On the Modal Analysis of a Cracking Compressor Blade The paper deals with issues related to reliable recognition and prediction of health of steel compressor blades. It outlines symptoms that indicate the blade cracking, and identification methods that can be used during operation, maintenance and repairs/overhauls of turbine jet engines of the SO-3 type. An optical method involving laser measurements of blade vibration while on the excitation source is presented in details as it is implemented in the WZL-3 (Military Aviation Works No. 3). The method has been verified in practice and some findings from the verification process are discussed. Next, new testing capabilities offered by the method are presented, including identification of early symptoms of material fatigue (the strengthening and the weakening of the material) and the crack initiation. The topics under discussion have been illustrated with some examples taken while measuring modal properties typical of both defect-free and faulty blades as well as from fatigue tests conducted according to the LCF and HCF profiles. It has been proved that the analysis of resonance responses for specific modes of operation allows of reliable differentiation between material's fatigue and cracking.

The diagnostic and research aspects of compressor blade fatigue detection have been elaborated in the paper. The real maintenance and overhaul problems and characteristic of different modes of metal blade fatigue (LCF, HCF, and VHCF) have been presented. The polycrystalline defects and impurities influencing the fatigue, along with their related surface finish techniques, are taken into account. The three experimental methods of structural health assessment are considered. The metal magnetic memory (MMM), experimental modal analysis (EMA) and tip timing (TTM) methods provide information on the damage of diagnosed objects, for example, compressor blades. Early damage symptoms, that is, magnetic and modal properties of material strengthening and weakening phases (change of local dislocation density and grain diameter, increase of structural and magnetic anisotropy), have been described. It has been proven that the shape of resonance characteristic gives abilities to determine if fatigue or a blade crack is concerned. The capabilities of the methods for steel and titanium alloy blades have been illustrated in examples from active and passive experiments. In the conclusion, the MMM, EMA, and TTM have been verified, and the potential for reliable diagnosis of the compressor blades using this method has been confirmed.

The intended aim of the paper was to present a short review of more than 15 years of experience of ITWL in the field of applying the signal of actual rotational speed (aperiodic and oscillation components thereof) to the expert diagnosing of aero-engines, including identification of low- and high-cycle fatigue (LCF, HCF) of critical structural members. What has been presented is some essential metrological bearings of the non-contact technique of measuring the engine’s rpm with some flexible key phasors (i.e. vibrating compressor/turbine blades). Also, methods of numerical analysis of measuring signals, in use nowadays, have been discussed. With the jet engine of the SO-3 type (in use on the TS-11 “Iskra” combat trainer) as an example, are discussed algorithms of both the identification of disadvantageous aeromechanical effects (energy state of the engine - i.e. the source of accelerated HCF wear of structural components) and the early detection of symptoms of fatigue failures to compressor blades and the bearing system. The discussed problems have been illustrated with examples selected as to emphasise practicalities of applying a new source of diagnostic information to ‘actively’ control the process of fatigue wear (HCF + LCF) of engine components and to forecast the engine health/maintenance status.

