Gotoh, Yuji

Radin, A.

Takahashi, N.

Okayama University Digital Information Repository

Evaluation of alternating flux leakage testing using 3-d non-linear eddy current analysis

Okayama University Scientific Achievement Repository

PhysicsElectricity & Magnetism fieldsOkayama University Year 2005Evaluation of alternating flux leakagetesting using 3-d non-linear eddy currentanalysisYuji Gotoh A. RadinOkayama Polytechnic College Okayama UniversityN. TakahashiOkayama UniversityThis paper is posted at eScholarship@OUDIR : Okayama University Digital InformationRepository.http://escholarship.lib.okayama-u.ac.jp/electricity and magnetism/1090-7803-5943-7 4/00/$10.00 02000 IEEE. DQ-10 EVALUATION OF ALTEWATING FLUX LEAKAGE TESTING USING 3-D NON-LINEAR EDDY CURRENT ANALYSIS Y.Gotoh', A.RadinZ and N.Takahashi* 'Dept. Electronics Eng., Okayama Polytechnic College, Kurashiki 710-0251, Japan 'Dept. Electrical and Electronic Eng., Okayama Univ., Okayama 700-8530, Japan -~ Introduction OAltemating flux leakage testing is one of the inspection method for detecting cracks on the surface of steel [l]. This method detects the leakage flux from the cracks in ferromagnetic material magnetized by an ac electromagnet. High frequency should be used to detect very small cracks in steel surface. In order to develop a precise inspection method, the flux and eddy current distributions should be investigated. But reports of such a calculation is very few, because the precise analysis of 3-D nonlinear steady state eddy current is not easy. UIn this paper, the property of leakage flux from the crack of steel under ac excitation is investigated using 3-D edge finite element method [2]. The experimental verification is also carried out. Model and Method of Analysis UFig.1 shows a model of altemating flux leakage testing. In order to examine the typical phenomenon of the testing, a large crack (the width, depth and length of crack is Zmm, 2mm and lOOmm, respectivity) is used. The steel is magnetized by an ac electromagnet (30 tnms for each coil, IA(rms), 1kHz). The z-component of the leakage flux near the crack is detected by the search coil (70 turns). The lift off between search coil and the surface of steel is 0.2nnn. The search coil is moved in the x-direction. The analyzed region is subdivided into 37,128 hexahedral edge elements. The flux and eddy current are analyzed by the step-by-step mcthod taking account of the non-linearity of stecl. The time interval Ut of the step-by-step method is chosen as 6.25~10" sec. In order to get the steady state result, calculation is carried out during 2.5 period (=39 steps). The yoke is assumed to he linear (relative permeability : 60,000). Results and Discussion Fig2 shows the flux distribution near the crack. The flux in the steel is not illustrated. Fig.3 shows the contour lines of the z-component Bz of flux density. Figs2 and 3 suggest that Bz at the search coil position (A) above the crack is nearly zero because the leakage flux flows almost in the x-direction. Bz is increased at the position (B) (the leakage tlux is directed toward the steel) and decreased at the position (C) (the amplitude leakage flux is reduced). Fig.4 shows the comparison between the measurement and calculation of the average flux density in the search coil. The figure illustrates the validity of calculation. The CPU time is 96 hours using VT-Alpha600 workstation (Spec ft95: 27.0). As it became possible to analyzed 3-D steady state nonlinear phenomenon of altemating leirkage flux testing, the obtained results will give useful suggestions to improve the inspcction method. References [I] Y.Gotob and M.Hashimoto, Non-Linear Electromagnetic Systems, V.Kose and JSievert IEds.l 10s Press.. DD'.l93-196. 1998. exciting coil z search 'Oil I QI A iyPk (b) x-y plane positions of search coil Fig.2. Flux distribution near crack. steel plate crack Fig.3. Contour lines of equi-flux density Bz [Gauss]. (c) enlarged (x-z plane) Fig.1. Analyzed model crack position position [mi] Fig.4. Results of experiment and analysis. . I  [2] A.Kameri, IEEE Trans. Mag'n., vo1.26, no.2, pp.466-469, 1990. 

Evaluation of alternating flux leakage testing using 3-d non-linear eddy current analysis

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