strength of the material would correspond to the stress at the red point shown on the curve. In a compression test, there is a linear region where the material follows Hooke's law. Hence, for this region, , where, this time, E refers to the Young's Modulus for compression. In this region, the material deforms elastically and returns to its original length when the stress is removed. This linear region terminates at what is known as the yield point. Above this point the material behaves plastically and will not return to its original length once the load is removed. There is a difference between the engineering stress and the true stress. By its basic definition the uniaxial stress is given by: where, F = Load applied [N], A = Area [m2] As stated, the area of the specimen varies on compression. In reality therefore the area is some function of the applied load i.e. A = f(F). Indeed, stress is defined as the force divided by the area at the start of the experiment. This is known as the engineering stress and is defined by, A0=Original specimen area [m2] Correspondingly, the engineering strain would be defined by: where l = current specimen length [m] and l0 = original specimen length [m] The compressive strength would therefore correspond to the point on the engineering stress-strain curve defined by where F* = load applied just before crushing and l* = specimen length just before crushing. Deviation of engineering stress from true stress In engineering design practice, professionals mostly rely on the engineering stress. In reality, the true stress is different from the engineering stress. Hence calculating the compressive strength of a material from the given equations will not yield an accurate result. This is because the cross sectional area A0 changes and is some function of load A = φ(F). The difference in values may therefore be summarized as follows: On compression, the specimen will shorten. The material will tend to spread in the lateral direction and hence increase the cross sectional area. In a compression test the specimen is clamped at the edges. For this reason, a frictional force arises which will oppose the lateral spread. This means that work has to be done to oppose this frictional force hence increasing the energy consumed during the process. This results in a slightly inaccurate value of stress obtained from the experiment. The frictional