Multiscale experimental investigations about the cyclic behavior of the 304L SS

This work follows a series of experiments carried out earlier at INSA of Rouen (Hassan, T., Taleb, L., Krishna, S., 2008. Influence of non-proportional loading paths on ratcheting responses and simulations by two recent cyclic plasticity models. Int. J. Plast. 24, 1863–1889). It investigates the elastoplastic cyclic behavior of a 304L stainless steel at room temperature. In a first step the cross path effect on ratcheting is confirmed, as well as the crucial role of the loading path non-proportionality. Strain controlled tests are also conducted for different strain amplitudes and loading paths. Cross-hardening effect appears more important when the shearing sequence is followed by the axial one. Moreover for alternating axial and shearing cycles, this phenomenon occurs after each crossing sequence leading to a very significant strain hardening, at least of the same order as the one obtained after a circular strain path. Yet, the magnitude of the observed over hardening does not necessarily seem a function of the cumulated plastic strain. The relative contributions of the isotropic and kinematic parts of the cyclic strain hardening are also very different in the axial direction. The material generally exhibits a transient period of cyclic hardening followed by cyclic softening. Under tension-compression, the importance of the cyclic strain hardening period seems to increase with the strain amplitude. Microstructural investigations have been performed in order to understand the main physical phenomena responsible for this macroscopic behavior. The additional cyclic hardening observed in cross paths may be explained by the high defect density generated: multiple slip systems, intersecting stacking faults and twins, formation of dislocation heterogeneous structures. Furthermore, it is shown that martensite nucleation takes place at the intersections between micro-shear bands or twin faults; the quantity of martensite being estimated through magnetic measurements. After the application of a magnetic field, the material anisotropy induced by the loading path is evaluated; the martensitic needles and platelets seem to develop in the axial direction.