Abstract
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.
The piezopolymer interdigital transducers have been characterized at low temperature (-80°C) and high temperature (+60°C) showing no significant change of behaviour, confirming that they can operate in the temperature range required for space application.
Different artificial defects have been detected successfully by signal processing with the developed laboratory system: defects with area of about 3cm2 made by an ultrasonic gel drop on the surface of an carbon overwrapped pressured vessel (COPV) has been detected successfully by using only two arrays offour transmitters and four receivers placed at distance 180 mm.
A search in the literature (Beard et al., 2005) (Champaigne and Sumners) (Chan) (Manka et al.) (Moll J. et al., 2002) (Park et al., 2006) (Prosser et al.) (Ross) (Staszewski et al, 2008) (Zhang et al., 2008) for methods and instruments that implement passive and active mode for structural health monitoring with acoustic and ultrasonic wave has been done to compare the development system with the state of the art.
