Dynamic yield point

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

The strength of the filler skeleton may be characterized by the complex dynamic modulus measured at low frequencies [24]. The authors note that when c > ccr the yield point should be viewed as a sum of two components ... 

In the case of thixotropic materials, the yield point depends on the conditions of measurement and on the previous history. At standstill, it acquires high values, while on stirring it may be close to zero. In practice, it means that vibration during handling of green ware may cause a decrease in yield point and subsequent deformation. Thixotropy is due to the formation of gel-type structures which are dcstro cd by dynamic effects. 

Because the recent experiments and simulations reviewed here concentrated on the universal aspects of the novel non-equilibrium transition, focus will be laid on the MCT-ITT approach. Reassuringly, however, many similarities between the MCT-ITT equations and the results by Miyazaki and Relchman exist, even though these authors used a different, field theoretic approach to derive their results. This supports the robustness of the mechanism of shear-advection in (7) entering the MCT vertices in (lid, 14), which were derived independently in [40, 41] and [43 5] from quite different theoretical routes. This mechanism had been known from earlier work on the dynamics of critical fluctuations in sheared systems close to phase transition points [61], on current fluctuations in simple liquids [62], and on incoherent density fluctuations in dilute solutions [63], Different possibilities also exist to include shear into MCT-inspired approaches, especially the one worked out by Schweizer and coworkers including strain into an effective free energy [42]. This approach does not recover the (idealized) MCT results reviewed below but starts from the extended MCT where no true glass transition exists and describes a crossover scenario without, e.g., a true dynamic yield stress as discussed below. 

Fig. 24 Flow curves O ( ) ) reaching from the supercooled to the glassy state of a simulated binary LJ mixture. The data points correspond to the temperatures T = 0.525,0.5, 0.45,0.44,0.43,0.42,0.4,0.38,0.3,0.2, and 0.01 in LJ-units (from bottom to top). Fj -model curves fitted by eye are included as lines. The inset shows the relation between the fitted separation parameters and temperature. Units are converted by cr =1.5c7(heo and y = 1.3ytheoT from [92]. The arrows mark the values of the extrapolated dynamic yield stresses C7 (fi)...

For room-temperature applications, most metals can be considered truly elastic. When stresses beyond the yield point are permitted in the design permanent deformation is considered a function only of applied load and can be determined directly from the usual static and/or dynamic tensile stress-strain diagram. The behavior of most plastics is much more dependent on time of application of the load, history of loading, current and past temperature cycles, and environmental... 

Tensile Properties Similar to polyethylene, the stress-strain curve of JSR RB has a yield point. Above the yield point, the stress-strain curve continues to increase with elongation, then breaks. This kind of stress-strain curve is similar to EVA and indicates a characteristic property lying somewhere between amorphous and crystalline polymers. The dynamic properties of JSR RB can be improved by stretch-... 

The simulations based on the point dipole model do exhibit a dynamic yield stress [173,292,296,315] and its dependence on the voliune fraction and electric field agrees with experiment [173]. However, the magnitude of the yield stress is severely underestimated in comparison with experiment, likely due to the neglect of multipolar and multibody interactions [173,243]. 

The protection potentials for seawater are described in Section 2.4. In pipelines and harbor installations, there is no limiting negative potential f/ for uncoated carbon steel or for steel provided with thick coatings over 1 mm, with yield points up to 800 N mm". With dynamically highly loaded structures, the protection potential ranges in Table 16-2 should be adhered to as in the regulations [1-3] because of the risk of hydrogen-induced stress corrosion (see Section 2.3.4). 

As we have seen, the strain rate dependence does suggest that yield behaviour often indicates the presence of two thermally activated processes, as discussed above. In some cases, notably polyethylene, a double yield point is observed. Ward and co-workers [64], Seguala and Darras [65] and Gupta and Rose [66] concur that these two deformation processes are essentially interlamellar shear and intra lamellar shear (or c-slip). They are akin to the dynamic mechanical relaxation processes identified in Chapter 10.7.1 for the specially oriented PE sheets, and Seguala and Darras have related them to the a and o 2 transitions reported by Takayanagi [67]. This establishes a direct link between yield and viscoelastic behaviour. 

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