Generation and detection of lamb waves for the characterization of plastic deformation
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In this thesis ultrasonic Lamb wave measurements are performed to detect material nonlinearity in aluminum sheets. When a Lamb wave propagates, higher harmonic wave fields are generated and under certain conditions the second harmonic is cumulative. When these conditions hold the Lamb waves are serviceable for material nonlinearity measurements. For generation, a wedge transducer combination is used. The detection of the Lamb wave are performed with either a laser interferometer or a second wedge transducer combination and the results are benchmarked. A short time Fourier transformation (STFT) is applied to the detected signal to extract the amplitudes of the first and second harmonics. A relative ratio of the first and second harmonics is deduced from nonlinear wave theory to assign the nonlinearity of the material. To verify the capability of the measurement setup and to show that cumulative second harmonics are generated, measurements for different propagation distances are performed. Further measurements on plasticly deformed specimens are carried out to examine the change of the material nonlinearity as a function of plasticity.