Plastic deformation processing

Let us examine this relation for typical values of the A/B interface = 1 (max energy dissipation in the A layer) I = 1 (max strength of interface with influxes) E = 12,000psi (Tq = 4000 psi /t = 30 mils (10 in) Lc = 4 x lO" in. We obtain for both terms G = 20 pli (energy dissipated) + 0.08 pli (true interface strength with max influxes), or G 20 pli, which says that the measured peel strength is dominated by visco-plastic deformation processes. 

Dislocations multiply in a facile manner during a plastic deformation process, and several mechanisms for this have been observed by electron miscroscopy. Dislocations are destroyed by the processes of recovery and recrystallization during annealing after plastic deformation. Since dislocations cause low-yield stresses in metals and other solids, solid strengthening is accomplished either by eliminating dislocations or by immobilizing them. 

The three principle methods of strengthening materials are grain-size reduction, solid-solution strengthening, and plastic-deformation processes, like strain (work) hardening. 

It has been shown that several polymers exhibit instabilities in their plastic deformation process. It should finally be mentioned that instabilities may also occur during the plastic deformation of metals This phenomenon which is called the Portevin-Le Chatelier effect, is generally interpreted in terms of different modes of dislocation movement depending on whether or not dislocations move by dragging along their atmosphere of impurities behind them. 

Whether there is loss of material from the system of contacting bodies as the direct result of plastic deformation processes depends on specific conditions such as the geometry of the contacting surfaces, the properties of the materials, the magnitude of the applied load, etc. 

Drawing is a plastic deformation process in which the cross-sectional area of a part is reduced by the contemporary action of a pulling force and a converging die. 

In conclusion, crack size can be estimated theoretically by Equation 9.2 when the load exceeds a certain critical value, which can be determined by Equation 9.3. The important parameters for the critical loads to propagate subsurface damage are presented in Table 9.1 (Bandyopadhyay et al., 1999). In achieving the plastic deformation process, the grain load for SiC and Si3N4 should be less than 0.2 N and 0.7 N, respectively. SEM and AFM techniques permit to evaluate the surface and subsurface fracture damage. 

The discussion of mechanical properties comprises the various contributions of elastic, viscoelastic and plastic deformation processes. Often two characteristic stress levels can be defined in the tensile curve of polymer fibers the yield stress, at which a significant drop in slope of the stress-strain curve occurs, and the stress at fracture, usually called the tensile strength or tenacity. In this section the relation is discussed between the morphology of fibers and films, made from lyotropic polymers, and their mechanical properties, such as modulus, tensile strength, creep, and stress relaxation. 

Before entering into a detailed discussion of the above list and based on what has been said thus far on the subject, briefly summarized (a) creep in materials (including ceramics), namely time-dependent plastic deformation, may occur during mechanical stresses well below the yield stress and (b) in general, two major creep mechanisms characterize the time-dependent plastic-deformation process-dislocation creep and diffusion creep. Now, a detailed discussion of paragraphs (a)-(d) follows. 

Axial compression of high modulus polymeric filaments results in the formation of so-called kinkbands. These are regions of sub-micron thickness where the compressive defonnation is concentrated. This mode of failure is characteristic for anisotropic materials. The processes involved in kinkband formation are not yet well understood. In this work the kinkband formation in single aramid filaments is measured as a function of the applied compressive strain. Above a critical compressive strain the kinkband density initially increases yery rapidly until eventually a maximum density is obtained. The experimental results are compared with an elastic stability model. The kinkbands form before elastic instability occurs and are therefore attributed to a plastic deformation process. A model is developed to describe the kinkband density as a function of the applied strain. 

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