tailieunhanh - Mechanics of Materials 2010 Part 11

Tham khảo tài liệu 'mechanics of materials 2010 part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 14 raft PLASTICITY Introduction 10 Hence when a shear stress is applied on a metal bond the atoms can slip and slide past one another without regard to electrical charge constraint and thus it gives rise to a ductile response. On the other hand in a ioninc solid each ion is surrounded by oppositely charged ions thus the ionic slip may lead to like charges moving into adjacent positions causing coulombic repulsion. This makes slipping much more difficult to achieve and the material respond by breaking in a brittle behavior Fig. . Ionic Brittle Material o - J J Metal - - J J O 0 _ o _ _ -- - - - - _ _ _ _ _ -- Figure Brittle and Ductile Response as a Function of Chemical Bond 11 In the preceding chapters we have examined the response of brittle material through fracture mechanics we shall next examine the response of ductile ones through plasticity. Causes of Plasticity 12 The permanent displacement of atoms within a crystal resulting from an applied load is known as plastic deformation. It occurs when a force of sufficient magnitude displaces atoms from one equilibrium position to another Fig. . The plane on which deformation occurs is the slip plane. Figure Slip Plane in a Perfect Crystal 13 Following a similar derivation as the one for the theoretical normal strength Eq. it can be shown that the theoretical shear strength to break all the atomic bonds across a slip plane is on the order of E 10. However in practice we never reach this value. 14 This can be explained through the a defect arising from the insertion of part of an atomic plane as shown in Fig. . This defect is called an edge dislocation. Dislocations are introduced into a crystal in several ways including 1 accidents in the growth process during solidification of the crystal 2 internal stresses associated with other defects in the crystal and 3 interaction between existing dislocations that occur during plastic deformation. 15 With respect to Fig. if a shear .

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