EXPERIMENTAL-STUDY AND PHENOMENOLOGICAL MODEL OF UNIDIRECTIONAL AND BIDIRECTIONAL RATCHET OF AUSTENITIC STAINLESS-STEEL AT HIGH-TEMPERATURE

The understanding and the quantitative description of the uni- and two-directional ratchet phenomena is one of the last aspects to be modeled by the phenomenological approaches employing models with internal variables. Towards this end, the first step consists in creating the most complete experimental basis as possible of unidirectional and two-dimensional ratchet. This has been done for an austenitic stainless steel at 600-degrees-C. The 1 D ratchet can be quantified as a function of the maximal stress and of the average stress with tensile and torsion tests. It is shown that the progressive strain exists only above 210 MPa and has a maximum value for an average stress between 25 and 50 MPa. The 2D tensile-torsion ratchet is examined in a close detail and the influence of both the primary (axial) and secondary (shear), loading parameters on the progressive strain rate is demonstrated. In order to be able to integrate the non-radiality effects present in this type of loading during the modeling, several cyclic out-of-phase tensile-torsion tests were performed (PHI=90 degrees). This set of tests provides the experimental basis necessary for modeling the ratchet phenomena. It is then shown set of experimental results can be described correctly by making a few modifications to the definition of the variation laws for the tensorial variables of kinematic hardening. The nature of the modifications introduced in the kinematic hardening variables depends on the type of ratchet to be modeled. For unidirectional loadings the progressive strain is governed by the average stress effect whereas for multiaxial loadings, it is essentially governed by directional flow effect.