NEW MEASUREMENT APPLICATIONS OF INDUCTIVE METHODS IN DIAGNOSTICS OF STEEL MATERIAL AND STRUCTURE

ELEKTRYKA 16 Zeszyt 3-4 (39-4) Rok LXII Zbigniew Hilary ŻUREK Silesian University of Technology, Gliwice NEW MEASUREMENT APPLICATIONS OF INDUCTIVE METHODS IN DIAGNOSTICS OF STEEL MATERIAL AND STRUCTURE Summary: This paper presents the theoretical foundations of magneto-inductive flaw detection and its modifications facilitating the study of changes in the structure and fatigue degradation. We have proved high measuring resolution in identification of metal element condition with analysis of operational changes of electric and magnetic parameters as a function of the measurement coil signal frequency. The paper presents practical solutions in the form of equipment and measuring methods making it possible to lower testing costs in relation to professional flaw detection equipment. Keywords: inductive converters, RLC bridges, impedance component measurements, impedance spectroscopy, resonance diagnostics, non-destructive testing, NDT NOWE ZASTOSOWANIA POMIAROWE METOD INDUKCYJNYCH W DIAGNOSTYCE MATERIAŁU I STRUKTURY STALI Streszczenie: W artykule przedstawiono podstawy teoretyczne defektoskopii magneto indukcyjnej oraz jej modyfikacje ułatwiające badania zmian struktury i degradacji zmęczeniowej. Wykazano wysoką rozdzielczość pomiarów w identyfikowaniu stanu elementów metalowych poprzez analizę zmian eksploatacyjnych parametrów elektrycznych i magnetycznych w funkcji częstotliwości sygnału w cewce pomiarowej. Przedstawiono rozwiązania praktyczne w postaci urządzeń i metod pomiarowych umożliwiające obniżenie kosztów badań w stosunku do profesjonalnych urządzeń defektoskopowych. Słowa kluczowe: Przetworniki indukcyjne, mostki RLC, pomiary składowych impedancji, spektroskopia impedancji, diagnostyka rezonansowa, badania nieniszczące, NDT INTRODUCTION From the viewpoint of operational safety, diagnostics and preventive measures, it is necessary to be acquainted with changes of physical parameters taking place during fatigue cycles of material operating in some mechanical element of the machine [1,, 3]. Fatigue operational loads are defined in accordance with Wöhler curve [4]. 1

74 Z. H. Żurek One possibility of demonstrating the destruction of the material is to measure its magnetic and electrical parameters (which change due to cyclically varied loads which lead to material fatigue and its eventual destruction [5, 6, 7, 8, 11]). This issue is important both at the laboratory and industrial level. Nowadays, we find many sensitive methods of determining the process of material degradation (X-ray or neutron camera); however, magnetic methods are the future. These methods are at present investigated by numerous scientific laboratories throughout the world (in industry and in universities). The increase in dynamic loading of mechanical constructions causes disastrous fatigue failures due to cumulation and propagation of microcracks developing until critical parameters are reached [4, 1]. This takes place (according to newest research) in the final stage of element s life. For most materials this stage does not exceed 5%. Changes in size of cracks or their detection by standard (conventional) flaw detection devices are no longer sufficient. The non-destructive methods of measuring changes in magnetic and electrical parameters accompanied by significant development in magnetic field measurement methods (MR, GMR and Hall transducers) go back to measurements using inductive transducers (converters). Such transducers are still characterized by some properties (excluding operational temperature) which so far have not been replaced. This is confirmed by their use in space research. Using basic relationships describing the behaviour of material sample placed in coil supplied by alternating current, a new course (and parameter of magnetic diagnostics) has been defined. This indicates possibilities of investigating the development of fatigue defects over time. The basic problem of detecting the progress of degradation of element material during operation lies in non-linear changes of measured electrical and magnetic parameters with distinct maximum or minimum in the curve. This means that the measured value of a given physical parameter may be due to either initial or final period of operation. 1. FATIGUE EFFECTS IN STEEL AND CHANGE OF MAGNETIC PARAMETERS Fatigue stresses [4, 14] are strains varying over time and due to typical loads occurring in different mechanical systems. The changes in physical material parameters [1,13,15] such as yield point, tensile strength, elasticity, attenuating coefficients of sound wave, thermal properties, magnetic and electric properties are all due to processes of fatigue loading. There were numerous attempts at correlating all specified changes of physical parameters with material s fatigue process; these attempts aimed at design of devices for detecting this process. Majority of existing designs use significant changes in attenuation of sound wave (e.g. rotor caps) in the material before and after service time. Correlation of changes in physical parameters with fatigue degradation process is often more subtle as well as less

