Plastic deformation polycrystalline materials

A typical shock-compression wave-profile measurement consists of particle velocity as a function of time at some material point within or on the surface of the sample. These measurements are commonly made by means of laser interferometry as discussed in Chapter 3 of this book. A typical wave profile as a function of position in the sample is shown in Fig. 7.2. Each portion of the wave profile contains information about the microstructure in the form of the product of and v. The decaying elastic wave has been an important source of indirect information on micromechanics of shock-induced plastic deformation. Taylor [9] used measurements of the decaying elastic precursor to determine parameters for polycrystalline Armco iron. He showed that the rate of decay of the elastic precursor in Fig. 7.2 is given by (Appendix)... 

Beyond point E, the material begins to plasticly deform, and at point Y the yield point is achieved. The stress at the yield point corresponds to the yield strength, Oy [see Eq. (5.20)]. Technically, point Y is called the upper yield point, and it corresponds to the stress necessary to free dislocations. The point at which the dislocations actually begin to move is point L, which is called the lower yield point. After point L, the material enters the ductile region, and in polycrystalline materials such as that of Eigure 5.26, strain hardening occurs. There is a corresponding increase in the stress... 

Figure 6.25 Stress-strain behavior (curve 1) for a polycrystalline material that demonstrates significant plastic deformation and necking. In addition, the true stress-true strain (curve 2) is shown schematically.

Yield criteria are mathematical tools to decide whether the stress state in a material will cause plastic deformation or not. In a polycrystalline metallic tensile test specimen, which can be assumed to be isotropic and uniaxially loaded, the material yields at a stress ... 

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