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

提案：DMS-0406004 PI：Chi-Sing Man机构：University of Buckingham题目：多晶材料的微观结构和残余应力：它们的非破坏性表征及其对机械性能的影响摘要许多材料（例如，岩石和金属片）是多晶的：它们是各种取向、大小和形状的微小微晶的聚集体。多晶材料的介观结构，包括微晶的体视学和取向，强烈地影响它们的宏观机械性能（例如，金属板的可成形性）。多晶材料也可以被预加应力。例如，通过表面增强处理，在飞机发动机的关键部件上人为地施加薄的亚表面压缩残余应力层，以改善其高周疲劳行为和材料损伤容限。介观结构对多晶固体各种宏观性质的影响往往通过一些粗略定义的微观结构变量来体现。识别此类微观结构变量并描述其对多晶材料力学行为的影响是材料设计和无损表征的先决条件。（1）通过只在水平面上进行均匀化处理，并使应力和织构基本上是深度的有界函数，推导出沿沿着织构多晶半空间表面各向传播的瑞利波频散的数学公式。(2)推导出金属板材塑性势的显式公式，其中包括取向分布函数（定义每个取向的晶体织构或微晶的体积分数）和二阶组构张量（给出晶粒尺寸和形状的粗略描述）作为独立变量。(3)发展一种数学理论，通过测量超声衰减的各向异性来确定晶粒形状。航空发动机制造商对将表面强化处理的有益效果纳入关键发动机部件的寿命预测非常感兴趣。这只能通过开发能够验证和量化残余应力水平的无损测量方法来实现。本项目的第1部分提供了一个可能的超声波无损检测的方法，引起低塑性抛光，一个新兴的表面增强技术的亚表面残余应力层的理论基础。第二部分和第三部分是由连续铸造铝合金板的质量改进和在线质量控制中的一些技术问题引起的，与由常规直接冷铸锭制成的对应物相比，连续铸造铝合金板提供超过25%的节能和超过14%的经济节约，但通常具有较差的成形性。第2部分是专门的数学工作，补充了一个联合研究项目之间的PI和他的同事在化学和材料工程，肯塔基州大学;联邦铝康卡斯特公司。（CACI）是该联合项目的工业合作伙伴。第3部分的最终目标是开发一种在线监测金属板材晶粒形状的超声技术。

Proposal: DMS-0406004PI: Chi-Sing ManInstitution: University of KentuckyTitle: Microstructures and Residual Stress in Polycrystalline Materials:Their Nondestructive Characterization and Effects on Mechanical PropertiesABSTRACTMany 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.