Improving the Lorentz Force Amplitude of an EMAT Using Stacked Coil Configuration

Transcription

1 Sensors & Transucers, Vol. 155, Issue 8, August 213, pp Sensors & Transucers 213 by IFSA Improving the Lorentz Force Amplitue of an MAT Using Stacke Coil Configuration 1 vans Ashigwuike, 1 Wamaeva Balachanran, 2 Saiq Thomas, 1 Naaraja Manivannan, 1 Ruth Mackay 1 Centre for lectronic System Research, Brunel University Uxbrige, Milesex, UB83PH 2 Herriot Watt University, School of Built ngineering, Institute for Infrastructure & nvironment, inburgh, 14 4AS, UK Tel.: +44() , fax: +44() mail: evans.ashigwuike@brunel.ac.uk, s.thomas@hw.ac.uk Receive: 16 May 213 /Accepte: 12 August 213 /Publishe:2 August 213 Abstract: This research presents a stuy of a novel metho of improving the Lorentz force amplitue of a planer spiral coil MAT configuration by stacking the coils. A 2D axisymmetric Finite lement Moel was evelope to stuy an compare the ensity of the Lorentz Force (LF) generate at the skin epth by the Stacke Spiral-MAT (SS-MAT) configuration on cs7 graes of pipe steel materials. The response of the novel MAT configuration to variation of parameters such as the static magnetic fiel, internal iameter, excitation frequency, lift-off, coil stacking, the thickness of the insulation layer an number of cycles per tone burst were also investigate to ascertain the viability of the Stacke Spiral-MAT. The results obtaine showe that there is a significant improvement in the ensity of Lorentz Force generate by the novel MAT when stacke. Hence a new MAT configuration is propose to enhance the performance of raially polarise bulk wave MATs. Copyright 213 IFSA. Keywors: Lorentz force, lectromagnetic acoustic transucer, y current. 1. Introuction lectromagnetic Acoustic Transucers (MATs) can generate an etect ultrasonic wave without making contact with the test material. This feature makes it an attractive option in contactless measurement such as when carrying out inspection in high temperature environment or taking measurement in a moving object [1-3]. MATs have also foun usefulness in many non-estructive evaluation applications, such as the etermination of physical parameters of conucting materials, flaw characterization, imension an thickness measurement. MATs consist of coils an permanent or electromagnet locate in proximity to the surface of the material. The coils are excite with a high frequency tone burst pulse, this creates a ynamic magnetic fiel which further inuces a high frequency ey current on the conucting material [1, 4]. The interaction of the ey current an the static magnetic fiel (bias fiel) gives rise to mechanical strain an hence prouces ultrasonic waves on the material [1, 4, 5]. The outcome of researches uring the last couple of years on the performance of MAT on conucting materials (ferro-magnetic an non-ferromagnetic material) have been reporte [6-9]. The reports have establishe that MAT has low transuction efficiency which gives rise to reuce signal to noise ratio (SNR). In other to increase the signal to noise 262 Article number P_SI_1313

2 Sensors & Transucers, Vol. 155, Issue 8, August 213, pp ratio of MAT, several attempts have been mae to improve the performance of MATs 1-18]. In [19] it was establishe that LF mechanism is the ominant transuction mechanism for normal biase MATs configuration on ferro-magnetic materials. Table 1 shows the measure electromagnetic properties of the grae of steel (pipe steel) use in this stuy. The research paper presents a 2 D axisymmetric finite element moel of a Stacke spiral MAT coil configuration an a novel stacke spiral MAT (SS-MAT) coil configuration to optimize the generation of ultrasonic wave on steel material base in LF mechanism. This is achieve by improving the strength of LF generate by the MAT system. The SS-MAT moel is an improvement on the conventional planner circular spiral coil configurations. Fig. 2: 2D Representation of SS-MAT showing y current, static fiel an effective Lorentz force; (a) single coil layer over pipe steel specimen; (b) stacke (two) coil layer over pipe steel specimen; (c) propose stacke (three) coil layer over pipe steel specimen. All rawings are to scale. Table 1. Measure lectromagnetic properties of CS7 grae of pipe steel material [6]. lectrical Material Relative Conuctivity esignation permeability µ r (S/m) CS Fig. 1. The structural iagram of a circular planner spiral coil. (a) (b) (c) Fig. 2. 2D Representation of SS-MAT showing y current, static fiel an effective Lorentz force. 263

