PSI - Issue 19

Yuya Tanaka et al. / Procedia Structural Integrity 19 (2019) 320–327 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

322

3

as smaller as the relative displacement of crack faces. In order to conduct the reasonable research, an unprecedented testing procedure, which can simulate the shear-mode fatigue crack faces, is needed. In this study, micro-scale frictional behavior of bearing steel was investigated with a new ring-on-ring testing method that makes it possible to quantitatively reveal the tribological characteristics of the shear-mode crack faces. 2. Experimental method The material used was a high-carbon-chromium bearing steel (JIS SUJ2), which was held at 840 ℃ for 30 minutes, oil hardened and then tempered at 170 ℃. Table 1 shows the chemical composition in mass%. The Vickers hardness, HV , measured with a load of 9.8 N was 753. Figure 1 shows the shape and dimensions of a specimen. The end surface of hollow cylinder was finished by polishing with an emery paper and then by buffing with an alumina paste. A servo-hydraulic fatigue testing machine (MTS: Series 809 Axial Torsional Material Testing system) was used to conduct the test. The capacities of this testing machine are 100 kN for the axial force and 1000 Nm for the torsional torque. In this study, the ring-on-ring test was performed in which the end surfaces of hollow cylinders were contacted mutually, and a static compressive force and a cyclic, sinusoidal, twisting displacement at an angle of constant amplitude were applied (cf. Figure 2). The cyclic reciprocating sliding motion was thus generated between the contact surfaces. Before the start of the test, a uniform contact condition was precisely adjusted by using an alignment fixture, which was developed specifically for this experiment. This fixture can effectively remedy the angular and axial misalignments by tuning the adjusting screws and by monitoring the contact condition with a pressure sensitive sheet. Figure 2 shows the configuration of the specimens and the alignment fixture. The tests were conducted at ambient temperature under dry condition up to 10 4 cycles. In this experiment, p was defined as: p = W A (1) A nominal area of 44.0 mm 2 was used for every calculation as a value of A . F was defined as: F = T r (2) A nominal mean radius of 7.0 mm was used for r (cf. Figure 1). S was defined as: S = S D − S F (3) The values of S D and S F were measured by using two laser displacement meters (KEYENCE: LK-H020). In order to put in a target of laser light made from a cermet tip on the specimen, a jig made by a 3D-printer was used (cf. Figure 3). Its inertia was negligibly small because the material of jig was made of light ABS resin. The jig was mounted on each specimen at a distance of about 1 mm from the edge of contact surface. A 4-point support was used to fix the jig to the specimen (cf. Figures 3 and 4).

Table 1. Chemical composition of SUJ2 in mass%. C Si Mn P S

Cu

Ni

Cr

Mo

O 2

Ti

1.01

0.25

0.34

0.017

0.007

0.10

0.05

1.41

0.03

5 ppm

20 ppm

Fig. 1. Shape and dimensions of a specimen (in mm).