ID 61
Title An arrayed uniform eddy current probe design for crack monitoring and sizing of surface breaking cracks with the aid of a computational inversion technique
Author Noritaka Yusa, Hidetoshi Hashizume, Ryoichi Urayama, Tetsuya Uchimoto, Toshiyuki Takagi, Kunihiro Sato
Published in NDT&E International 61 (2014), 29-34.
Keywords electromagnetic nondestructive evaluation, reconstruction, inversion, fatigue crack, finite element method
Abstract This study demonstrates that eddy current testing can be an effective method for monitoring the growth of surface breaking cracks with the aid of computational inversion techniques. A uniform eddy current probe with 23 arrayed detectors was designed, and pseudo monitoring tests were carried out to measure signals due to six mechanical fatigue cracks introduced into type 316L austenitic stainless steel plates. In the test the position of the probe was fixed to simulate monitoring. The depths of the cracks were evaluated using a computational inversion method developed on the basis of -nearest neighbor algorithm. The depths of the mechanical fatigue cracks whose actual depths were 1.1, 2.1, 3.1, 5.5, 6.7, and 8.5 mm were evaluated to be 0.9, 1.9, 3.8, 4.3, 7.0, and 5.7 mm, respectively. Additional simulations were conducted to demonstrate the stability of the method.

ID 62
Title Evaluation of the electromagnetic characteristics of type 316L stainless steel welds from the viewpoint of eddy current inspections
Author Noritaka Yusa, Hidetoshi Hashizume, Tetsuya Uchimoto, Toshiyuki Takagi, Kunihiro Sato
Published in Journal of Nuclear Science and Technology 51 (2014), 127-132.
Keywords inspection; numerical simulation; finite element method; electromagnetic nondestructive evaluation; conductivity; permeability; vibrating sample magnetometer
Abstract This study evaluated the electromagnetic characteristics of austenitic stainless steel welds from the viewpoint of eddy current testing. Seven welded plate specimens, which were welded using JIS Z3221:2010 YS316L welding metals, were prepared. Two welding metals and several welding conditions were adopted to discuss the generality. The results of eddy current examination of the specimens using three different types of eddy current probes, i.e. an absolute pancake probe, a differential plus point probe, and a uniform eddy current probe, confirmed that the welds cause a large noise especially when the absolute pancake probe was used. The analysis of the signals through finite element simulations revealed that the magnetic property of the welds is not negligible from the viewpoint of eddy current testing. Complementary experiments were conducted using a vibrating sample magnetometer to validate the results.

ID 63
Title 拡散接合を用いた複雑形状模擬きず製作技術の開発
Author 遊佐訓孝, 王晶, 橋爪秀利
Published in 非破壊検査 63(3)(2014), 148-152.
Keywords electromagnetic non-destructive evaluation, eddy current testing, direct current potential drop testing, skin depth, depth of penetration, finite element simulation
Abstract This study demonstrates that an artificial flaw that has an arbitrary boundary profile and the response to non-destructive testing similar to a natural crack can be fabricated by bonding surfaces with artificial grooves using diffusion bonding. Artificial flaws with a small openings and widely spread boundary profiles inside can be realized by bonding Type 316L stainless steel blocks with artificial grooves. The artificial flaw is designed so that its surfaces have a 20% physical contact. The conditions for bonding are based on the results of preliminary experiments. Validations are carried out using two electromagnetic non-destructive testing methods: direct current potential drop and eddy current tests, whose results are well reproduced by finite element simulations. Subsequent destructive testing confirms that the actual profile of the artificial flaw agrees well with the designed one.

