福岡 克弘

2020年度日本AEM学会技術賞 受賞

<p>Mechanical parts, plants and cross-linkages inspected with MT are typically complex 3D shapes. In the complex 3D shape portion, because a magnetizer often cannot be configured to the inspection portion and the test object cannot be appropriately magnetized, there is a possibility of overlooking a crack in such an instance. Thus, MT system development that was successfully able to detect omnidirectional cracks in 3D shape portions was considered in this study's trials. Two magnetizers were hence arranged face-to-face, and the magnetization of omnidirectional scenarios for all surfaces of 3D shape test object (arranged in between both magnetizers) was evaluated.</p>

The maximum magnetic flux leakage from a crack is obtained when the direction of the magnetic flux is orthogonal to the longitudinal direction of the crack. In magnetic-particle testing with a yoke method, since we usually cannot predict the direction of the crack to be detected, it is necessary to perform the testing at least two times by changing the direction of magnetization. In a rotating magnetic field type magnetizer using three-pole coils (three-phase alternating current), omnidirectional crack can be detected by a single testing. However, directions of the weak magnetic flux density appear at positions far from the center of the magnetizer, and the rotating magnetic field becomes no homogeneous distribution. In this research, it was considered to split each magnetic pole in the magnetizer to generate the uniform rotating magnetic field. The distribution of the rotating magnetic flux density was evaluated with a finite element method analysis, and an optimal disposition angle of the split coil was discussed. In addition, a multi coil magnetizer was developed to generate the uniform rotating magnetic field more widely.

Because magnetic particle testing (MT) can detect microcracks by a simplified method, it is applied in non-destructive inspections of ferromagnetic materials in various industrial fields. Recently, establishing a technology that quantitatively evaluates crack shapes as well as detects cracks with high-precision has become an important topic in the non-destructive inspection. We consider developing such a quantitative evaluation technique that employs magnetic particle pattern of a crack in MT. In this research, the process of magnetic particle adherence to a crack was observed with a high-speed video camera and the change in the magnetic particle amount was evaluated at each instant. In addition, the technique for evaluating small magnetic flux leakage (MFL) density accurately was discussed with the measurement and the numerical analysis. From these results, the relationship between the MFL density and the magnetic particle amount was evaluated.

The establishment of non-destructive inspection technology for plant structures is necessary, since the occurrence of cracks has been reported in some nuclear power plants. In this research, a uniform eddy current multi-probe to inspect cracks on a curved structure was developed. We designed exciting coils of this probe, considering the shape of the curved structure, so that the eddy current flows uniformly. Pick-up coils were arranged on a flexible printed circuit board to fit on the curved surface shape portion. The detection characteristics for EDM (electrical discharge machining) slits provided on the curved surface shape portion of the specimen were evaluated. The clear signals for the EDM slits provided on the curved surface which had a curvature radius of 25 mm were obtained by this probe. We confirmed that the crack shape could be estimated by detecting the signals from the developed probe.

In magnetic-particle testing, the maximum leakage flux from a flaw is obtained when the direction of the magnetic flux is normal to the longitudinal direction of the flaw. Since we usually cannot predict the direction of the flaw to be detected, it is necessary to perform the testing at least two times by changing the direction of magnetization. In a rotating field type magnetizer using a three-phase alternating current (AC), any directional flaw can be detected by a single testing. However, until now, the details of the rotating magnetic flux density in a specimen and its change with time have not been understood. In this research, the distributions of the rotating magnetic flux density were evaluated by measuring this density in three directions using Hall elements and by numerical analysis. The distributions of the magnetic flux density around the flaw were also evaluated by numerical analysis. The distribution of the rotating magnetic flux density near the central part of the magnetizer was made clear, and it was confirmed that the distribution became more uniform by equalizing the impedance of each coil of the magnetizer.

High-T_c superconductors (HTS), which have a characteristic of the critical current density over 3&times;10⁴ A/cm² in liquid nitrogen temperature (77K) and 1T, can be produced. Thus, they are promising for many practical applications such as a magnetic bearing, a magnetic levitation, a flywheel, a magnetic shielding and etc.. Since the HTS characteristics are not homogeneous in some specimens due to grain boundaries and cracks, the distribution of magnetic characteristics should be assessed. Thus, we have measured the distribution of the magnetic flux density on the surface of the HTS using a hall element, and have evaluated its magnetic characteristics. The measurement of magnetic characteristics using a hall element is difficult for measuring the distribution of the magnetic flux density on the actual surface and the inside of the HTS sample. In this research, we examined a quantitative evaluation of the magnetic shielding characteristics of the HTS including weak links under the static magnetic field with the three-dimensional finite element method analysis.

