PSI - Issue 13

Taro Suemasu et al. / Procedia Structural Integrity 13 (2018) 1088–1092 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Nomenclature FIB

Focused ion beam

JIS

Japanese industrial standards

PSB RCF

Persistent slip band Rolling contact fatigue

RD Rolling direction RISS Roughness-induced stress shielding SEM Scanning electron microscopy

1. Introduction In rolling contact fatigue (RCF), a work hardening layer and texture are formed by cyclic rolling contact [Garnham et al. (2008)], and the fatigue crack propagates in the texture via in-plane shear type Mode II loading [Beynon et al. (1996) and Garnham et al. (2008)]. As the crack propagates inside the material, its direct observation is difficult, so RCF crack propagation has not been clarified. To enable safe and economical material/structure designs for RCF, it is necessary to clarify the factors that affect fatigue crack propagation. Roughness-induced stress shielding (RISS) effect, which reduces the driving force for fatigue crack propagation owing to the frictional force between the fatigue crack surfaces, is a factor affecting fatigue crack propagation in RCF. However, as fatigue crack propagation due to RCF occurs inside the material, RISS has not been quantitatively evaluated. In this study, we aimed to quantitatively evaluate the effects of RISS. The fatigue test was performed using textured materials as in an actual machine under RCF. We used a fatigue test method [Hamada et al. (2018a, 2018b, 2018c)] by which we could realize Mode II loading, and observe the fatigue crack shape directly. A small film specimen with a pre-crack was attached to a round bar with an adhesive, the bar was subjected to cyclic torsional loading, and shear stress was applied to the small film specimen. After the fatigue test, the RISS was quantified using a method that derives the contact force between the fatigue failure surfaces by using the observed fatigue crack shape and the calculated deformed shape of the fatigue crack. In this study, we focused on the extent to which RISS reduces the Mode II stress intensity factor K II . Further, the RISS was quantified by assuming that the shape of the observed fatigue crack on the side of the specimen and the internal crack shape were identical. The thickness of the thin-film sheet used in the test must be uniform. Therefore, a JIS-SUS430 ferritic staineless-steel cold-rolled sheet with a uniform thickness of 30 µm was used in the fatigue test. The specimen was cut out from the sheet. Fig. 1 shows the microstructure of the specimen. The base microstructure is ferrite, and it exhibits an extended texture morphology. It is not a layered structure owing to rolling contact loading. However, we assumed that the texture formed by cold-rolling is the same as that formed by rolling contact loading. A thin disk-shaped specimen was cut out from the sheet by using a disk punch for transmission electron microscopy analysis. The diameter of the thin disk-shaped specimen was 3 mm, and the surface of the specimen remained as rolled. To assist fatigue crack propagation, a pre- crack with a length of 600 μm was introduced parallel to the rolling direction using a focused ion beam (FIB) device (Quanta 200 3D manufactured by FEI). Figure 2 shows the scheme of specimen. 2.2. Fatigue test and analysis method Fig. 3 presents the schematic of the specimen and round bar jig. The specimen was adhered to the round bar jig, to which cyclic torsional loading was applied. The round bar jig was made of a titanium alloy (Ti-6Al-4V). An instantaneous adhesive CC-36 (Kyowa Electronic Instruments Co., Ltd.) was used for adhering the specimen to the round bar jig. The fatigue tests were performed in air by using a cyclic torsional fatigue testing machine, TB-10 (Shimadzu Co.), at room temperature. The test frequency was 33.3 Hz, the stress ratio was R = −1, and the nominal shear stress loaded on the thin-film disk specimen was ± 150 MPa. After completion of the fatigue test, the thin-film disc specimen was peeled from the jig using dropped acetone, and the remaining adhesive on the specimen surface was removed with acetone. Subsequently, the fatigue crack shape and failure surface were observed using optical microscopy and scanning electron microscopy (SEM). SEM was conducted at an accelerating voltage of 15 kV. 2. Experimental method 2.1. Material and specimen