Đang chuẩn bị liên kết để tải về tài liệu:
Mechanical Properties of Engineered Materials 2008 Part 5

Tham khảo tài liệu 'mechanical properties of engineered materials 2008 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 4 Introduction to Elastic Behavior 4.1 INTRODUCTION Elastic deformation by definition does not result in any permanent deformation upon removal of the applied loads. It is induced primarily by the stretching or bending of bonds in crystalline and noncrystalline solids. However in the case of polymeric materials elastic deformation may also involve the rotation of bonds in addition to the stretching and unwinding of polymer chains. Elastic deformation may be linear or nonlinear in nature Fig. 4.1 . It may also be time dependent or time independent. When it is time independent the strains are fully instantaneously recovered on removal of the applied loads Fig. 4.1 a . However in materials e.g. polymeric materials some time may be needed for the viscous flow of atoms or chain stretching rotation to occur to return the atoms to their initial configurations. The elastic behavior of such materials is therefore time independent i.e. viscoelastic. Also the elastic strains in polymers can be very large typically 5-1000 compared to relatively low elastic strain limits 0.1-1.0 in metallic and nonmetallic materials Figs 4.1 . This chapter presents an introduction to basic concepts in elasticity. Following a brief description of the atomic displacements that are responsible for elastic deformation the anisotropic elasticity of crystalline materi- Copyright 2003 Marcel Dekker Inc. Figure 4.1 Schematic illustration of a linear elastic time-independent elastic deformation of 1020 steel at room temperature and b nonlinear elastic deformation of rubber at room temperature. Adapted from McClintock and Argon 1966. Courtesy of Addison-Wesley. als is described before presenting an overview of continuum elasticity theory. The final sections of the chapter contain advanced topics that may be omitted in an introductory course on mechanical properties. These include sections on tensors and generalized elasticity theory using shorthand tensor nomenclature. 4.2 REASONS for elastic behavior .