PSI - Issue 19

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

321

2

Nomenclature HV

Vickers hardness

A

nominal contact area [mm 2 ] test frequency [Hz] tangential force [kN] number of cycles [cycles]

f

F N p

nominal contact pressure [MPa] mean radius of a hollow cylinder [mm]

r

R a

arithmetic mean roughness relative displacement [µm]

S

S D S F

displacement of driving side specimen [µm] displacement of fixed-end side specimen [µm]

t

time [sec]

T

twisting moment [Nm] static compressive force [kN] coefficient of kinetic friction

W μ k

1. Introduction

Rolling contact machine elements such as bearing, railway rail/wheel, gear, etc. have a common problem of delamination failure like flaking, shelling and spalling. These failures, called rolling contact fatigue failure, are caused by the propagation of fatigue crack initiated inside the material beneath the contact surface. The crack grows mainly in shear-mode (modes II and III) rather than opening-mode (mode I) due to the compressive stress applied to the crack (Kida (2002) and Matsunaga (2010)). Currently, for example, the quality and reliability of bearings are ensured by the statistical and empirical method (ISO 281 (2007)) on the basis of the durability test with a great number of real products. Thus, this design procedure takes a great deal of time and cost inevitably. Furthermore, for such large bearings as used in wind power generators, statistical analysis based on the endurance test results of large and expensive actual products is virtually impossible, as pointed out by M. H. Evans (2012) and A. Greco et al. (2013) that this type of design procedure is not appropriate for a massive bearing. Therefore, any better alternative to the conventional evaluation method has been demanded. In specific, a new design method using fracture mechanics is required. In the literature, the mode II threshold stress intensity factor ranges, Δ K IIth , of bearing steel (JIS SUJ2) are reported. Otsuka et al. (2004) conducted the fatigue test by applying the cyclic shear stress, static compressive stress parallel to the crack face and static bending stress to open the crack to the specimen. They measured 3 MPa √ m as the value of Δ K IIth , which is the result without crack face interaction. Murakami et al. (2002) developed a unique double-cantilever-type testing method to investigate mode II fatigue crack propagation characteristics. They obtained a value of Δ K IIth (13-15 MPa √ m ) by FEM assuming the friction coefficient between crack surfaces to be 1.0. Also, Kida et al. (2004) indicated that the maximum shear-mode stress intensity factor range, Δ K IImax , is related to the friction coefficient. Matsunaga et al. (2010) and Okazaki et al. (2014) clarified that the value of Δ K IIth depends on the fraction of interfering crack area to the total area of defect plus crack through the experiments using the specimens containing artificial defects with the various fraction. Moreover, Okazaki et al. (2017) has shown that as the crack opening stress increased , the value of Δ K IIth decreased. These results indicate that shear-mode fatigue crack growth and its threshold behavior are essentially related to the frictional resistance against the relative slip of the crack faces. Consequently, in order to understand the mechanism of shear-mode fatigue, it is necessary to reveal the tribological properties of crack faces being in the cyclic sliding motion. However, it is difficult to carry out this research by the conventional tribological testing method, in which the friction and wear phenomena for sliding motion in one direction have been investigated by using the most common procedure, such as the pin-on-disk test. However, there are few studies on the reciprocating sliding of the amplitude