3 Sensors & Transucers, Vol. 155, Issue 8, August 213, pp Theory This analysis is base on the fact that in normal bias MAT structure, LF has been proven to be the ominant transuction mechanism for both Ferromagnetic an non ferro-magnetic materials [2]. Lorentz force is generate as a result of the interaction between the magnetic flux ensity an the ey current ensity J inuce by a coil excite with a sinusoial current. The sinusoial excitation current generate ynamic flux ensity B as well as J y current in the steel plate. The y current experiences a Lorentz force uner the interaction of the ynamic magnetic fiel from the excitation current an the static magnetic fiel B s from the permanent or electromagnet. The LF causes the vibration of the atomic structure of the material, which then leas to the generation of acoustic wave insie the conucting material. Generally, the irection an strength of the LF is governe by the following vector equations [21]. H = J, (1) B = r H, (2) = B /t, (3) J =σ, (4) consist of a rectangular meaner-line coil, Fig. 2 (b) consist of a planer spiral coil while Fig. 2 (c) an Fig. 2 () consist of the novel single an ouble layer SS-MAT coil respectively. ach structure consists of ientical cross-sectional area of thickness t c, with w c, a permanent magnet an pipe steel specimen of 1mm thick. The lift off istance between the rectangular coils an the steel specimen is h an the istance between each coil turn is enote as. In Fig. 2 (b) an Fig. 2 (c), the internal iameter of the coil is enote as in. A 2D axis symmetrical finite element moel was establishe using a commercial software calle ComsolMultiphysics, as shown in Fig. 5. The coil was excite with a transient current [11]: i k t, w t n k 1 I 1 cos cosw t, for t 2n for / w (9) t 2n / w where I, w an n are the amplitue of the current, the angular centre frequency an the number of cycles respectively. w =2πf (1) Fig. 3 shows the time history of external current ensity of the SS-MAT at 3 A. f = J B, (5) s f = J B, (6) s f L = f + f s, (7) F L = f L V, (8) where μ r an σ, are the relative permeability an conuctivity of CS7 grae of pipe steel;, H are the electric fiel intensity an ynamic magnetic intensity respectively; f an f s are the Lorentz force ensities ue to the ynamic an static magnetic fiel respectively; f L is the composite Lorentz force ensities of f an f ; an s F L is the Lorentz force on a given volume, where ρ is mass volume ensity an τ an μ are the Lamé constants. 3. Finite lement Moelling The structure of the MAT consiere in this research is shown in Fig. 2. In Fig. 2 (a), the structure Fig. 3. Time history of the xternal current ensity of three layer SS-MAT (I =3 A) Moelling of Single Layer PS-MAT The parameters use for the simulation of the 1 layer SP-MAT in Fig. 2 (a) are: f =5 khz, n=2, I =3 A, B =1.2 T, W c =.8 mm, in =5 mm, c =.1 mm an h=.5 mm. 264

