عنوان مقاله [English]
In this paper, Lamb waves (also called plate waves) are used for inspection of two plates which are joined by an adhesive bond. The most attractive feature of Lamb waves is that they can travel long distances with little attenuation. However, the dispersive and multi-modal nature of Lamb waves makes their usage in nondestructive testing of materials difficult. Lamb waves can propagate in either symmetric or asymmetric modes designated as $S_i$ and $A_i$, respectively, with i=0,1,2,... . The most suitable Lamb wave mode for inspection of adhesively bonded plates was found to be the first symmetric mode, S$_0$. A number of good and defective specimens were prepared from aluminum plates bonded together by epoxy resin adhesive. Using two variable-angle-beam ultrasonic transducers in pitch-catch configuration, the desired plate wave mode was generated in specimens. The incidence angle required for generating a specific mode was obtained by examining the dispersion curves of the plate. Dispersion curves show the dependence of the wave velocity on the frequency for each mode. A number of defective samples were prepared with different defect sizes. The defects were in the form of disbonds implanted in the joint by placing a piece of paper between the adhesive and adherent (see the following figure). The ultrasonic signals transmitted through the adhesive bond of each samplewere analyzed and compared. The results indicate that Lamb waves can efficiently detect anomalies in the bonded joints and provide quantitative measures of defect size.