Real-time health monitoring of composite and metal structures has attracted considerable attention over the past years. Lamb waves were first described by Horace Lamb [1] for homogeneous isotropic material. They are elastic waves that can be generated in a solid plate with free boundaries and are also known as plate waves. They can be divided in symmetric (S_n) and anti-symmetric (A_n) modes according to their displacement pattern. Their unique properties are widely recognised [2-4]. Lamb waves have the ability to test the whole volume of the structure along the line between the transmitter and receiver and can propagate over considerable distances. Thus substantial time saving is achieved over conventional ultrasonic techniques, where point scanning of the whole structure is required. However, the dispersive nature of Lamb waves and the existence of many modes simultaneously can complicate the interpretation of the acquired signal. It is therefore important to work below a cut-off frequency, that is specific for each material, so as to excite only the fundamental symmetric S_0 and/or anti-symmetric A_0 Lamb modes and select a frequency region where the desired mode is non-dispersive, [5,6].
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