Abstract: A two-surface constitutive model, which considers the effects of cyclic hardening / softening, plastic strain amplitude history and nonproportional cyclic loading, is presented. In the model, the evolution equations of limit and yield surfaces are introduced, the nonproportionality of cyclic strain paths is denned, and a rational method to determine the modal parameters is proposed. The predictions of the model, for 42CrMo steel with quenched and tempered treatment, agree with the experimental results very ...