Determining texture in materials using laser ultrasonic methods
Lindamood, Lindsey R.
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When the microstructure of a material has directional variation in elastic stiffness, such as a transversely isotropic material, ultrasonic waves propagate at different speeds when traveling along different directions through the material. Much of the research using ultrasound to measure the texture of materials relies not only on the polarization of the traveling wave but also on the direction of propagation relative to the preferred orientation of the material. The research in this thesis focuses on changing the polarization direction of the ultrasonic pulse, specifically the shear wave, and maintaining the wave’s propagation direction. The key experimental arrangement uses a laser line source to generate longitudinal and shear waves simultaneously in the through-thickness direction of a plate-like sample. The line source enables us to give directionality to the ultrasonic pulse and change the polarization by rotating the line. We show how the texture of the material influences the propagation of the ultrasound. We also have experimental evidence that early fatigue damage can cause shear birefringence. Finding the exact locations of microcracking in fatigued materials with the traditional ultrasonic methods is difficult. Understanding how texture may alter results will help to better identify the microscopic changes that are occurring within these fatigued materials. The experimental results show that changes in wavespeed relative to the rolling direction provide information on preferred orientation and stiffness. This dissertation shows how the derivation of velocities in a single cubic crystal can be used in the orientation distribution function to generate the velocities in a material with cubic crystallites and orthorhombic symmetry. Comparing experimental velocity results to theory allows for the generation of texture coefficients using those velocity measurements.