4 Sensors & Transucers, Vol. 155, Issue 8, August 213, pp Moelling of Propose Stacke SP-MAT The simulation was carrie out using the same coil parameters as in 3.2 above but with ajustment in the number of layers of the coil to two in Fig. 2 (b) an three in Fig. 2 (c). An insulation layer thickness (t in ) of.1 mm was also inclue in the moel. The insulation layer was moelle as a composite polymer (Z1258 anti rust silicone baffle paint) with performance inex as presente in Table 2 [23]. symmetrical moel was revolve about its axis, an a 3D surface plot was obtaine. A line extrusion plot of the Lorentz force obtaine at the skin epth of the steel material for both the single layer an multilayer SS-MAT as shown in Fig. 6, Fig. 7 an Fig. 8. Table 2. Performance inex of Z1258 anti rust silicone baffle paint [23]. No. Inex name Unit Inex 1. Appearance The film becomes bright after rying 2. Viscosity S 4 3. Content of soli (1 g paint, (23±2 C/h) % 55±5 4. Drying time uner room temperature H Dielectric strength MV/m 3 6. Volume resistivity Ω-m Fineness (scrapper fineness meter) µm Ahesion coil grae 1~2 Fig. 4. Finite element mesh of the moel use Finite lement Mesh The finite element moel was obtaine by iscretising the physical moel in to numerous secon orer quaratic triangular elements consisting of 6555 elements. The mesh element were refine at about.2 mm aroun the skin epth surface of the steel material an aroun the excitation coil with minimum element quality of This refinement of the moel significantly improves its accuracy. Though the number of elements as wells as the solution time was greatly increase. This is achieve by continuously comparing the calculate result at ifferent egree of refinement until when the result harly changes with refinement as shown in Fig. 4. To obtain a more precise result, the time steps, relative tolerance an the absolute tolerance of then finite element moel were ajuste. 4. Simulation an Analisis Applying the external source current in equation (9) an the static magnetic fiel in the finite element moel, equation (8) is solve to obtain the Lorentz force ensity in the steel as shown in Fig. 5. Also to obtain an accurate recor of the time variation of the Lorentz force from the simulation, the 2D axis Fig. 5. Axi-symmetrical F moel of a stacke three layer SS-MAT Influense of Coil Stacking on the Lorentz Force Density As shown in equation (5), the ynamic magnetic fiel generate by the MATs coil is a very important factor that influence the performance of MATs in general. To improve the strength of the ynamic Lorentz force, the coils were stacke in layers an connecte in series. The stacke coils were excite with a tone burst sinusoial pulse as in equation (9). Fig. 6, Fig. 7 an Fig. 8 shows a 3D surface plot of a single, two an three layers SS-MAT. The values of the ensity of LF were obtaine by measuring the peak to peak value of the surface plot. This proceure was repeate to obtain the LF ensity for various number of coils in a stack as shown in Fig. 9. The values of the LF ensity obtaine was N/m 3, N/m 3 an N/m 3 for the single layer, two layers an three layers respectively. 265

5 Sensors & Transucers, Vol. 155, Issue 8, August 213, pp Fig. 6. 3D line extrusion surface plot of the Lorentz force generate by a single layer PS-MAT. an increase in the number of coils in a stack, translates to an increase in the total ynamic magnetic flux. Beyon the ighth coil, the istance between the coil an the specimen (lift off) will be enough as to obscure the effect of the aitional coil, hence no significant increment was observe beyon nine coil per stack. Also, it worthy to note that in practice, an increment in the number of coil per stack oes not lea to excessive heating ue to the fact that the uration of the excitation current is typically at micro-secon level [21]. Therefore, the SS-MAT configuration performs optimally when the number of coil in a stack oes not excee 8. Fig. 7. 3D line extrusion surface plot of the Lorentz force generate by a Two layer SS-MAT. Fig. 8. 