摘要:

Results of macroscopic experiments on the inelastic deformation behavior of solid polymers are used to establish general properties. They include nonlinear rate sensitivity in the inelastic range in monotonic loading and the transition from primary, to primary and secondary, and to primary and secondary and tertiary creep as the creep stress level increases beyond the quasilinear region. Consistent with the notion of a solid it is presumed that a nonzero stress can be sustained at rest. The rest stresses at which relaxation tests terminate form the relaxation boundary. It is suggested that this relaxation boundary has the appearance of a nonlinear stress–strain curve. These properties are used as criteria in evaluating some types of “clock” models where ordinary time in the kernels of linear viscoelastic integral representations is replaced by a transformed time. It is a function of either the invariants of strain or the invariants of stress. It is shown that these models cannot reproduce the change in the creep properties as stress level increases. Creep always terminates. For large times asymptotic solutions hold linear rate sensitivity and a linear relaxation boundary are predicted. Nonlinear rate sensitivity can be modeled before the asymptotic solution is reached.

ISSN:0148-6055    

DOI:10.1122/1.550914

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

We describe six different tests used to obtain a consistent set of viscoelastic properties for yogurt. Prior to yield, the shear modulus μ and viscosity η are measured nondestructively using the speed and damping of elastic waves. Although new to foodstuffs, this technique has been applied to diverse materials from metals to the earth’s crust. The resultant shear modulus agrees withμ∼E/3for incompressible materials, where the Young’s modulusEis obtained from a stress–strain curve in compression. The tensile yield stressτois measured in compression and tension, with good agreement. The conventional vane and cone/plate rheometers measured a shear stress yieldτos∼τo/3,as expected theoretically, but the inferred “apparent” viscosity from the cone/plate rheometer is much larger than the wave measurement due to the finite yield(τos≠0).Finally, we inverted an open container of yogurt for106 s≫η/μand observed no motion. This demonstrates unequivocally that yogurt possesses a finite yield stress rather than a large viscosity. We present a constitutive model with a pre-yield viscosity to describe the damping of the elastic waves and use a simulation code to describe yielding in complex geometry.

ISSN:0148-6055    

DOI:10.1122/1.550915

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

A Stokesian dynamics model is presented to predict suspension viscosity and microstructure for concentrated suspensions. The influence of electrostatic repulsive forces, London–van der Waals attractive forces, and boundary interactions on the degree of shear-thinning is reported. Three different mechanisms for shear-thinning are described and quantified. The degree of shear-thinning caused by Brownian motion is well predicted by using random and layered structures. Shear-thinning for repulsive forces between particles is caused by the “melting” or breakup of an ordered or semicrystalline configuration as the shear rate increases. For suspensions flocculated into a secondary minimum, shear-thinning is produced by breakup of particle aggregates as the hydrodynamic forces dominate the interparticle forces. At high concentrations, the dispersed and flocculated suspensions can form slip planes that reduce suspension viscosity. Rough walls increase the viscosity predictions by an order of magnitude and cause the shear thinning prediction to compare well with experimental results from the literature.

ISSN:0148-6055    

DOI:10.1122/1.550910

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The dynamic viscoelastic behavior of suspensions with bimodal particle radius distributions was examined varying the ratio of the radii of large and small particles(rL/rS)up to 3.3. Monodisperse silica spherical particles with radii ranging from 65 to 215 nm were used. The medium was ethyleneglycol, possessing a refractive index very close to that of the used silica particles. The medium reduced effectively interparticle potentials due to dispersion forces, and it provided bimodal suspensions with only the hard core interparticle potential. The total weight fraction of the suspended particles was kept at 52 wt %, while the weight composition of the small particles(XS)to the total particulate mass was increased from 0 to 1 with a 0.2 interval. The bimodal suspensions showed viscoelastic behavior with a frequency dependence similar to that of a unimodal (monodisperse) suspension, which was attributed to the contribution of Brownian motion of suspended particles. The zero-shear viscosities(η0)of the bimodal suspensions showed minima atXSvalues dependent onrL/rS,whereas the high frequency limiting viscosities showed no obvious minima. A simple model is proposed to interpret these viscoelastic features in the bimodal suspensions. In the model, we assumed that in the suspension a Brownian motion of a hypothetical particle with an average radius governed the whole viscoelastic features. Values ofη0predicted by the model and experiments agreed fairly well.

