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Mechanical Properties of Engineered Materials 2008 Part 7

Tham khảo tài liệu 'mechanical properties of engineered materials 2008 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 6 Introduction to Dislocation Mechanics 6.1 INTRODUCTION Early in the 20th century a number of scientists tried to predict the theoretical strength of a crystalline solid by estimating the shear stress required to move one plane of atoms over another Fig. 6.1 . They found that the predicted theoretical strengths were much greater than the measured strengths of crystalline solids. The large discrepancy an order of magnitude or two between the theoretical and measured shear strengths puzzled many scientists until Orowan Polanyi and Taylor 1934 independently published their separate classical papers on dislocations line defects . The measured strengths were found to be lower than the predicted theoretical levels because plasticity occurred primarily by the movement of line defects called dislocations. The stress levels required to induce dislocation motion were lower than those required to shear complete atomic planes over each other Fig. 6.1 . Hence the movement of dislocations occurred prior to the shear of atomic planes that was postulated by earlier workers such as Frenkel 1926 . Since 1934 numerous papers have been published on the role of dislocations in crystalline plasticity. A number of books Hirth and Lothe 1982 Hull and Bacon 1984 Weertman and Weertman 1992 have also been written on the subject. This chapter will therefore not attempt to Copyright 2003 Marcel Dekker Inc. Figure 6.1 Shear of one row of atoms over another in a perfect crystal. present a comprehensive overview of dislocations. Instead the fundamental ideas in dislocation mechanics required for a basic understanding of crystalline plasticity will be presented at an introductory level. The interested reader is referred to papers and more advanced texts that are listed in the bibliography at the end of the chapter. 6.2 THEORETICAL SHEAR STRENGTH of a CRYSTALLINE SOLID Frenkel 1926 obtained a useful estimate of the theoretical shear strength of a crystalline solid. He considered the shear stress .