摘要:

The flow of RX-08-FK, a mostly organic, high-energy, paste-extrudable explosive containing about 75% suspended solids, was experimentally studied. The paste was forced through long capillary tubes in a specially designed, double-piston rheometer at temperatures ranging from −54 to +74 °C, and at nominal shear rates at the tube walls from 50 to 40 000s−1.Paste was preconditioned for many hours to achieve the desired experimental temperature and the rheometer itself was enclosed in a preconditioned temperature-controlled chamber. We found the paste to be pseudoplastic with significant yield stress and entrance effects. The results of 176 runs at temperatures of −35 °C and higher were fitted by an empirical model much like theoriginalmodel suggested by Herschel and Bulkley, modified to include a factor correcting the stress for the substantial entrance loss experienced with this paste. The observed stresses at −54 °C were substantially higher and less shear dependent than those extrapolated from the above model; these differences may relate to the fact that the glass-transition temperature of RX-08-FK is approximately −60 °C.

ISSN:0148-6055    

DOI:10.1122/1.550866

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The rheology of a discontinuous fiber filled polypropylene in squeeze flow between parallel plates is studied. The material has an initial anisotropic fiber orientation distribution and therefore displays a strongly anisotropic in-plane flow behavior with predominant flow transverse to the axis of principal orientation. The kinematic field is computed using a linear, orthotropic constitutive model, where the fibers are assumed to move affinely with the surrounding fluid. The fiber orientation distribution is updated in each timestep thus coupling orientation and flow. Two different orientation descriptions are evaluated: orientation tensors with closure approximations, and a technique based on direct solution of the orientation of a set of test fibers. The two methods are first compared to exact solutions of the orientation distribution function in simple shear and pure extension; the direct solution is exact within numerical error, whereas the methods based on orientation tensors and quadratic and hybrid closure fail to correctly describe any transient fiber orientation evolution. Finally, the orientation representations are implemented in the kinematic model and compared to the experimental data; the direct solution method is found to give a very accurate prediction of the observed flow kinematics, whereas the other techniques result in substantial errors.

ISSN:0148-6055    

DOI:10.1122/1.550833

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

We develop a Rayleigh-type approach to the analysis of compressible granular flow. In this approach, a (possibly negative) influx of energy from the walls is taken into account. This is greatly advantageous in granular flow rheology, since such influx of energy arises spontaneously as soon as the balance between the energy influx (due to whatever mechanism keeps the unperturbed system in a thermalized state) and the dissipation rate is violated. We show that standing waves, although waves of a rather peculiar type, may well exist, and that such waves have thicknesses of several hundreds particle diameters. We also give an analysis of the stability of compressible granular flow, which exhibits some peculiar characteristics.

ISSN:0148-6055    

DOI:10.1122/1.550834

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Nanometric silica particles, dispersed in water, have been bridged into long necklaces by adsorbing macromolecules of poly(ethylene oxide). At high coverage of particles by macromolecules the necklaces repel each other and the dispersions are homogeneous solutions; at low coverage the necklaces bind to each other and a concentrated phase separates from excess water. It is reported here that shear induced gelation and shear induced flocculation are observed near the boundary of the phase separation region. The structures of these dispersions under shear have been observed through small angle neutron scattering. It has been found that, above a critical shear rate, the necklaces connect to each other to form threadlike objects which align along the velocity. At higher shear these objects associate sideways to form three-dimensional flocs.

ISSN:0148-6055    

DOI:10.1122/1.550874

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Fibers of linear polymers below the glass transition show a strong nonlinear deformation behavior even at small strains. The elastic modulus is a fast increasing function of the strain. A new model for the description of the deformation of these fibers is presented. It is assumed that the fiber consists of long and continuous polymer chains which do not break during the extension of the fiber. The deformation of the fiber is calculated for finite strains and arbitrary values of the orientation parameter. Equations for the elastic stress versus strain curve and modulus versus strain curve have been derived. The model can be used for the calculation of the modulus of isotropic fibers and sheets with uniplanar orientation as well.

ISSN:0148-6055    

DOI:10.1122/1.550859

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The results of the continuous chain model for the elastic deformation of polymer fibers in the glassy state, which has been presented in the previous article, are compared to the results from experiments on poly(p-phenylene terephthalamide) (PpPTA), poly(ethylene terephthalate) (PET), cellulose, and PE fibers, and PpPTA and PET sheets. The calculated elastic stress versus strain curve is compared to the experimental curve of PpPTA fibers. The predicted relation between the elastic modulus versus strain curve and the orientation of the fiber during extension are compared to the results obtained by x-ray diffraction. The effect of plastic deformation on the experimental modulus versus strain curve is considered, and a new method for the experimental determination of the shear modulusgfrom the elastic modulus versus strain curve is presented. The calculated elastic modulus versus strain curves are compared to the experimental curves of PpPTA, PET, cellulose, and PE fibers with widely divergent values for their orientation parameter. The shear moduli for these fibers have been determined and are compared with literature values, the relation between the experimentally determined shear modulus and the structure of the fiber is discussed. The shear moduli of a series of PET fibers are compared to the values obtained by birefringence. The modulus of PpPTA and PET sheets are compared to the calculated values. Good agreement between the experimental and calculated results has been obtained.

