Torsion creep apparatus

Two types of measurements were made on these samples. In the region where moduli are higher than 109 dynes/sq. cm., a Clash-Berg torsional creep apparatus (7) was used. For moduli below 109 dynes/sq. cm., a modified Gehman apparatus (14) was employed. In both cases shear creep compliance, Je(t), was obtained. To convert this to relaxation modulus, Gr(t), the following equation was used ... 

Measurements of creep in torsion can be made very accurately. The reason is that deformation can be measured by measuring the large deflections of a light beam. A convenient way to simultaneously obtain shear dynamic and transient data is to combine both types of measurements in the same equipment (4). Usually this requires only small modifications of the experimental device. For example, the cross bar in a torsion pendulum can be removed and replaced by weights and pulleys to apply a constant torque to the upper clamp. In this way, a torsion creep apparatus is obtained (Fig. 7.8). The... 

Simpler torsional creep apparatus used in studies on oriented polymers has been described by Raumann and by Ladizesky and Ward. ... 

The time dependence of the shear compliance 544(f) was studied over a range of draw ratios in cold-drawn LDPE by Qayton using torsional creep apparatus. In general, good agreement was obtained between this directly measured shear compliance and that calculated by inserting the tensile creep data above into eqn. (8) (see Qayton et al ). It should be mentioned that the measurement of S44. for a material with fibre symmetry is one of the cases where the difficulties mentioned in Section 10.3.2 are least severe, since torsion occurs about an axis which is perpendicular to a plane of isotropy in the specimen. 

A detailed examination of the problems of direct measurement of 544(f) and 566(f) in highly cold-drawn LDPE with fibre symmetry was carried out by Ladizesky and Ward using torsional creep apparatus. They found only a small dependence of 544(t) upon time at high draw ratio, in agreement with the above mentioned studies on tensile and torsional creep. The direct determination of 566(fX by experiments in torsion was, however, shown to require a complicated double extrapolation procedure reasonable agreement then being obtained between the measured values and those derived from the measurements of 5t 1 and 5i2 during tensile creep. 

Torsional creep apparatus used by Kolrausch (1863) to study viscoelasticity in glass fibers and rubber threads. 

There are a large number of designs for all types of torsional apparatus, mostly home made. In apparatus for torsional creep the measurement of displacement may be made by a variety of methods e.g. optical lever and spot-following recorder, or electrical sensing. The application of the torque may be made by means of weights or electrically. 

Generally, the operative frequency range for the torsional pendulum method is 0.01 to 50 Hz, the upper limit of the frequency defined by the dimensions of the oscillatory frequency relative to the dimensions of the sample. At higher investigative temperatures, the polymeric materials may undergo extensional deformation (creep) due to the weight of the inertia bar. Under such circumstances, a modified torsion pendulum apparatus may be used in which an inertia disk is attached directly onto the end of the sample (12). In addition, this method is frequently employed to measure the torsion modulus at low frequencies. 

Collected results for the nine compliance constants are shown in Table 8.9. The 3 axis is the initial draw direction and the 1 axis lies in the plane of the sheet, following the convention indicated in Figure 8.2. in and 533 were obtained from measurements of extensional creep in a dead loading creep machine and refer to the 10s response at 0.1% strain, was obtained from the deformation of an electron microscope grid printed on the surface of the sample [97], and 12 and S23 by the Michelson interferometer method [12]. S22 was determined by increasing the compressive strain of strips under load in a compressional creep apparatus [7]. S55 was determined by the torsion of rectangular samples cut with their long axes parallel to 3 and 1 respectively [18], 44 and see were also determined in this way... 

Thus 8.2 x 103 kg (roughly 9 tons) placed on the sample pan in Figure 2-9 would cause the pointer to move down 0.40 cm in KT4 s. Clearly all the apparatus is assumed to have an infinite modulus and inertial effects are ignored. Clearly, this is not the preferred way to run a shear creep experiment on such a polymer. Torsional deformation of a rod would be a much better choice. 

Numerous less elaborate devices with parallel plate or cone and plate geometry have been described in the literature. " Application of torque by a torsion wire from which the rotating element of the apparatus is suspended can be quite satisfactory for creep if the torsional stiffness of the wire is small and the top is turned through a large angle then the torque remains essentially constant despite the small angular displacement at the bottom of the wire. It is difficult to perform creep recovery experiments with a torsion wire, but these can be accomplished if the... 

An example of a free oscillation device whose deformation geometry is torsion of a circular cylinder is shown in Fig. 6-7. The apparatus of Plazek, Vrancken, and Berge mentioned above under creep is equipped here with a moment arm to provide an adjustable moment of inertia. The oscillations are followed by reflection of a light beam from a mirror attached to the rotating unit onto a photocell, whose output is automatically recorded. From the trace (Fig. 6-8), the characteristic frequency cOc and the decrement A can be obtained it is convenient to match with an adjustable enlarger to standard exponential envelopes. The decrement is proportional to the slope of a plot of the logarithm of the maximum amplitude... 

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