Ultrasonic Interrogation of Polycrystalline Materials

Abstract

It is accepted that the same features of microstructure that dominate α, attenuation of ultrasonic waves also determine mechanical properties of industrial materials. For example, in polycrystalline metals the grain size greatly influences both ultrasonic attenuation [1–4] and material strength, ductility, toughness and formability [5]. Since ultrasonic inspection is less expensive than the destructive tests required to assess mechanical properties many analytical and experimental studies have been directed at establishing whether and how features of microstructure may be inferred from ultrasonic inspection data. One significant contribution to attenuation in polycrystalline materials is scattering by the grains [1,2] and precipitates [6]. This results from interaction with material defects comparable to one wavelength λ in size, such as grain boundaries. Scattering depends on size, shape, orientation and anisotropy of the grains, and the structure, thickness and chemistry of their boundaries. The standard assumptions used when modeling grain scattering are that the discontinuity of the grain boundary is of elastic nature; an individual grain scatterer has a simple shape with the mean grain size D; the grains are randomly located and randomly oriented; the number of grains is large; and the scatter from individual grains is not coherent.