ID 64
Title 磁性を有する鋼材に発生した表面開口割れの深さ評価のための渦電流探傷技術の開発
Author 遊佐訓孝, 橋爪秀利
Published in 鉄と鋼 100(2014), 1339-1346.
Keywords nondestructive inspection, sensor, simulation, process control, maintenance
Abstract This study develops an eddy current testing probe whose signals due to a surface breaking crack appearing in a ferromagnetic material exhibit clear change with respect to the depth of the crack. Experimental verifications are conducted using a type 430 ferritic stainless plate specimen with five artificial slits whose depths are 1, 3, 5, 10, and 15 mm. Although exciting frequencies adopted were 25, 50 and 100 kHz, there is clear difference between signals due to the 10 mm deep slit and those due to the 15 mm deep one. Subsequent finite element simulations are conducted to discuss the physical background of the difference on the basis of the reciprocity theorem and electromagnetic fields.

ID 65
Title Optimized microwave excitation probe for general application in NDT of wall thinning in metal pipes
Author Kota Sasaki, Linsheng Liu, Noritaka Yusa, Hidetoshi Hashizume
Published in NDT&E International 70(2015), 53-59.
Keywords pipe-flaw inspection, electromagnetic field analysis, probe optimizing parameter, microwave probe design
Abstract Generalized approaches to developing a microwave NDT for flaws inside an arbitrary diameter pipe through optimizing a microwave-exciting probe were investigated. A microwave probe obtained from a parameter-optimizing scheme based on transmission characteristics is proposed. Three-dimensional finite element simulation of five microwave probes indicates that a larger optimization parameter enables reduced microwave reflection, as well as improved single-mode propagation inside the pipe as compared with conventional probes. Experimental verification, using straight brass pipes of various diameters confirmed that the optimized microwave probe exhibits a larger signal-to-noise ratio for internal flaws when compared with conventional probes, thereby validating the new optimization parameter.

ID 66
Title Evaluation of physical properties of the molten salt mixture flinabe for a fusion blanket system using molecular dynamics simulation  
Author Hiroki Shishido, Noritaka Yusa, Hidetoshi Hashizume, Yoshiki Ishii, Norikazu Ohtori
Published in Fusion Science and Technology 68 (2015), 669-673.
Abstract This study evaluates the physical properties of the molten salt Flinabe, using molecular dynamics simula-tions to discuss its applicability to a fusion blanket sys-tem. More specifically, the simulations calculate the den-sity and viscosity of Flinabe to facilitate further discus-sion of the applicability from the viewpoint of the heat removal of the first wall. The results of the simulations are compared with data reported in earlier publications, which support the validity of the simulations. This study reveals that Flinabe tends to have lower viscosity than Flibe even when they contain almost the same BeF2. Ana-lyzing the results of the simulations confirms that the degree of polymerization in Flinabe correlates with its viscosity, as that in Flibe does. The analyses also re-vealed, however, that the correlation in the case of Flibe is not directly applicable to the case of Flinabe.

ID 67
Title Evaluation of Probability of Detection (POD) studies with multiple explanatory variables
Author Noritaka Yusa, Jemery Knopp
Published in Journal of Nuclear Science and Technology 53 (2016), 574-579.
Keywords inspection, evaluation, safety assessment, cracking, flaw, probability of detection, numerical simulation, finite element method, nondestructive testing and evaluation, eddy current testing
Abstract This study proposes a new method to evaluate a probability of detection (POD) as a function of more than one flaw parameter. The main idea of the method is to assume that signals due to flaws described with a given parameter x have a normal distribution of a mean of μ(x) and a standard deviation of σ(x), and to use a combination of signals calculated by numerical simulations and experimental data to evaluate μ(x) and σ(x), respectively. The method does not postulate a closed-form of μ(x) found in conventional approaches, and it evaluates a few parameters that characterize the distribution using maximum likelihood analysis to calculate POD. This allows POD evaluation for data that does not satisfy linearity or constant variance assumptions without transformation. The proposed method is demonstrated through analyzing simulated eddy current signals due to flaws appearing in type 316L stainless steel welds. The results of the demonstration confirm that the proposed method can provide the POD with its confidence bounds as a function of the depth and the length of a flaw. The results also showed that the proposed method does not require a large amount of experimental data compared to conventional a-hat vs a analysis.

