Creep life prediction of 9Cr-1Mo steel under multiaxial state of stress

Creep rupture life of 9Cr-1Mo steel under multiaxial state of stress has been assessed. Stress multiaxiality in cylindrical specimens during creep tests has been introduced by incorporating circumferential U-notches of different notch root radii. Creep tests were carried out on both smooth and notched specimens of the steel at 873 K over the net applied stresses in the range of 110-210 MPa. The creep rupture life of the steel was found to be higher in the presence of notch than that of smooth specimen indicating 'notch strengthening' behavior under multiaxial state of stress. The extent of strengthening tends to saturate for relatively sharper notches. Finite element analysis of stress and strain distributions across the notch was carried out to assess the notch strengthening behavior observed in the steel. The reduction in the von-Mises stress, extent of which increased and tends towards saturation with increase in notch root radii, resulted in increase in rupture life under multiaxial state of stress. Estimation of the creep rupture life under multiaxial state of stress has been assessed based on the different models, invoking the concept of skeletal point for calculating the representative stress. It has been observed that the creep rupture behavior of the steel under multiaxial state of stress is predominantly governed by the von-Mises stress. The creep rupture life has been predicted using finite element analysis coupled with continuum damage mechanics.