An ultrasonic tomographic signature approach for material property gradient analysis in powder metals and natural wave guide structural health monitoring

Computerized tomography (CT) is a cross-sectional image reconstruction technique via collecting energy data (X-ray, radioisotopes, ultrasound, etc.) along the perimeter of an object in some pattern. First introduced to the field of diagnostic medicine with an X-ray source, CT technology has been successfully exploited in many areas. In this thesis work, two engineering applications of the CT technology were investigated: ultrasonic bulk wave tomography for powder metals and ultrasonic guided wave tomography for natural wave guide structural health monitoring. An ultrasonic topographic approach to material property gradient analysis in powder metals and natural wave guide structural heath monitoring is developed.; Powder Metallurgy (P/M) is an attractive metalworking technology that shapes systems of metal particles directly into near net-shaped parts by compaction and sintering. Despite its success, one technological limitation of the P/M fabrication process is the shape distortion of the final compacts, which, if successfully overcome, would lead to a much broader application of the P/M technology.; In search of an effective nondestructive way to facilitate the study of the powder metallurgy manufacturing process, the first part of this thesis is focused on design and realization of a laser-based ultrasonic bulk wave tomography system for material property gradients testing. Laser beams are applied to excite ultrasonic waves in cylindrical powder metal samples, and air transducers are exploited to receive the transmitted ultrasonic waveforms. Results show that this tomography technique is a good candidate of nondestructive study of powder compacts with the potential of being applied for in-situ process monitoring.; The second part of the thesis focuses on ultrasonic guided wave tomography for natural wave guide structural health monitoring. Sparse array guided wave tomography was investigated as a low resolution imaging methodology for health monitoring which provides many advantages.; Several guided wave features were extracted for image reconstruction including ultrasound group velocity and wave attenuation coefficients. Experiments were conducted on aluminum plates with various artificial defects. Results show that sparse array guided wave tomography has the potential of providing a reliable practical tool for structural health monitoring. In addition, this study investigates the critical role of the system matrix condition number in controlling the resulting image quality when algebraic CT reconstruction algorithms are applied.