PSI - Issue 2_A

Nobuo Nagashima et al. / Procedia Structural Integrity 2 (2016) 1435–1442 Author name / Structural Integrity Procedia 00 (2016) 000–000

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at certain test temperatures. Pseudoelastic strain has a significant effect on low-cycle fatigue properties. For example, generally, the total strain amplitude ( Δεt ) for 1000 cycles to failure is about 3%, while that of a Ni-Ti alloy, for the same number of cycles to failure, is 10%. The index ( K p ) in the Manson-Coffin law is 0.5 to 0.6 for common materials, while the index for the crack initiation life of a Ti-24%V alloy is about 0.8. Thus, the fatigue strength of a titanium alloy with high shape memory effect can be increased by pseudoelastic strain. High manganese austenitic steel is non-magnetic, and has high strength and toughness at low temperatures. However, there are only a few reports on the fatigue properties of high manganese steel (Nishida 1995). A high manganese steel with added silicon showed shape memory effect associated with martensitic transformation from γ austenite with a FCC structure to ε -martensite with a HCP structure (Sato 1982). In recent years, Fe-Mn-Si alloys have been actively studied worldwide, as Fe-based shape memory alloys (Sawaguchi 2008). Fe-Mn-Si shape memory alloys have been shown to exhibit a partial pseudoelasty due to reverse transformation of the deformation induced ε -martensite during unloading (Sawaguchi 2005). Figure 1 shows stress-stain curves for a Fe-28Mn-6Si 5Cr-0.5NbC alloy, obtained under cyclic loading and unloading. The load was increased to 1.0% in steps of 0.2%. In the figure, PE 0.2 to PE 1.0 represent the pseudoelastic strain that appears as an inelastic deformation component of springback, at applied strains of 0.2 to 1.0%. It was also found that further reverse martensitic transformation of the tensile deformation-induced martensite occurred by subsequent compression (Sawaguchi 2006, Sawaguchi2008). Such pseudoelastic and reversible plastic deformation mechanism may reduce accumulation of plastic strain and improve the low cycle fatigue life. Based on these findings, the authors investigated the low cycle fatigue properties of various Fe-Mn alloys and showed that Fe-30Mn-4Si-2Al alloy and Fe-15Mn-10Cr-8Ni-4Si alloy have high fatigue resistance, attributable to reversible ε -martensite transformation. In recent years, Fe-15Mn-10Cr-8Ni-4Si alloy has been commercially used as a seismic vibration control damper for very tall buildings (Nikulin 2013, Sawaguchi 2015, Nikulin2015, Li 2015, Sawaguchi 2015). The low cycle fatigue properties of Fe-28Mn-6Si-5Cr-0.5NbC alloy have, however, not yet been reported. Since Fe-28Mn-6Si-5Cr-0.5NbC alloy exhibits considerable pseudoelasticity, the relationship between the pseudoelasticity and low cycle fatigue properties of the alloy deserves attention. Thus, the present study investigated the low cycle fatigue properties of Fe-28Mn-6Si-5Cr-0.5NbC alloy at different total strain amplitudes, and this paper reports on the relationship between these low cycle fatigue properties and the pseudoelastic behavior of the alloy. 2. Experimental Method The specimen material was Fe-28Mn-6Si-5Cr-0.5NbC alloy (mass%) ( σ 0.2 =270MPa, σ B =950MPa) (hereafter, ‘FMS alloy’). Before the test, the alloy structure was single- phase austenitic (γ), with a crystal grain size of about 30 μm. SUS 304 steel ( σ 0.2 =275MPa, σ B =618MPa), also single-phase γ , was used for comparison. When subjected to cyclic strain, SUS304 steel is partially transformed into a strain- induced martensite (α’) phase. A fatigue test was performed, using a 50 kN servohydraulic fatigue testing machine, under fully reversed loading (stress ratio R = -1). The test was performed at four maximum total strain amplitudes ( ε ta = Δε t /2) (2.0%, 1.4%, 0.9%, and 0.6%), and at a strain rate of 5×10 -3 s -1 , using 8-mm-diameter specimens with parallel sections. An extensometer with a gauge length of 8 mm was attached to the specimen, to measure the axial strain.

Figure 1 Tensile stress-strain curves for Fe-28Mn-6Si-5Cr-0.5NbC alloy at room temperature (Sawaguchi 2008). The PE 0.2 to PE 1.0 are pseudo elastic strains at the applied tensile strains of 0.2 % to 1.0 %, respectively.