3D cross section linear extrusion surface plot of the Lorentz force generate by a Three layer SS-MAT. It was observe that the ensity of the LF in the materials increases exponentially with the increase in the number of coils in the stack/layers an saturates when the number of coil increases beyon eight coils per stack. Any further increment in the number of coil beyon this point yiele no significant increase in the Lorentz force ensity. This is expecte since Fig. 9. LF strength uner ifferent number of coil layers Influence of Lift-off Distance on the Lorentz Force Density The performance of the single, two an three layer stacke SS-MAT configuration on CS7 grae of pipe steel was investigate, by varying the lift-off istance from.5 mm to 8 mm above the surface of the steel specimen. The measurement of LF ensity was taken within the skin epth (at about.1 mm below the surface) of the material specimen at the secon turn of the coil. The results shown in Fig. 1 inicates that the LF ensity ecreases exponentially with the lift-off istance with the three layer SS-MAT configuration having the best lift off characteristics, followe by the two Layere configuration. This phenomenon also confirms the correlation between the coil layers, the ynamic magnetic fiel, an the LF an is in goo agreement with finings of [22]. The results also inicates that for single an two layers SS-MAT configuration, any increase beyon 4. mm an 4.5 mm respectively of lift off, the LF generate in the specimen is almost zero. While for three layer SS- MAT, the same effect was observe beyon 5.5 mm of lift-off. This further inicates that there is a significant improvement in the lift off istance from 1 mm-3 mm obtaine in most commercial MAT [1] to 1 mm-5 mm. 266

6 Sensors & Transucers, Vol. 155, Issue 8, August 213, pp varying the static magnetic fiel from.5 T to 2. T. The result shows that by increasing the static magnetic flux, the LF generate in the material specimen also increases an vice versa. At.5 T, the peak to peak value of the LF is N/m 3 while the value of 2. T prouce the highest LF of N/m 3. The time variation of the LF was observe not be affecte by the variation of the magnetic flux ensity. The result is in goo agreement with publishe work of [9]. Fig. 1. Variation of Lift off istance with the number of coils per stack, on CS7 grae of pipe steel Influense of Coil Internal Diameter ( in ) on the Lorentz Force Density Fig. 11 shows a correlation between the internal iameter of the three layer SS-MAT coil an LF ensity generate on the surface of the pipe steel specimen. As the internal iameter is increase the LF also increases up to about 14 mm, at this point any further increment has no noticeable effect on the LF ensity. This phenomenon is because as the internal iameter is increase, the interaction between opposing ynamic magnetic fiel in the coil is reuce. This continues up to a point where there will be little or no effect of the opposing fiel. Therefore, for optimal performance of this stacke PS-MAT configuration, the internal iameter must be greater than 14 mm. Fig. 12. Time variation of static flux ensity B from (.5 T to 2. T) ffect of Variation of Insulation Layer Thickness (t in ) on the Lorentz Force Density As observe in section 3.4, stacking coils in layers improves the performance of the SS-MAT. To prevent short circuiting an bining the coils tightly together, they are coate with insulation material. The insulation material use for this simulation was Z1258 Anti-rust silicone baffle paint an has the following properties [23]: (i) (ii) (iii) (iv) Goo heat resistance Firm ahesion Short rying perio High ielectric strength of 31 6 V/m. Fig. 11. LF strength uner ifferent internal iameter of SS-MAT ffect of Variation of Static Flux Density (B ) on the Lorentz Force Density The static flux ensity is another important factor that affects the performance of MATs, hence the nee to investigate its effect on the LF generate by the three layer SS-MAT, on the material specimen. Fig. 12 shows the result of the simulation obtaine by Fig. 13 shows the result of the simulation obtaine by varying the insulation thickness from.1 mm to 2.5 mm. The result implies that, the LF strength ecreases exponentially with increasing insulation thickness. The highest value of LF strength of N/m 3 was obtaine at insulation thickness of.1 mm while the least value of N/m 3 was obtaine at a thickness of 2.5 mm. Therefore, for optimal performance of the KT coil, the insulation thickness must not excee 1 mm. The performance inex of Z1258 anti rust silicone baffle paint is given in Table

