PSI - Issue 19

Fumiyosi Yoshinaka et al. / Procedia Structural Integrity 19 (2019) 214–223 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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seismic damage in structures such as high-rise buildings. There are several types of seismic dampers according to their mechanism of absorbing the vibration energy induced by seismic motion. Steel dampers can be classified as hysteresis dampers that absorb seismic energy through cyclic elasoplastic deformations. These dampers are most widely used due to their high cost performance. However, their durability is not very high since the low-cycle fatigue (LCF) is inevitable due to their operation principle. Therefore, alloys with a high fatigue resistance designed specifically for seismic dampers are required to ensure long-term reliability of high-rise buildings. Fe-high Mn austenitic steels/alloys have attracted a lot of attention due to their special mechanical performance (e.g., De Cooman et al. (2018)). Their deformation behavior can be controlled by the stacking-fault energy (SFE) of face-centered cubic (FCC) γ -austenite. In particular, the dominant plastic deformation mode of the γ -phase changes from perfect dislocation glide, extended dislocation glide, γ -twinning, and the hexagonal-close- packed (HCP) ε martensitic transformation with a decreasing SFE (Remy and Pineau (1977)). Fe-Mn-Si alloys are designed to have a quite low SFE to demonstrate the ε -martensitic transformation as a dominant deformation mode. This type of alloys is known for its shape memory effect, which emerges upon the reverse transformation from deformation- induced ε martensite to γ -austenite triggered by heating (Cladera et al. (2014)). It has recently been demonstrated that the rev erse ε→γ transformation can also happen due to mechanical loading that is opposite in sign to that causing the deformation-induced forward γ→ε transformation (Sawaguchi et al. (2006)). Furthermore, the highly reversible nature of dislocation in Fe-Mn-Si alloy can improve its fatigue durability, especially under the condition, where cyclic plastic deformations are experienced (Sawaguchi et al. (2015)). Based on this idea, we developed a Fe-15Mn 10Cr-8Ni-4Si (wt%) alloy with the intend to use it in a steel damper (Sawaguchi et al. (2016)). This paper examines the fatigue life properties and mechanical response during strain-control fatigue tests in the Fe-15Mn-10Cr-8Ni-4Si alloy. In addition to LCF properties, the high-cycle fatigue (HCF) behavior is also investigated by load-control fatigue tests. Since the fatigue mechanism of this alloy seems to be strongly related to the plastically deformed microstructure developed during cyclic loadings, the microstructure after the fatigue tests is investigated using electron-backscatter diffraction (EBSD) microscopy. The material used in this study is an Fe-15Mn-10Cr-8Ni-4Si (wt%) seismic damping alloy. To investigate its LCF and HCF properties, the strain- and load-control tests (hereinafter referred to as “LCF test” and “HCF test” ) were conducted, respectively. Table 1 lists the mechanical properties of the material used for the LCF and HCF tests. Note that the samples used for the LCF tests were obtained from several lots produced at different times. However, the mechanical properties of a sample obtained from one lot are shown here as a typical case since there were little differences across the lots. The material used for the LCF test was manufactured using induction furnace melting followed by the heat treatment of annealing at 1000°C, 1 h → water cooling at the National Institute for Materials Science. The material used for the HCF test was melted in a 10-ton-class electric furnace in air at Nippon Koshuha Steel Co., Ltd. and then heat treated at 1100°C, 1 h → water cooling. A low strength is required for seismic damping alloy to preferentially yield compared to other components. To meet this requirement, a coarse-grain microstructure with an average grain size of around 100 µm was prepared. Table 1. Mechanical properties of the Fe-15Mn-10Cr-8Ni-4Si alloy used for the low- and high-cycle fatigue (LCF and HCF) tests. Test type Young’s modulus E 0.2% proof stress σ 0.2 Tensile strength σ B Elongation λ LCF 184 GPa 230 MPa 660 MPa 67% HCF 187 GPa 274 MPa 687 MPa 66% The LCF tests were conducted under the axial strain control with a strain ratio of − 1. The strain rates ranged between 0.05 and 0.5%/s; however, this work did not distinguish the results obtained with different strain rates since no apparent difference in the fatigue life was recognized. The total strain range Δ ε t was between 0.5% and 6%. Tests with Δ ε t = 2% were conducted on ten specimens to roughly estimated the variability of the fatigue life. The HCF tests were conducted under the loading control with a stress ratio of − 1. The test frequency ranged between 1.5 Hz to 25 Hz depending on the stress amplitude to prevent over-heating of the specimen; lower frequencies were selected 2. Material and experimental procedure