ISSN:0148-6055    

DOI:10.1122/1.550860

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

We have characterized various steady and time-dependent material functions of suspensions of a non-Newtonian binder, poly(dimethyl siloxane), incorporated with 10%–60% by volume of hollow and spherical glass beads. The material functions included storage and the loss moduli, shear stress and first normal stress difference growth and relaxation, relaxation modulus upon step strain and creep and recovery behavior. Both constant shear stress and shear rate experiments were carried out using multiple rheometers over a broad temperature range (−35to 40 °C) while following sample fracture and wall slip effects. With increasing volume fraction, φ, of the noncolloidal particles, the strain range, over which linear viscoelastic behavior is observed, became narrower and the relaxation time of the suspension increased. Increasing solid content gave rise to the development of the yield stress and the dependence of large amplitude oscillatory shear properties on time and deformation history. The yield stress values increased with φ, but were not sensitive to temperature. Forφ⩾0.3the first normal stress difference values reached reproducible and steady negative values (with tensile force positive). Larger negative values of the first normal stress difference were observed with increasing deformation rate and solid concentration. The manifestations of the material functions resulting from the incorporation of the solids into the non-Newtonian binder included the suppression of the extrudate swell and the dipping of the free surface in the Weissenberg experiment.

ISSN:0148-6055    

DOI:10.1122/1.550841

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

This study focuses on the demixing of neutrally buoyant suspensions of spheres during slow, pressure driven flows in circular conduits. Distributions of the solid fraction of particles, φ, and the suspension velocity, ν, are measured at different lengths from a static in-line mixer. Experiments were conducted over a range of volume average solids fractions,φbulk (0.10⩽φ⩽0.50),and at two different ratios of the particle radius,a,to the radius of the circular conduit,R(a/R=0.0256anda/R=0.0625). Atφbulk⩾0.20,the particles rapidly migrate to the low-shear-rate region in the center of the conduit. This migration results in a blunting of the ν profile, relative to the parabolic profile observed in homogeneous Newtonian fluids. For the flow geometry with the smaller ratio ofa/R,the φ profile builds to a sharp maximum or cusp in the center. Particle structures are observed in the experiments with the highera/R.The entrance lengths for the development of the φ and ν fields,LφandLν,respectively, are strong functions ofa/Randφbulk.LφandLνrapidly decrease as φ anda/Rincrease. Over the range of our data, the ν profiles are observed to develop more rapidly than the φ profiles. The experimental results are compared with fully developed flow predictions from the shear-induced migration (SIM) model and the suspension balance (SB) model. At the smallera/R,the SIM model more accurately predicts the experimental results. At largera/R,some qualitative features of the experimental results are better predicted by the SB model, however, neither model provides good quantitative predictions, especially at lowφbulk.

ISSN:0148-6055    

DOI:10.1122/1.550863

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

We present a new molecular-level picture of chain dynamics for describing the viscoelasticity of crosslinked polymers. The associated mathematical model consists of a time-dependent momentum balance on a representative polymer segment in the crosslinked network, plus phenomenological expressions for forces acting on the segments. These include a cohesive force that accounts for intermolecular attraction, an entropic force describing the thermodynamics governing chain conformations, and a frictional force that captures the temperature dependence of relative chain motion. We treat the case of oscillatory uniaxial deformation. Solution of the model equations in the frequency domain yields the dynamic moduli as functions of temperature and frequency. The model reproduces all of the qualitative features of experimental dynamic modulus data across the complete spectrum of time and temperature, spanning the glassy zone, the β transition, the dynamic glass transition, and the rubbery zone. All of the model parameters can be evaluated through the use of independent experimental data. Comparison of model predictions with experimental data yields good quantitative agreement outside of the glass transition region.

ISSN:0148-6055    

DOI:10.1122/1.550828

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

When an alternating stack of molten polymer sheets is pulled in uniaxial tension parallel to the layers, interfacial area per unit volume increases, amplifying interfacial effects. Multilayers of several polymer pairs were prepared by lamination (up to 100 layers). Pairs with low and high interfacial tension, Γ, and pairs with functional groups which could react were selected. These were stretched at 220 °C at various constant extension rates in a rotating clamp extensional rheometer. At low rates the extra measured stress could be related to Γ and the number of interfaces. At higher extension rates extra stress exceeded Γ. This method for measuring interfacial tension can be applied to opaque samples and does not require knowing sample density or viscosity. For the reactive pairs there was pronounced strain hardening caused by coupled and even cross linked chains at each interface. The contribution of the cross linked interface was shown to follow rubber elasticity theory. As a result of the reaction the measured stress was two orders of magnitude higher than for the nonreactive case. Extra stress due to the grafting reaction was also measured. It increased with extension rate, presumably due to entanglements of the grafts across the interface. No evidence of graft reaction reducing Γ was found.

ISSN:0148-6055    

DOI:10.1122/1.550829

出版商:The Society of Rheology    

年代:1997

数据来源: AIP