Lamb wave ultrasonic system for active mode damage detection in composite materials

The work describes a new approach for the development of a SHM system based on flexible piezopolymer transducers which have some advantages over the piezoceramic thin disk transducers in space applications. The flexible piezopolymer transducers made with thin 100 μm PVDF film were proposed by the authors in previous works (Capineri et al., 2002) (Bellan et al., 2005a) (Bellan et al., 2005b). The proposed technology is based on a network of transducers arranged in array configuration, designed to excite particular types of ultrasonic Lamb waves in laminate materials (metallic or composite). This type of transducers are called interdigital transducers because the comb geometry of the electrodes. The finger to finger distance of interdigital electrode patterns determine the central wavelength of the transducer and then the corresponding Lamb wave’s mode. The interest for this technology comes from the characteristics of this piezoelectric polymer film that can operate in a broader range of temperature (-80°C to +60°C) respect to piezoelectric ceramics transducers and its high mechanical compliance allows being adapted to curved surfaces. A laser based micro fabrication design process developed by the research group allows to “tune” for and efficient excitation and reception of selected Lamb waves, depending on the characteristics of laminate material and defect types; in our case we adopted 8mm and 16 mm fingers distance and four pairs of electrodes.