Creep apparatus

The data for the samples outside of the radiation field were obtained from the same experimental creep apparatus used to measure creep rates during irradiation. The water-cooling system was operated in the same manner as during radiation tests. 

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 ... 

The critical stress can now be seen to be essentially an experimental limitation. The smallest strain reading on our present creep apparatus is 5x 10 . The anticipated plateau strain rate of 10" s for a typical sample with a critical stress of 0.2 GPa would therefore only produce a measurable creep response after 500 days, which is on the limit of the time scale of present creep tests at Leeds University. 

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... 

The basics of a creep apparatus are simple enough a means of applying a load and a means of monitoring the deformation with time. In practice, considerable care has to be taken to design an apparatus that gives the necessary accuracy and stability over long... 

The following constraints are therefore relevant to the design of tensile creep apparatus if it is to be versatile in application to a wide range of materials and orientations ... 

Buckley and McCrum have recently published work on the anisotropy of creep obtained using a tensile creep apparatus based on precision measurement of clamp displacement. Creep strains up to a maximum of 0 1% were used. The creep compliance was subject to error limits of 5% but the scatter of points on a given creep curve was always less than 0-5% provided creep was terminated after 60 s below room temperature or after 180 s above room temperature. Details of the apparatus have not yet been published. 

Reference has been made earlier to the well known creep apparatus developed by Turner and used in many laboratories for accurate creep... 

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. 

The stiffness C22 normal to the plane of the sheet has been determined by compressing narrow strips in a compressional creep apparatus [6]. A lever device is used to determine the deformation, and precision can be improved by compressing a sandwich of sheets separated by effectively rigid spacers. Wilson et al. [6] found that for sheets of polyethylene terephthalate frictional constraints prevented strain in the c or z directions. In this case e = 6 = 0 and Oy = 22, or... 

The stiffness normal to the plane of the sheet C22 has been determined by measuring the compressional strain of narrow strips under load in a compressional creep apparatus [7], The load is applied to the compression cage A (Figure 8.4) via two level arms pivoted about a common fulcrum B. The load is placed on the weight pan at the end of the larger arm C, and supported by the rod D. This rod is held in position by an electromagnet E, and until released, prevents the load from being applied to the samples. 

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... 

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

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