Yield behavior stress

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. 

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. 

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. 

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,... 

Double-grooved specimens were used to study the failure of PC, PC/PE, PET, ABS, and HIPS during transitions from plane stress to plane strain. The yield behavior of PC is consistent with a von Mises-type yield criterion plane strain reduces its elongation. The yield behavior of PC/PE is consistent with a Tresca-type yield criterion plane strain appears to be relieved by voiding around the PE particles. PET undergoes a ductile-to-brittle transition its behavior is consistent with a von Mises-type yield locus intersected by a craze locus. The yield behavior of ABS and HIPS is not significantly affected by the plane-stress-to-plane-strain transition. Plane strain alone does not necessarily cause brittleness. 

However, not being able to independently control e3, our investigation is limited only to observations of yield behavior made during a controlled plane-stress-to-plane-strain transition. 

We have observed four distinct types of plane stress/plane strain yield behavior (a) PC, (b) PC/PE, (c) PET, and (d) ABS and HIPS. These are discussed separately. 

We have used double-grooved specimens to study systematically the yield behavior of the polymers PC, PC/PE, PET, ABS, and HIPS as they undergo transitions from plane stress to plane strain. Four distinct types of behavior have been observed ... 

Since spallation is controlled by the response to tensile stress pulses, the measurements of yield behavior were performed in uniaxial tension rather than in shear, and a tensile yield stress criterion was required. Bouwens-Crowet et ah (6) rearranged Equation 1 to give an expression for the uniaxial-tension yield stress [Pg.201]

Jenike developed the idea that no single line represents the yield but rather a curve called the yield locus. The yield behavior depends on the packing density of the powder when it is caused to flow under the action of normal and shear stress. Figure 12.36 shows a yield locus for a given porosity, e. A Mohr circle for the stage when yielding starts is characterized by the principal stresses i and 2-The points at the end of the yield locus lies on the Mohr circle pertains to... 

For macroscopically isotropic polymers, the Tresca and von Mises yield criteria take very simple analytical forms when expressed in terms of the principal stresses cji, form surfaces in the principal stress space. The shear yield surface for the pressure-dependent von Mises criterion [Eqs (14.10) and (14.12)] is a tapering cylinder centered on the applied pressure increases. The shear yield surface of the pressure-dependent Tresca criterion [Eqs (14.8) and (14.12)] is a hexagonal pyramid. To determine which of the two criteria is the most appropriate for a particular polymer it is necessary to determine the yield behavior of the polymer under different states of stress. This is done by working in plane stress (ct3 = 0) and obtaining yield stresses for simple uniaxial tension and compression, pure shear (di = —CT2), and biaxial tension (cti, 0-2 > 0). Figure 14.9 shows the experimental results for glassy polystyrene (13), where the... 

The behavior of real component powders may be estimated using a yield criterion (1), which requires a knowledge of the magnitudes of the directional response to stress of the particle and knowledge of the particle size distribution and particle orientation. Much of this information is not readily available for molecular organic solids and is the subject increasing attention of academic materials scientists. Even knowing the crystal structure, the dynamic response to mechanical stresses is not currently predictable without exhaustive effort (3). However, yield behavior may be measured and correlated to the... 

Yield Behavior of Powders The yield behavior of a powder depends on the existing state of consolidation within the powder bed when it is caused to flow or yield under a given state of stress, defined by the acting normal and shear stresses. The consolidation state controls the current bed voidage or porosity. Figure 21-34 illustrates a times series of shears occurring for the BCR116 hmestone standard for a rotary shear cell. For each shear step, torque is applied... 

This paper, the third in a rheotechnics series, reports on rheological studies performed at y higher than 10 1 sec-1. In two-phase polymer systems, Rosen and Rodriquez (3) hypothesized anomalous behavior (yield shear stress) at y = 10 1. Recently, Zosel (9) verified this hypothesis experimentally for ABS systems. A study of the rheological behavior of ABS resins at very low y is now in progress at our laboratories. The correlation between rheological behavior and moldability will be discussed in detail in a subsequent paper. 

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