The backstress tensor for bilinear kinematic hardening evolves so that the effective stress versus effective strain curve is bilinear. The initial slope of the curve is the elastic modulus of the material and beyond the user-specified initial yield stress , plastic strain develops and the backstress evolves so that stress versus total strain continues along a line with slope defined by the user- specified tangent modulus . This tangent modulus cannot be less than zero or greater than the elastic modulus.