Experimental study and phenomenological modelization of ratchet under uniaxial and biaxial loading on an austenitic stainless steel

Abstract

Abstract The comprehension and the quantitative description of the uni- and bidirectional ratchet phenomena remains one of the final aspects to be correctly modelized by the phenomenological approaches employing a model with internal scalar and tensorial variables. Toward this end, the first step consists in creating a set of experimental basis of unidirectional and bidirectional ratchet. This has been done for an austenitic stainless steel at 600°C. With the aid of alternating tension and torsion tests, the unidirectional ratchet can be quantified as a function of the maximal and average stresses. It is shown that the progressive 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 maximum 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 modelization, the nonradiality effects present in this type of loading, several cyclic out-of-phase tension-torsion tests are performed ( Φ = 90°). At ambient temperature, several axial-internal pressure ratchet tests agree with the results obtained from tension-torsion tests. However, if ratchet tests were to be performed with two cyclic components (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 cyclic components. This set of tests constitutes the experimental basis necessary for the modelization of the ratchet phenomena. It is then shown that is possible to reasonably describe this set of experimental results after taking into account a few modifications in the definition of the evolutionary 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 modelized. For uniaxial loadings the progeessive strain is governed by average stress effects, whereas for multiaxial loadings it is essentially governed by directional flow effects.