ISSN:0148-6055    

DOI:10.1122/1.549917

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.549918

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

The cell theory has been previously employed to compute the equation of state of a disordered condensed system. It is now generalized to include anisotropic stresses. We adopt the condition of affine deformation, transforming an originally spherical into an ellipsoidal cell. With a Lennard‐Jonesn−mpotential between non‐bonded centers, the formal expression for the deformational free energy is derived. It is to be evaluated in the limit of the linear elastic range. Since the bulk modulus in this limit is already known, it is convenient to consider a uniaxial deformation. To begin with, we restrict ourselves to the low temperature limit in the absence of entropy contributions. Young's modulus and Poisson's ratio then follow.

ISSN:0148-6055    

DOI:10.1122/1.549919

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

We consider the extension of the preceding theory to finite temperatures, while retaining the previous assumptions of affine deformation, of a linear elastic range, and of central interparticle forces. At sufficiently low temperatures, a quasi‐harmonic approximation with volume and strain dependent frequencies ensues. It accounts for an initial linear increase of the Poisson constant μ and a similar decrease of Young's modulusYwith increasing temperature. Numerical evaluation of the complete cell potential and of the free‐volume integral shows that this is followed by plateau regions for both functions, whereas the bulk modulus is but weakly dependent on temperature over the same range of temperatures. This includes a third region of more rapidly decreasingYand increasing μ. However, these latter results require further examination. For upon a further increase in temperature, the validity of the model breaks down, when it would predict a reversal in the sign of the temperature coefficient ofYand impermissible values for μ. This is qualitatively similar to the performance of the cell theory in predicting the thermal expansivity of glasses at low temperatures. Examination of experimental results for some inorganic and organic glasses indicates the existence of the three regions referred to above, or at least the last two.

ISSN:0148-6055    

DOI:10.1122/1.549920

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

A capillary extrusion rheometer was used to evaluate the flow behavior of mixtures of nitrocellulose (12.2% nitrogen) and nitroglycerine when processed using solvent blends of different strengths. The aim of the work was to quantify the effects on flow behavior of “gelatinization,” that is, the degree of breakdown of the fibrous structure of the nitrocellulose. The flow behavior was measured over a range of extrusion temperatures. The results indicate that the materials behave as Herschel‐Bulkley fluids, that the shear stress decreases as gelatinization increases, and that viscous heating is more apparent in poorly gelatinized doughs. Surface temperatures of the extrudate determined experimentally are compared with computer‐modelled values. Die swell measurements decreased with decreasing degree of gelatinization and with increasing extrusion temperature.

ISSN:0148-6055    

DOI:10.1122/1.549921

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

A theoretical model for foam rheology that includes viscous forces is developed by considering the deformation of two‐dimensional, spatially periodic cells in simple shearing and planar extensional flow. The undeformed hexagonal cells are separated by thin liquid films. Plateau border curvature and liquid drainage between films is neglected. Interfacial tension and viscous tractions due to stretching lamellar liquid determine the individual film tensions. The network motion is described by a system of nonlinear ordinary differential equations for which numerical solutions are obtained. Coalescense and disproportionation of Plateau borders results in the relative separation of cells and provides a mechanism for yielding and flow. This process is assumed to occur when a film’s length reduces to its thickness. The time and position dependence of the cell‐scale dynamics are computed explicitly. The effective continuum stress of the foam is described by instantaneous and time‐averaged quantities. The capillary number, a dimensionless deformation rate, represents the relative importance of viscous and surface tension effects. The small‐capillary‐number or quasistatic response determines a yield stress. The dependence of the shear and normal stress material functions upon deformation rate, foam structure and physical properties is determined. A plausible mechanism for shear‐induced material failure, which would determine a shear strength, is revealed for large capillary numbers. The mechanism involves large cell distortion and film thinning, which provide favorable conditions for film rupture.

ISSN:0148-6055    

DOI:10.1122/1.549940

出版商:The Society of Rheology    

年代:1987

数据来源: AIP