I’ve found this very interesting webpage. I re-post here its content, slightly modified and re-arranged, since I think many people dealing with mechanics could find it useful 🙂
A number of terms have been defined for the purpose of identifying the stress at which plastic deformation begins. The value most commonly used for this purpose is the yield strength (YS). The YS is defined as “the stress at which a predetermined amount of permanent deformation occurs“. The graphical portion of the early stages of a tension test is used to evaluate YS, that can be found with this method:
- the predetermined amount of permanent strain is set along the strain axis of the graph (the horizontal one), to the right of the origin. This starting point is indicated in the figure as point (D);
- a straight line is drawn through (D) at the same slope as the initial portion of the stress-strain curve;
- the intersection point (point 3) between the straight line and the stress strain curve is projected to the stress axis. The stress value measured at point 3 is the YS and is expressed in psi (pound-force per square inch), or bar, MPa, and so on.
This plotting method allows to subtract the elastic strain from the total strain, leaving the predetermined permanent offset as a remainder. Such offset, when reporting YS values, should be stated, i.e.: “Yield Strength (at 0.2% offset) = 51,200 psi“.
Some hints about the other points that appear in the figure above:
- the proportional limit (point 2) is defined as “the stress at which the stress-strain curve first deviates from a straight line“. Below this limit value of stress, the ratio of stress to strain is constant, and the material is said to obey Hooke’s Law (stress is proportional to strain). The proportional limit usually is not used in specifications because the deviation begins so gradually that controversies are sure to arise as to the exact stress at which the line begins to curve;
- the ultimate tensile strength (UTS, point 4) is “the maximum stress that a material can withstand while being stretched or pulled“. Basically, during a tensile test, the UTS is the final amount of stress sustained by the material at the exact moment preceding the object rupture. Rupture corresponds to the fracture point (point 5).