Modeling and simulation of temperature-dependent cyclic plastic deformation of austenitic stainless steels at the VHCF limit

Abstract

The exploration of fatigue mechanisms in the VHCF regime is gaining importance since many components have to withstand a very high number of loading cycles due to high frequency or long product life. In this regime, the period of fatigue crack initiation and thus the localization of plastic deformation play an important role. The material that was investigated in this study is the metastable austenitic stainless steel AISI 304 in the initially purely austenitic condition. The experimental investigations during quasi-isothermal fatigue tests revealed that a moderate increase of temperature from room temperature up to 150°C led to a reduced VHCF strength. At both temperatures the 304 grade still undergoes a pronounced localization of plastic deformation in shear bands accompanied by a deformation-induced martensitic phase transformation from the γ-austenite to the α’-martensite during VHCF loading. In the present study, the experimental study is extended by modeling and simulation of the relevant temperature-dependent VHCF deformation mechanisms in order to provide a more profound understanding of the observed cyclic deformation. For this purpose, two-dimensional (2-D) morphologies of microstructures are modeled in the mesoscopic scale by the use of the boundary element method (BEM), and cyclic plastic deformation is considered by certain mechanisms defined in a simulation model. It describes the localization of plastic deformation in shear bands taking into account the formation, plastic sliding deformation and cyclic slip irreversibility of each shear band. The deformation-induced martensitic phase transformation is represented by introducing martensitic nuclei into the modeled microstructure depending on the plastic deformation in shear bands. The influence of temperature is incorporated into the simulation model by the use of empirical data and a kinetic model, each related to the tensile test. The simulated cyclic deformation and phase transformation is compared to experimental observations and allows for assessing the individual influence of deformation mechanisms on the temperature-dependent fatigue behavior. Finally, a temperature-independent ‘limit curve’ for the accumulated irreversible plastic sliding deformation regarding failure in the VHCF regime is proposed.