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Essais & Simulations n°114

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Mesures et Methodes de

Mesures et Methodes de Mesure Compte tenu du caractère international des secteurs abordés, certains articles sont écrits et publiés en anglais. Etude Détection d’endommagement dans les composites par vibrothermographie et résonances locales Damage detection in composites by vibrothermography and local resonances Résumé La vibrothermographie est une technique de thermographie active capable de détecter des singularités de type délaminage ou décollement dans un matériau composite comme les laminés en fibres de carbone très utilisés actuellement en construction aéronautique (CFRP). Les causes physiques du mécanisme d’échauffement interne, suite à l’interaction avec les ondes élastiques, peuvent être la friction, l’hystérésis viscoélastique ou encore la déformation plastique. La méthode est efficace également sur des géométries complexes et requiert uniquement que les deux faces internes du délaminage soient libres de vibrer. Cette étude consiste à déterminer la relation entre l’échauffement observé et la fréquence d’une excitation sinusoïdale entre 8 et 27 kHz. Il apparaît clairement des pics d’échauffement pour certaines fréquences particulières. La comparaison avec les fréquences de résonance locale du défaut modélisé par éléments finis montre des similitudes avec l’apparition de certains de ces pics. Une étude expérimentale visant à exploiter la présence de ceux-ci montre l’intérêt d’un test vibrothermographique de type balayage rapide (chirp), dans l’optique de mettre sur pied un test court et fiable pour le contrôle non destructif de matériaux composites. 1 Introduction Vibrothermography (abbreviated VT) is an active non destructive technique able to detect singularities like delamination, whether the defect internal faces are in contact or not. Under sonic or ultrasonic mechanical vibrations, a delamination may behave like an internal heat source due to friction between the rubbing faces and/or viscoelastic hysteresis heating in the defect area. Local plastic deformation losses are discarded provided the stress level is kept well within the point of yield [1-3]. Despite numerous finite element simulations and theoretical explanations, there is still a lack of understanding concerning the physics governing the heat generation mechanism and no definitive experimental validation of either theory has been presented and accepted to date to explain the source or sources of heat generation in vibrothermography [4-11]. The present article first focuses on the research of links or evidences concerning a dominant heat generation mechanism in polymer composite related to possible «local resonances» phenomenon produced on one side or the other of the delamination [12]; that is to say the resonances of one or the other laminated sub-plate created and delimited by the delamination. High stress concentrations due to the dynamic amplification at local resonance may be correlated with the presence of corresponding heating peaks observed on the surface right above the defect by an infrared camera. For this purpose, a finite element modal analysis of the delamination has been proposed and compared with the heating rate versus frequency curve established by a series of consecutive short sine tests with a 200 Hz frequency interval. According to the comparison, vibration damping related to viscoelatistic effects are expected to play a significant role, especially when matching or close to a local resonance frequency. Secondly, linear and logarithmic sine sweep and chirp testing results will be presented and discussed exploiting the presence of the specific peaks revealed in the spectrum. Finally, it should be noted that this paper presents an alternative method to the common usage by the nature of the contact between the vibration transducer and the specimen. Indeed, vibrothermographic devices usually has a pneumatically-driven coupling system pressing the transducer against the sample. The hammering of the surface generates non linearities induced by the acoustic chaos. It follows that the coupling pressure has a strong influence on the detection efficiency and poor coupling creates unwanted heat in the vicinity of the contact. Moreover, Mots clés vibrothermographie, thermosonique, délamination, composite, CFRP Key-words vibrothermography, thermosonic, delamination, composite, CFRP. Essais & Simulations • JUIN 2013 • PAGE 12

Mesures et Methodes de Mesure a little Kapton® tape may be placed on the plate at the location of the coupling, the shaker head being bounded to the tape. This situation is different than usual VT setup which consists in pressing the excitation device by a controlled (or not) static force against the plate, causing some beneficial acoustic chaos but inducing major problems like non reproducibility or fretting of surfaces [13-15]. the risk of damaging the composite is always present [13-16]. For the purpose of this study, reproducible pure sine testing are performed by the way of a rigid tip-specimen coupling obtained by the gluing of the specimen directly on the vibration transducer head (larger plates may rest on a vibration isolation material like Teflon pieces). 3 Laminated plate theory and finite element models 3.1 Local resonance model hypothesis For sake of simplicity, delamination may be modelled as two separated small sub-plates, one on either side of the detachment, both free to resonate with their own dynamics but clamped together along the boundary, disregarding the remainder of the panel (Fig. 2). This idea 2 Experimental setup The measurement set-up used for this study is detailed in Fig. 1. A piezoelectric shaker (Wilcoxon model F7) is driven by a Data Physics (DP) interface. A thermal camera (50 mK NEDT, 8-14 μm range) is triggered at the DP start-up signal by a function generator. The excitation signal is monitored via a force sensor located in the shaker head. An additional vibration sensor or a laser vibrometer may be used to provide FRF’s (Frequency response Function). The CFRP (Carbon Fiber Reinforced Plastic) sample is an eight plies symmetric laminated plate [90/03]s, which contains a 25x25 mm Teflon® insert between the third and the fourth ply. The sample plate has been bounded to the shaker head with high performance cyanoacrylate glue. No other contact than this rigid coupling is produced on the plate. Note that for a short time testing, Essais & Simulations • JUIN 2013 • PAGE 13

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