AERolling element bearings are widely-used machine components and their failure can result in damage to the whole machine. A bearing failure can be caused by many factors, such as improper lubrication, the presence of abrasive particles, overloading, moisture, a corrosive environment, improper mounting, electrical discharges and material defects. These causes can be eliminated by observing the correct operating conditions, except for rolling contact fatigue (RCF). In this case, full-scale bearing life tests, or material tests for RCF resistance, are performed. Various methods of condition monitoring are used to detect damage to these components or specimens in industry or during testing in laboratories. An analysis of vibrations from the tested bearings/specimens is the most widely used method for damage detection and is based on vibration acceleration measurement and its analysis in the time or frequency domain. A more sensitive method for surface damage or subsurface crack detection is the acoustic emission (AE) method. Over the past three decades, AE-based monitoring has developed as a potential tool for rolling bearing diagnostics. This paper is aimed at the detection and monitoring of the onset and propagation of natural defects of steel specimens using AE technology. The experiments were carried out under various loading conditions on specimens of case-hardened 16MnCr5 steel. The AE signal parameters, such as count rate, cumulative count rate and RMS, were compared with vibration levels and temperature. In conclusion, the results of this study suggest that the AE monitoring method can be employed as an evaluation tool for rolling contact fatigue testing of material specimens.

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This paper deals with the basic research of cyclic damage during the initial stages of fatigue process using the non-destructive testing methods. The acoustic emission method was used for monitoring of the microstructure changes during fatigue loading. The electrical potential measurements of specimen and microscopic observation were used mainly to detect the first short cracks and their propagation. The fatigue tests at room temperature were conducted on titanium alloy and creep-resistant steel specimens under bending and tension loading. The aim of the study was to compare the acoustic emission signal at different types of loading until fracture and to analyze in detail the signal changes in initial stages of fatigue process. This analysis was primarily based on the waveform similarity and division into classes. The results show the high sensitivity of the acoustic emission technology in the transition from the stage of surface relief evolution to the stage of crack nucleation and propagation.