Yield stress

According to EN ISO 527-1 (determination of tensile properties in plastics), the stress at yield is the first stress value in the stress-strain diagram at which an increase in elongation occurs without an increase in stress ( ). It is generally expressed in megapascals (MPa) and can be less than the maximum stress at breakage of the sample. ${\displaystyle \partial \sigma /\partial \epsilon =0}$

In contrast to the yield point in metallic materials, a permanent deformation takes place in plastics even at stresses below the yield stress . It is therefore not an equivalent dimensioning parameter. Instead, the stress at x% elongation or a value determined from creep tests is often used.