Microstructures and Residual Stress in Polycrystalline Materials: Their Nondestructive Characterization and Effects on Mechanical Properties

University Of Kentucky Research Foundation, Lexington KY

Investigators

Abstract

Proposal: DMS-0406004 PI: Chi-Sing Man Institution: University of Kentucky Title: Microstructures and Residual Stress in Polycrystalline Materials: Their Nondestructive Characterization and Effects on Mechanical Properties ABSTRACT Many materials (e.g., rocks and sheet metals) are polycrystalline: they are aggregates of tiny crystallites of various orientations, sizes, and shapes. The mesoscale structure of polycrystalline materials, which includes the stereology and orientations of the crystallites, strongly affect their macroscopic mechanical properties (e.g., the formability of sheet metals). A polycrystalline material may also be prestressed. For instance, through surface-enhancement treatments a thin subsurface layer of compressive residual stress is artificially imparted on critical components of aircraft engines to improve their high-cycle fatigue behavior and material damage tolerance. The effects of mesoscale structure on various macroscopic properties of polycrystalline solids are often manifested through some coarsely defined microstructural variables. Identifying such microstructural variables and delineating their effects on mechanical behavior of polycrystalline materials are prerequisites for material design and nondestructive characterization. The objectives of this project are threefold: (1) By implementing homogenization only over horizontal planes and letting stress and texture be essentially bounded functions of depth, derive mathematical formulae for the dispersion of Rayleigh waves propagating in various directions along the surface of a textured polycrystalline half-space which carries an inhomogeneous subsurface layer of residual stress. (2) Derive explicit formulae of plastic potentials of sheet metals that include the orientation distribution function (which defines crystallographic texture or the volume fraction of crystallites in each orientation) and the second-order fabric tensor (which gives a rough description of grain size and shape) as independent variables. (3) Develop a mathematical theory for determination of grain shape from measurements of anisotropy in ultrasonic attenuation. Manufacturers of aircraft engines are very interested in incorporating the beneficial effects of surface-enhancement treatments into life predictions of critical engine components. This can be accomplished only through development of nondestructive measurement methods capable of verifying and quantifying residual stress levels. Part 1 of this project provides the theoretical basis of a possible ultrasound method for nondestructive inspection of the subsurface residual stress layer induced by low plasticity burnishing, an emerging surface-enhancement technique. Parts 2 and 3 are motivated by some technical problems in quality improvement and on-line quality control of continuous cast aluminum alloy sheets, which, as compared with their counterparts made from conventional direct chill cast ingots, provide energy savings of over 25% and economic savings of over 14% but often have inferior formability. Part 2 is devoted to mathematical work that supplements a joint research project between the PI and his colleagues at Chemical and Materials Engineering, University of Kentucky; Commonwealth Aluminum Concast, Inc. (CACI) is the industrial partner of the joint project. Part 3 has its ultimate goal to develop an ultrasound technique for on-line monitoring of grain shape in sheet metals.