The magnetic field created by technical devices is a source of information. This information could be used in contactless diagnostics and predictive maintenance or for resolving problems along with standard NDT (nondestructive testing) methods, especially if we consider large, slow-speed devices, such as electromotors, transmissions, or generators. Identification of causalities of device failure processes with near magnetic field is one of the suitable NDT methods improving sustainability of systems. The measurements presented in the article were performed with the VEMA 04 fluxgate vector magnetometer with the DC-250 Hz bandwidth and 2 nT sensitivity. Postprocessing of the results was performed in the means of standard methods of discrete Fourier Transform, spectrogram creation and Wavelet Transform. The article presents data gathered during the measurement of a pair of extraction fans with power of 140 kW each and maximum revolutions up to 740 rev/min controlled by frequency converters and a single semi-Kaplan water power plant with 400 kW peak power at 1005 rev/min maximum generator speed. The measurements were performed before and after repairs of one of the ventilators in the ventilation system at 60% and 100% of maximal output power. The rotating magnetic fields of the fan electromotor stator, fan rotor revolutions, rotor slip frequency and ball-bearing frequencies were identified in frequency spectrums in the distance of 700 mm from fan electromotor axis in both cases. During the measurements on the semi-Kaplan turbine, the changes in states of mechanical and electrical components of the machine were monitored in the magnetic fields with increase of the power in the range of 0–95%, before and after phasing to the electrical grid. Standard processing methods, Discrete Fourier Transform, spectrograms and Discrete Wavelet Transform were used. In the spectrograms of the measured magnetic fields, the 1st–4th harmonics of the turbine shaft, generator shaft and also their side frequencies were identified. Significant changes of magnetic fields in time were identified in the area of 60–95% power. With the help of the Wavelet, transform intervals were identified where it is desirable to operate the turbine. The analyses of magnetic fields measurements performed on the power plant were compared with vibro-diagnostic principles.

This paper investigates the diagnostic and research aspects of the compressor blade fatigue. The authors have reviewed the characteristics of different modes of metal blade fatigue (LCF, HCF, VHCF). The polycrystalline defects and impurities influencing the fatigue, along with their related surface finish techniques, have been taken into account. The experimental methods of structural health assessment have been considered. The Tip Timing (TTM), Experimental Modal Analysis (EMA) and Metal Magnetic Memory (MMM) provide information on the damage of the diagnosed object (compressor blade). It has been proven that the shape of resonance characteristics gives an ability to determinate if fatigue or a blade crack is concerned. Early damage symptoms, i.e. modal properties of material strengthening and weakening phases have been described. The experimental verification of the FEM model is presented based on a large body of experimental data collected by the author.

This paper concerns the idea of using magneto-mechanical effects in prophylactic activities to reduce the risk of occurrence of an extensive blackout on high voltage (HV) lines. The subject matter of the paper is related to the event from April 8, 2008, which deprived approximately 600,000 recipients from 18 cities of the West Pomeranian voivodeship of power and caused millions in losses for the national economy. In theoretical considerations, the idea of diagnosing the technical state and level of effort of steel power line towers using the metal magnetic memory method (MMM) is presented, with a consideration of the methodology for obtaining a reference signal from the external magnetic field. The capabilities of the MMM method have been illustrated by examples of active and passive experiments. Finally, the authors indicated the need of IT support of MMM data analysis using a 3D model of the predicted distribution of the magnetic field in the vicinity of the tested object.

The first step of a methodological approach to the validation of the metal magnetic memory (MMM) method in non-destructive testing (NDT) applications and in systems used for the diagnosis of early stages of material fatigue in mechanical constructions (structural health monitoring, SHM, and prognosis health management, PHM) has been presented in the paper. The study is focused on the properties of the external natural source of magnetisation of the object under MMM examination and the impact of its components. The precise data of the Earth's geomagnetism measurements (from ground stations and satellites) and the revised model of the Earth's magnetism can be applied in order to calibrate high sensitive magnetic field sensors, validate measurement results and extend the functional capacity of the MMM method.

The influence of mechanical stress on low frequency AC magnetic permeability was studied. The cold-drawn bars with C45 steel were subjected to investigation. The tensile stress (within elastic regime) was applied by means of material testing machine. Simple measuring system was assembled, which consisted of: function generator with magnetizing coil, detection coil and precise AC voltmeter. The registered changes of induced voltage were proportional to the change of magnetic permeability of the stretched rods. The obtained results were almost frequency-independent due to low frequency limit (250-500 Hz, weak eddy currents, no spin-origin energy dissipation). A significant magneto-mechanical hysteresis was observed slightly evolving from cycle to cycle with tendency of stabilization. The extension of basic Stoner-Wohlfarth model of magnetic permeability allowed to fit the data reproducing hysteretic behavior and considering the relaxation of the internal stress. The proposed, low-cost method is suitable in the industrial applications for stress control in large-sized steel elements.