Ultrasonic Reflection Tomography of Inhomogeneous Materials

Abstract

<p>Low-contrast inclusions in polycrystalline graphite, particle-filled elastomers, and other inhomogeneous media are typically undetectable with conventional radiographic techniques, but may affect the machining, service life, and performance of these materials. Traditional ultrasonics are also limited in the detection of these defects due to high internal scattering, attenuation, and wavespeed variations (refraction) arising from grain structure, porosity, and viscoelasticity. The use of array techniques improves detection of lowcontrast defects by using several ultrasonic paths to probe the region at various angles of incidence and reflection. The use of multiple measurements significantly increases both signal-to-noise and location accuracy, thereby abating attenuation and edge-on defect problems. Array methods include computed tomography (CT) and phased array imaging. Whereas phased array methods are primarily hardware-based (for beam focusing), CT relies on post-acquisition signal processing (software) for generating images.</p>