Plastic body deformation

With higher forces the friction between particles is overcome and the particles slide with respect to one another. This behavior is referred to as a plastic body deformation. The start of this particle movement... 

Plastic Forming. A plastic ceramic body deforms iaelastically without mpture under a compressive load that produces a shear stress ia excess of the shear strength of the body. Plastic forming processes (38,40—42,54—57) iavolve elastic—plastic behavior, whereby measurable elastic respoase occurs before and after plastic yielding. At pressures above the shear strength, the body deforms plastically by shear flow. 

A plastic material is defined as one that does not undergo a permanent deformation until a certain yield stress has been exceeded. A perfectly plastic body showing no elasticity would have the stress-strain behavior depicted in Figure 8-15. Under influence of a small stress, no deformation occurs when the stress is increased, the material will suddenly start to flow at applied stress a(t (the yield stress). The material will then continue to flow at the same stress until this is removed the material retains its total deformation. In reality, few bodies are perfectly plastic rather, they are plasto-elastic or plasto-viscoelastic. The mechanical model used to represent a plastic body, also called a St. Venant body, is a friction element. The... 

The following analysis of stresses assumes that the green body is purely elastic. Certainly this is not the complete picture because wet, sticlq powders or gels are not elastic but plastic, showing deformation of the particle network by the frictional movement of particles against each other. We have discussed these mecheinical properties in Sections... 

When attempting to describe more accurately the rheological behaviour of ceramic plastic mixes, one should also take into account the elastic behaviour above the yield point. If a plastic body is abruptly stressed by a constant load, there first occurs rapid clastic deformation followed by delayed elastic deformation and irreversible flow. Similarly, instant as well as delayed relaxation take place after stress relief. If a formed product has only a limited possibility to relax, it retains some interna stress w hich may be the cause of drying defects. 

In terms of rheology ceramic bodies hold a special position between ideal elastic and ideal plastic bodies, as they exhibit Bingham behaviour. Plotted on a shear stress/shearing speed graph, ceramic plastic bodies start to deform only after having reached a certain shear stress tq, the so-called yield point. 

Highly plastic bodies are accordingly those which offer little resistance to deformation, but nevertheless stiU have a high tear resistance. [11,12]... 

Plastic bodies only show irreversible deformation above a given shear stress (see Section 7.5.1). 

At solid body deformation the heat flow is formed, which is due to deformation. The thermodynamics first law establishes that the internal eneigy change in sample dU is equal to the sum of woik dW, carried out on a sample, and the heat flow dQ into sample (see the Eq. (4.31)). This relation is valid for any deformation, reversible or irreversible. There are two thermo-d5mamically irreversible cases, for which dQ = -dW, uniaxial deformation of Newtonian liquid and ideal elastoplastic deformation. For solid-phase polymers deformation has an essentially different character the ratio QIW is not equal to one and varies within the limits of 0.35 0.75, depending on testing conditions [37]. In other words, for these materials thermodynamically ideal plasticity is not realized. The cause of such effect is thermodynamically nonequilibrium nature or fractality of solid-phase polymers structure. Within the frameworks of fractal analysis it has been shown that this results to polymers yielding process realization not in the entire sample volume, but in its part only. 

Plastic bodies are those in which the apphed shear stress has exceeded the yield point. Under these conditions, the deformation of the plastic body is progressive with time until, at some point, it is able to sustain a constant applied shear stress. On the removal of the shearing stress, any elastic component of the total strain is recovered however, the plastic component of the strain is not recovered. When the body deforms over a measurable period of time, i.e., flows under the action of a constant stress, one speaks of creep occurring. An elastic-plastic body having a near-zero yield point can be called a viscous body. Such a body can not sustain shear stress over a finite period of time, since the stress relaxes with time in other words, the body flows to relieve shear stresses. At constant strain, the stress could initially build up instantaneously as an elastic response, however, such stress would ultimately decay through internal relaxation mechanisms. If the externally applied stress is maintained, the deformation will continue to occur with time, since the applied stress is never balanced. The slower the rate of stress relaxation, the more viscous the body is. 

True vs. apparent strength the visco-plastic energy dissipation dominated the magnitude of the peel strength. However, little dissipation occurs if the interface becomes very weak. In other words, some influxes are necessary to produce sufficient stress to activate the viscoelastic deformation in the body of the A. 

From the results obtained in [344] it follows that the composites with PMF are more likely to develop a secondary network and a considerable deformation is needed to break it. As the authors of [344] note, at low frequencies the Gr(to) relationship for Specimens Nos. 4 and 5 (Table 16) has the form typical of a viscoelastic body. This kind of behavior has been attributed to the formation of the spatial skeleton of filler owing to the overlap of the thin boundary layers of polymer. The authors also note that only plastic deformations occurred in shear flow. 

When a plastic material is subjected to an external force, a part of the work done is elastically stored and the rest is irreversibly (or viscously) dissipated hence a viscoelastic material exists. The relative magnitudes of such elastic and viscous responses depend, among other things, on how fast the body is being deformed. It can be seen via tensile stress-strain curves that the faster the material is deformed, the greater will be the stress developed since less of the work done can be dissipated in the shorter time. 

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