![]() ![]() In engineering design practice we mostly rely on the engineering stress. The compressive stress would therefore correspond to the point on the engineering stress strain curve defined byį * = load applied just before crushing and l * = specimen length just before crushing.ĭeviation of engineering stress from true stress L = current specimen length and l 0 = original specimen length This is known as the engineering stress and is defined by,Ĭorrespondingly, the engineering strain would be defined by: Indeed, we can however say that the stress is defined as the force divided by the area at the start of the experiment. In reality therefore the area is some function of the applied load i.e. By its basic definition the uniaxial stress is given by:Īs we said, the area of the specimen varies on compression. There is a difference between the engineering stress and the true stress. Above this point the material behaves plastically and will not return to its original length once the load is removed. This linear region terminates at what is known as the yield point. Hence for this region σ = Eε where this time E refers to the Young's Modulus for compression. ![]() Even in a compression test, there is a linear region where the material follows Hooke's Law. The compressive strength of the material would correspond to the stress at the red point shown on the curve. A Stress–strain curve is plotted by the instrument and would look similar to the following: As can be imagined, the specimen (Usually cylindrical) is shortened as well as spread laterally. However, rather than applying a uniaxial tensile load, a uniaxial compressive load is applied. The apparatus used for this experiment is the same as that used in a tensile test. The compressive strength is usually obtained experimentally by means of a compressive test. Of course, the major difference between the two types of loading is the strain which would have opposite signs for tension (positive) and compression (negative).īy definition, the compressive strength of a material is that value of uniaxial compressive stress reached when the material fails completely. On a macroscopic scale, these aspects are also reflected in the fact that the properties of materials in tension and compression are quite similar, at least for most materials. The phenomena prevailing on an atomic level are therefore similar. Since atoms in solids always try to find an equilibrium position and distance between other atoms forces arise throughout the entire material which oppose both tension or compression. On an atomic level, the molecules or atoms are forced apart when in tension whereas in compression they are forced together. On the other hand if the material compresses and shortens it is said to be in compression. When a specimen of material is loaded in such a way that it extends it is said to be in tension.
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