ISSN:0148-6055    

DOI:10.1122/1.550911

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The steady and transient rheological behavior of three mesophase pitches (two of them obtained from catalytic polymerization of naphthalene and methyl naphthalene, respectively, and one from supercritical extraction of petroleum pitch) was investigated. Steady shear flow curves were generated and showed that all pitches exhibited a shear thinning behavior at low rates of shear (region I flow). For higher shear rates, the viscosity became constant (region II flow). Similarly to observations on liquid-crystal polymers (LCPs), this behavior was attributed to the polydomain structure that mesophase pitches exhibit. Reflective polarized light microscopy on quenched samples showed that the mesophase pitches’ domain size decreased with increasing rates of shear. This domain shrinkage was successfully predicted using a model developed for LCPs [Reference 1984]. The transient rheological behavior associated with inception and cessation of steady shear flow also was studied for the three mesophase pitches. The results showed that, upon start-up of flow, the shear stress exhibited an overshoot, and its magnitude greatly depended on the shear history of the mesophase pitches. This behavior, already reported for LCPs, is probably a result of the polydomain structure of mesophase pitches.

ISSN:0148-6055    

DOI:10.1122/1.550912

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The interfacial characteristics of a variety of polymer/wall interfaces were measured by using the sessile drop method in order to calculate the work of adhesion. Polymers included linear low-density as well high-density polyethylenes, while wall substrates included clean stainless steel and modified stainless steel by applying two different fluoropolymers in order to alter its surface energy. A linear correlation is found between the critical shear stress for the onset of slip and the work of adhesion of the corresponding polymer/wall interface, in agreement with earlier publications of Hill et al. (1991) and Hatzikiriakos et al. (1993). In the present work, the experimental results are interpreted in terms of parameters defined by these two theories. It is suggested that small deviations from the no-slip boundary condition in the case of polymer melt flow are due to a stress-induced chain detachment/desorption of polymer chains from the wall.

ISSN:0148-6055    

DOI:10.1122/1.550909

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

A comprehensive study of sharkskin behavior in linear polyethylene extrusion is carried out to explore its molecular origin. Experimental characteristics are analyzed as a function of temperature, applied stress, and die surface condition. The experimental data favor an interfacial molecular instability (IMI) mechanism for sharkskin formation over a noninterfacial continuum mechanical mechanism. The effect of a local cooling of the die exit is demonstrated to be predictable by the proposed IMI mechanism. The IMI mechanism states that sharkskin occurs because of a local conformational transition at the die exit wall where the adsorbed chains entrap a layer of interfacial chains. This layer oscillates between entanglement and disentanglement states due to a reversible coil↔stretch transition. The corresponding oscillation of the exit wall boundary condition leads to cycles of local stress relaxation and growth and to periodic perturbation of the extrudate swell in the form of sharkskinlike surface roughening on the extrudate.

ISSN:0148-6055    

DOI:10.1122/1.550902

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

A number of laboratories are currently using sliding plate rheometers equipped with shear stress transducers to study the nonlinear viscoelasticity and slip of molten polymers. In making such measurements, it is sometimes essential to know to what degree the dynamic response of the shear stress transducer itself is influencing the output signal. Experiments should be designed to minimize this effect, but some attenuation and phase shift is inevitable because of the presence of polymer inside the transducer. We have measured the dynamic response of a shear stress transducerin situin a sliding plate rheometer for two molten polyethylenes. We have also developed a model for the transducer response. In oscillatory shear experiments above a frequency of 1 Hz, the amplitude ratio and phase lead are significant for both materials studied.

ISSN:0148-6055    

DOI:10.1122/1.550903

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

In this paper we calculate numerically the shear induced distortion of the equilibrium microstructure in a dilute colloidal suspension. In the low density limit, our predictions both with and without hydrodynamic interactions follow the form of the Ree–Eyring equation for the shear rate dependence of the rheology. For low to intermediate shear rates, rescaling the low density solutions by appropriate volume fraction dependent scale factors captures many of the qualitative features observed in simulation and experiment on systems at high concentration, including shear thinning and normal stresses. At high shear rates, more detailed consideration of many-particle interactions are required to account for the observed phenomenology and the use of the mean-field approximation inherent in the rescalings fails. Comparison of systems with and without hydrodynamic interactions suggests that both hydrodynamic and thermodynamic forces act to increase the stress relaxation time in concentrated suspensions.

ISSN:0148-6055    

DOI:10.1122/1.550904

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Data are presented to confirm that a simple analysis based upon the Bingham model correlates slump data for a wide variety of materials, including concrete, silt, and mayonnaise.

ISSN:0148-6055    

DOI:10.1122/1.550905

出版商:The Society of Rheology    

年代:1998

数据来源: AIP