New measurement application... 75 strenuous when metallographic methods are used (this has been proven in numerous references [16-38]). The test detection of physical parameters is less destructive to material surface. If the detection is to be conducted on site, it is much cheaper than X-ray or neutron methods. The greatest problem lies in significant diversity in the directions of fatigue evolution of material physical parameters. Examples illustrating this issue will be discussed in subsequent sections. 1.1. Contribution of phase transitions Phase transitions are among the most transparent effects of changes in material's physical parameters. The relationships of plastic strain to change in structure and simultaneous change in material's physical parameters are particularly distinct in case of austenitic stainless nickelchromium steel (type: AISI - 34, DIN - 1.431). The measurements have been carried out using a tube segment (fragment of cooling system of electric motor manufactured by KOMEL). As a result of deformation, one millimetre thick tube wall (Fig.1) has experienced phase changes (mechanical and thermal perturbance of austenite s stability) dependent on the rate of plastic strain. The map of phase structure for segment marked in Fig.1 and situated in the deformation zone of tube segment is shown in Fig.. Fig. 1. Location of sector subjected to tests of phase structure and Euler s angles Rys. 1. Lokalizacja wycinka poddanego badaniu struktury fazowej i kątów Eulera Blue colour marks face-centred cubic structure (FCC), i.e. austenite, red colour is ferrite. The most deformed zones (compressed on the left, stretched on the right) exhibit a significant change in the austenite stability; austenite is subjected to phase transformation into ferrite. Austenite is prevalent in the central and least deformed zone (see Fig. a). Image of Euler s angles (Fig.b) displays the texture and most refined zones (result of plastic strain). These are extreme areas at the left and right side of the tube, in accordance with area marked in Fig.1.

76 Z. H. Żurek a) Fig.. Distribution of austenite and ferrite phases (a), view of Euler s angles (b) of the sector shown in Fig.1 Rys.. Rozkład fazy austenitu i ferrytu (a), obraz kątów Eulera (b) z wycinka na rys.1 Investigation of Euler s angles is just one of the non-destructive ways of imaging the scope of structure deformation (areas of fatigue degradation). Green colour in phase maps marks places which are not indexed by measurement system. b) 1.. Participation of structure degradation In some types of steel we may identify the history of increasing fatigue changes related to linear or pointwise defects. The simplest ones are glide lines constituting glide bands. Good example illustrating this issue is chromium-manganese steel (ASTM A89 class C); its physical parameters have been selected in such a way by setting chemical composition and processing, that electric and magnetic parameters (of a paramagnetic) should not vary during operation, on account of high alternating field. X8CrMn1818 material is parametrically stable steel of chromium-manganese type. In this steel we can observe the growth and movement of glide band lines [17, 18, 19] in relation to the initial structure, which has been developed in the material during plastic working carried out in production process.

New measurement application... 77 a) b) Rys. 3. Struktura stali A89 class C. Struktura wejściowa a) i po mln cykli obciążenia b) Fig. 3. Structure of A89 class C steel. Initial structure a) and after million loading cycles b) When number of loading cycles is increased, the number of glide bands rises and their direction in relation to image due to forging and rolling process is changed; physical parameters change as well. The increase in number of glide bands changes electric conductivity and it is also accompanied by attenuation of sound wave (this was the basis of investigation method abandoned in power engineering).. IMPACT OF OPERATION ON CHANGES IN MAGNETIC PARAMETERS OF STEEL (THERMAL POWER ENGINEERING) Character of material loading influences qualitative and quantitative image of changes in magnetic parameters. Input steels used in power engineering have been compared [18, 19, 3]. Even during initial comparisons (coal power engineering, Bełchatów power plant) of 13HMF, 15HM and H1M1F steels (Figs. 4, 5), we can observe significant differences in magnetic permeability before (initial state) and after operation. The measurements have been carried out with HP bridge owned by University of Silesia laboratory. 35 Re r 3 5 15 1 5 specimen material before use 15HM before use 13HMF before use H1M1F effect of operation 15HM effect of operation 13HMF effect of operation H1M1F,,5 1, 1,5 f, khz, Fig. 4. Curves of reactive permeability changes versus material grade and magnetic field frequency Rys. 4. Krzywe zmian przenikalności biernej w funkcji gatunku materiału i częstotliwości pola magnetycznego

78 Z. H. Żurek 1 Im r 1 8 specimen material before use 15HM before use 13HMF before use H1M1F effect of operation 15HM effect of operation 13HMF effect of operation H1M1F 6 4,,5 1, 1,5 f, khz, Fig. 5. Curves of active permeability changes versus material grade and magnetic field frequency Rys. 5. Krzywe zmian przenikalności czynnej w funkcji gatunku materiału i częstotliwości pola magnetycznego Significant changes in inductance which are related to operation have been observed for steel grades 15HM and H1M1F. Steel 13HMF has not displayed major operational changes of magnetic parameters in the assumed parameter measurement range. The reason for very minor inductance changes may be traced to differences in character of element loading or to element s chemical composition. 3. METAL CYLINDER IN LONGITUDINAL MAGNETIC FIELD NORMALIZED IMPEDANCE COMPONENTS For a long cylindrical sample (Fig.6) subjected to longitudinal and variable magnetic field, distribution of the field and eddy currents changes in accordance with sample radius r [5, 6, 9, 17-19]. Fig. 6. Cylindrical sample in the longitudinal magnetic field of excitation coil Rys. 6. Próbka cylindryczna w podłużnym polu magnetycznym uzwojenia wzbudzającego

