Experimental Methods for Compression Test

Most important factor that has to be controlled in compression test is the application of load. The axiality of load has to be ensured. The platen, normally the one in contact with the moving cross-head is made self aligning with a spherical head on the top of the plate which is directly in contact with the specimen. The centre of the spherical head coincides with the centre of the specimen top surface through which must pass the axis of the machine. Great care needs to be exercised for aligning the specimen axially with machine (Figure - a).

Because of friction between the loading plate and top of specimen the material immediately below the top surface is subjected to near hydrostatic stress and avoids failure. Though the compressive stress in direction perpendicular to load axis decreases away from loading surface it does not vanish before distance is at least diameter of the specimen. For this reason the specimen length of 3d is normally recommended. The top surface is also recommended to be smooth and friction free. In some specimens close cut circular concentric grooves and created to hold lubricant, particularly in specimens of ductile materials. The problem of bulging is minimal in concrete and hence l/d = 2 is often used for this material. In yet another method the ends are made conical hollow to receive similar loading plate as shown in Figure 1(b). The angle of cone is equal to angle of friction between the materials of platen and specimen, and it is not readily known. It is important that compression force must be applied such that rate of straining is constant. Most materials required to be tested in compression are viscoelastic in nature and do not reach equilibrium readily. Prescribed strain rates are, therefore, maintained.

                   Figure: (a) Self Aligning Platen; and (b) Conical Hollow End of Compression Specimen