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A Compression Test

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Discussion

        A compression test is any test in which a material experiences opposing forces that push inward upon the specimen from opposite sides. We placed the material sample in between two plates that distribute the applied load across the entire surface area of two opposite faces of the test sample and then the plates were pushed together by a universal test machine causing the sample to flatten. Through this test the compressive strength, yield strength, ultimate strength, elastic limit, and the elastic modulus were determined for the sample material. By definition, the compressive strength of a material is that value of uniaxial compressive stress reached when the material fails completely. The compressive strengthis usually obtained experimentally by means of a compressive test. In a stress-strain graph, the modulus of elasticity, EC, can be determine. Modulus of Elasticity is the rate of change of strain as a function of stress. It can be found by using linear regression on the semi-linear first portion of the data up to about one half of the maximum load. The modulus of elasticity should be compared with the empirical approximate modulus, EAPPROXIMATE. This is just an approximation for the modulus that resulted from a series of tests and a very simplified curve-fitting. The maximum stress applied during the test is referred to as f´C.

        Hardness testing is often used for material assessment, in research and development work, and in quality control of production processes. It indicates the properties of a material such as wear resistance, ductility, and strength. Hardness defines the ability of a material to resist plastic deformation, most often by penetration. It is normally tested on a basis by resistance to scratching or cutting. Relative hardness tests are limited in practical use and do not provide accurate numeric data or scales particularly for modern day metals and materials. The more accurate method for obtaining hardness value is by use of an indenter of a specified shape and applying force to make an indentation within a specific time frame. The depth or area of the indent is then measured. The relationship of the hardness and size of the impression are then expressed. Rockwell hardness testing is a general method for measuring the bulk hardness of metallic and polymer materials. Although hardness testing does not give a direct measurement of any performance properties, hardness of a material correlates directly with its strength, wear resistance, and other properties. Hardness testing is widely used for material evaluation because of its simplicity and low cost relative to direct measurement of many properties. Rockwell hardness testing is an indentation testing method we used in our experiment.To start the test, the indenter was set into the sample at a prescribed minor load. A major load was then applied and held for a set time period. The force on the indenter was then decreased back to the minor load. The Rockwell hardness number was calculated from the depth of permanent deformation of the indenter into the sample. The minor and major loads were applied using dead weights or springs. The indenter position was measured using an analog dial indicator or an electronic device with digital readout.

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