PSI - Issue 2_B

P.-M. Hilgendorff et al. / Procedia Structural Integrity 2 (2016) 1156–1163 Hilgendorff et al./ Structural Integrity Procedia 00 (2016) 000–000

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1. Introduction Many structural components such as railway wheelsets and automotive rolling bearings are cyclically loaded up to a very high number of loading cycles beyond 10 7 . Although many alloys undergo fatigue failure above 10 7 cycles (Bathias 1999), the classical durability limit is still mostly based on fatigue tests between 10 6 and 10 7 cycles. Therefore, the exploration of very high cycle fatigue (VHCF) behavior is of particular importance and has become a subject of growing interest in research today. In the present study the VHCF deformation mechanisms of a metastable austenitic stainless steel (AISI 304) considering the influence of a moderate temperature increase is investigated by the use of material modeling and simulation. Based on experimental observations (shown in section 2) a simulation model is proposed that can describe the characteristic deformation mechanisms in the mesoscopic scale (see section 3). After introducing the boundary element method for solving numerically the model, simulations of cyclic plastic deformation within 2-D morphologies of microstructures are carried out and results are compared to experimental observations. 2. Experimental results Fatigue tests on a resonant testing machine (stress ratio R =-1) led to the observation that AISI 304 in the initially purely austenitic condition (without predeformation) has a VHCF strength of about 240 MPa at room temperature (25°C, up to 10 9 cycles) and a reduced value of about 190 MPa at 150°C (up to 10 7 cycles). Efforts were made to keep the specimen temperature approximately constant during fatigue tests by cooling with a fan in case of room temperature and continuous heating with a heating coil at the elevated temperature of 150°C. In this study, primarily the fatigue behavior of AISI 304 at 25°C (case I) and at 150°C (case II) at the respective VHCF loading are compared. Investigations by means of a confocal laser microscope revealed a strong localization of plastic deformation in shear bands during fatigue. Fig. 1a and b show emerging slip markings on the specimen surfaces and indicate that in case I (Fig. 1a) and in case II (Fig. 1b) the plastic deformation extended to more or less the same progress after about 10 7 cycles.

Fig. 1. (a) Confocal laser microscope image of the fatigued specimen surface of AISI 304 at 25°C and loading amplitude Δσ /2=240 MPa after 2·10 7 cycles and (b) at 150°C and loading amplitude Δσ /2=190 MPa after 10 7 cycles.

The global α’-martensite content measured by a feritscope in the fatigued volume of the specimens continuously increased during fatigue loading and reached a saturated value of about 4% in case I and about 2.75% in case II after 10 7 cycles. Based on the aforementioned experimental results and further results (Grigorescu et al. 2016), in the following section a simulation model is proposed that takes into account the deformation mechanisms of the metastable austenitic stainless steel under influence of a moderate temperature increase. 3. Simulation Model In this study, the fundamental idea is to represent the relevant cyclic plastic deformation within 2-D morphologies of microstructures in the mesoscopic scale and validate the results with experimental data. A microstructure is modelled by a continuum mechanical approach with considering the individual orientation of each grain and the