Volume 31, Issue 1, January 1987
Index of content:
31(1987); http://dx.doi.org/10.1122/1.549946View Description Hide Description
This work presents an experimental investigation on the creep behavior of an epoxy resin with emphasis on characterizing the effects of physical aging, temperature, sorbed moisture, and applied stress. A numerical method was used for determining the stress dependent parameters of Schapery's nonlinear creep equation. The of the resin investigated here was close to ambient temperature (46°C) so that the creep effects are rather pronounced without having to heat the resin to higher temperatures. The effects of moisture, temperature and of physical aging on the resin are therefore very strong. The effect of physical aging can be clearly seen in the transient compliance while the initial compliance is less affected. It was observed that the creep exponent n seems not only to be dependent on the physical age of the polymer and on the absorbed moisture but also on the applied stress. It was observed that log n and the logarithms of the initial and transient compliances change linearly with log (time). This makes it easy to extend Schapery's equation to include the effects of physical aging.
A Study of Non‐Isothermal Non‐Newtonian Extrudate Swell by a Mixed Boundary Element and Finite Element Method31(1987); http://dx.doi.org/10.1122/1.549915View Description Hide Description
Using a combination of boundary element methods,finite element methods and streamline tracking for stresses, we explore the coupling between temperature‐induced swelling and stress‐induced swelling for materials with temperature‐dependent properties and a single relaxation time. One sees an interactive effect; the temperature‐ and stress‐induced swellings are not simply additive. The change of swelling shape is also noticeable. Calculations are reported for a temperature‐dependent Maxwellmodel up to a Weissenberg number of 1.5, and Weissenberg numbers up to 4.0 with the PTT model; Péclet numbers up to 200 are considered.
31(1987); http://dx.doi.org/10.1122/1.549950View Description Hide Description
Equibiaxial, hydrostaticdeformation of isotactic polypropylene (i‐PP) of an intermediate weight average molecular weight has been achieved by uniaxial compression between two axially aligned circular cylinders at the desired draw temperature Independent measurements of load (i.e., stress) and displacement (i.e., strain) are made during the course of deformation. A systematic variation in (from 30–130°C) at a ram speed of 0.25 in./min. has been studied. The stress‐strain data corrected for machine compliance has been compared to a theoretical model. This model assumes a rigid‐plastic behavior with a hydrostatic pressure effect. The shear stress at the wall, caused by metal‐polymer contact, is coulombic friction. As the compressive force (and, hence the hydrostatic pressure) is increased, the frictional shear stress (in the central portion of the compressed sample) exceeds the maximum shear stress (also changing with hydrostatic pressure), causing the i‐PP to yield on the surface (referred to as “stiction”). The friction to “stiction” transition is continuous on the stress‐strain curve, but is explicitly observed as a “skin” under an optical microscope with crossed polars. The model quantifies the onset of “stiction” and gives a threshold friction coefficient (depending on material properties) below which the polymer will not shear at the wall! The static friction coefficient between metal and polymer is the only empirical parameter to fit the stress‐strain data (within 5%) up to compression A quantitative measurement of the Bauschinger effect is obtained by simultaneous measurement of yield stress under compression and tension in a single experiment.
Anisotropic Particle Distribution in Dilute Suspensions of Solid Spheres in Cylindrical Couette Flow31(1987); http://dx.doi.org/10.1122/1.549916View Description Hide Description
The pair distribution function of the particles in a noncolloidal suspension was for the first time measured experimentally. Evidence of anisotropy in dilute suspensions of solid spheres is presented. Suspensions of polystyrene spheres in silicone oils with volume fractions were subjected to cylindrical Couette flow in a Weissenberg Rheogoniometer. The experimental conditions were such that the Reynolds number based on the particle radius was very small, and the particle Péclet number very large, so that inertial effects and Brownian motion were negligible relative to viscous effects. Suspended spheres on a plane perpendicular to the axis of rotation were observed using a video camera interfaced with a VICOM image processing computer. Using automatic data gathering and processing techniques specially developed for this purpose, an analysis of the relative positions of the spheres showed that (under shear) the spheres were not uniformly distributed in the suspension, but rather were arranged anisotropically. Specifically, the probability of finding a sphere at a given location relative to a reference particle was found to be a maximum when the spheres were nearly touching and the line joining their centers was parallel to the direction of undisturbed flow. The experimental phenomena observed are consistent with the nature of two‐body hydrodynamic interactions between spheres.