Yield behavior

Foam rheology has been a challenging area of research of interest for the yield behavior and stick-slip flow behavior (see the review by Kraynik [229]). Recent studies by Durian and co-workers combine simulations [230] and a dynamic light scattering technique suited to turbid systems [231], diffusing wave spectroscopy (DWS), to characterize coarsening and shear-induced rearrangements in foams. The dynamics follow stick-slip behavior similar to that found in earthquake faults and friction (see Section XU-2D). 

Fig. 4. Tensile stress—strain curves for polysulfone showing yield behavior at A, 20°C B, 99°C and C, 149°C. To convert MPa to psi, multiply by 145.

Controlled Stress Viscometer. Most rotational viscometers operate by controlling the rotational speed and, therefore, the shear rate. The shear stress varies uncontrollably as the viscosity changes. Often, before the stmcture is determined by viscosity measurement, it is destroyed by the shearing action. Yield behavior is difficult to measure. In addition, many flow processes, such as flow under gravity, settling, and film leveling, are stress-driven rather than rate-driven. 

To illustrate the effect of radial release interactions on the structure/ property relationships in shock-loaded materials, experiments were conducted on copper shock loaded using several shock-recovery designs that yielded differences in es but all having been subjected to a 10 GPa, 1 fis pulse duration, shock process [13]. Compression specimens were sectioned from these soft recovery samples to measure the reload yield behavior, and examined in the transmission electron microscope (TEM) to study the substructure evolution. The substructure and yield strength of the bulk shock-loaded copper samples were found to depend on the amount of e, in the shock-recovered sample at a constant peak pressure and pulse duration. In Fig. 6.8 the quasi-static reload yield strength of the 10 GPa shock-loaded copper is observed to increase with increasing residual sample strain. 

It has been a persistent characteristic of shock-compression science that the first-order picture of the processes yields readily to solution whereas second-order descriptions fail to confirm material models. For example, the high-pressure, pressure-volume relations and equation-of-state data yield pressure values close to that expected at a given volume compression. Mechanical yielding behavior is observed to follow behaviors that can be modeled on concepts developed to describe solids under less severe loadings. Phase transformations are observed to occur at pressures reasonably close to those obtained in static compression. 

Necking occurred in samples of polymer E and of the thermoplastic Phenoxy. The other, more crosslinked polymers failed before necking started. The results on the post yield behavior are included in Table 2.1. Apparently, a process such... 

Anh, T.H. and Vu-Khanh, T. Fracture and Yielding Behaviors of Polystyrene/Ethylene-Propylene Rubber Blends Effects of Interfacial Agents, Polym. Eng. Set 41(12), 2073-2081, December 2001. 

Figure 5.5 Fractional yield behavior of primary, secondary, and tertiary products...

A unified approach to the glass transition, viscoelastic response and yield behavior of crosslinking systems is presented by extending our statistical mechanical theory of physical aging. We have (1) explained the transition of a WLF dependence to an Arrhenius temperature dependence of the relaxation time in the vicinity of Tg, (2) derived the empirical Nielson equation for Tg, and (3) determined the Chasset and Thirion exponent (m) as a function of cross-link density instead of as a constant reported by others. In addition, the effect of crosslinks on yield stress is analyzed and compared with other kinetic effects — physical aging and strain rate. 

The yield behavior is also a kinetic phenomenon and has been treated as the nonlinear cooperative deformation (21). Consider a... 

The role of crosslinks in the yield behavior of thermosets is also discussed. 

The variety of epoxy resins offers a wide range of molecular structures that exhibit different yield behavior at the macroscopic level. The study of plastic deformation in different epoxy resins can help understand the structure/property relationship of plasticity in thermoset resins. 

The Argon theory has successfully interpreted the yield behavior of a large number of amorphous thermoplastic polymers (3,4). For thermosets,... 

This paper rerports an investigation of the yield behavior of several amine and anhydride cured DGEBA resin systems. The Argon theory is used to assess the controlling molecular parameters from the experimental results. Such parameters are then compared with the known chemical structures of the resins. The mechanisms of plastic flow in thermoset polymers such as epoxies is demonstrated. 

The Argon theory, therefore, consistently interprets the yield behavior of both thermosets and thermoplastics. This indicates that crosslinks in thermosets do not introduce appreciable deviation to the kink formation process described. This point is also supported by Ygmani and Young s finding of the molecular parameters, z and a, being insensitive to crosslinking density for DGEBA cured with different amount of TETA. 

Such effects are likely to be important. The use of SP interactions to create bioinspired material properties (e.g., see Chap. 9) implies that the ultimate yield behavior of SP materials could depend on the mechanical response of supramolecular interactions. Paulusse and Sijbesma (2004) have also shown that ultrasound-generated shear stresses can mechanically tear apart coordination SPs, damage that is subsequently repaired during dynamic equilibration once the shear stresses are removed. The mechanical response of supramolecular interactions within materials has potentially important consequences in the context of self-repairing materials, where the mpture of sacrificial supramolecular interactions protects a permanent, underlying materials architecture. The dynamic repair of the SP component in... 

To assess the yield behavior of a sample, plot the data as viscosity versus stress. 

In conclusion, the yield behavior of thermosets is similar to that found for other glassy polymers. The presence of crosslinks does not basically affect the yield behavior of polymer networks. 

The ductile-brittle transition is clearly related to the yielding behavior of the thermoset in static experiments, (see Fig. 12.5). 

These relaxation time equations together with Eqs. (2), (15), and (19) can be utilized in analyzing the experimental measurements of volume relaxation and recovery, of linear and nonlinear viscoelastic relaxations, and of yield behavior and stress-strain relationships. 

In the solid state deformation, the nonlinear viscoelastic effect is most clearly shown in the yield behavior. The type of stresses applied to a system has little effect on the linear viscoelastic relaxation, but becomes very important as the stress level increases. At high stress levels, the contribution from the external work done on a lattice cell has to be included in the nonlinear viscoelastic analysis. By taking into account the long range cooperative interaction, the external work can,... 

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