Viscoelastic tests/parameters

The data were collected by Brookfield EZ-Yield software as shown in Figs. 8 and 9. The test parameters were as follows Spindle - 72, Immersion - Secondary, Zero Speed - 0.1 rpm. Wait Time - 30 s. Run Speeds - 0.05/0.5 rpm. The tests were carried out to either 105 % or 100 % torque reduction. Those tests carried out to 105 % torque reduction were trimmed down to include only those data points leading to and including the peak value of measured shear stress. The test data were exported as a Microsoft Excel spreadsheet. The data from the spreadsheet were then copied into a template developed by David Moonay to calculate the viscoelastic properties of the material. 

Because plastics are temperature and time dependent, by virtue of their viscoelasticity, both parameters must be defined where comparisons between materials are to be made. The test temperature is typically the standard one as noted in Chapter 6. although nonstandard conditions are used to measure the effect of temperature. The test is normally carried out at one of the standard test speeds, chosen from a set of values given in the standard. These recommended speeds are shown in Table 2. Note that for modulus measurement the test speed is normally I mm/min. 

J Mechanical testing parameters (a) A representative strain-stress curve in tensile testing. Yield stress (o-yg) and yield point strain (eyp) can be obtained by recording values at the point where the curve transitions from a linear relationship between stress and strain (elastic deformation) to a non-linear relationship (plastic deformation). Ultimate tensile strength (fr ,s) is the maximum stress in the curve, and the corresponding strain is called uniform strain (cp). The strain at fracture (eO can also be obtained from the curve, (b) When the transition point between elastic and plastic deformation is difficult to identify, a 0.2% strain offset line parallel to the elastic portion is drawn to obtain the <7ys or 0.2% offset o-ys. (c) Schematic of the deformation that occurs when shear force is applied to a viscoelastic polymer. 

From the previous discussions and Equation 7.5 it is evident that the measured value of adhesive fracture energy or fracture toughness typically reflects the extent of plastic and viscoelastic deformations in the region of the crack tip. In polymeric materials the extent of such deformations is highly dependent upon the test rate and temperature and, therefore, the measured values of Gc and Kc are often dependent upon these test parameters. 

To test the foregoing dimensionless relationship, two powders (Avicel Pl 1101, a ductile, viscoelastic material, and Emcompress, a brittle material, blended with 0.5% magnesium steaiate) were compressed on the PressterTw, a single-station mechanical replicator of rotary tablet presses. In the first set of experiments, a 16-station Manesty Betapress (a research-scale press) was simulated at two speeds, 60 and 100 rpm. In the second set, a 36-station Fette P2090 (a medium-scale production press) was simulated at two speeds, 55.8 and 70 rpm. It should be noted that 100 rpm of the Beta-press corresponds to 55.8 rpm of the Fette 2090 in terms of the linear speed of the turret. Basic parameters for the two tablet presses arc presented in Table 3. 

Frequency sweep studies in which G and G" are determined as a function of frequency (o)) at a fixed temperature. When properly conducted, frequency sweep tests provide data over a wide range of frequencies. However, if fundamental parameters are required, each test must be restricted to linear viscoelastic behavior. Figure 3-31... 

The creep test is a simple and inexpensive test for viscoelastic foods which provides valuable information on the rheological parameters. Davis (1973) pointed out that indeed too much information can be obtained from the creep test. For example, an eight-parameter rheological model defined by eleven parameters was required for shortening and lard. One drawback with creep studies using concentric cylinder systems is that the materials structure is disturbed when the sample is being loaded. 

The calculation from dynamic flexural experiments of elastic or viscoelastic functions is subject to errors arising from clamping (Ref. 6, p. 23). In the case of test samples whose section has dimensions of the order of magnitude of the free length, such a free length must be replaced by an effective length that represents that parameter in a more realistic way (see Fig. 7.7). 

Materials can show linear and nonlinear viscoelastic behavior. If the response of the sample (e.g., shear strain rate) is proportional to the strength of the defined signal (e.g., shear stress), i.e., if the superposition principle applies, then the measurements were undertaken in the linear viscoelastic range. For example, the increase in shear stress by a factor of two will double the shear strain rate. All differential equations (for example, Eq. (13)) are linear. The constants in these equations, such as viscosity or modulus of rigidity, will not change when the experimental parameters are varied. As a consequence, the range in which the experimental variables can be modified is usually quite small. It is important that the experimenter checks that the test variables indeed lie in the linear viscoelastic region. If this is achieved, the quality control of materials on the basis of viscoelastic properties is much more reproducible than the use of simple viscosity measurements. Non-linear viscoelasticity experiments are more difficult to model and hence rarely used compared to linear viscoelasticity models. 

The same parameters can also be determined by applying a constant shear stress to the interface and measuring the resulting shear strain as a function of time (see fig. 3.40), so-called interfacial creep tests. At t = 0, a shear stress is suddenly applied, and kept constant thereafter. For ideally viscous monolayers a steady increase of the shear strain with t will be observed, while for an elastic material the observed strain will be instantaneous and constcmt in time. For a viscoelastic material, as in fig. 3.40, there is first am Instantaneous increase AB in the strain, the elastic response followed by a delayed elastic response BC and a viscous... 

The outline of the review is as follows. First the microscopic starting points, the formally exact manipulations, and the central approximations of MCT-ITT are described in detail. Section 3 summarizes the predictions for the viscoelasticity in the linear response regime and their recent experimental tests. These tests are the quantitatively most stringent ones, because the theory can be evaluated without technical approximations in the linear limit important parameters are also introduced here. Section 4 is central to the review, as it discusses the universal scenario of a glass transition under shear. The shear melting of colloidal glasses and the key physical mechanisms behind the structural relaxation in flow are described. Section 5 builds on the insights in the universal aspects and formulates successively simpler models which are amenable to complete quantitative analysis. In the next section. 

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