東京大学 学位記番号：第15158号

Strong magnetic fields can be generated by magnetization of the high-T_c superconductor(HTS)with pulsed magnetic fields using a small exciting coil. The magnetization using the pulsed magnetic field is performed on the zero field cooled state for HTS bulk samples. It is necessary to evaluate the property of the pulsed field magnetization of it on the pulsed field magnetization process. Thus, we measured the magnetic flux density on the pulsed field magnetization process and evaluated the property of the magnetization of single crystal YBCO bulk superconductor. The optimum conditions were studied for the pulsed field magnetization of the HTS. In addition, we examined the improvement of the magnetization characteristic on repeatedly impressing the pulsed magnetic field and deterioration in the material because of the pulsed magnetization.

YBCO superconductors, which have the characteristic of critical current density over 1×10^4A/cm^2 in liquid nitrogen temperature (77K) on 1T, can be produced. Thus, they are promising for many practical applications such as the magnetic shielding. The magnetic property of the high-Tc superconductors (HTSCs) don't uniform in the specimen, they have discontinuous distribution by grain boundaries and cracks. It is important in order to design applied HTSC to develop an evaluation method of the magnetic property distribution. Thus, we had developed the magnetic field visualization system, which enables the measurement of the magnetic field using a Hall device and produced the visual images of the magnetic field on a computer display. The magnetic characteristics of the HTSC on the DC magnetic field and the AC magnetic field with the frequency of 50 Hz have been evaluated using this visualization system. However, the magnetic characteristic evaluations of various frequency elements are necessary to use the HTSC by actual applications.Thus, the magnetic properties of the HTSC on the AC magnetic field from 1 to 250 Hz were evaluated in this study. From the experimental results, we are able to understand that the polycrystal HTSC has good magnetic shielding performance on higher frequency, even if it contains many weak links.

Strong magnetic fields can be generated for magnetization of the high-T_c superconductor (HTS) with pulsed magnetic fields using a small exciting coil. In the present study, magnetization using the pulsed magnetic field is performed on HTS bulk samples in the zero-field cooled state. In order to saturate the magnetization, it was necessary to optimize the time constant of the magnetic flux invasion and the flux flow during the increasing and decreasing processes of the pulsed magnetic field. Therefore, we measured the magnetic flux density on the pulsed field magnetization process and explained the relation between the pulsed magnetic field and the time constant of the magnetic flux invasion and flux flow. An empirical model of the pulsed field magnetization process was also proposed and optimum conditions were studied for pulsed field magnetization of the HTS. From this study, it was confirmed that an optimum pulsed waveform existed for pulsed field magnetization.

Bulk-type YBCO superconductors prepared by the MPMG process exhibit the high critical current density Jc, and are promising for many practical applications such as magnetic levitation. A magnetic levitation system is to use as a strong superconducting bulk magnet by magnetizing the high-Tc superconductor (HTSC). When superconducting bulk magnets are activated using the field cooling procedure under a DC magnetic field, a large electromagnet system or a superconducting coil is needed. In contrast, a pulsed magnetic field using a small coil is applied a strong magnetic field for magnetizing the HTSC. Therefore, it is important to evaluate the pulsed field magnetization properties of the HTSC. Since superconducting properties are not homogeneous in some specimens due to grain boundaries and cracks, spatial distribution of magnetization properties should be assessed. In the present research, the distribution of the pulsed field magnetization was measured with a magnetic field visualization system.<br>The magnetization with the pulsed magnetic field is done in the state of the zero field cooling. However, it is necessary for enough magnetization of the HTSC to consider the time constant on the magnetic flux invasion and the flux flow, when the applied magnetic field decreases. Therefore, we examined the optimum pulsed magnetization for the HTSC. The optimum value of the pulsed magnetization existed.

The bulk-type YBCO superconductor prepared by the MPMG process characterizes high critical current density Jc over 3&times;104A/cm2 at 77K and 1T. Therefore it is expected to be applied to various fields such as magnetic bearing, magnetic levitation, flywheel for electricity storage, and etc.. In these applications, evaluation of magnetic characteristics of the high-Tc superconductor is an important problem. It is necessary to value not only DC magnetic properties but also AC magnetic properties. Especially, in real applications it is important to value the AC quench properties of it. So we had developed the magnetic field visualization system, which enables measurement of magnetic field using a Hall device and produced the visual images of magnetic field on a computer display. We measured the AC quench properties of the YBCO ring specimen under the AC magnetic field with the frequency of 50Hz. From the experimental results we can understand that the superconductor results in the quench by the heat under the some AC magnetic fields, and that the quench properties of the ring specimen are ruled by its weak links.

Because the bulk-type YBCO superconductor prepared by the MPMG process has high critical current density J_c. It is expected to be applied to various fields such as magnetic bearing, magnetic levitation, magnetic shielding, and etc. In such applications it is important to develop measurement methods of magnetic property to high-T_c superconductor. It is necessary to value not only DC magnetic property but also AC magnetic property. So we measured magnetic pmperty of the high-T_c superconductor under the AC magnetic field (5-500Hz).<br>From the experimental results we can understand that there are two different kinds of elements in AC magnetic property: the one is the first order lag element by the flux flow, and the other is the applied magnetic flux that go around the ring specimen into that inside. The magnetic property in low frequency is ruled by the flux flow.

学位授与機構 学位記番号：424番