Measuring Acoustic Nonlinearity of Elastic Materials Using Thermal Modulation of Ultrasonic Waves

Nonlinear acoustic techniques have been used to determine the nonlinear properties of materials. Existing methods either require complex equipment to measure absolute nonlinear coefficients or can only be used on laboratory-sized specimens. A recently developed thermal modulation method addresses the limitations of existing methods, but further theoretical analysis and validation are required. In this dissertation, theoretical analyses were first conducted to study the mechanically and thermally induced acoustoelastic effect. Beginning with the wave equation, the relationship of the ultrasonic wave velocity with respect to mechanical strain and the thermal strain was derived in detail. These analyses provided theoretical support for subsequent validation experiments and applications. Mechanical and thermal modulation tests on aluminum and concrete were performed to validate the theory of thermally induced acoustoelasticity. The stretching technique was applied in calculating ultrasonic wave velocity changes, helping reach a high resolution and accuracy in measuring small wave velocity changes. Acoustoelastic coefficients obtained from the mechanical and thermal modulation methods showed good agreement. Owing to the simple test setup and high measurement sensitivity, the thermal modulation test is a potential experimental method to determine absolute acoustic nonlinearity parameters. The thermal modulation method was then applied to evaluating nonlinear parameters in different materials, and the values were consistent with those from the literature. In addition, the acoustoelastic coefficient, obtained using the thermal modulation method, was used to evaluate stress change in a full-scale prestressed concrete girder. The predicted stress change was verified by direct strain measurement. Adviser: Jinying Zh