Issue 38

S. Averbeck et alii, Frattura ed Integrità Strutturale, 38 (2016) 12-18; DOI: 10.3221/IGF-ESIS.38.02 16 Microhardness measurements in the white etching zone varied from 753HV to 1246HV, with an average value of 986HV (five measurements). This is significantly harder than the original martensitic microstructure, which has a hardness of 700- 750HV. Out-of-phase experiments In contrast, no microstructurally changed areas could be found after the out-of-phase tests, independently of the surrounding medium (oil or air). The OOP-A cycle was only used for very few tests, as it became apparent that specimens fractured very early in the high cycle fatigue regime or even in the low cycle fatigue regime (N B < 10 5 ). The OOP-B tests showed no signs of microstructural change, although the cracks propagated in the same manner like the in-phase fatigue cracks. In some specimens, no crack formation was observed at all during the 10 6 cycles of testing. D ISCUSSION hite etching microstructural changes are not an exclusive feature of White Etching Cracks, as the white layers found on rail steel (e.g. [14]) or in dry sliding contacts (e.g. [15]) demonstrate. It is thus necessary to carefully examine whether the zone formed during the compression-torsion experiments really is similar to WEC microstructural change, and to eliminate any other formation possibilities and error sources. For example, the fracture stage which followed the testing cycle could have caused the microstructural changes. However, when comparing the in- phase results with those of the OOP-B tests, this becomes very unlikely. If the white etching layer was formed due to the torsional fracturing stage, it should appear on all specimens, not only on those tested in-phase. Many formation mechanisms for white etching microstructure rely on a thermal component. This is especially true for the examples in dry sliding contacts mentioned above. No signs for thermal influences have been found in the experimental results. The fact that the white layers formed regardless of whether the specimen was tested in air or in oil rules out a mechanism like those in dry sliding contacts. Aside from these aspects ex negativo , the white etching surface layer produced by in-phase testing is in several aspects similar to the altered microstructure found in WECs. The FIB images, for example, closely resemble those presented by Franke et al. [16]. When viewed in the light optical microscope, the appearance of the white etching structure is very similar to WEC found in the literature. The hardness values also closely match those reported for WEC. The thickness of the white etching structures never exceeded 1µm in the specimen, mostly amounting to only a few hundred nanometres. This is well within the range reported for bearing WECs; however, the volume of white etching microstructure can vary widely in bearings [8]. In most cases, the cracks did not seem to develop branches. Small cracks at an angle to the fracture surface could be found in some specimens, one example being shown in Fig. 2. Whether this is a genuine WEC-type branch or simply a secondary crack from fatigue and fracture presently cannot be answered. An additional FIB cut might serve to clarify this. The cracks never seemed to nucleate at inclusions or voids; at least, no traces of such were found in the vicinity. It is therefore assumed that either surface or microstructural inhomogeneities were responsible for crack initiation. The most ambiguous aspect of the results is the question of carbides. While they are usually absent or in the process of dissolution in bearing WECs, their presence is no decisive argument against WECs [6]. Judging from the light optical microscope observations, numerous carbides were interspersed in the white etching areas found in this study. The limited magnification of the LOM makes it impossible to assess whether they are affected by the surrounding microstructural change. Nor could the FIB investigation deliver a conclusive result. While the carbide in Fig. 3 might be dissolving, others retained a largely globular shape with sharp borders to the surroundings. Generally, it should be remembered that the testing time of 10 6 cycles was moderate. A longer test, e.g. up to 10 7 cycles, could serve to clarify whether carbides dissolve or not. This could also provide insights to whether the thickness of the white structure will further increase over time or not. The early failures under load spectrum OOP-A were probably a result not only of the higher equivalent stresses, but also of the 90° phase shift. Fatemi and Shamsaei [13] cite several reports that out-of-phase loads have a significantly more detrimental effect on fatigue properties than in-phase loads. The higher compressional loads themselves probably would not be as problematic, as they would tend to close any incipient cracks. No satisfactory explanation could be found for the behaviour of the OOP-B tests, in which no observable microstructural change was found despite the fact that the crack paths were very similar to those from in-phase testing. The most obvious explanation is that the equivalent stresses were too low in the crucial points in each cycle, i.e. at the time of maximum W