摘要:

General expressions for the yield stress of a magnetorheological substance are derived in this work from consideration of unsymmetric elastic and magnetic stress states of a continuum. Geometrical models of composites illustrate use of the general expressions to obtain closed‐form analytical formulas exhibiting the dependence of the shear stress on shear strain, magnetic susceptibility and volumetric loading of the composite materials, demagnetization coefficient, and the applied field intensity over the full range of composition and shear strain. The model prediction recovers scaling behavior known from laboratory experiments, and values of shear modulus that compare in order of magnitude and trend with existing values determined from dipole–dipole and finite element analyses. Duality of electro‐ and magnetopolarized systems permits the results to be applied to the polarization component of the electrorheological response.

ISSN:0148-6055    

DOI:10.1122/1.550699

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Numerical simulations have been undertaken for the flow of an IUPAC‐LDPE melt used previously in an international experimental study (Meissner, 1975). The flow geometry corresponds to an axisymmetric 4:1 contraction equipped with capillary dies of different length/radius (L/R) ratios, ranging from orifice dies (L/R=0) to infinitely long ones (L/R=∞). The constitutive equation used is an integral‐type K‐BKZ model with a relaxation spectrum, which fits well experimental data for the shear and elongational viscosities and the normal stresses as measured in shear flow. The simulations have been performed for the full range of experimental measurements in this system, where the apparent shear rates reach 10 s−1at 150 °C, but the viscoelastic character of the melt is very strong corresponding to a stress ratio of about 2 and a Trouton ratio of about 50. Stable solutions have been obtained for the whole range of experimental values. They show a dramatic vortex growth in the contraction with increasing flow rate, as expected for LDPE melts. New results concerning extrudate swell from orifice dies (L/R=0) are obtained and are in excellent agreement with the experimental data. Results for different ratiosL/R≥4 corroborate previous findings by Luo and Tanner (1988), who predicted a decrease in swelling with increasingL/Rvalues, also observed experimentally. The present simulations are also in good agreement with the experiments for the low range ofL/Rratios (0≤L/R≤4), which was not the case in the previous simulations. The pressures from the simulations have been used to compute the excess pressure losses in the system (end or Bagley correction). The results are also in good agreement with the experimental values at least in a qualitative sense.

ISSN:0148-6055    

DOI:10.1122/1.550700

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The effect of time‐varying electric fields on the stresses in model and commercial electrorheological (ER) fluids in steady shear were surveyed. The dependence of the time‐averaged shear stress with the frequency of an applied alternating (ac) square‐wave electric field reveals two classes of fluids: those that exhibit substantial ER shear stresses with static (dc) and low frequency ac fields, and those that perform poorly in dc fields but show substantial ER activity in ac fields. These contrasting behaviors are consistent with the existence of two different means by which ER activity is generally produced: mismatches of the conductivities and dielectric constants, respectively, between the suspended particles and suspending fluids. Sudden changes in the applied electric field were found to induce a variety of transient stress phenomena; while the detailed response of the disparate materials studied did differ, many similarities were observed. The characteristic times for the growth and decay of the stress were found to depend inversely on the shear rate; they varied only weakly with the amplitude of the applied field. Rise and decay times as short as a few ms were observed at the highest shear rates utilized. The inverse relationship between the characteristic rheological response times and the shear rate is consistent with the necessary roles of both field‐induced structure formation and shear‐induced strain growth in producing an ER shear stress. Implications of this work include that high shear rates must be designed into ER devices in order to achieve short response times.

ISSN:0148-6055    

DOI:10.1122/1.550682

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Complex materials such as liquid crystals present complicated rheological responses when they are subjected to shear flows [Oswald and Allain (1988); Matsumotoetal. (1989); Paaschetal. (1989)]. In particular, AOT–water lamellar liquid crystals behave as weak gels when they are subjected to oscillatory shear [Robles‐Vázquezetal. (1993),(1994)] but when the applied stress surpasses the yield stress, they exhibit viscoelasticity [Soltero (1994)]. Also, for AOT–water biphasic dispersions at concentrations below the phase inversion concentration (≊8 wt % AOT) [Alexopoulosetal. (1989)], the formation of shear‐induced microstructures at high shear rates causes shear thickening [Valdésetal. (1993)]. Upon decreasing shear rates, the dispersion still preserves much of its structure and consequently its viscosity remains higher than the values measured in the increasing shear rate mode. This behavior is referred as rheopexy or antithixotropy [Mewis (1979)]. Here we examine in detail the rheological response of lamellar liquid crystals under shear flows using a controlled‐stress rheometer. We consider two double tail surfactants, one anionic (AOT) and another cationic (DDAB), whose phase diagrams have been studied extensively [Fontell (1973); Franses and Hart (1983); Fontelletal. (1986)]. Particular attention is given to the hysteresis detected when the samples are sheared under increasing–decreasing shear stress modes which result in thixotropic and antithixotropic behavior.

ISSN:0148-6055    

DOI:10.1122/1.550683

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550684

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550685

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550686

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.4765670

出版商:The Society of Rheology    

年代:1995

数据来源: AIP