7 Sensors & Transucers, Vol. 155, Issue 8, August 213, pp Fig. 13. Variation of insulation layer thickness from (.1 mm to 1 mm) ffect of Variation of xcitation Perio on the Lorentz Force Density It has been shown in [22] that the perio number of a tone burst signal, etermines the quantity of energy transferre from the power source to the coil, which has a irect effect on the LF strength an mechanical energy generate. This implies that more perio number gives rise to more excitation energy. In the previous results (Fig. 5 to Fig. 13) the SS-MATs were excite with a 2 perio tone burst signal. To unerstan the effect of variation of the number of perios on the SS- MAT uner a given excitation current amplitue an frequency of 3 A an 5 khz respectively, the number of perio of the tone burst signal was varie in the following sequence (2 to 12) with all other parameters remaining unchange. The result shows that an increase in the perio, leas to an increase in the with of the LF wave packet (Fig. 14 (a) an Fig. 14 (b)). Analysing the result further, a linear relationship was establishe between the LF strength an the variation in the number of perios of the tone burst signal, see Fig. 14 (c). Fig. 14 (a). Lorentz force ensity for three layer SS-MAT uner ifferent perio number: 2 perios. Fig. 14 (b). Lorentz force ensity for three layer SS-MAT uner ifferent perio number: 1 perios. Fig. 14 (c). Variation of Lorentz force ensity with the number of perios from (2 to 12) ffect of Variation of xcitation Frequency on the Density of Lorentz Force To investigate the effect of variation of excitation frequency on the ensity of the LF generate by the propose Three layer SS-MAT, the static fiel an the excitation current were maintaine at.7 T an 3 A respectively. The excitation frequency was varie from 5 khz to 8 MHz, an the corresponing change in LF generate in the material specimen was calculate. Fig. 15, shows that the LF ensity increases as the excitation frequency varies from.5 MHz an reache a peak value a at 4.5 MHz, thereafter it starts ecreasing with a more pronounce effect seen in the three layer PS-MAT. This implies that any increase in frequency beyon 4.5 MHz has a negative effect on the ensity of the LF generate in the material specimen. Hence for optimal performance of this MAT, the right excitation frequency between 2.5 an 5 MHz shoul not be exceee. This result is in goo agreement with work publishe by [18]. 268

8 Sensors & Transucers, Vol. 155, Issue 8, August 213, pp Fig. 15. Variation of LF ensity with excitation frequency. 5. Conclusion In this research, a novel stacke PS-MA transucer was presente an stuie. Also, a 2D axis symmetrical finite element moel in the time omain was successfully evelope an implemente, we have use the moel to stuy the effect of some parameters on the performance of a novel SS-MAT configuration. The stuy carrie out in the first stage inclues calculating an comparing the LF ensity generate by the three SS-MAT configuration (single, two an three layers) on CS7 grae of pipe steel material. The secon stage of the stuy investigate the effect of variation of some MAT parameters (such as coil internal iameter, lift off, coil with, excitation frequency, coil stacking, insulation layer thickness an excitation perio) on the LF generate by the SS-MAT configuration. The numerical results were in goo agreement with previous work one by other researchers. Accoring to the stuy, the propose SS-MAT is effective in improving the Lorentz force ensity, which is a very goo parameter require for non estructive testing of steel materials. This is so because an increase in the Lorentz force ensity leas to a corresponing increase in the strength of the acoustic signal generate on the material specimen. The