Propagation of ultrasonic Lamb waves in adhesively bonded lap joints

This paper considers the bonded joint as a multilayer structure that is analysed using the Transfer Matrix method. In this particular case three layers (two adherents and the adhesive) are considered. The study of this propagation problem may be developed from matrix formulations which describe elastic waves in layered media. This technique combines the theory of the dynamics of the continuum within each layer with the conditions for the interaction at the interfaces between layers. The behaviour of the different modes which propagates in the overlap region is obtained, and is found that the relative amplitudes can be estimated based on the properties of the incident wave mode. It was verified that the excitation of these modes is ruled by the degree to which their mode shapes match the mode shapes of the incident wave. This allows us to explain the physical phenomena that are behind the mode conversion, which could be used to a correct selection of modes for non-destructive evaluation of the bond region. Another result which must be emphasized consists is the possibility to determine by this method the attenuation of both longitudinal and transversal waves in plates what usually is a difficult by using conventional pulse-echo technique, especially in thin plates. Using an immersion pitch and catch setup, the total attenuation, that is composed by the losses due to the leaking in the fluid and material damping, can easily be obtained doing two measurements at different distances. The leaking losses can be obtained by knowing the bulk properties of both fluid and plate. So, attenuation of longitudinal and transversal waves (material damping) can be evaluated. In the experimental work two sets of lap joints built from 1mm and 4 mm thickness aluminium plates are tested using the fundamental S0 mode as incident wave. Very good agreement is found when compared with the theoretical predictions.