New measurement application... 79 This type of rod sample is frequently controlled with induction method utilizing two coils [5, 6] (see Fig.7). Fig. 7. Configuration of two coils with tested sample [5, 6] Rys. 7. Układ dwóch cewek pomiarowych z badaną próbką [5, 6] If external magnetic field H contains axial component only, then eddy current density in the sample responds with angular component only. This is expressed by formula [5,6,9]: where: J J1 J ( kr) j r k H (1) J kr 1 ( ), ( ) kr - Bessel function of the first kind for integer order zero, kr - Bessel function of the first kind for integer order one. Complex coefficient of electromagnetic wave propagation in a conductor where wave damping coefficient k j μ k j k, () k r o μ 1 r o (3) and Δ is the depth of wave penetration into conductive environment. In the discussed case, magnetic flux density in the cylindrical sample is characterized by axial component only, this is described with formula [5,6,9]: J ( kr) B r H (4) J kr ( ) ro. ( ) F. Förster and K. Stambke introduced uniform magnetic field in the entire cross-section of the sample, described by magnetic flux density: B F-S r o H sk, where is the so-called effective magnetic permeability [5, 6]. sk (5)

8 Z. H. Żurek This magnetic flux density takes into account the skin effect present in the conducting cylinder of radius r, if the magnetic flux calculated for this density is equal to magnetic flux calculated for density given by formula (4), i.e. if the following equation is true: r r B F-S r r B( r r r (6) π d π ) d. Hence we obtain complex effective magnetic permeability as: The solution is r sk J kr r r r J kr d. (7) J kr 1 sk. kr J kr (8) Effective permeability depends on the sample diameter Dp r, specific conductivity γ, relative magnetic permeability of sample material r and frequency of the excitation field f. In case of low frequencies ( kr 1), eddy currents in the sample are negligible. The magnetic field H( r) is then constant and equal to H across the entire cross-section. Moreover, J kr and 1 kr J1 kr, which implies that μ sk kr 1, is called cut-off frequency. Remembering that 1. Frequency f g, where absolute value of kr = r jπf μ j r f μ 1, (9) g r o g r o and taking into account equality j 1, we obtain: π r f μ 1, (1) g r o and hence f g. π D μ (11) p r o In case of non-magnetic materials ( μr 1) the cut-off frequency depends solely on the sample diameter and conductivity. When cut-off frequency for a given sample is equal to the supply frequency, then in order to calculate effective permeability with the help of (8), we substitute kr 1. For operating frequency f fg, absolute value kr may be expressed with a multiple of f g : kr f kr f r πf μr f kr r f μ f f g r g g (1)

New measurement application... 81 Equation (1) implies the dependence of effective permeability on the ratio f / f only. Hence it arises that distribution of field and eddy currents in cylindrical samples and their effective permeability for identical ratios f / f are equal. This is the so-called Law of Similarity formulated by Friedrich Förster and his team. Effective permeability g sk g expressed by (8) is a basic quantity characterizing the condition of sample in variable magnetic field. Since effective permeability is a function of complex variable ( kr ), it is also a complex quantity: Re[ ] j Im [ ]. (13) sk sk sk The detailed information on material parameters may be obtained by determining impedance and measurement voltage of test coil. Voltage E induced in the measurement winding, when the coil is empty, and assuming that H H, may be calculated from the relationship: where π f, designate d d j μ e t E z z H Ss z j μ Ss H ( t ), dt dt (14) z number of turns, S s π 4 D s, DS diameter of test winding. Let us E jzμ π Ds H ( t) je. (15) 4 Since sample diameter D p is smaller than sample coil diameter, then cross-section area of the air-gap between sample and coil is equal to S L π( Ds Dp ). (16) 4 When sample is introduced into the coil, the induced voltage E is equal to geometric sum of voltages induced in the air-gap and sample Therefore E L jzμ π Dp sk H ( t) r (17) 4 E p jzμ π Dp sk H ( t) r. (18) 4 π E E L E p jzμ H ( t) Ds Dp Dp rsk, (19) 4 p p ( ) π D D E jzμ H t D s 1 rsk. () 4 Ds D s

8 Z. H. Żurek If we define coil stacking factor as then we obtain the relationship: Dp, (1) Ds r sk E E 1. () When voltages E and E are interpreted as voltage drops for empty coil and coil stacked with sample, respectively, we obtain: where: E I Z I j L, (3) E I Z I R j L, (4) I excitation current, L inductance of empty coil, L inductance of coil stacked with sample, Z impedance of empty coil, Z impedance of coil stacked with sample. When we divide both sides of (4) by (3), then we obtain the relationship: E E Z Z Therefore we arrive at the conclusion that voltage induced in excitation coil is changed when sample is introduced, and this change corresponds exactly to change in excitation winding impedance. Since E je and Z j L then E Z j1 rsk, E L and hence result the following dependencies for normalized impedance components [5, 6]:. For non-magnetic samples Re [ E] R r Im [ sk ], E L Im[ E] L 1 r Re [ sk ]. E L formula is simplified into the following form: r. (5) (6) (7) (8) ( 1), which stack the the coil completely ( 1), this Re [ E] R E L Im[ E] L E L Im [ ], sk Re [ ]. In this case the curve of normalized coil voltage (impedance) coincides with effective permeability curve sk of the sample. In case of magnetic materials, when sample diameter is sk (9) (3)

