American Journal of Applied Sciences 6 (1): 109-113, 2009 ISSN 1546-9239 © 2009 Science Publications Corresponding Author: Dr. Monther Moh'd Krishan, The Dean of Ma'an University College, Al-Balqa Applied University, P.O. Box: 194 Ma'an, Jordan 71111 109 Impedance Method Application for Multi-Layer Structure Testing Monzer Moh`D Krishan Ma'an University College, Al-Balqa Applied University, Jordan Abstract: Multi-layer materials and structures are widely used in the modern aircraft industry. The main defect of these structures is delamination. The most appropriate method for detection of such kind flaws is the impedance testing method. This research is devoted to the features of the impedance method for multi-layer panel testing. The theoretical and experimental investigations of mechanical impedance are presented for honeycomb panels. The theoretical investigation provided the analytic expression, which enabled us to determine object parameters such as honeycomb density and thickness, covering thickness and cylindrical rigidity on a test result. The assembly of a combined probe was proposed for experimental investigations of mechanical impedance. An amplitude of the output signal of this probe is depended on the testing object mechanical impedance and flexibility of contact area. Numerical simulation results demonstrate the influence of the object parameters on the effectiveness of the defect detection in honeycomb structure. Key words: Multilayer structure testing, impedance method, aircraft industry, modeling and simulation, honeycomb structure INTRODUCTION Honeycomb structures are widely used in aerospace industry. The fuselage and wings of some aeroplanes consist of multi-layer structure such as honeycomb one. These materials have high toughness and little specific weight. Various types of defects can appear during operation period of time in honeycomb structures. The most common defects of honeycomb structures are delamination of honeycombs from coating and contortion of walls. Presence of such kind of defects has unfavourable influence on any device with mechanical properties. So, the task of finding defects in honeycomb structures is very important. Acoustical methods of NDE are also used for multi-layer structure testing. The most sensitive method for little defect detection is impedance method [1] . The elastic vibrations of low frequency is the base of this method. Impedance method is used for glued junction detection and also exfoliation defect disclose [2] . Impedance method is universal and suitable for use. In general, the honeycomb structure impedance can be expressed as [3] : j H H F Z R jX Ze v ϕ = = + =   (1) Where: Z = The mechanical impedance of structure F = Complex amplitude of applied force v = Vibrating velocity in the applied force point R H , X H = Active and reactive components of the impedance and ( ) H H arctg X /R ϕ= Mechanical impedance of multi-layer structure in the area of applied force is defined by the all components of this structure connected in the common mechanical system. This system vibrates as a single whole when all layers band each other and the module Z has maximum value. If there is a defect in the place of layers joint, rigidity of this place will be less then flaw-free one. Therefore, the module Z of a defect place will be less then flaw-free one and Z rotates through some angle. Changing Z leads to changing of a transducer acoustical channel [3] : j  B U P Pe U ϕ = =    (2) where, U Π , U B -received and emited signal amplitudes accordingly, ϕ-phase shift between amplitudes U Π and U B . So, defected joint can be located using changing mechanical impedance Z (its amplitude P or phase shift ϕ). There could be some simplifications in the description of mechanical impedance of honeycomb