The usual yield function proved in Eq. (42) does not produce precise
physical response while linked with the phase-field model for crack
growth. In the process of material failure, the damaged elastic reaction
pulls the stress back within the yield surface and any further
deformation up to complete failure is purely
elastic.27 That is opposite to what is noticed
physically for ductile materials where the deformation is controlled by
plastic strain. To compensate for this behavior, plastic degradation
function should take place in the yield surface. 46