tailieunhanh - Mechanical Properties of Engineered Materials 2008 Part 6

Tham khảo tài liệu 'mechanical properties of engineered materials 2008 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 5 Introduction to Plasticity INTRODUCTION After a high enough stress is reached the strain no longer disappears on the release of stress. The remaining permanent strain is called a plastic strain Fig. . Additional incremental plastic strains may also be accumulated on subsequent loading and unloading and these can lead ultimately to failure. In some cases the dimensional and shape changes associated with plasticity may lead to loss of tolerance s and premature retirement of a structure or component from service. An understanding of plasticity is therefore important in the design and analysis of engineering structures and components. This chapter presents a basic introduction to the mechanisms and mechanics of plasticity in monolithic materials. Following a simple review of the physical basis for plasticity in different classes of monolithic materials ceramics metals intermetallics and polymers empirical plastic flow rules are introduced along with multiaxial yield criteria. Constitutive equations of plasticity are then presented in the final section of the chapter. Copyright 2003 Marcel Dekker Inc. Figure Schematic illustration of plastic strain after unloading. PHYSICAL BASIS for plasticity Plasticity in Ceramics Most ceramics only undergo only elastic deformation prior to the onset of catastrophic failure at room temperature. Hence most reports on the mechanical properties of ceramics are often limited to elastic properties. Furthermore most ceramists report flexural properties obtained under three- or four-point bending. Typical strength properties of selected ceramic materials are presented in Table . Note that ceramics are stronger almost 15 times stronger in compression than in tension. Also the flexural strengths are intermediate between the compressive and tensile strength levels. Reasons for these load-dependent properties will be discussed in subsequent chapters. For now it is simply sufficient to state that the trends are due .

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