Plastic deformation bond characteristics

The initial part of the curve, OA in Fig. 1, is the characteristic linear-elastic behavior of the material, i.e., the extension that occurs is fully reversible and the relationship between the force and the extension is linear. At an atomic level the bonds between the atoms of the crystal structure are just flexing. The extension in this region is however very small and can only be measured using special extensometers. This linearity ceases at point A and the material starts to behave irreversibly, i.e., permanent or plastic deformation occurs. This phenomenon is known as yielding. In this region the atoms take up new position relative to each other by the mechanism of dislocation activation. 

Plastic deformation does not occur by dislocation motion for noncrystalline ceramics because there is no regular atomic structure. Rather, these materials deform by viscous flow, the same manner in which liquids deform the rate of deformation is proportional to the applied stress. In response to an applied shear stress, atoms or ions slide past one another by the breaking and re-forming of interatomic bonds. However, there is no prescribed manner or direction in which this occurs, as with dislocations. Viscous flow on a macroscopic scale is demonstrated in Figure 12.32. viscosity The characteristic property for viscous flow, viscosity, is a measure of a noncrystal-... 

Fig. 7.10. Stress versus strain characteristics of three lap joints and an unbonded test coupon. Joint A, 25 mm X 12 5 mm overlap B, 25 mm X 190 mm overlap C, 25 mm X 25 0 mm overlap. Adherend mild steel, 16 gauge. Adhesive heat cured toughened epoxide (Permabond ESP105). Test coupon 25 X 75 mm, 16 gauge mild steel of the type used to fabricate the joints represented by continuous line. The classical form of the elastic/plastic deformation of the unbonded test coupon is clearly seen. (A) This specimen does not fail until after the test coupon has become plastic. (B) Although possessing 50% more bond area, the load required to fail specimen B is not very much greater than that needed to fail A however, the toughness of the adhesive and the spare capacity of the initially unloaded central area are clearly illustrated by the ability of the joint to sustain a load even though it has cracked. (C) This example emphasises the point made with specimen B. A performance such as this makes it difficult to say whether the adhesive has failed the steel or the steel has failed the adhesive.

Three special characteristics of gold are beneficial for diffusion bonding, namely, the absence of an oxide skin when heated in air which might act as a barrier, the metal s low elastic modulus, and its rapid self-diffusion. As a consequence, diffusion bonding of gold can be achieved at room temperature with plastic deformations of as little as 20%. These favorable properties are widely exploited for gold wire interconnection in microelectronics assembly. 

Metals, both pure and alloyed, consist of atoms held together by the delocalized electrons that overcome the mutual repulsion between the ion cores. Many main-group elements and all the transition and inner transition elements are metals. They also include alloys—combinations of metallic elements or metallic and nonmetallic elements (such as in steel, which is an alloy of primarily Fe and C). Some commercial steels, such as many tool steels, contain ceramics. These are the carbides (e.g., FeaC and WgC) that produce the hardening and enhance wear resistance, but also make it more brittle. The delocalized electrons give metals many of their characteristic properties (e.g., good thermal and electrical conductivity). It is because of their bonding that many metals have close packed structures and deform plastically at room temperature. 

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