result also shows that close attention shoul be pai to the these MAT parameters in other to obtaine accurate measurement using the SS-MAT configuration. References [1]. M. Hirao an H. Ogi, MATs for Science an Inustry: Noncontacting Ultrasonic Measurements, Springer, 23. [2]. B. Maxfiel an C. Fortunko, The esign an use of lectromagnetic Acoustic Wave Transucers (MATs), Mater. val., Vol. 41, 1983, pp [3].. Dobbs, lectromagnetic generation of ultrasonic waves, Physical Acoustics, Vol. 1, 1973, pp [4]. T. Kunu, Ultrasonic nonestructive evaluation: engineering an biological material characterization, CRC Press, 23. [5]. G. Alers, A history of MATs, in Proceeings of the AIP Conference, 28, p. 81. [6]. R. Ribichini, F. Cegla, P. Nagy an P. Cawley, xperimental an numerical evaluation of electromagnetic acoustic transucer performance on steel materials, NDT Int., Vol. 45, 212, pp [7]. R. Jafari-Shapoorabai, A. Konra an A. Sinclair, The governing electroynamic equations of electromagnetic acoustic transucers, J. Appl. Phys., Vol. 97, 25, pp [8]. R. Dhayalan, V. Satya Narayana Murthy, C. Krishnamurthy an K. Balasubramaniam, Improving the signal amplitue of meanering coil MATs by using ribbon soft magnetic flux concentrators (MFC), Ultrasonics, Vol. 51, 211, pp [9]. S. Thomas, S. Obayya, R. Taneja an W. Balachanran, A Couple lectromagnetic an Mechanical Analysis of lectromagnetic Acoustic Transucers, International Journal for Computational Methos in ngineering Science an Mechanics, Vol. 1, 29, pp [1]. H. Ogi, Fiel epenence of coupling efficiency between electromagnetic fiel an ultrasonic bulk waves, J. Appl. Phys., Vol. 82, 1997, pp [11]. R. Jafari-Shapoorabai, A. Konra an A. Sinclair, Improve finite element metho for MAT analysis an esign, I Transactions on Magnetics, Vol. 37, 21, pp [12]. R. Luwig, Z. You an R. Palanisamy, Numerical simulations of an electromagnetic acoustic transucer-receiver system for NDT applications, I Transactions on Magnetics, Vol. 29, 1993, pp [13]. M. Kaltenbacher, K. ttinger, R. Lerch an B. Tittmann, Finite element analysis of couple electromagnetic acoustic systems, I Transactions on Magnetics, Vol. 35, 1999, pp [14]. R. B. Thompson, A moel for the electromagnetic generation of ultrasonic guie waves in ferromagnetic metal polycrystals, I Transactions on Sonics an Ultrasonics, Vol. 25, 1978, pp [15]. R. S. wars, X. Jian, Y. Fan an S. Dixon, Signal enhancement of the in-plane an out-of-plane Rayleigh wave components, Appl. Phys. Lett., Vol. 87, 25, pp [16]. X. Jian, S. Dixon an R. S. wars, Ultrasonic generation an optimization for MAT, in Proceeings of the AIP Conference, 25, p [17]. X. Jian an S. Dixon, nhancement of MAT an ey current using a ferrite back-plate, Sensors an Actuators A: Physical, Vol. 136, 5/1, 27, pp [18]. M. Kaltenbacher, R. Lerch, H. Lanes, K. ttinger an B. Tittmann, Computer optimization of electromagnetic acoustic transucers, in Proceeings of the I Ultrasonics Symposium, 1998, pp [19]. Ribichini, F. Cegla, P. B. Nagy an P. Cawley, Quantitative moeling of the transuction of electromagnetic acoustic transucers operating on ferromagnetic meia, I Transactions on 269

9 Sensors & Transucers, Vol. 155, Issue 8, August 213, pp Ultrasonics, Ferroelectrics an Frequency Control, Vol. 57, 21, pp [2]. R. Ribichini, P. Nagy an H. Ogi, The impact of magnetostriction on the transuction of normal bias fiel MATs, NDT Int., 212. [21]. S. Wang, L. Kang, Z. Li, G. Zhai an L. Zhang, 3-D moeling an analysis of meaner-line-coil surface wave MATs, Mechatronics, Vol. 22, 9, 212, pp [22]. Hao Kuang Sheng, Huang Song Ling, Zhao Wei an Wang Shen, Multi-belt coil longituinal guie wave magnetostrictive transucer for ferromagnetic pipes testing, Science China Technological Science, Vol. 54, No. 2, February 211. [23]. Transformer coil coating - Technical manual, Zhuzhou Insulation Material, Co. Lt. 213 Copyright, International Frequency Sensor Association (IFSA). All rights reserve. ( 27