ID 68
Title Mechanical and electrical characteristics of a bridge-type mechanical lap joint of HTS STARS conductors
Author Satoshi Ito, Noritaka Yusa, Nagato Yanagi, Hitoshi Tamura, Akio Sagara, Hidetoshi Hashizume
Published in IEEE Transactions on Applied Superconductivity 26 (2016), 4201510.
Keywords contact resistance, fusion reactors, high-temperature superconductors, superconducting magnets, superconducting tapes
Abstract Segmented fabrication of high-temperature superconducting (HTS) coils has been proposed for the helical fusion reactor, FFHR-d1; the joint-winding and remountable (demountable) helical coils concept are the primary and advanced options, respectively. This paper introduces recent research activities on a bridge-type mechanical lap joint of HTS Stacked Tapes Assembled in Rigid Structure (STARS) conductors especially to be used for joint-winding of the helical coils, and also discusses some technical issues to develop a reliable joint. First, the required joint performance is discussed from the viewpoints of operation cost and mechanical behavior, taking the present design of joint-winding of the helical coils for FFHR-d1 into consideration. Then, mechanical and electrical characteristics of a prototype STARS conductor joint, which achieved a joint resistance of 1.8 n at a current of 100 kA, are analyzed based on small and large-scale joint tests. It is suggested that the joint performance of the prototype joint can be improved more according to tests of small-scale conductor joints. Finally, the study introduces a fabrication procedure for a reliable joint. We need to develop non-destructive testing for a joint at room temperature. For this purpose we also perform an X-ray Computed Tomography inspection as a first step. The inspection can roughly judge whether a fabricated joint is acceptable.

ID 69
Title Numerical investigation of the ability of eddy current testing to size surface breaking cracks
Author Noritaka Yusa, Hidetoshi Hashizume
Published in Nondestructive Testing and Evaluation 32 (2017), 50-58.
Keywords electromagnetic non-destructive evaluation, sizing, maintenance, stress corrosion cracking, fatigue crack
Abstract This study attempts to estimate the limitations of eddy current testing for evaluating the depth of a crack with the aid of numerical simulations. Three-dimensional finite element simulations were conducted to obtain eddy current signals resulting from a crack in a plate made of Ni-based alloy. The simulations modeled the crack as a sufficiently long rectangular region with a constant width and depth. A uniform conductivity was given inside the modeled crack because actual cracks do not always behave as electrically insulating walls. The conductivity of the modeled crack was chosen on the basis of a recent study reporting how real cracks can be modeled from the viewpoint of electromagnetic non-destructive evaluation. This study considered three conventional eddy current probes: a pancake probe, a vertical pancake probe, and a plus point probe. The results of the simulations reveal that lower frequency does not always enhance the evaluation of a deeper crack if the crack is conductive; moreover, the maximum depth of a stress corrosion crack that conventional eddy current testing can evaluate quantitatively should be limited to a few mm at most. In contrast, the simulations also demonstrate that the ability of eddy current testing to size a fatigue crack would not differ significantly from that to size an artificial slit.

ID 70
Title Demonstration of low-frequency vibration tests to evaluate bolt loosening
Author Noritaka Yusa, Sakuo Sawada
Published in International Journal of Applied Electromagnetics and Mechanics 52 (2016), 89-94.
Keywords non-destructive testing and evaluation, vibration, accelerometer, impulse test, frequency
Abstract This study proposes a simple and cost-effective method to detect bolt loosening on the basis of low-frequency vibration signals as low as several hertz. Laboratory tests were conducted using M16 bolts connecting two SS400 plates. The bolts were vibrated by external impulse using a test hammer, and their vibrations were measured using accelerometers attached to the nuts holding the bolts. The accelerometers had the lowest measuring frequency of 0.5 Hz, and no high-pass filter was used. The results of the tests show that one can confirm whether or not bolts have been tightened to the rated torque quite clearly in time domain signals. Subsequent field tests were conducted to evaluate the M42 anchor bolts in a factory, and the results demonstrate the effectiveness of the method.

