Index of content:
Volume 13, Issue 1, March 1969
13(1969); http://dx.doi.org/10.1122/1.549158View Description Hide Description
We here explore the possibility that, in viscometry of liquid crystals of nematic type, non‐Newtonian effects stem from a competition between boundaries and flow, each striving to produce a different orientation.
13(1969); http://dx.doi.org/10.1122/1.549155View Description Hide Description
The stress‐optical properties of aluminum dilaurate‐toluene solutions were investigated over a wide range in shear rates,D. Flow curves of 1, 2, and 3 wt‐% of the aluminum soap dispersed in toluene displaced the type of behavior previously reported by Weber and Bauer: i.e., Newtonian behavior at low shear rates, pronounced shear thinning at D greater than and dependence of apparent shear stress, τ, on length‐to‐radius ratio, of the capillary tubes used at high shear rates.Flowbirefringence behavior was similar. For example, at low shear rates the degree of birefringence, increased in proportion to D and the extinction angle, χ, was nearly 45°. Above a critical shear rate, increased overproportionally, finally leveling out at a high value. Simultaneously χ decreased equally rapidly to about 1°. Despite these dramatic changes in dependence of τ, and χ on D, the stress‐optical coefficient remained constant over the entire range of variables. Elastic recoverable shear strains, or simply “recoverable shears,” s, were calculated from the variance of τ with These were found to be in good agreement with data obtained from normal stress measurements in a Weissenberg Rheogoniometer. At low shear stress s increased linearly with τ. Above a critical shear stress the recoverable shear increased rapidly with small increases in τ finally leveling off at a value of approximately 300 shear units and remaining constant with additional increases in shear stress. This behavior is similar to that observed with polyethylene in the region where melt fracture occurs. In these experiments, however, the transition takes place in a rotational instrument under continuous shear in which no “entrance effects” can be postulated as causing melt fracture. During the progress of this investigation it was ascertained that the rotational viscometer could be used to determine elasticity in aluminum soap solutions under dynamic conditions. Quantitative evaluation of the results produced data which correlated with dynamic test data obtained by a variety of techniques. The authors believe that this is a unique and novel method for measuring the dynamic rheological properties of viscoelastic materials.
13(1969); http://dx.doi.org/10.1122/1.549156View Description Hide Description
Measurement of the total normal thrust generated in steady torsional flow between parallel plates enables one to determine the normal stress quantity as a function of shear rate γ̇ for polymer melts. This measurement has been carried out for two linear polyethylene melts of different molecular weight using a modified Maxwell orthogonal rheometer. It is found that the normal stress curves have similar shape but are shifted from one another both horizontally along the γ̇ axis and vertically. A strong correlation exists between and where is the dynamic storage modulus in shear, and the two quantities have identical molecular weight dependencies. The manner of treatment of normal stress data for polymer melts of differing molecular weights is explored, with particular regard to the application of the method of reduced variables. In addition, the predictive ability of the Springs four‐constant equation as regards normal stress behavior of polyethylene melts is discussed.
13(1969); http://dx.doi.org/10.1122/1.549128View Description Hide Description
The description of the flow mechanism of nonlinear viscoelasticity is based on what is known and on what can be measured by experiment. Such materials harden or soften under stress. These processes cannot go on indefinitely, but must result finally in failure. Their deformations are both reversible and irreversible, hence stresses and entropy changes at constant temperature consist also of reversible and irreversible parts. Experiments are generally carried out by applying an external force and measuring the resulting deformation. For the nonlinear case, these quantities are not known a priori in terms of stress and strain components, but the problem can be simplified by treating nonlinear behavior as a special case of linear behavior. It is shown that, where the normal stress‐strain or strain rate relationships are constant for the linear case, they are also constant for the nonlinear case if the strain or stress rate consists of the sum of reversible and irreversible parts. The upper limit of this constant relationship coincides with the stress at which a nonlinear material has its ultimate strength. At greater stresses, the material is in a region of instability. The validity of this approach is demonstrated by data on a bituminous paving mixture (typical of hardening) and on several molten polyethylenes.
13(1969); http://dx.doi.org/10.1122/1.549157View Description Hide Description
In this paper there is introduced a strain energy function which describes a class of materials that includes elastic animal tissue as well as other highly distensible materials. The functional form of this representation is general enough to suggest other forms that may encompass additional classes of material. The theoretical stress‐strain curves are shown to correlate well with experimental data obtained upon loading (as distinguished from unloading) different animal tissues such as frog’s striated muscle, human papillary muscle, and cat’s and rabbit’s mesentery, as well as synthetic materials such as latex rubber. There is also developed a thinness theory which can be used to calculate the deformation field in the case of applied plane stress for the more complicated geometry of a wedge‐shaped (arcuate) specimen. This thinness theory has been applied to a fan‐shaped specimen with tangential and radial loading.
13(1969); http://dx.doi.org/10.1122/1.549151View Description Hide Description
A diameter effect is observed in calculating normal stress differences from observation of the expansion of viscoelastic jets. The possibility that the effect is due to an elastic reaction to the deceleration which accompanies expansion of the jet is raised, and a simple analysis shows this may be plausible.
13(1969); http://dx.doi.org/10.1122/1.549152View Description Hide Description
13(1969); http://dx.doi.org/10.1122/1.549153View Description Hide Description
The creep and dynamic mechanical properties are reported for a series of filled and unfilled polythylenes. The fillers (untreated and treated with silanes) were kaolin and wollastonite. The variables studied include filler concentration, silane treatment, water soaking, temperature, and load. Most of the creep data can be explained, and semiquantitatively predicted, in terms of the increase in elastic modulus of the materials as a result of the incorporation of the filler. However, there are some secondary effects due to filler‐polymer interactions which apparently change the properties of the polymer phase and result in a further reduction of creep and an increase in damping. The presence of the filler greatly reduces creep, increases the stiffness, and increases the loss modulus of polyethylenes. Silane treatment generally reduces creep and increases modulus somewhat. An unexpected long‐time aging effect at room temperature was found which makes the polyethylenes more creep resistant. If allowance is made for the aging effect, soaking in water increases the creep of filled polyethylenes.
13(1969); http://dx.doi.org/10.1122/1.549154View Description Hide Description