Compression Test of materials

Compression Test
The goal of a compression test is to determine the behavior or response of a material while it experiences a compressive load by measuring fundamental variables, such as, strain, stress, and deformation. Uniaxial compression tests provide much of the same information about material properties as tension tests. By testing a material in compression, the compressive strength, yield strength, ultimate strength, elastic limit, and the elastic modulus all be determined. With the understanding of these different parameters and the values associated with a specific material it may be determined whether or not the material is suited for specific applications or if it will fail under the specified stresses. 
The compression test specimen is comparatively simple in shape, and the length of the test piece should not be too great, because it is necessary to avoid buckling. The compression test specimen either a cylinder with a ratio of length to diameter L/D < 2 to avoid non-axial motion, or the specimen may be in the form of a cube.
Compression tests are used if in-service forces are of this type. Also, they are used when the material is brittle in tension (such as concrete, Gray cast iron), or when a material’s behavior under large and permanent strains is desired, as in metal-forming applications.
Simple tensile testing usually produces sufficient data to determine the mechanical properties of ductile materials. In those materials, the yield limits under tension and compression are generally the same. Therefore, it is not necessary to perform the compression test on highly ductile materials such as mild steel or most Al-alloys. On the other hand, in some materials such as brittle and fibrous ones, the tensile strength is considerably different from compressive strength as seen in Figure 1. Therefore, it is necessary to test them under tension and compression separately.

Figure 1: Compression and Tension stress-strain curves for (a) GCI and (b) Concrete
Brittle materials, such as cast iron and concrete, are often weak in tension because of the presence of submicroscopic cracks and faults. However, these materials can prove to be quite strong in compression, due to the fact that the compression test tends to increase the cross sectional areas of specimens, preventing necking to occur, and cracks tend to remain closed in compression. In compression, a single large flaw is not fatal (as it is in tension). Often, it is found that brittle materials fail at much higher compressive stresses than tensile stresses (Table 1), although ductile materials such as metals may have tensile and compressive strengths that are nearly equal.
Table 1: Comparison of the tensile, compressive, and flexural strengths of selected ceramic materials

A compression test is conducted in a manner similar to the tensile test, except that the force is compressive and the specimen contracts along the direction of the stress.
Brittle materials in compression typically have an initial linear region followed by a region in which the shortening increases at a higher rate than does the load. Thus, the compression stress – strain diagram has a shape that is similar to the shape of the tensile diagram. However, brittle materials usually reach much higher ultimate stresses in compression than in tension. Brittle materials in compression behave elastically up to certain load, and then fail suddenly by splitting or by cracking in the way as shown in Figure 2.
Figure 2: Compression stress-strain curve
Caution must be taken during compression testing to minimize friction between the loading platen and the specimen because friction will provide an artificial resistance to ΔA and will therefore make the material appear stiffer and stronger than it actually is. Even after plastic deformation has begun, the true stress -true strain curve from a well-run compression test of a metal should closely match that of tensile test, although the engineering curve will not because of tensile necking. The true stress-strain curve for a given polymer in tension is always lower than in compression since the chain are more mobile under tensile condition.
One potential advantage of compression testing is the avoidance of necking instability, so larger strains can often be imposed than are possible under tension. This can also can be seen as a drawback if aspects of the necking behavior and ensuring tensile fracture are of interest. Compression testing also avoids early failure due to brittle cracking in ceramic materials.