Elastic and viscous properties

This method applies a constant force to the sample and monitors the strain (deformation) as a function of time This is called the creep test and gives the elastic and viscous properties of substances. 

The starting time for rheological measurements correspond to t = 120 min. Indeed, the rheological parameters were only recorded during the rennet-induced coagulation phase to avoid structural modifications during the acidification phase which may consequently influence the gelation process. Elastic and viscous properties of reconstituted milks... 

In this introduction, the viscoelastic properties of polymers are represented as the summation of mechanical analog responses to applied stress. This discussion is thus only intended to be very introductory. Any in-depth discussion of polymer viscoelasticity involves the use of tensors, and this high-level mathematics topic is beyond the scope of what will be presented in this book. Earlier in the chapter the concept of elastic and viscous properties of polymers was briefly introduced. A purely viscous response can be represented by a mechanical dash pot, as shown in Fig. 3.10(a). This purely viscous response is normally the response of interest in routine extruder calculations. For those familiar with the suspension of an automobile, this would represent the shock absorber in the front suspension. If a stress is applied to this element it will continue to elongate as long as the stress is applied. When the stress is removed there will be no recovery in the strain that has occurred. The next mechanical element is the spring (Fig. 3.10[b]), and it represents a purely elastic response of the polymer. If a stress is applied to this element, the element will elongate until the strain and the force are in equilibrium with the stress, and then the element will remain at that strain until the stress is removed. The strain is inversely proportional to the spring modulus. The initial strain and the total strain recovery upon removal of the stress are considered to be instantaneous. 

Figure H3.1.2 The oscillatory response of a food material that possesses both elastic and viscous properties can be represented by a complex variable G. This variable has two components that can be expressed as either the Cartesian coordinates G and G or the polar coordinates IG I and 8. IG I is the magnitude of the imaginary G and is measured as the ratio of the amplitudes of stress and strain.

Theoretical analysis indicates that occurrence of such convective instabilities depends on anisotropy of electrical conductivity and dielectric properties in the initial aligned nematic material. That is, conductivity parallel to the direction of alignment must differ from conductivity perpendicular to this direction. Calculation of the stability condition requires knowledge not only of these anisotropic electrical properties but also of anisotropic elastic and viscous properties which oppose disruption of the alignment and flow. 

Fibrin is a viscoelastic polymer, which means that it has both elastic and viscous properties (Ferry, 1988). Thus, the properties of fibrin may be characterized by stiffness or storage modulus (representing its elastic properties) and creep compliance or loss modulus/loss tangent (representing its inelastic properties). These parameters will determine how the clot responds to the forces applied to it in flowing blood. For example, a stiff clot will not deform as much as a less stiff one with applied stress. 

To measure elastic and viscous properties which are characteristic of the material under consideration and independent of the nature of the apparatus employed, the applied stress and the resulting deformation must be uniform throughout the sample. Concentric cylinder and cone and plate methods approximate these requirements. For materials which are self-supporting, measurements on, for example, the shearing of rectangular samples are ideal. 

Polyurethanes have a combination of elastic and viscous properties that can be explained in standard engineering terms using DMA methods. Information can be obtained on the properties of polyurethanes that relates to the storage and dissipation of energy applied during use. 

Determination of Elastic and Viscous Properties of Vessel Wall... 

Oldroyd, J.G. The elastic and viscous properties of emulsions and suspensions. Proc. Roy. Soc. London 1952, A218, 122-132. 

Here we report on mesoscopic studies carried out on the mixture Beta-picoline (BP or 3-methylpyridine)/heavy water mixture (BP-D2O) aiming to get detailed information on the elastic and viscous properties of the two external single- phase and the internal two-phase states in such mixtures by means of analysis of the optical Rayleigh-Brillouin spectra which have been measured over a wide range of temperatures. 

Viscoelasticity. Viscoelastic materials are characterized by a combination of elastic and viscous properties. Thus, the shear stress is not only dependent on the rate of shearing but on the strain 7 as well. In the simplest case, the viscoelastic behavior is governed by... 

The gel point in materials represents the point where behavior changes from viscous (liquid-like) to elastic (solid-like). The conditions under which this occurs are critical to such food constituents as wheat-soya solutions used as setting agents within reconstituted meat products. In this case, oscillatory-controlled stress experiments, in which a small sinusoidal stress is applied to the material, provide a convenient method for evaluating elastic and viscous properties without destroying the delicate structure of soft semisolids. 

The biomechanical response of the body has three components, (1) inertial resistance by acceleration of body masses, (2) elastic resistance by compression of stiff structures and tissues, and (3) viscous resistance by rate-dependent properties of tissue. For low-impact speeds, the elastic stiffness protects from crush injuries whereas, for high rates of body deformation, the inertial and viscous properties determine the force developed and limit deformation. The risk of skeletal and internal organ injury relates to energy stored or absorbed by the elastic and viscous properties. The reaction load is related to these responses and inertial resistance of body masses, which combine to resist deformation and prevent injury. When tissues are deformed beyond their recoverable limit, injuries occur. 

Oldroyd, J.C. (1955) The effect of interfacial stabilizing films on the elastic and viscous properties of emulsions. Proc. R. Soc. London, Sen A, 232,567-577. 

Sinusoidal time-varying (STV) flow demonstrates differences between elastic and viscous properties, and it demonstrates viscoelastic behavior in more complex time-varying flow [14]. 

Another method for measuring mechanical properties on the macroscopic scale uses the relation between mechanical properties and the propagation of acoustic waves [89]. The velocity of sound waves and also the damping thereof can be directly deduced from the elastic and viscous properties. For polymers, ultrasound can be used since the damping of the acoustic waves is decreased at high frequencies. However, this method seems not to have been applied to fuel cell-related membrane materials so far. 

Perhaps the most general mathematical treatment of the surface mass loading effect on bulk shear wave resonators has been presented by Kanazawa (13). In this work, a wave equation was developed for acoustic wave propagation within the deposited layer, assuming the material had both elastic and viscous properties. Boundary conditions between crystal and deposited mass were established by assuming shear forces and particle displacements were equal for both materials at the interface plane. This approach results in a fairly complex mathematical model, but simplified relationships were derived for purely elastic and purely viscous behaviour. 

A minimal model for interpreting the main features of the static and dynamic aspects of EOM effects [31] is introduced in this section. To reduce complexity, this model does not explicitly consider the anisotropic effect on elastic and viscous properties. More realistic expressions considering the anisotropy [47-50] are available, but the limited experimental data makes it difficult to unambiguously determine a larger set of material parameters. 

For the AB zone, the Rouse model [11] accounts for the viscoelasticity. The model is composed of springs and beads, expressing elastic and viscous properties, respectively. The relaxation time t for slip of a bead becomes proportional to n, whereas the number of springs or elasticity is inversely proportional to n n is a chain length expressed by the number of segments in a chain. As a result, E is proportional to or co°, namely. 

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