NTN-SNR (France)

Among the prevalent tribological failures affecting rolling element bearings, an unconventional rolling contact fatigue mode has been identified as white etching cracks. Those correspond to three-dimensional branching crack networks partially bordered by white etching microstructure, eventually leading to premature and unpredictable failure. Recent work supports that this failure mode may be associated with various combinations of operating conditions depending on the application or test rig, but that all seem to converge towards similar tribological drivers related to surface-affected hydrogen evolution at asperity scales, which is known to embrittle the bearing steel. Nevertheless, as white etching cracks remain delicate to reproduce without artificial hydrogen charging, the underlying formation mechanisms remain unsettled. The present work aims to better understand how some of the main tribomechanical and tribochemical drivers may trigger white etching cracks and premature failures. In this study drivers such as sliding kinematics, water contamination, and electrical potential and lubricant additives are progressively transposed on a twin-disc machine that provides an enhanced control of contact parameters. Various attempts advocate that the tested drivers are not self-sufficient to reproduce the failure mode in such apparatus, but confirm that specific lubricant additives may reduce the fatigue life by promoting surface-initiated embrittled cracking similar to white etching cracks. A local criterion accounting for the local sliding frictional power dissipation and the lubrication regime is further proposed to assess the risk of white etching cracks based on the analysis of various reproduction and occurrences.

In radio astronomy, the radio spectrum is used to detect weak emission from celestial sources. By spectral averaging, observation noise is reduced and weak sources can be detected. However, more and more observations are polluted by man-made radio frequency interferences (RFI). The impact of these RFIs on power spectral measurement ranges from total saturation to subtle distortions of the data. To some extent, elimination of artefacts can be achieved by blanking polluted channels in real time. With this aim in view, a complete real-time digital system has been implemented on a set of FPGA and DSP. The current functionalities of the digital system have high dynamic range of 70 dB, bandwidth selection facilities ranging from 875 kHz to 14 MHz, high spectral resolution through a polyphase filter bank with up to 8192 channels with 49 152 coefficients and real-time time-frequency blanking with a robust threshold detector. This receiver has been used to reobserve the IIIWZ35 astronomical source which has been scrambled by a strong satellite RFI for several years.