In this paper, the attenuation of lamb waves in three-layer adhesive joints, including two elastic plates bonded together by a viscoelastic adhesive layer, is investigated using the global matrix method and then a suitable incidence angle is calculated to generate a low-attenuation lamb wave using an angle beam transducer. The theoretical boundary value problem in adhesive joints with a perfect bond and traction-free boundary conditions on their outer surfaces is solved to find a combination of frequencies and modes with lowest attenuation. A characteristic equation is derived by applying continuity and boundary conditions in adhesive joints using the global matrix method. Phase velocity and attenuation dispersion curves are obtained with numerical solution of this equation by a computer code for a three-layer joint, including an aluminum repair patch bonded to the aircraft aluminum skin by a layer of viscoelastic epoxy adhesive. To validate the numerical solution results, wave structure curves are plotted for a special mode in two different frequencies in the adhesive joint. Also, the transducer incidence angle is calculated in terms of frequency to generate lamb wave modes with low attenuation level using the theoretical method.

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In this paper, at first the attenuation of Lamb waves in threelayer adhesive joints, including two elastic plates bonded together by a viscoelastic adhesive layer, is investigated using Global matrix method and then suitable incidence angle is theoretically calculated to generate low-attenuation Lamb waves using angle beam transducer. Theoretical boundary value problem in threelayer adhesive joints with perfect bond and traction-free boundary conditions on their outer surfaces is solved to find a combination of frequencies and modes with lowest attenuation. Characteristic equation is derived by applying continuity and boundary conditions in three-layer joints using Global matrix method. Phase velocity dispersion curves and attenuation intensity plot in high and low frequencies are obtained with numerical solution of this equation by a computer code for a three-layer joint, including an aluminum repair patch bonded to the aircraft aluminum skin by a layer of viscoelastic epoxy adhesive. To validate the numerical solution results of characteristic equation, wave structure curves are plotted for a special mode in two different frequencies in the adhesive joint. Also, transducer incidence angle is calculated in terms of frequency for different modes using theoretical method to generate Lamb wave modes with low attenuation level by angle beam transducer. These modes are recognizable by transducers in inspections with Lamb waves because of low attenuation level.