New measurement application... 83 increased, test voltage also rises. Increase in voltage is due to prevalence of variable magnetic field over eddy currents. In non-destructive testing, in order to neglect successive power losses due to coupling, we operate in a circuit where the supply coil acts as a test coil. This facilitates direct application of RLC measurement bridge as well. 4. THEORETICAL ANALYSIS OF THE COIL MEASUREMENT The research thus far and results of measurements of electric and magnetic parameters of material subjected to operational fatigue loading related to electric parameters (specific conductivity) and magnetic parameters (magnetic permeability) treated separately. Uniting both classes of parameters into a function of frequency during entire service life has eliminated the onerous metrological problem of equalizing results of electric and magnetic parameters in differing time intervals (until total destruction of the element). A new direction for magnetic diagnostics has been defined (new diagnostic parameter); this indicates a possibility of quantitative investigation of the progress in fatigue defects (material degradation) over time. Using Manipulate commands of MATHEMATICA software for presentation of calculation results and analysis of changes in impedance components in different ranges of operational frequency of inductive converter has made it possible to acquire data necessary for design of measurement coil more quickly and to select optimum ranges of measurement frequencies. The range of optimum frequency ratios f / g f lies in 3 7 interval. In this range we may detect subtle changes of material parameters. Frequency ranges used in eddy current devices for material flaw defection (cracks) are not suitable for detection of fatigue processes and specific stresses. Quickening of detectability analysis of changes in material physical parameters using changes in coil s real and imaginary impedance components provides an opportunity of selecting the optimum measurement frequency range (Figs. 8 and 9) for each steel type. In case of ferromagnetic steel (practical construction) with relative magnetic permeability much greater than 1, we observe change in the imaginary component caused by change in the material conductivity and permeability. Change in the output real component (Eqs. 7 and 8) is mostly caused by operational changes in magnetic permeability and impact of plastic effects (Villari effect). Application of determined measurement frequency ranges will shorten the time interval between lab tests and on site tests. Possibility of quick testing of changes in electric and magnetic parameters for different types of steel has been illustrated by different Manipulate examples in MATHEMATICA. In case of parametrically stable paramagnetic steel (e.g. generator caps), where relative magnetic permeability is only slightly greater than

84 Z. H. Żurek one, we observe mostly a change in imaginary component caused by changes in material and supplied coil conductivity (Eqs. 9 and 3). Fig. 8. Comparison of influence of two coil stacking coefficient values on the change of Re[ μ ] sk and Im[ μsk ] components of coil with paramagnetic material sample Rys. 8. Porównanie wpływu dwóch wartości współczynnika wypełnienia na zmianę składowych Re[ μ ] oraz Im[ μsk ] cewki z próbką materiału paramagnetycznego sk Fig. 9. Comparison of influence of two coil stacking coefficient values on the change in Re[ μ ] sk and Im[ μsk ] components of coil with ferromagnetic material sample Rys. 9. Porównanie wpływu dwóch wartości współczynnika wypełnienia na zmianę składowych Re[ μ ] oraz Im[ μsk ] cewki z próbką materiału ferromagnetycznego sk

New measurement application... 85 Basing on theoretical curves, we may select an approximate frequency interval, where it will be possible to detect changes in magnetic and electrical parameters of tested material with maximum resolution. In paramagnetic material the scope of frequencies useful from diagnostic viewpoint is much higher. In higher frequency range it is possible to detect changes in electrical and magnetic parameters; this means that detection of subtle processes of structure destruction taking place in the material is possible (the destruction is reflected in the changes of magnetic permeability and electric conductivity of tested element s steel). 5. RLC MEASUREMENT BRIDGES RLC measurement bridges are manufactured in many groups and classes. In the current paper, we have focused on two groups of RLC bridges. The first one includes professional bridges characterized by wide range of frequency, supply voltage and measurement current control and automation of measurements and recording. The second group contains bridges with one level of test voltage and current and limited range of measurement frequency, usually these are frequencies such as 1 Hz, 1 Hz, 1 khz, 1 khz (1 khz 1 MHz). This second group of simplified bridges has been described in order to prove that limitation of their measurement range does not reduce their utility scope, while sensitivity and resolution of detecting changes in electrical and magnetic parameters on the basis of changes in normalized impedance components is adequate and acceptable. Test procedure must be run with a single measurement instrument and in accordance with established bridge measurement rules; this determines the basic measurement circuit and maximum value of coil resistance. The ac RLC bridges operate in two basic configurations: serial (s), where quantities Ls, Rs, Cs are measured, or parallel (p), when quantities Lp, Rp and Cp are measured. The basic schemes are shown in Fig.1. Equivalent parallel circuits are used to measure high impedance elements, while series circuits are applied in measurements of low impedance elements, in order to limit power losses and errors. Coils used in diagnostics are characterized by impedance ranging from.5 to 5, that is why series circuit is used to measure Ls and Rs parameters. Example of measurement circuit and levels of measurement signal are shown in Fig.11. Application of test coils in improper series or parallel bridge configuration or change of this configuration during tests may lead to substantial errors [17-19]. This case is illustrated by test result of ferromagnetic samples connected into equivalent series measurement circuit (Rs, Ls) or parallel measurement circuit (Rp, Lp). In either case a test coil has been used. Results of sample tests in both measurement circuits are shown in Fig.1.

