Compression tests

Compression tests, in which agglomerates ate cmshed between parallel platens, ate probably most universal. To obtain reproducible and accurate results, the rate of loading and method of load appHcation must be strictly controUed. A variety of commercial testers ate available to allow this needed control over the compression process. Several means of distributing the load uniformly at the point of contact ate used, including covering the platen... 

Plastic deformation is commonly measured by measuring the strain as a function of time at a constant load and temperature. The data is usually plotted as strain versus time. Deformation strain can be measured under many possible loading configurations. Because of problems associated with the preparation and gripping of tensile specimens, plastic deformation data are often collected using bend and compression tests. 

Deformation Under Loa.d. The mechanical behavior of coal is strongly affected by the presence of cracks, as shown by the lack of proportionahty between stress and strain in compression tests or between strength and rank. However, tests in triaxial compression indicate that as the confirming pressure is increased different coals tend to exhibit similar values of compressive strength perpendicular to the directions of these confining pressures. Except for anthracites, different coals exhibit small amounts of recoverable and irrecoverable strain underload. 

Adiabatic compression test. High pressure is applied r idly to a liquid in a U-shaped metal tube. Bubbles of hot compressea gas are... 

Most ceramics have enormous yield stresses. In a tensile test, at room temperature, ceramics almost all fracture long before they yield this is because their fracture toughness, which we will discuss later, is very low. Because of this, you cannot measure the yield strength of a ceramic by using a tensile test. Instead, you have to use a test which somehow suppresses fracture a compression test, for instance. The best and easiest is the hardness test the data shown here are obtained from hardness tests, which we shall discuss in a moment. 

Fig. 17.2. Tests which measure the fracture strengths of ceramics, (a) The tensile test measures the tensile strength, CTj. (b) The bend test measures the modulus of rupture, o , typically 1.7 x CTj. (<) The compression test measures the crushing strength, a, typically 15 x...

The third test shown in Fig. 17.2 is the compression test. For metals (or any plastic solid) the strength measured in compression is the same as that measured in tension. But for brittle solids this is not so for these, the compressive strength is roughly 15 times larger, with... 

Under compression loading, the long flexible tension specimens would simply buckle. Thus, lateral support to prevent buckling is necessary as shown in the compression test fixture with side-support plates in Figure 2-24. There, the specimen is essentially as long as the fixture is tall, and only a small portion of the specimen can be seen where it is not supported. 

Pipes and Cole [2-25] measured the interaction term F,2 in various off-axis tests for boron-epoxy. They reported significant variation of F,2 for off-axis tension tests and acceptable variation for off-axis compression tests. However, compression tests are much more difficult to perform than simple off-axis tension tests on a flat specimen with a high length-to-width ratio. A compression specimen with a high length-to-... 

Fig.4. Sample hot deformed by 25% a) SEM microstructure after compression test at the room temperature and fracture, b) electron microstructure of the y precipitate.

As for the compression test, the lowest stress amounted to 0.314 kp/mm at the lower deformation rate at 503K and to 0.535 kp/mm for the higher rate at the same temperature. For the lower deformation temperatures the values of the flow stress increased sharply and at 373K reached 2.3 and 4.5 kp/mm" for the lower and higher deformation rates, respectively At room temperature they were 9.0 kp/mm and 11.28 kp/mm . 

Fig.4. The dependence of flow stress on deformation temperature in tension test (left) and in compression test (right) of the A12n78 alloy after heat treatment.

Confined compression tests are used to determine information pertaining to the behavior of foundations where large volume changes of soil can occur under compression but in the vertical dimension only. 

Unconfined compression tests are used to estimate the shearing strength of cohesive soils. 

Triaxial compression tests are another means of determining shearing strength of a soil. A complex device is used to apply pressure along the sides of a cylindrical specimen and axially down the axis of the cylindrical specimen. In general, triaxial tests are superior to direct shear tests since there is better control over intake and discharge of water from the specimen. 

In the compression test, four or five sample cubes of the slurry are allowed to cure for a specified period of time. The cement cubes are placed in a compression testing machine and the compressive strength of each sample cube obtained experimentally. The average value of the samples is obtained and reported as the compressive strength of the set cement. 

Unlike the methods for tensile, flexural, or compressive testing, the typical procedure used for determining shear properties is intended only to determine the shear strength. It is not the shear modulus of a material that will be subjected to the usual type of... 

The compressive data are of limited design value. They can be used for comparative material evaluation and design purposes if the conditions of the test approximate those of the application. The data are of definite value for materials that fail in the compressive test by a shattering fracture. On the other hand, for those that do not fail in this manner, the compressive information is arbitrary and is determined by selecting a point of compressive deformation at which it is considered that a complete failure of the material has taken place. About 10% of deformation are viewed in most cases as maximum. 

The test can provide compressive stress, compressive yield, and modulus. Many plastics do not show a true compressive modulus of elasticity. When loaded in compression, they display a deformation, but show almost no elastic portion on a stress-strain curve those types of materials should be compressed with light loads. The data are derived in the same manner as in the tensile test. Compression test specimen usually requires careful edge loading of the test specimens otherwise the edges tend to flour/spread out resulting in inacturate test result readings (2-19). 

Results of uniaxial strain static and gas gun compression tests on syntactic foam have been conducted. The foam was buoyant and composed of hollow glass microspheres (average diameter 100 microns) embedded in an epoxy plastic. Static testing consists of compressing a 0.25 cm x 2.5 cm dia. wafer between carefully aligned 2.5 cm dia. steel pistons. Lateral expansion of the wafer is... 

Combined liquid crystalline polymers, 49 Combustion testing, 245 Composites, thermoplastic, 32 Compression force deflection (CPD), 244 Compression tests, 242 Condensation... 

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