Creep measurements compressive

As with other plastics materials, temperature has a considerable effect on mechanical properties. This is clearly illustrated in Figure 13.5 in the case of stress to break and elongation at break. Even at 20°C unfilled PTFE has a measurable creep with compression loads as low as 3001bf/in (2.1 MPa). 

Foamed blends of ethylene-styrene interpolymer and LDPE were subjected to a range of mechanical tests, including compressive impact testing, Instron compression and Poisson s ratio measurements, compressive creep measurements and compression set and recovery measurements. The data obtained were compared with those for EVA and the suitability of these foamed blends as replacements for EVA in the manufacture of soccer shin guards and midsoles for sports shoes was evaluated. 20 refs. 

Wang and Campbell studied the stress relaxation curves for their samples by applying a 25 % constant strain and measuring the normalized stress relaxation for 30 s. They also performed creep measurement. The initial load was applied by compressing the samples at 4 mm/s ( 33 %/s) up to 25 % strain or when it reached 223 N and holding the force for 30 s [54]. 

Creep measurements bring in the dimension of time by characterizing the extension with time of a test specimen subjected to a constant load. Loads are applied in tension, fiexure, or compression. Classical creep is extremely time consuming because it involves subjecting the test specimens to the desired load for the period of time of interest, which can be several months or even years. Consequently, creep data are not common. Sometimes, it is possible to apply time-temperature superposition to generate a master curve... 

The original height of the two layers of cheese is recorded. Then the plate and weight are placed on top of the cheese. Its new height is recorded immediately and each succeeding 5 minutes thereafter. This experiment measures creep in compression. 

Reciangular column, 0.190 in. X 0.190 in. in cross section and 2 in. high, was loaded in column compresion after insertion in a closely filling (to the comers of the column) cylindrical steel tube which prevented buckling. See A Method for Measuring Compressive Creep of Thermoplastic Materials , by E. D. Jones, G. P. Koo, and J. L. OToole, MaUnals Research and Slandards. 6,(5 May 1966). 

Generally speaking, creep may be measured at tension, flexure, and compression. Compression of cylindrical samples is the easiest way to estimate creep. Creep has three stages (Fig. 1.16) first, nonpermanent second, permanent and third, nonpermanent (breakage). The typical time for creep measurement is 4 h, but within 4 h the permanent stage of deformation may not be achieved. So the time of creep testing may vary from 4-10 h, but sometimes it may be extended up to 50 h. 

The time-dependent reduction in thickness at a given load may be measured in a way similar to tension creep and extrapolated to longer times. Testing of the core after lamination with geotextiles or other adjacent materials facihtates the measurement of increasing time-dependent intrusion. In a typical compression creep measurement, a square of drainage product is placed between two flat plates and a load is applied. Compression is measured by sensitive extensometers attached to the plates. The rate of creep reduces with time and is generally plotted as percent compression or retained thickness versus log time, as shown in Fig. 9.15. 

Abstract Polymeric solids have tensile creep compliance, compression creep compliance, flexural creep compliance, and tangential or incremental tensile and compressive compliance. While these compliance values would all be numerically the same in a given metal, th will all be numerically distinct in polymeric solids. This paper investigates vriiy these compliance values vary in polymeric solids and presents experimental data to indicate the magnitude of this variation between several of these compliance values in iso-polyesters and vinylesters. The tensile creep compliance is found to be 15% greater than the flexural creep compliance in vinylesters while the incremental compliance at 3700 hours in a creep test is found to be approximately 25% less than the initial compliance on loading for iso-polyesters. The measured tensile and flexural creep compliances may be used to calculate the compressive creep compliance. 

Figure 2 Collection of test pieces made of unshaped refractories. Description from left to right (Top) Two bricks for measuring thermal conductivity (230 x 114 x 76 mm) Shape A according to EN cup for slag tests (100 x 100 x 100 mm, hole 50 mm in diameter and depth). (Middle) Test piece for measuring abrasion resistance according to ASTM C 704 (114 x 114 x 40 mm) test piece according to special specifications for petrochemical industry (230 x 50 x 50 mm) drilled 50 mm cylinders out of a shape B (one with 12.5 mm hole for determination of Refractoriness Under Load or Creep Under Compression). (Bottom) Shape B according to EN 3 cubes for petrochemical specifications (50 x 50 x 50 mm) Shape C according to EN.

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

Creep tests require careful temperature control. Typically, a specimen is loaded in tension or compression, usually at constant load, inside a furnace which is maintained at a constant temperature, T. The extension is measured as a function of time. Figure 17.4 shows a typical set of results from such a test. Metals, polymers and ceramics all show creep curves of this general shape. 

This test on rigid plastics indicates their ability to withstand continuous short-term compression without yielding and loosening when fastened as in insulators or other assemblies by bolts, rivets, etc. It does not indicate the creep resistance of a particular plastic for long periods of time. It is also a measure of rigidity at service temperatures and can be used as identification for procurement. Data should indicate stress level and the temperature of the test. 

Some viscoelasticity results have been reported for bimodal PDMS [120], using a Rheovibron (an instrument for measuring the dynamic tensile moduli of polymers). Also, measurements have been made on permanent set for PDMS networks in compressive cyclic deformations [121]. There appeared to be less permanent set or "creep" in the case of the bimodal elastomers. This is consistent in a general way with some early results for polyurethane elastomers [122], Specifically, cyclic elongation measurements on unimodal and bimodal networks indicated that the bimodal ones survived many more cycles before the occurrence of fatigue failure. The number of cycles to failure was found to be approximately an order of magnitude higher for the bimodal networks, at the same modulus at 10% deformation [5] ... 

Although creep, stress relaxation, and constant-rate tests are most often measured in tension, they can be measured in shear (19-22), compression (23,24), flexure (19), or under biaxial conditions. The latter can be applied... 

---