Some Studies on Microtexture and Acoustoelasticity in Polycrystalline Media

To the naked eye, the polished surface of a metal sheet appears as a homogeneous continuum. Under the metallographer's microscope the same surface reveals, after etching, an underlying polycrystalline structure. Many materials, including metals, ice and rocks, are aggregates of tiny crystals or grains, which assume different orientations in space and are separated by interfaces called grain boundaries. Comprehensive mapping of grain boundaries and individual grain orientations in polycrystals has recently become possible with the emergence of orientation imaging microscopy. Details about grain orientations, e.g., the misorientations of neighboring grains, are called microtexture. Microtexture could exert a strong influence on the mechanical behavior of polycrystalline materials (e.g., on the volume of backscattered ultrasonic noise, which at sufficiently high levels renders ultrasound totally useless for material flaw detection in aircraft engines; an undetected metallurgical defect in the No. 2 engine was the culprit for the crash of a United Airlines DC-10 in 1989). The main objectives of the present project are as follows: (i) to reexamine the mathematical foundations and properties of several theoretical constructs which have been proposed by material scientists for describing microtexture in various degrees of detail and sophistication; (ii) to delineate the effects of microtexture on the mechanical anisotropy of sheet metals in forming operations; (iii) to study the effects of microtexture on the acoustoelastic behavior of stressed polycrystals; (iv) to develop further an earlier study that concerns using the dispersion of Rayleigh waves for the nondestructive inspection of surface layer of residual stress, which is imparted on critical components of aircraft engines to enhance their high-cycle fatigue performance.