ID 71
Title Reducing joint resistance by heat treatment during fabrication of a mechanical joint of high-temperature superconducting conductors
Author Tatsuki Nishio, Satoshi Ito, Noritaka Yusa, Hidetoshi Hashizume
Published in IEEE Transactions on Applied Superconductivity 26 (2016), 4800505.
Keywords Fusion reactors, high-temperature superconductors, power cable connecting, superconducting magnets
Abstract A mechanical joint of high-temperature superconducting (HTS) conductors has been investigated for segmented-fabrication of HTS magnets proposed for a future fusion reactor. In a previous study, joint resistance of 1.8 n &Omega at 100 kA was achieved for a large-scale simple-stacked Gadolinium Barium Copper Oxide (GdBCO) conductor having a bridge-type mechanical lap joint with indium foils inserted between joint surfaces. The obtained joint resistivity (the product of joint resistance and contact area) was, however, ten times larger than that of a single mechanical lap joint achieved in previous studies, which may be caused by non-uniform distribution of contact pressure. In order to reduce this joint resistance, therefore, we propose a new method by introducing heat treatment in fabricating process of the joint. By applying the method, the joint resistance became the lowest at heating temperatures of 75- 90 °C, which was reduced to 60 % of the value obtained without heat treatment. The reduction of joint resistance was due to a decrease in thickness of the inserted indium and an increase in true area of the contact surface. On the other hand, relatively large voids were observed in the indium by X-ray Computed Tomography when the heating temperature exceeds 120 °C. The voids were caused by emission of the gases absorbed in the tape and foil, and the voids decreased by baking the conductor tapes and indium foil before the heat treatment. Applying the heat treatment to a large conductor joint, the voids can be formed due to the temperature exceeding 120 °C since the temperature control in the large joint becomes more difficult. Therefore the baking process before the joint fabrication with heat treatment is one of the options to avoid the void formation.

ID 72
Title Demonstration of probability of detection taking consideration of both the length and the depth of a flaw explicitly
Author Noritaka Yusa, Weixi Chen, Hidetoshi Hashizume
Published in NDT&E International 81 (2016), 1-8.
Keywords crack, surface examination, statistical analysis, eddy current testing, finite element method, nondestructive testing and evaluation
Abstract This study proposes the construction of the probability of detection (POD) as a function of both the depth and length of a flaw. In addition, this study discusses how to censor signals in constructing the POD. The general effects of the flaw parameters on signals are evaluated by numerical simulations, and the scattering of signals, which is critical to the POD, is estimated by signals obtained in experiments. A new likelihood function is introduced, and the proposed method is demonstrated using eddy current signals caused by various artificial flaws on a flat type 316L stainless steel plate obtained in a laboratory test. The demonstration confirms that the proposed method can provide a reasonable POD with a small amount of experimental signals, and reveals that proper censoring significantly decreases the detrimental effect of noise on the POD.

ID 73
Title Fabrication of imitative cracks by 3D printing for electromagnetic nondestructive testing and evaluations
Author Noritaka Yusa, Weixi Chen, Hidetoshi Hashizume
Published in Case Studies in Nondestructive Testing and Evaluation 5 (2016), 9-14.
Keywords electromagnetic nondestructive evaluation, eddy current testing, artificial defect, performance demonstration, additive manufacturing
Abstract This study demonstrates that 3D printing technology offers a simple, easy, and cost-effective method to fabricate artificial flaws simulating real cracks from the viewpoint of eddy current testing. The method does not attempt to produce a flaw whose morphology mirrors that of a real crack but instead produces a relatively simple artificial flaw. The parameters of this flaw that have dominant effects on eddy current signals can be quantitatively controlled. Three artificial flaws in type 316L austenitic stainless steel plates were fabricated using a powderbed-based laser metal additive manufacturing machine. The three artificial flaws are designed to have the same length, depth, and opening but different branching and electrical contacts between flaw surfaces. The flaws were measured by eddy current testing using an absolute type pancake probe. The signals due to the three flaws clearly differed from each other although the flaws had the same length and depth. These results were supported by subsequent destructive tests and finite element analyses.