86 Z. H. Żurek Fig. 1. Basic equivalent series and parallel RLC bridge circuits Rys. 1. Podstawowe zastępcze szeregowe i równolegle układy pracy mostka RLC Fig. 11. Levels of measuring signals V at constant voltage level CV and constant current level CC Rys. 11. Poziomy sygnałów pomiarowych V przy stałym poziomie napięcia CV i przy stałej wartości prądu CC 15 14 13 1 11 1 9 8 7 6 5 4 3 1 L/L rdzeń 1, p rdzeń 1, s rdzeń, p rdzeń, s (R-R )/L 4 6 8 1 1 Fig. 1. Comparison of measurements of impedance components for two ferromagnetic cores in series (s) and parallel (p) circuits Rys. 1. Porównanie pomiaru składowych impedancji dwóch rdzeni ferromagnetycznych w układzie szeregowym s i równoległym p The curves shown in Fig.1 differ widely on account of coil parameters and measurement method (parallel or series circuit).

New measurement application... 87 6. METHOD OF IMPEDANCE SPECTROSCOPY, VOLTAGE OR CURRENT RESONANCE IN NDT The assessment of changes in physical parameters of metals and their alloys is often used in technical diagnostics, for detecting development in degradation changes in these materials. Such diagnostic method may be for instance (Fig.13) applied to aluminium alloys with admixture added for abrasibility sake (tribologic effects). Chemically pure aluminium is a 6 paramagnetic with magnetic susceptibility equal to x1 and electric conductivity (lower than that of copper by 3%) equal to 37.67 MS/m. 18 16 14 1 1 Z, cewka pusta,388e6 Hz probki stopu Al w cewce p1al pal p3al p4al 8 6 4 3,36E6 Hz 3,59659E6 Hz f, Hz 1 3 4 5 Fig. 13. Curve of impedance changes of empty coil and coil stacked with material samples vs. measuring frequency Rys. 13. Wykres zmian impedancji cewki pustej i z próbkami materiału w funkcji zakresu częstotliwości pomiarowej Possibility of distinguishing subtle changes in chemical composition hastens the investigations and makes it possible to analyse the changes (Fig.14) at once. 4 L/L 3 p1al pal p3al p4al punkt pom. 78 3,49658E6 Hz (3,4159, 3,58) 1 punkt pom. 53 -,3883E6 Hz (-3,6198,,9841) punkt pom.73 3,361E6 Hz (,9913,,57456) (R-R )/(L ) -3,61 - -1 1 3 4 5 6 Fig. 14. Dependence of the calculated normalized components of impedance vs. measurement frequency Rys. 14. Zależność wyliczonych unormowanych składowych impedancji w przedziale częstotliwości pomiarowej

88 Z. H. Żurek In professional measurement bridges, the condition of electronic subassembly (in our case it is coil with core made up of tested steel) is assessed on the basis of changes in coil core impedance Z in relation to the standard associated with service time. In the range of set alternating measurement current frequencies, the lengths of current and voltage vectors are measured. From the vector lengths the processor calculates value of impedance Z and phase angle ; hence we obtain resistance R, reactance X and impedance module Z indispensable for constructing curves. Numerous other parameters measured by RLC bridges are not used in such diagnostics. The professional measuring instruments are characterized by wide range of operational frequency, voltage and current and high stability of these quantities over time and temperature range. During our research we have proved test usefulness of cheap RLC measurement bridges. 7. MEASUREMENTS WITH BRIDGE OF SIMPLIFIED DESIGN RLC bridges of simplified design usually possess four to five frequency ranges, one current range and one voltage range. Switching of measurement ranges into series or parallel circuit, change of measured quantity and change of frequency range is achieved manually. In order to obtain high sensitivity and resolution of basic RLC meter, we have conducted comparison measurements for different coil parameters. Carbon steel used in transport for wheelset construction has been tested. Test results of changes in impedance components are shown in Fig.15; tests have been run with RLC meter operating in five frequency ranges. The appropriate selection of coil resistance and number of turns gives measurement results which are close to values obtained theoretically. The convergence of measurement results (at few frequency values, limited by RLC bridge type) with theoretical curve (MATHEMATICA Manipulate command) has been achieved by using measurement coil with very low winding resistance. A characteristic feature of theoretical curve is that measurement results approach zero, when frequency increases to infinity. Test results of samples made of carbon steel are shown in Fig.16; RLC bridge with unchanged configuration has been used, but with two measurement coils differing in number of turns, resistance and impedance. In both cases the applied ranges of measurement frequency tend to change shapes of obtained curves; coil with high resistance produces worst results. This high resistance changes the course of curve linking measurement points and values of point coordinates in the range of normalized impedance components.

