Pressure dependent yield behaviour

We will see that this simple form of pressure-dependent yield criterion is more satisfactory than the Coulomb criterion when a representation is developed which includes the effects of temperature and strain rate on the yield behaviour. In physical terms, the hydrostatic pressure can be seen as changing the state of the polymer by compressing the polymer significantly, unlike the situation in metals where the bulk moduli are much larger ( 100 GPa compared... 

The dependence of yield behaviour of PMMA-tubes in shear in internal pressure was studied by Rabinowitz et al. (1970). Results for pressures from 0.1 to 703 MPa are presented in Fig. 13.72. It appears that... 

All photolytic systems showed a Stem-Volmer type pressure dependent behaviour. The photolytic lifetimes were calculated on the basis of the cited overall quantum yields 4 day for MEK, 14 h for MVK, 22 h for MACR and 35 min for MGLY. 

There have been a number of detailed investigations of the influence of hydrostatic pressure on the yield behaviour of pol3maers. Because it illustrates clearly the relationship between a yield criterion, which depends on hydrostatic pressure, and the Coulomb yield criterion, an experiment will be discussed where Rabinowitz, Ward and Parry [23] determined the torsional stress-strain behaviour of isotropic PMMA under hydrostatic pressures up to 700 MPa. The results are shown in Figure 11.17. 

Lazzeri and Bucknall [131] have proposed that the pressure dependence of yield behaviour caused by the presence of microvoids can explain the observation of dilatation bands in rubber-toughened epoxy resins [132], rubber-toughened polycarbonate [133] and styrene butadiene diblock copolymers [134]. These dilatation bands combine in-plane shear with dilatation normal to the shear plane. Whereas true crazes contain interconnecting strands, as described in Section 12.5.1 above, dilatation bands contain discrete voids that, for rubber-toughened polymers, are confined to the rubber phase. 

The photoablation behaviour of a number of polymers has been described with the aid of the moving interface model. The kinetics of ablation is characterized by the rate constant k and a laser beam attenuation by the desorbing products is quantified by the screening coefficient 6. The polymer structure strongly influences the ablation parameters and some general trends are inferred. The deposition rates and yields of the ablation products can also be precisely measured with the quartz crystal microbalance. The yields usually depend on fluence, wavelength, polymer structure and background pressure. 

Phosphorescence of s-triazine has been observed by Ohta et al. following excitation of the 6o band of the Si — So transition. Values for the phosphorescence lifetime and quantum yield were reported. The effects of rotational excitation on the yields and decays of the fast and slow components of Si state s-triazine fluorescence have been studied. Excitation along the rotational contours of the 6j and 6o bands revealed that the fast component showed little rotational level dependence in contrast to the slow component. This behaviour was interpreted in terms of an increase in the number of triplet levels coupled to the optically prepared singlet levels with increasing angular momentum quantum number, J. A broad emission feature present in addition to narrowline fluorescence from rovibronic levels of 6 or 6 in S, s-triazine has been observed and the rotational level dependence of its quantum yield and decay over a range of pressures reported... 

I This step represents the photochemical initiation of the reaction. The quantum yield (366 nm) for the formation of phosgene has been determined as 6244 at 25 C [308], and its dependence on light intensity and reactant pressure investigated [674]. Simiiar behaviour is obtained if the reaction is initiated with a-particies (from radon) instead of photons, and the same form of kinetic dependence is found [37] the photochemical yield per quantum is approximateiy equai to the radiation yield per ion pair [37]. 

Bulk and pack densities are known to be dependent upon powder feed rates, height of fall and particle morphology when directed into specific volume containment. Thus powder flowability, cohesiveness, impact pressure and powder yield strength together with other physical particle properties such as particle size and particle shape contribute influentially to powder bulk behaviour. 

Many factors influence the accuracy of experimental data and each experimental run could be described by a different set of parameters n, K, Ty. Since the flow models strongly depend on input data and their evaluation, a sensitivity analysis was used to find effect of value of flow behaviour index n on accuracy of laminar and turbulent flow models. Dependency of slurry/water pressure gradient ratio i / io on mean slurry velocity V of the measured slurries for both tested turbulent models and yield power-law model is shown in Fig. 4, where also a role of parameter n is illustrated. The value of n given by best fitting of laminar data by Eq. (4) represents quite well laminar region. For turbulent data it is not valid (see dashed line). The best fitting value n for turbulent data varies not only with kind of solid material, but depends also on concentration. 

Biological macromolecules in solutions can be distinctly characterized from their transport behaviour in solution phase. The study of transport processes yields physical parameters like the diflusion coefficient, sedimentation coefficient, intrinsic viscosity, friction constant etc. of the dissolved solute molecule. These coefficients are dependent on two parameters. First, is the size and shape of the solute particle Second, is the type of the solvent medium and its environment (pH, temperature, pressure, ionic strength etc.). The solvent medium can force the diffusing particles to assume a special shape and/or to get distributed in a special fashion in space through solvent-solute interactions. At the same time a pair of solute molecules will also influence each other s behaviour and/or their physical shape and size. This process may or may not be mediated by the solvent. To account for all these mechanisms, we need to discuss the solute-solvent, solvent-solvent and solute-solute interactions. Interestingly enough, much of this information is contained in the transport coefficients of a solute and physical parameters describing a solvent. 

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