ID 74
Title A survey of attitudes toward energy and electricity of undergraduate students studying in the Faculty of Engineering, National University of Laos
Author Noritaka Yusa
Published in Journal of Energy and Environmental Education 11 (2017), 59-64.
Abstract ラオス国立大学工学部学生を対象としたアンケート調査を行った。掲示により募った400名(うち女性85名)に対し、ラオス語に よる計15問の選択式の質問に対する回答を求めた。調査の結果、ラオス国立大学工学部学生は高いエネルギーに対する意識を持っ ていること、エネルギー源の選択においては安全性が第一と考えている一方で温暖化とコストへの関心が低いこと、再生可能エネ ルギーに対する期待が高いこと、そして原子力に対しては否定的であることが明らかとなった。また、将来の自国の主たるエネルギー 源とすべきものは何かとの質問に対する回答率(複数選択可)は、水力91%、太陽光70%、風力43%、天然ガス32%、石油23%、石炭16%、 原子力7%と、現状水力で自国消費の数倍の電力を生産しておりかつ更なる水力発電開発の余地も大きいという自国のエネルギー事情 を理解した上で、再生可能エネルギーは魅力的であるが現実的には将来も水力を主たるエネルギー源とするべきであるとの合理的な 判断をしていることをうかがわせるものであった。

ID 75
Title Demonstration of the applicability of nondestructive microwave testing to the long-range inspection of inner-surface cracks in tubes
Author Kota Sasaki, Takuya Katagiri, Noritaka Yusa, Hidetoshi Hashizume
Published in Materials Transactions 58 (2017), 692-696.
Keywords electromagnetic nondestructive evaluation, time of flight, localization, signal processing, finite element simulation
Abstract This study evaluated the applicability of nondestructive microwave testing method for the long-range detection of inner-surface cracks in metallic tubes. Two seamless straight tubes 7 m in length and 23.0 mm in inner diameter were prepared, and artificial circumferential slits penetrating the tube walls were introduced. A probe to propagate microwave inside the tube, designed based on three-dimensional finite element simulations, was attached to an end of the tube, and the reflections of the microwave propagating inside the tube were measured using a network analyzer. The experiments confirmed clear reflections due to the slits situated 6 m away from the probe. Furthermore, imposing a signal processing method to compensate for the dispersion of the microwave clarified the reflections and enabled the slit to be localized quantitatively from the time-of-flight of the reflections.

ID 76
Title Thermal design investigation for a flinabe blanket system
Author Hiroki Shishido, Noritaka Yusa, Hidetoshi Hashizume, Yoshiki Ishii, Norikazu Ohtori
Published in Fusion Science and Technology 72 (2017), 382-388.
Keywords Liquid blanket, Molten salt, Flinabe, Molecular dynamics
Abstract The present study evaluates the thermal design of a blanket system using Flinabe in order to facilitate further discussions on its applicability as a self-cooled liquid blanket system. Molecular dynamics simulations were performed to evaluate the Prandtl number of mixtures in five compositions (LiF-NaF-BeF2 = 31-31-38, 36-27-37, 42-22-36, 49-16-35, and 67-0-33). Thermofluid analysis was carried out to estimate the temperature margin and pressure drop per unit length in a simple geometry model of the blanket system. The Prandtl number of Flinabe is above 100 at 400 °C. The present study reveals that as a coolant Flinabe remarkably relaxes the design conditions compared to Flibe owing to its low melting point. In contrast, the pressure drop per unit length of Flinabe is higher than that of Flibe because the viscosity exponentially increases at low temperature. The temperature margin is quite dependent on the heat load on the first wall. If the pressure drop per unit length is around 1.0 MPa/m, the heat load value must be approximately below 0.7 MW/m2.

