The stress value obtained by dividing the instantaneous value of area into the applied load is defined as the true stress:Ī = actual (instantaneous) area resisting the load, mm2 (in2). If the actual area were used, the calculated stress value would be higher. Thoughtful readers may be troubled by the use of the original area of the test specimen to calculate engineering stress, rather than the actual (instantaneous) area that becomes increasingly smaller as the test proceeds.
Lf = specimen length at fracture, mm (in), measured as the distance between gage marks after the two parts of the specimen have been put back together. The relationship is defined by Hooke’s law,Į = modulus of elasticity, MPa (lb/in2 ), a measure of the inherent stiffness of a material.ĮL = elongation, often expressed in percent. In the elastic region, the relationship between stress and strain is linear, and the material exhibits elastic behavior by returning to its original length when the load (stress) is released. The stress–strain relationship has two regions, indicating two distinct forms of behavior: Representing elongation per unit length, without units. The units of engineering strain are given as mm/mm (in/in), however think of it as The engineering strain at any point in the test is given by, The engineering stress at any point on the curve is characterised as the force divided by the original area.į = applied force in the test, N (lb), andĪo = unique area of the test specimen, mm2 (in2). The components are intended to withstand the expected stresses experience in service. These quality are of interest in design because the designer expects that the strains experienced by any part of the product won’t altogether change its shape. The engineering stress and strain in a tensile test are characterised comparative to the first region and length of the test specimen. The first is more significant in design, and the second is more significant in assembling. There are two different types of stress–strain relationship: