The respective influences of the Von-Mises equivalent stress and of the maximum principal stress on the high temperature creep damage of two industrial alloys (INCO 718 and 17–21 SPH stainless steel) are pointed out in a quantitative way through tensile-torsion biaxial tests. Through inversions of the shear component, the important part taken by the principal direction corresponding to the maximum principal stress is also shown. Opposite results are observed according to whether the alloy suffers cyclic hardening as 17-12 SPH does or cyclic softening which is the case of INCO 718. These results are supported by metallographic observations. They demand an anisotropic form for the damage variable D, while besides a time dependence kinetics equation must include the part taken by the strain. The bases of the formulation are described in Part II.

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