EXPERIMENTAL-STUDY AND PHENOMENOLOGICAL MODELIZATION OF RATCHET UNDERUNIAXIAL AND BIAXIAL LOADING ON AN AUSTENITIC STAINLESS-STEEL

The comprehension and the quantitative description of the uni- and bid irectional ratchet phenomena remains one of the final aspects to be co rrectly modelized by the phenomenological approaches employing a model with internal scalar and tensorial variables. Toward this end, the fi rst step consists in creating a set of experimental basis of unidirect ional and bidirectional ratchet. This has been done for an austenitic stainless steel at 600 degrees C. With the aid of alternating tension and torsion tests, the unidirectional ratchet can be quantified as a f unction of the maximal and average stresses. It is shown that the prog ressive strain only exists when the maximum stress is greater than 210 MPa and has a maximum for an average stress around 25 MPa and fixed m aximum stress of 300 MPa. The tension-torsion ratchet is examined in a detailed fashion, and the influence of both primary (axial direction) and secondary (shear direction) loading parameters on the progressive strain rate is demonstrated. To be able to integrate, during the mode lization, the nonradiality effects present in this type of loading, se veral cyclic out-of-phase tension-torsion tests are performed (Phi = 9 0 degrees). At ambient temperature, several axial-internal pressure ra tchet tests agree with the results obtained from tension-torsion tests . However, if ratchet tests were to be performed with two cyclic compo nents (in or out of phase cyclic tension-torsion plus a static stress due to internal pressure), it can be shown that the rate of diametral ratchet is an increasing function of the phase angle between the cycli c components. This set of tests constitutes the experimental basis nec essary for the modelization of the ratchet phenomena. It is then shown that it is possible to reasonably describe this set of experimental r esults after taking into account a few modifications in the definition of the evolutionary laws for the tensorial variables of kinematic har dening. The nature of the modifications introduced in the kinematic ha rdening variables depends on the type of ratchet to be modelized. For uniaxial loadings, the progressive strain is governed by average stres s effects, whereas for multiaxial loadings it is essentially governed by directional flow effects.