The failure criteria for isotropic materials is based on the von Mises stress (or equivalent stress in Mechanical):

(5–124)

or on the von Mises strain (or equivalent strain in Mechanical):

(5–125)

where and are the first and second principal strains. For isotropic material, the stress failure function is defined as:

(5–126)

and the strain failure function is defined as:

(5–127)