5K-6 Experimental and Numerical Studies on Lamb Waves Conversion in a Waveguide with Gaussian Section Variation

This work deals with the conversion of a Lamb wave propagating in a Duraluminium elastic waveguide, when it reaches a Gaussian thickness variation area located between two areas of constant and equal thickness. The varying section area is wide compared to the used wavelengths. Two kinds of waveguides having the same Gaussian thickness variation width but of different height (strong and slow section variation) are studied, allowing the comparison for a given incident Lamb wave. Experimental and numerical studies are performed to provide the behaviour of the guided wave inside the area of varying thickness, and the conversion of this wave into other Lamb waves that are transmitted outside this area. The experimental study is conducted with a contact transducer for the excitation. For several positions on the waveguide, a laser velocimeter is used for the measurement of the time depending normal displacement on the plate surface due to the wave travelling. The signal processing used double space-time FFT and spatial sliding windows FFT. Experimental results show that the incident S0 Lamb wave is converted into adiabatic wave inside the Gaussian varying thickness zone for the both waveguides. Outside this area, the transmitted Lamb waves are strongly depending on the waveguide shape and on frequency. This transmission into Lamb waves is experimentally quantified versus frequency for the strong varying section waveguide. These experimental results are successfully compared to a numerical study based on the Finite Element Method