical model for the improvement of laser-ultrasonic generation

Abstract The new laser-based ultrasound model presented in this paper takes into account the layered structures generally encountered in aeronautic materials (painted metals, composites, lapped joints…) which are subjected to impacts, fatigue and corrosion. The structure is supposed to be a plate made of two layers of different materials. The sample presents a cylindrical orthotropy. Validations of the model are performed by comparing the results it provides with experimental measurements using a Nd:YAG pump laser and an interferometric detection. The symmetry of the model allows fast calculation and prediction of the displacements over a long time period. These advantages are used to study the improvement of the laser-ultrasonic generation, by varying the thickness of the first layer in the case of a NDT strippable paint layer covering a metal layer.

Equation generalisee du touchau amelioration de la methode de mesure de l'effusivite thermique

Resume La methode de mesure de leffusivite thermique la plus courante est la methode du touchau. Une generalisation de la theorie sur laquelle repose cette methode est proposee. Les transferts thermiques existants entre la sonde et lexterieur ne sont plus negliges. Dans ces conditions, la methode du touchau deja modifiee precedemment par lutilisation des coincidences locales, peut alors etre utilisee avec une precision accrue. Lutilisation de sondes miniaturisees de technologie relativement rustique est rendue possible. Dans le domaine bio-medical, la nouvelle methode a permis de montrer que, lors de la mesure in vivo de leffusivite de la peau, letat circulatoire sous-cutane netait pas perturbe par le signal thermique genere par lapplication de la sonde.

Mesure de l'effusivite par un appareil du type touchau

Resume On procede a une etude detaillee du touchau, appareil simple permettant de determiner leffusivite dun materiau isolant par une methode de contact. Un mode de depouillement de cette experience, ameliore et adapte au traitement sur ordinateur, est presente. La methode decrite permet non seulement de deduire leffusivite du materiau, mais, par la combinaison de deux mesures successives, toutes les proprietes thermiques. Elle permet egalement de deceler la presence de defauts internes et devaluer la profondeur a laquelle ils se situent. Un appareil miniaturise et entierement automatise est presente ainsi que les premiers resultats obtenus.

Ultrasound Generation in Composites via Embedded Optical Fibers

The laser based ultrasound generation is now a well known and established technique used in NDE and material characterization [1]. The use of embedded optical fiber sensors in structures made of composite materials is of great interest for smart structures allowing an integrated health monitoring. The combination of both techniques could lead to an optical fiber based ultrasonic embedded system comprising both ultrasounds sources and detectors.

Laser-ultrasonics semi-analytical model for two-layered samples

The early detection of corrosion in aging structures is probably one of the most important challenges of the aeronautic maintenance services. Laser-ultrasonics offers interesting characteristics to become an industrial technique able to solve this problem. Neverless, to become quantitative, this non-destructive method requires a precise description of the laser-ultrasonic generation. This paper presents a new and original model which takes into account the layered structure which is generally encountered in aeronautic materials subjected to impacts, fatigue and corrosion. This model solved the Christoffel equations in an axisymmetrical configuration over an infinite plate of finite thickness presenting a cylindrical orthotropy. The sample is a flat plate made of two layers of different materials and the laser impinges the sample normally to the surface. The method of resolution used allows fast calculation and observation of the displacements over a long time period. This is very useful in NDT, especially in the case of thick samples. Validations were conducted by comparing the results calculated by this model to the ones obtained with a previous model and with experimental measurement using a Nd:YAG pump laser and an interferometric detection.

Thermal Diffusivity in Situ Measurements of Carbon/Carbon Composite Reinforcements

The diffusivity of directional reinforced composites (3-D C/C) was measured using the flash method. Problems related to the heat losses and the pulse duration were considered. The variation in the diffusivity determined at various moments during the heating confirms the numerical simulations: instead of being a nearly constant quantity as it is true for a homogeneous medium, the apparent diffusivity for these materials is a monotonic function that decreases with time and depends on the sample thickness.

Editorial: Letter from the Guest Editor

Since 1997 and every two years, the International Workshop on Structural Health Monitoring created and organized by Professor Fu-Kuo Chang has become the main international meeting in the field. Nevertheless, the geographical location of Stanford does not facilitate the venue of researchers and industrials from Europe. To remedy this situation, the idea of a European Workshop held every two years alternating with the Stanford Workshop took shape. In July 2002, the idea became a reality. The French National Aerospace Research Establishment (ONERA), in a joint effort with the Ecole Normale Supérieure of Cachan (near Paris) and the Laboratoire National des Ponts et Chaussées (LCPC), organized in Cachan the First European Workshop on SHM (‘‘SHM 2002’’). ‘‘SHM 2002’’ was a success, since near 240 attendees coming from 30 countries listened to more than 150 communications. Although each of the two workshops attracts people from the whole world, the analysis of the geographical origin of the speakers and attendees clearly demonstrates the harmony of the two meetings and the fact that their addition is the only way to get a global vision of the advancement of the R&D in the field. On behalf of the International Scientific Committee of ‘‘SHM 2002’’, I would like to thank the SHM Journal for having invited us to propose a selection of papers based on the communications presented in Paris. A balance between fundamental work and applications in the two fields in which SHM is emerging – civil engineering and aerospace industries – has been respected. Most of these papers emanate from European countries and I hope they will give a good idea of the variety and quality of the work in progress on this side of the Atlantic.