ID 77
Title Assessment of local conductivity distribution in stress corrosion crack region using direct current potential drop method
Author Wenlu Cai, Shejuan Xie, Cherdpong Jomdecha, Xiaojuan Wang, Cuixiang Pei, Yong Li, Zhenmao Chen, Noritaka Yusa
Published in Corrosion Science 123 (2017), 197-208.
Abstract As an effort to enhance the precision of quantitative nondestructive evaluation on stress corrosion crack (SCC) sizing with eddy current testing (ECT) technique, the electrical conductivity distribution in the SCC region was investigated in this work through measurement and inversion of the direct current potential drop (DCPD) signals with a sliced specimen strategy. An inversion scheme, consisting of an efficient forward DCPD signal simulator and the conjugate gradient optimization algorithm, was proposed and implemented for the reconstruction of the two-dimensional (2D) conductivity distribution inside the SCC. The reasonable reconstruction results from the measured DCPD signals of the practical SCC specimens proved the validity of the proposed scheme. The 2D distribution obtained in this paper shows that the conductivity becomes larger when approaching the crack tip and is the minimum in the crack opening position. The obtained SCC conductivity information gives a good reference to enhance the crack sizing accuracy of ECT.

ID 78
Title 配管側面入射マイクロ波による配管内壁面広域一括探傷技術の検討
Author 佐々木幸太、遊佐訓孝、片桐拓也、橋爪秀利
Published in 日本AEM学会誌 25 (2017), 197-208.
Keywords electromagnetic nondestructive inspection, time of flight, maintenance, piping system, network analyzer
Abstract This study proposes long-range nondestructive inspection of the inner surface of a pipe without open end using microwave emitted by a probe attached to the pipe wall. Three-dimensional finite element simulations were conducted to evaluate how to propagate microwave effectively inside a pipe having an inner diameter of 19 mm. The simulations considered a coaxial cable penetrating the pipe wall to emit microwave inside the pipe. Experimental verifications were then carried out using a brass pipe with a total length of 4.5 m and a probe fabricated on the basis of the results of the numerical simulations. The experiment confirmed clear reflections due to artificial wall thinning 1 m away from the probe situated approximately at the center of the pipe. The experiment also revealed that directivity of the microwave depended on the frequency, which agrees with the results of the numerical simulations.

ID 79
Title ラオス国高校生の原子力に対する知識調査
Author 遊佐訓孝
Published in 保全学 16 (2017), 82-88.
Keywords Education, Public, International relations, Sociology, Questionnaire survey, Regional characteristics, Public acceptance, High school student
Abstract This study conducted a survey to gather information on the knowledge of Laotian high school students on nuclear energy. This study prepared an original 11-pages textbook that briefly introduced nuclear energy and related matters in Lao. One hundred high school students belonging to a public high school with a good reputation in Savannakhet Province were requested to read the textbook approximately in a half day to tell subjective impression on the amount of prior knowledge on the contents of each section. The survey revealed the knowledge of the high school students on the nuclear energy and related matters was quite limited although the official textbooks used in the secondary education in Laos contain quite a few explanations on structure of atom, chemical and nuclear reaction, structure of nuclear power plants, the chain and fusion reactions, and so on.

ID 80
Title Application of low-frequency eddy current testing to the inspection of a double-walled tank in a reprocessing plant
Author Naoki Tajima, Noritaka Yusa, Hidetoshi Hashizume
Published in Nondestructive Testing and Evaluation 33 (2018), 189-197.
Keywords electromagnetic nondestructive evaluation, corrosion, finite element simulation, double tank, austenitic stainless steel
Abstract This paper discusses the applicability of simple low-frequency eddy current testing to the detection of deeply embedded flaws. The study specifically considered a double tank in a reprocessing plant for extracting plutonium-uranium from spent nuclear fuels. The tank was modeled by two type 304 austenitic stainless steel plates situated with an air gap of 80 mm, and the change in the thickness of one of the plates was detected through the other plate and the air gap. Axisymmetric two-dimensional finite element simulations were conducted and found that a simple circular coil with a large diameter enabled to detect the thickness based on the magnetic flux density at the center of the coil although the plates were as thick as 30 mm. The results of the numerical simulations were validated by experiments.