Plastic deformation, excessive

The Kirkendall effect alters the structure of the diffusion zone in crystalline materials. In many cases, the small supersaturation of vacancies on the side losing mass by fast diffusion causes the excess vacancies to precipitate out in the form of small voids, and the region becomes porous [11], Also, the plastic flow maintains a constant cross section in the diffusion zone because of compatibility stresses. These stresses induce dislocation multiplication and the formation of cellular dislocation structures in the diffusion zone. Similar dislocation structures are associated with high-temperature plastic deformation in the absence of diffusion [12-14]. 

Recrystallization occurs when a crystalline material is plastically deformed at a relatively low temperature and then heated [1]. The as-deformed material possesses excess bulk free energy resulting from a high density of dislocations and point-defect debris produced by the plastic... 

If the stress is further increased, eventually a point is reached when the straight-line relationship is lost. This is termed the elastic limit. If stresses in excess of the elastic limit are applied and then removed, the spring will not return to its original length. Thus, a fraction of the change in length is permanent or irreversible, and this is termed plastic behavior. Further increase in load will result in more and more plastic deformation... 

Punch displacement measurements are easily done on a single station press by attaching LVDT to the punch. On a rotary press, such measurements can be done by means of slip ring, telemetry, or instrumented punch. Punch displacement profiles may be used in conjunction with compression force to estimate work of compression and work of expansion (measure of elasticity). Because capping tendency increases with the punch penetration depth, it may be desirable to monitor actual punch movement into the die. The shape of a force-displacement curve is an indication of the relative elasticity or plasticity of the material whereas plastic deformation is desirable for stronger tablets, excess plasticity usually results in tablets that tend to cap and laminate. ... 

Specimen preparation involves wet grinding to remove the surface layer, which may be decarburized or otherwise nonrepresentative of the bulk of the specimen, followed by standard metallographic polishing and etching. This procedure ensures a flat, reproducible surface for the x-ray examination, and allows a preliminary examination of the specimen to be made with the microscope. In grinding and polishing, care should be taken not to produce excessive heat or plastic deformation which would cause partial decomposition of both the martensite and austenite. 

The nature of the interaction between two surfaces is determined by the real area of contact, especially by the size of the real area. With a simple limit analysis assuming ideally plastic deformation, we can try to calculate a minimum value for Ar. If the surfaces that are placed in contact are rough, but not excessively rough, a typical junction will be induced as shown in Figure 2.4. Then the interface will be in a state of triaxial constraint. Thus the value of the real area of contact, Ar, is given by ... 

In this case, the load-displacement record may be somewhat nonlinear near the maximum load point, i.e., near the fracture load (see Fig. 4.5c). Most valid tests of practical test specimens exhibit this behavior. The nonlinearity represents plastic deformation around the crack border, and slight (irregular) crack extension during the last stage of the test. If the extent of the nonlinearity is not excessive, then it can be ignored and Kic can be calculated from the maximum (or fracture) load and the initial crack diameter lug. 

The question now is how much nonlinearity is considered to be not excessive. Formally, the nonlinearity should not exceed that which would correspond to an increase in the initial (or physical) crack diameter (2ao) by the plane strain plastic zone correction factor i.e., by Iriy (see Eqn. (3.49)). Physically, it is acceptance of the fact that a plastically deformed zone would develop at the crack tip, and its presence is equivalent to a change in the effective crack length at the onset of fracture from Itto to Itto -I- 2riy i.e.,... 

To further discuss failure, one must first decide what constitutes failure. Although it is common to associate failure with a part breaking into two or more pieces, conditions other than fracture can make a part inoperative or render its operation unsatisfactory. Other forms of failure include excessive elastic deformation, plastic flow and deformation, wear, or even loss of attractive appearance. At times several of these modes may combine to produce the resultant failure. For example, plastic deformation often precedes fracture and may, in fact, nucleate the microscopic cracks that ultimately coalesce into a macroscopic fracture. This chapter will concentrate largely on fracture and will only touch briefly on other types of failure. 

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