Of concern in plasticity theory is the yield strength, which is the level of stress that causes appreciable plastic deformation. It is tempting to define yielding as occurring at an elastic limit (the stress that causes the first plastic deformation) or at a proportional limit (the first departure from linearity). However, neither definition is very useful because they both depend on accuracy of strain measurement. The more accurately the strain is measured, the lower is the stress at which plastic deformation and non-linearity can be detected.

To avoid this problem, the onset of plasticity is usually described by an offset yield strength that can be measured with more reproducibility. It is found by constructing a straight line parallel to the initial linear portion of the stress strain curve, but offset from it by a strain of Δe = 0.002 (0.2%). The yield strength is taken as the stress level at which this straight line intersects the stress strain curve (Figure 2.1). The rationale is that if the material had been loaded to this stress and then unloaded, the unloading path would have been along this offset line resulting in a plastic strain of e = 0.002 (0.2%). This method of defining yielding is easily reproduced.