Issue 38

S. Averbeck et alii, Frattura ed Integrità Strutturale, 38 (2016) 12-18; DOI: 10.3221/IGF-ESIS.38.02 13 are caused by the cyclic Hertzian stresses in the contact area. This complex superposition of compressional and shear stresses changes over time as a rolling element passes over the raceway. In this area, defects, such as non-metallic inclusions or voids, can act as stress concentrators and enable crack initiation. A phenomenon called ‘butterflies’ which occurs in this region has been linked to White Etching Crack formation [9]. These are cracks around inclusions which are accompanied by white etching areas with virtually the same properties. It has to be considered, though, that butterflies are always oriented at 20-30 degrees to the contact surface, whereas WECs are not specifically oriented in any direction [1, 8]. E XPERIMENTS his study differs from earlier WEC investigations in its approach to WEC reproduction. Instead of testing bearings, simple hourglass-shaped specimens made from 100Cr6 steel were used (Fig. 1). The specimens were heat treated at the SKF GmbH bearing manufacturing plant in Schweinfurt, Germany, according to the regular bearing heat treatment standards of SKF. After heat treatment, the specimens were ground to a roughness R z of 2µm and subsequently polished. Figure 1: Specimen geometry. Load-controlled testing was carried out with superimposed axial and torsional loads on a servohydraulic MTS 858 testing machine. Two different approaches to the reproduction of rolling contact fatigue were used: in-phase and out-of-phase loading. For both load spectra, tests were performed in air and in partially synthetic SAE 75/W80 transmission oil with a specific additive package. This is a typical gearbox oil which has been found to promote WEC formation, most likely due to its additives [10]. All experiments were carried out at room temperature. In-phase loading Transfer between RCF conditions and in-phase loading was based on achieving a similar highest equivalent stress. Although the stress state in a bearing constantly changes over time, it is possible to identify a time and location where the equivalent stress is highest during one load cycle. This stress state can be modelled by in-phase compressional and torsional load, as demonstrated by Burkart et al. [11]. The equivalent stresses were determined using von Mises’ criterion and applied such that the ratio between the principal stresses’ difference, |σ I -σ II |/|σ II -σ III |, was the same in the experiment as in a real bearing. There remains, of course, one main difference between a bearing and the experiment: while the maximum stress is located beneath the surface in the former, it occurs at the surface in the specimen. Out-of-phase loading This load spectrum was based on work by Beretta and Foletti [12], who used out-of-phase compression-torsion loads to study coplanar crack propagation from artificial defects. They tested bearing steel specimens with two different load patterns, which were proposed by bearing manufacturer SKF. The first pattern aimed at reproducing subsurface conditions, while the second should reproduce the conditions deeper under the bearing surface. It was found that only the T