New measurement application... 89 35 3 L/L Hz 1 Hz 5 1 Hz stal P35/3 1 Hz 15 1 1 khz 5 1 khz Hz (R-R )/L 1 1 3 3 4 4 5 5 Fig. 15. Convergence of theoretical curve from MATHEMATICA Manipulate with measurements in five frequency sub-ranges Rys. 15. Nałożenie przebiegu teoretycznego z manipulatora MATHEMATICA i z pomiarów w pięciu podzakresach wartości częstotliwości 45 P35/3 cewka 1 4 zw., 7,73 P35/3 cewka 4 zw.,,1 4 35 3 5 L/L 1 Hz 1 Hz 15 1 1 khz 5 1 khz (R-R )/L -1, -7,5-5, -,5,,5 5, 7,5 1, Fig. 16. Comparison of standard steel sample measuring results, for two measurement coils designs Rys. 16. Porównanie wyników pomiaru wzorcowej próbki stalowej, wykonanego z użyciem dwóch konstrukcji cewek pomiarowych In comparative structuroscopy, both coils are useful, but the one with increased winding resistance is better. If coil resistance is too high, then ferromagnetics cannot be tested. Resistance value should be selected keeping in view magnetic and electrical parameters of the material and cut-off frequency. The range of coil parameters given in Fig.16 is correct for ferromagnetic steels. 8. INVESTIGATION OF VILLARI EFFECT IN THE CO-ORDINATE SYSTEM OF NORMALIZED IMPEDANCE COMPONENTS Tests of changes in initial magnetization curve of material magnetic permeability have been run for most steel types used in transport. The initial magnetization curve and permeability have been charted for P35 steel rod sample. The curves for non-loaded and loaded rod are shown in Figs. 17 and 18 (load has been equal to 5 MPa, tensile force).

9 Z. H. Żurek 1,4 1, B, T 1,,8 P35 producent 1,6 MPa 5 MPa,4,, H, A/m 1 3 4 5 6 Fig. 17. Initial magnetization curves of P35 steel, without and with loading Rys. 17. Krzywe pierwszego magnesowania stali P35 bez obciążenia i z obciążeniem 4 r 3 P35, 1 MPa 5 MPa 1 H, A/m 1 3 4 5 6 Fig. 18. Magnetic permeability curves of P35 steel, without and with loading Rys. 18. Krzywe przenikalności magnetycznej stali P35 bez obciążenia i z obciążeniem Fig.19. Villari effect recorded in the image of normalized impedance components is shown in 6 L/L P55AT - MPA P55AT - 4 MPA po 1 min P55AT - 4 MPA po 4 H P55AT - 4 MPA po 168 H P35_3 - MPa po 1 min P35_3-4 MPa 5 4 3 1 R/L - -15-1 -5 5 1 15 Fig. 19. Villari effect recorded by changes of normalized impedance components for five basic ranges of RLC bridge Rys. 19. Efekt Villariego zarejestrowany przez zmienności unormowanych składowych impedancji dla pięciu zakresów podstawowych miernika RLC

New measurement application... 91 Creeping and relaxation changes of P55AT steel have been recorded in the time interval range.6 h 168 h. Legibility of changes in impedance components (Fig.19) in relation to standard measurements (initial magnetization curves) is high even when bridges of simplified design are used. 9. INVESTIGATION OF TECHNICAL CONDITION OF GENERATOR STATOR SILICON STEEL LAMINATIONS The feasibility of using test procedures in power engineering diagnostics has been verified with tests of parameter changes of stator laminations in turbogenerator after a certain service life. The end laminations of generator s stator core belong to those elements of generator s end region which are subjected to impact of leakage magnetic field due to currents in coil overhangs of the stator and rotor windings. This field rotates synchronously in relation to stator. Intensity of additional heating of end laminations depends on generator load character (this in turn influences the shape of leakage flux path). The most unfavourable effects occur when generator operates at underexcitation, with capacitative load and armature current close to rated current. In high-power generators different numerous methods reducing the consequences of this effect are used. Nevertheless, the overheating of end laminations is the cause of multiple failures and damages in generators [ 18, 1, 5]. Stator scheme and example of end laminations are shown in Figs. -1. Pakiety skrajne Pakiety środkowe Fig.. View of generator stator core end laminations and their location Rys.. Widok pakietów skrajnych stojana generatora i schemat lokalizacji

9 Z. H. Żurek Fig. 1. View of stator core end laminations Rys. 1. Widok na pakiety skrajne rdzenia stojana Samples for testing have been cut out from end and centre laminations. The measurements have been run with RLC bridge Agilent 463B. The coil of test probe has been supplied with voltage (1 mv) at frequencies ranging from 1 Hz to 1 Hz (bridge ranges). From the measurements we have obtained Z, R and L parameters of the measurement coil; the real and imaginary components of effective permeability have been calculated, for frequencies equal to 1 Hz, 1 Hz, 1 Hz, Hz i 1 Hz (Fig. ). Fig.. Measurement results of changes in normalized impedance components of electromagnetic steel sheets for selected frequencies Rys.. Wyniki pomiaru zmian unormowanych składowych impedancji blach elektrotechnicznych dla wybranych częstotliwości The curves of normalized impedance components, which have been determined for five frequency ranges (f = 1, 1, 1, and 1 Hz) are characterized by dynamic changes in material permeability and specific conductivity. The significant changes in parameters after service life are displayed by samples taken from the end laminations, from short-circuit location marked as E and samples of steel subjected to regeneration D.

