To relate the parameters in a plasticity model to an experimental stress±strain curve, the effective
stress and eective strain are usually introduced so that a single eective stress±eective
strain curve can be obtained. This curve should reduce to an experimentally observed uniaxial
stress±strain curve, and is generally referred to as the (unique) universal stress±strain curve.
There is some arbitrariness in choosing general forms for the eective stress and eective strain.
The de®nition of the eective stress is usually made in accordance with the loading (yield)
function employed. For example, if the von Mises yield criterion is used in the formulation, then
re takes the form: