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The sensitivity of ultrasonic measurements can be increased significantly by using nonlinear techniques instead of conventional linear ultrasonics. The nonlinear ultrasonics, based on a harmonic generation technique also known as collinear wave interaction, is used widely in practice due to its simplicity in implementation. The amplitudes of harmonics which are monitored with this technique not only originate in a test specimen, but also in electronics, the acoustic channel and the surrounding medium. This makes interpretation of the measurement results extremely complex. Noncollinear wave mixing is an alternative technique to harmonic generation. This technique has multiple advantages in comparison with harmonic generation, and it is easy to implement in industrial applications when in-line and real-time measurements are required. With a proper selection of measurement conditions for noncollinear wave mixing it is possible to measure a nonlinear material response without an influence of nonlinearities in the electr5onics, the acoustic channel and the surrounding medium. A mathematical model for prediction of nonlinear wave amplitude coefficients is presented in this thesis. All possible noncollinear wave interactions can be analyzed with this model and the nonlinear wave amplitued coefficients in isotropic solids can be predicted. Based on this model, a procedure is proposed for the selection of the optimal measurement conditions for noncollinear wave mixing experiments. Three measurement techniques were developed based on noncollinear wave mixing. Firstly, a method was developed for the phase velocity measurement in an isotropic solid. The phase velocity in a solid can be determined with this method when one of the wave velocities (shear of longitudinal) is known. This method does not require an estimation of the phase time-of-flight and wave propagation path. It is only necessary to measure the wave incident angles. Secondly, a method was developed to monitor the curing process of epoxy and experimental results were analyzed. This method enables in-line and real-time measurements of the epoxy cure dynamics in a thin layer (thickess about 0.2 mm), with detection of the minimum viscosity, the gel onset, the gel peak and vitrification points. Thirdly, a method was developed and the experimental results were analyzed for physical ageing measurements in glassy thermoplastics. The physical ageing dynamics and the current physical age in thermoplastic can be determined with this method. The results of this study demonstrate that more accurate and more sensitive characterization of engineerig materials can be achieved by making use of advanced ultrasonic measurement techniques

Development and analysis of noncollinear wave mixing techniques for material properties evaluation using immersion ultrasonics