New measurement application... 93 In case of two samples selected on account of their location and degradation processes influencing parameter changes, full-range measurements with professional RLC bridge HIOKI 353-5 have been run for frequency range 1 Hz to 1 Hz. 1. DESIGN OF MEASUREMENT CONVERTER ON THE BASIS OF TI MEASUREMENT MODULE A substantial acceleration of tests may be achieved with the help of measurement module made by Texas Instruments (see Fig.3). This module and modified test procedures have been described in Ph.D. thesis currently under preparation by Mr. Dariusz Baron (EthosEnergy Poland S.A., previously TurboCare Poland S.A.). TI module has made it possible to construct a miniature tester for operational degradation of material (samples as well as surfaces of ferromagnetic and paramagnetic materials). Module used for testing the surfaces is shown in Figs. 3 and 4. Fig. 3. Measuring module with modified position of measurement coil Rys. 3. Moduł pomiarowy z modyfikowaną pozycją cewki pomiarowej Fig. 4. Prototype of detecting probe Rys. 4. Prototyp sondy detekcyjnej Device shown in Fig.4 has been used for non-destructive testing of wear degradation of turbogenerator rotor caps.

94 Z. H. Żurek 11. CONCLUSION Method based on historic procedures introduced by Förster into non-destructive testing of cracks and material selection (e.g. after hardening) has acquired a new application field in diagnostics of fatigue degradation process. This usage has made it possible to assess quantitatively the wear development in element over time. For the scientific circles working directly on the non-destructive testing of fatigue degradation effects, the possibility of relating the degradation process to non-linear changes in physical parameters over time provides a multitude of new research opportunities. It is now possible to evaluate the material with analysis of impedance components of measurement coil sample or material surface. This method additionally lowers the costs of both lab and on-site tests. The dissemination of testing procedures will allow and accelerate the research into development of new generation flaw detection devices and provide new measurement prospects. Miniaturization of professional, cheap and generally available measurement modules during last few years helps to increase the number of possible applications of this measurement method. REFERENCES 1. Baron D., Żurek Z.H.: Spektroskopia impedancji metoda oceny stopnia wyeksploatowania kołpaków wirników generatorów. Przegląd Elektrotechniczny 14, nr 3, s. 55-59.. Bozorth R.: Ferromagnetism. The Institute of Electrical and Electronics Engineers. IEEE Magnetics Society, Sponsor, Inc., New York 1936, An IEEE Press Classic Reissue, Magnetic Society, 1993, 1. 3. Brailsford F.: Materiały magnetyczne. PWN, Warszawa 1964. 4. Dietrich L., Rutecka A., Kowalewski Z.: Mechanical characterization Fatigue and Creep of A356+TiB based MMC. IPPT Report 9 for INASMET w San Sebastian, Hiszpania. 5. Förster F.: Theoretische und experimentelle Ergebnisse des magnetischen Streufluseverfahrens. Materialprüfung 1981, Vol. 3, No. 11, p. 37-378. 6. Heptner H., Stroppe H.: Magnetische und Magnetoinduktive Werkstoffprüfung, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig 1969 197. 7. Janeczek T.: Diagnostyka eksploatacyjna kolejowych zestawów kołowych metodą magnetyczną. Politechnika Śląska, Wydział Transportu, Katowice 9 (rozprawa doktorska). 8. Kocańda S.: Zmęczeniowe niszczenie metali. WNT, Warszawa 1978. 9. Krakowski M.: Elektrotechnika teoretyczna. PWN, Warszawa 1983.

