Uniaxial mean stress relaxation of 9-12% Cr steel at high temperature: Experiments and viscoplastic constitutive modeling

Symmetrical and asymmetrical strain cycling tests of X12CrMoWVNbN10-1-1 steel were conducted at 873 K. Significant rate dependence, mean stress relaxation and continuously cyclic softening behavior were observed during the fatigue process. In addition, a strain-amplitude dependent competition between cyclic softening and mean stress relaxation was revealed. When the strain amplitudes were larger than the initial plastic point, the cyclic softening was dominated with a rapidly relaxed mean stress and a significantly altered hysteresis loop during the primary cycles. Whilst for the strain amplitudes being less than the initial plastic point, the continuously relaxed mean stress with unclearly altered hysteresis loops was observed during the whole lifetime. Accordingly, a new cyclic viscoplastic constitutive is proposed through the combination of a new nonlinear kinematic hardening rule and the Abdel-Karim-Ohno model. The strain-amplitude dependent cyclic softening and mean stress relaxation behavior were finely reproduced by the proposed model, which was achieved by introducing a mean stress relaxation parameter as a function of the maximum plastic strain and the accumulated cyclic plastic strain.