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 modele d by the phenomenological approaches employing models with internal va riables. Towards this end, the first step consists in creating the mos t complete experimental basis as possible of unidirectional and two-di mensional ratchet. This has been done for an austenitic stainless stee l 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 MP a.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 o f loading during the modeling, several cyclic out-of-phase tensile-tor sion 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 correctl y by making a few modifications to the definition of the variation law s for the tensorial variables of kinematic hardening. The nature of th e modifications introduced in the kinematic hardening variables depend s on the type of ratchet to be modeled. For unidirectional loadings th e progressive strain is governed by the average stress effect whereas for multiaxial loadings, it is essentially governed by directional flo w effect.