New measurement application... 95 1. Kurzydłowski K.J.: Metody monitorowania degradacji struktury materiałów. Badania mechanicznych właściwości materiałów i konstrukcji. Wykłady, Zakopane 1-13 grudnia 3, IPPT, Warszawa 3. 11. Markuszewicz M., Mierzejewski A.: Materiały magnetyczne. Wydawnictwo Górniczo- Hutnicze, Katowice 1954. 1. Rasek J.: Materiały amorficzne i ich właściwości. W kręgu krystalografii i nauki o materiałach. Wydawnictwo Uniwersytetu Śląskiego, Katowice, s. 7-45. 13. Sikora R.: Elektromagnetyczne metody testowania materii. Instytut Naukowo-Badawczy ZTurek, Warszawa 3. 14. Socha G.: Experimental investigation of fatigue cracks nucleation, growth and coalescence in structural steel. Int. J. Fatigue, 5, 3, pp. 139-147. 15. Starke P., Walther F., Eifler D.: PHYBAL - A new method for lifetime prediction based on strain, temperature and electrical measurements. International Journal of Fatigue 6, Vol. 8, No. 9, p. 18-136. 16. Żurek Z.H.: Badanie stanu ferromagnetyków w polu magnetycznym dla diagnostyki technicznej, ELEKTRYKA. Prace Naukowe 1(6), Politechnika Radomska, Radom 3, VI Konferencja Systemy TRANSCOMP, Zakopane, 4-6.1.3. 17. Żurek Z.H.: Wprowadzenie do elektromagnetycznej diagnostyki degradacji stali. Wyd. Politechniki Śląskiej, Gliwice 1. 18. Żurek Z.H.. Piotr D.: Obwody RLC w diagnostyce i eksploatacji maszyn Adres wydawniczy: Gliwice, Wydawnictwo Politechniki Śląskiej, 14, PL ISBN 978-83-788-53- 19. Żurek Z.H. Piotr D.: RLC circuits for material testing and NDT Adres wydawniczy: Katowice 15, KOMEL, PL ISBN 978-83-931-99-8-. Żurek Z.H.: Aplication of flaw detection methods for detection of fatigue processes in low-alloyed steel. European NDT Days in Prague, 7, November 5-9, 7, Czech Republic. http://www.cndt.cz/endtd7/content/documents/program_nde.pdf 1. Żurek Z.H.: Metoda diagnostyki stanu stalowych (paramagnetycznych i ferromagnetycznych) elementów maszyn elektrycznych na przykładzie bandaży i kap wirników generatorów, N N51 38538 (raport).. Żurek Z.H.: Opracowanie metody magnetycznej do wczesnej detekcji procesów zmęczeniowych w stalach niskostopowych niskowęglowych, N N57 87 33 (raport). 3. Żurek Z.H., Janeczek T., Maciejewski J.: Steel Magnetic Parameters as Material Fatigue Diagnostics Criterion. NDT.net, Issue 9-3, p. 51-57. 4. Żurek Z.H., Szudyga M.: Magnetometric diagnostics of constructional steels. Electrical Review. Przegląd Elektrotechniczny 9, no. 1, p. 118-1. 5. Żurek Z.H., Baron D.: Measurement of changes of magnetic permeability and electric conductivity values in generator rotor retaining rings, Oficyna Wyd. Politechniki Wrocławskiej, Wrocław 1, p. 183-188. 6. Żurek Z.H.: Ocena możliwości bezstykowej diagnostyki stanu zestawu kołowego metodą magnetyczną. Przegląd Elektrotechniczny 5, nr 6, s. 51-56. 7. Żurek Z.H., Sieradzki S., Adamek J.: Ocena stanu technicznego kołpaków czół uzwojeń turbogeneratorów na podstawie magnetycznych niestabilności austenitu G18H18. Przegląd Elektrotechniczny 1, nr 7a, s. 18-. 8. Żurek Z.H., Kukla D., Kurzydłowski J.: Wybrane metody wykrywania degradacji zmęczeniowej w stalach ferromagnetycznych. Przegląd Elektrotechniczny 1, nr a, s. 86-9.

96 Z. H. Żurek 9. Żurek Z.H., Baron D.: Niestabilność eksploatacyjna parametrów magnetycznych i elektrycznych blach rdzenia stojana generatora TWW--A. Przegląd Elektrotechniczny 14, nr 1, s. 1-3. 3. Żurek Z.H., Baron D.: Spektroskopia impedancji metoda oceny stanu technicznego kołpaków wirników generatorów. XVIII Konferencja Energetyki, Gniew 13. 31. Żurek Z.H., Baron D.: Badanie stabilności eksploatacyjnej parametrów magne-tycznych i elektrycznych blach rdzenia stojana generatora typu TWW--A. XVIII Konferencja Energetyki, Gniew 13. 3. Żurek Z.H., Baron D.: Spektroskopia impedancji uniwersalny parametr diagnostyki elementów maszyn i urządzeń. XLIX Sympozjum SME, Gdynia/Gdańsk 13. 33. Żurek Z.H., Baron D.: Pomiar zmian wartości przenikalności magnetycznej i przewodności elektrycznej właściwej kołpaków generatorów. XLVIII Sympozjum SME, Książ 1. 34. Żurek Z.H., Kukla D., Baron D.: Materiałowe warunki brzegowe blach elektrotechnicznych krzemowych. XLVIII Sympozjum SME, Książ 1. 35. Żurek Z.H, Baron. D. Niestabilność eksploatacyjna parametrów magnetycznych i elektrycznych blach rdzenia stojana generatora TWW--A. Przegląd Elektrotechniczny 14, nr 1, s. 1-3. 36. Żurek Z.H, Baron. D. Diagnostyka maszyn w transporcie i energetyce metodą spektroskopii impedancji. KOMEL Maszyny Elektryczne - Zeszyty Problemowe 13, nr 1, cz. 1, s. 195-198. 37. Żurek Z.H, Kurzydłowski J. K., Kukla D., Baron D. Material edge conditions of electromagnetic silicon steel sheets. Przegląd Elektrotechniczny 13, nr b, s. 11-115. Dr hab. inż. Zbigniew Hilary ŻUREK, Prof. Pol. Śl. Silesian University of Technology Faculty of Transport Department of Automotive Vehicle Construction ul. Krasińskiego 8 4-19 Katowice e-mail: zbigniew.zurek@polsl.pl