Impact identification on a sandwich plate from wave propagation responses M. Meo a, * , G. Zumpano a , M. Piggott a , G. Marengo b a Crashworthiness, Impact and Structural Mechanics Group, School of Engineering, Cranfield University, Cranfield, Bedford MK43 0AL, UK b GKN Aerospace Services, Cowes, Isle of Wight, PO31 6RH, UK Abstract This study investigates the use of an impact detection algorithm to locate a potentially damaging impact on an orthotropic plate by detecting the stress waves generated by such an event. The proposed algorithm was tested experimentally on a sandwich plate by using ultrasonic signals. The arrival times of stress waves at different frequencies at the sensor locations were determined by analyzing the recorded signals using the wavelet transforms. The stress wave propagation phenomenon was characterized by measuring the propaga- tion speeds along different directions. This data along with the sensor co-ordinates were input into the impact detection algorithm, which uses the difference in time of flight to the sensors and trigonometric identities to locate impact source locations. The accuracy of the method was demonstrated by the close agreement observed between the estimated locations for the three impact locations studied with the actual locations of the impact loads applied. In particular, maximum error in the estimation of the co-ordinates of the impact location was less than 9% for all different types of loading considered. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Group velocity; Wavelets; Impact location identification 1. Introduction Modern structures, especially in aerospace application, make increasing use of fibre reinforced plastic composites, taking advantage of their material properties. Fibre rein- forced plastics have a number of advantages over metallic materials, as they have high strength for their density, and their material properties can be designed by varying the makeup. However, they are brittle and capable of with- standing a strain of less than 2% before breaking, and a small notch caused by such breakage can reduce the ulti- mate tensile strength by almost 50% [1]. This leads to a high susceptibility to low velocity impact damage, which is exacerbated by the limited visual signature that this dam- age displays on the struck surface. In order to develop safe structures, especially in safety critical areas such as aircraft, it is necessary to be able to detect and locate in real-time impact damage in the composite material structures. This study looks at the use of a network of sensors embedded within or bonded to the component that can measure stress waves propagating through the structure to determine the impact occurrence. Impact location is an inverse problem that uses the arrival time of stress waves to the sensors and their co-ordinates to locate the impact location. For the arrival time measurements, a joint time– frequency approach was investigated. Since the composite manufactured components are orthotropic, the impact localisation algorithm investigated took into account the change of the wave propagation speed with the propagation direction. 2. Experimental data The test piece was a sandwich section of a firewall of heli- copters, which consisted of a Nomex 0.635 thick and Hexcel AS4-8552 laminates. The laminate skin dimensions were 0263-8223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2005.09.028 * Corresponding author. Tel.: +44 1234 750111x5220; fax: +44 1234 752149. E-mail address: m.meo@cranfield.ac.uk (M. Meo). www.elsevier.com/locate/compstruct Composite Structures 71 (2005) 302–306