Yield strength of steel
Introduction
The elastic limit or yield limit is the maximum stress that an elastoplastic material "Plasticity (solid mechanics)") can withstand without suffering permanent deformation. If stresses greater than this limit are applied, the material experiences plastic behavior with permanent deformations and does not spontaneously recover its original shape when the loads are removed. In general, a material subjected to stresses lower than its elastic limit is temporarily deformed according to Hooke's law.
Materials subjected to stresses greater than their elasticity limit have plastic behavior. If the stresses exerted continue to increase, the material reaches its fracture point. The elastic limit therefore marks the transition from the elastic field to the yield zone. More formally, this means that in a uniaxial stress situation, the elastic limit is the admissible stress from which the yield surface of the material is entered.
Determination of elastic limit
If the stresses are arranged as a function of the deformations in a graph, it is observed that, initially and for the majority of materials (elastomers do not comply, for example), an area appears that follows an almost linear distribution, where the slope is the modulus of elasticity E. This area corresponds to the elastic deformations of the material up to a point where the function changes regime and begins to curve, an area that corresponds to the beginning of the plastic regime. That point is the elastic limit.
Due to the difficulty in locating it exactly and with complete fidelity, since in the experimental graphs the straight line is difficult to determine and there is a band where the elastic limit could be located, in engineering a conventional criterion is adopted, established by the ASTM A370 standard and known as the parallel method, offset method or compensation method (offset method in English), and the elastic limit is considered the tension at which the material has a plastic deformation of 0.2% (or also ε = 0.002). In this case, the yield strength should be reported, for example, as: yield strength (for 0.2% compensation) = R = 360 MPa. It should be noted that other compensation values such as 0.1% or others can be used: it all depends on the type of material.