Materials creep resistance is usually determined under uniaxial constant load condition while the state of stress in real components is multiaxial in general. How to correlate multiaxial creep behavior to uniaxial creep data is still an open issue. Most of the theories available in the literature are phenomenological in nature and limited to few classes of metals and stress triaxiality ranges. From the computational point of view, stress field relaxation depends on the creep model formulation. Therefore, it is necessary to use a refined creep model capable to reproduce the effective state of stress in the material. In the present work, a mechanism based creep model proposed by Bonora and Esposito [1] was used to predict creep life of P91 high chromium steel under multiaxial state of stress. This creep model is capable to reproduce all creep stages and was validated for different classes of metals and alloys. The performance of different reference stress definitions (i.e. the principal facet stress, skeletal stress and a new formulation) was investigated. Results were compared with available creep life data obtained on round notched samples at different temperatures.

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