Rheological material behavior

With plastics there are two types of deformation or flow viscous, in which the energy causing the deformation is dissipated, and elastic, in which that energy is stored. The combination produces viscoelastic plastics. See Chapter 2, MATERIAL BEHAVIOR, Rheology and Viscoelasticity, regarding their effects on fabricated products. 

Rheological Behavior, Rheometry, and Rheological Material Functions of Polymer Melts, 80... 

RHEOLOGICAL BEHAVIOR, RHEOMETRY, AND RHEOLOGICAL MATERIAL FUNCTIONS OF POLYMER MELTS... 

The material behaviors considered will include linear elasticity plus linear or nonlinear creep behavior. The nonlinear case will be restricted to power-law rheologies. In some cases the elasticity will be idealized as rigid. In ceramics, it is commonly the case that creep occurs by mass transport on the grain boundaries.1 This usually leads to a linear rheology. In the models considered,... 

If we interpret this question as asking whether models exist for the general class of complex/non-Newtonian fluids that are known to provide accurate descriptions of material behavior under general flow conditions, the current answer is that such models do not exist. Currently successful theories are either restricted to very specific, simple flows, especially generalizations of simple shear flow, for which rheological data can be used to develop empirical models, or to very dilute solutions or suspensions for which the microscale dynamics is dominated by the motion deformation of single, isolated macromolecules or particles/drops.24... 

In the area of liquid state rheology there is also considerable research in progress on the development of mathematical models that predict material behavior during composite fabrication cure processes. For example, the viscosity of a thermosetting matrix can be predicted for any cure cycle by using a mathematical model developed from kinetic and rheological data (26). 

The DNF model incorporates the experimentally observed characteristics by using a micromechanism-inspired approach in which the material behavior is decomposed into a viscoplastic response, corresponding to irreversible molecular chain sliding due to the lack of chemical crosslinks in the material, and atime-dependent viscoelastic response. The viscoelastic response is further decomposed into the response of two molecular networks acting in parallel the first network (A) captures the equilibrium response and the second network (B) the time-dependent deviation from the viscoelastic equilibrium state. A onedimensional rheological representation of the model framework and a schematic illustrating the kinematics of deformation are shown in Fig. 11.6. 

The LCP rheology usually describes the material behavior in a specific form, mainly nematic, which shows flow behavior similar to that of CPNC. This phase morphology is characterized by local orientation, evident in rheooptical studies. As... 

The flow behavior of silicone oils [5] and silicone oil/glass sphere suspensions [14] was studied by several authors. One of the most used rheological material parameters to characterize the flow behavior is the zero-shear-rate viscosity tIq. The t]o value of the linear silicone oils studied are correlated with the relevant weight-average molecular weight Af by Eq. 3 [16], where a = 3.58. 

However, amorphous water-soluble materials, such as food materials, deform viscoelastically. The deformation and relaxation behavior of such materials can be described by means of various viscoelastic models. Depending on the nature of the stress/strain applied, either the storage and loss modulus or the elasticity and the viscosity are included as material parameters in these models. These rheological material parameters depend on the temperature and the water content as well as on the applied strain rate. The viscoelastic deformation enlarges the contact area and decreases the distance between the particles (see Fig. 7.3). If the stress decreases once again, the achieved deformation is partially reversed (structural relaxation). 

A rheology experiment behavior of a material subjected to shear... 

In rheology we generally assume that a material is incompressible. The deviations from simple Hookean or Newtonian behavior due to nonlinear dependence on deformation or deformation history are usually much greater than the influence of compressibility. We discuss the influence of pressure briefly in Chapters 2 and 6. For incompressible materials the overall pressure cannot influence material behavior. In other words, increasing the barometric pressure in the room should not change the reading from a rheometer. For incompressible materials the isotropic pressure is determined solely by the boundary conditions and the equations of motion (see Sections 1.7 and 1.8). 

The term viscoelasticity combines viscous and elastic stress-strain flow characteristics. If materials behavior is dominated by viscous flow it is generally referred to as a fluid, whereas if the elastic properties dominate the mechanical properties of a material it is considered to be solid. Most adhesives are applied in a liquid or pasty condition to allow wetting and promote spreading and then are required to phase change into a solid. In the liquid state, rheology provides the methods to differentiate between elastic and viscous flow characteristics while, for example, dynamic mechanical analysis of cured adhesive polymers uses similar principles to access elastic and viscous parameters of the stress-strain response. 

Caruthers and co-workers Model. The group of J.M. Caruthers at the Chemical Engineering Department of Purdue University has, over the last decade or so, been developing a set of unified constitutive equations that aim to realistically model a wide range of rheological and mechanical properties (41,42). A detailed description of the model is beyond the scope of this article and we mention here only the main ideas behind the development of the model (see also Viscoelasticity). The model addresses the time, temperature, and history (thermal and mechanical) dependence of the material behavior using a set of... 

Currently, there is no theory for concentrated suspensions that accounts for fiber contact. However, semidilute theory has been used to some extent to model their stress behavior, in which case A is fit to the rheological material functions of a fluid instead of calculating A from theory. In the majority of fiber composites of industrial significance, the matrix is polymeric and exhibits nontrivial viscoelastic behavior, which increases the complexity of modeling such suspensions. The first attempts ignore the fiber and treat the suspension as a homogeneous viscoelastic fluid. 

However, for non-Newtonian fluids, even though continuity equation and the equation of motion written as Equation 11.2 remain valid, the Newtonian constitutive equation is not correct and a different constitutive equation is needed. To find constitutive equations, experiments are performed on materials using standard flows described above. The functions of kinematic parameters that characterize the rheological behavior of fluids are called rheological material functions. Standardized material functions are shown in Table 11.1 [2-4]. 

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