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Characterization of Guided Waves Propagation in a Composite Skin-stringer Assembly

M. H. SHERAFAT, R. GUITEL, N. QUAEGEBEUR, L. LESSARD, P. HUBERT, P. MASSON

Abstract


Adhesive bonding of composite structural elements are intensively employed in the aerospace industry to assemble sub-structures. However, this structural feature is prone to degradation of the adhesive over time or disbonding when submitted to fatigue, or extreme loads. These types of damage can significantly jeopardize the performance and safety of a structure with little advanced warning. The objective of this research is to evaluate the integrity of a composite skin-stringer joint using Lamb wave propagation. The structure of interest is made of two carbon fibre reinforced polymer (CFRP) plates bonded together with an adhesive film as a skin-stringer assembly panel. The two plates are 8 plies of woven out-of-autoclave material with a quasi-isotropic lay-up. A rectangular piezoceramic is used to generate plane guided waves and non-contact measurement is performed using a 3-D Laser Doppler Vibrometer (LDV) to extract the required information for evaluation of the bonded joint. Plane wave generation by the thin slender rectangular transducer allows for directional generation of the Lamb waves and avoids geometrical spread, minimizing attenuation levels. The robustness of the set-up is validated for a simple composite plate and dispersion curves are extracted using spatial Fourier Transform. An acrossthe- bond-line strategy is then used to extract the reflection and transmission of energy through the bonded joint. The results include dispersion curves as well as reflection and transmission coefficients using a transfer function in the frequency domain. It was found that the adhesive bonded joint affects wave behaviour at low frequencies (below 100 kHz). Moreover, the transmission level of symmetric mode through the joint is larger than for the anti-symmetric level below 100 kHz, while it is almost zero at higher frequencies (above 400 kHz). The results verify that guided wave propagation is very effective method for structural health monitoring of composite skin-stringer bonded joints.

doi: 10.12783/SHM2015/227


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