Viscoelastic fluid models, plastic

As mentioned above, interfacial films exhibit non-Newtonian flow, which can be treated in the same manner as for dispersions and polymer solutions. The steady-state flow can be described using Bingham plastic models. The viscoelastic behavior can be treated using stress relaxation or strain relaxation (creep) models as well as dynamic (oscillatory) models. The Bingham-fluid model of interfacial rheological behavior (27) assumes the presence of a surface yield stress, cy, i.e.. 

Creep is related to plastics viscoelastic behavior and can be explained with the aid of a Maxwell model such as that shown in Figure 3-55 [12, 287]. When a load is applied to the system, shown diagrammatically, the spring will deform to a certain degree. The dashpot will first remain stationary under the applied load, but if the same load continues to be applied, the viscous fluid in the dashpot will slowly leak past the piston, causing the dashpot to move. Its movement corresponds to the strain or deformation of the plastic material. 

Rheology is the study of the deformation and flow of fluids. Four different models are used to characterize the flow of fluids Newtonian, Bingham plastic, power law, and viscoelastic In the characterization, models have been developed to relate the observed effects that shear rate has on foam. Several scientists who have studied foamed fluid rheology categorize foam into various models. 

The plastic at its forming temperature can behave as an elastic solid, a viscoelastic fluid, or a combination of the two. Modeling of thermoforming has been done using all of these models. On a molecular scale, things are just as complex. For an amorphous material, such as polystyrene or PMMA, the forming temperature determines the chain mobility and the ease of flow. For semicrystalline materials, such... 

Plasticity and Viscoplasticity and Other Models. As discussed above, the alternative representation of the nonlinear viscoelastic response of polymers is that of plasticity and viscoplasticity. In some respects, these models could be recast as viscoelastic models and they would be equivalent to some of the models discussed above. However, the perspective that glassy polymers are really fluids and do follow time-temperature superposition is lost with these models. Hence, the physical interpretation of material parameters, in this author s opinion, becomes very qnestionable. Therefore, only references to the major papers on polymer plasticity and viscoplasticity are given (174-177). 

This model assumes that lubricant behaves as a viscoelastic-plastic fluid, in agreement with the rheological law proposed by Bair and Winer [11]... 

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