Showing papers by "Tetsuya Uchimoto published in 2020"

Features extraction and discussion in a novel frequency-band-selecting pulsed eddy current testing method for the detection of a certain depth range of defects

Abstract: Local wall thinning defects are unavoidable defects in actual engineering structures and in some cases it occurs in a certain depth range of the object structures. In this study, a novel frequency-band-selecting pulsed eddy current testing (FSPECT) has been proposed for the detection of local wall thinning defects in a certain depth range. Feature of peak value has been extracted and analyzed. The results demonstrate that the FSPECT possess the comparable performances in terms of detection sensitivity over the traditional square wave pulsed eddy current testing (PECT). In addition, other fruitful detailed features extraction in the numerical calculation and experimental signals of FSPECT are deeply studied. The features obtained by simulation and experiment mainly include lift-off point of intersection (LOI) and zero-crossing time. Furthermore, the influences of the depth of local wall thinning defect on features of LOI and zero-crossing time have been explored, which enhance the accuracy and reliability of FSPECT method for the evaluation of local wall thinning defects.

New Combination of Magnet and Coil of Electromagnetic Acoustic Transducer for Generating and Detecting Rayleigh Wave

Abstract: Nondestructive testing for identifying defects on the surface of metal materials is important for industries and infrastructures. The Rayleigh wave is widely used for detecting these surface defects. For replacing piezoelectric transducers with electromagnetic acoustic transducers (EMATs) for the surface inspection of metal materials, this article proposes a new magnet and coil combination consisting of a periodic-permanent-magnet (PPM) and a returned dislocation meander line coil. The returned dislocation meander line coil was developed using a traditional meander line coil, whose wires return from one side to another and shift for a certain distance. A 2-D finite-element simulation was conducted to analyze the performance of the proposed Rayleigh wave EMAT. The simulation results revealed that, compared with a large conventional magnet, the PPM increased the maximum magnetic flux density, and made the magnetic flux density distribution more concentrated on the specimen’s surface, particularly below the coil. In the middle part of the coil, the PPM greatly increased the intensity of the horizontal magnetic field. Additionally, the returned dislocation meander line coil made full use of the strong magnetic field below the center of each small magnet and at the adjacent magnets. The designed Rayleigh wave EMAT was fabricated, and the experimental results revealed that the new design of the Rayleigh wave EMAT increased the received signal by 57.9% compared with the conventional Rayleigh wave EMAT.

Evaluation of detectability of differential type probe using directional eddy current for fibre waviness in CFRP

Abstract: This paper describes the detectability of eddy current testing (ECT) using directional eddy current for detection of in-plane fibre waviness in unidirectional carbon fibre reinforced plastic (CFRP)...

Small electromagnetic acoustic transducer with an enhanced unique magnet configuration

Abstract: We describe a small electromagnetic acoustic transducer (EMAT) that is different from traditional large EMATs. The maximum vertical magnetic flux density decreases quickly, and its distribution becomes very uneven with increasing lift-off distance of the permanent magnet. To increase the magnetic field strength of a small magnet without increasing the thickness of the magnet configuration, we propose a different permanent magnet configuration. With the same lift-off distance of the magnet, the increase in the maximum vertical magnetic flux density is about 20% when using this configuration. When this lift-off distance is 1.5 mm, the configuration increases the amplitude of the received signal by 71.4% for an aluminium specimen; in contrast, only a 29.0% increase is achieved with a low-carbon steel specimen. This is because, in the latter, the vertical magnetic flux density is less than 0.66 T. The interplay between the Lorentz force and magnetostrictive force leads to a decrease in the efficiency of both forces in generating ultrasonic waves.

Improved Dynamic Magnetostriction measurement method based on M-EMAT for the characterization of residual strain

Abstract: The Dynamic Magnetostriction (DM) measurement method as a nondestructive testing method of ferromagnetic materials, measures the DM curve using electromagnetic acoustic transducer (EMAT) to characterize material and predict the mechanical properties. Lorentz forces and magnetostriction forces contribute mainly to the ultrasonic generation. Only the range of low field, in which the magnetostriction force mechanism is predominant can be used to measure the DM curve, but the boundaries of this range are uncertain. The improved DM measurement method based on shear wave Magnetostriction EMAT (M-EMAT) is proposed and the theory of generation of shear waves is derived. For the M-EMAT, only magnetostriction force mechanism contributed to the generation of shear waves. A theoretical model for the optimization of the transducer is developed, the model is in good agreement with the experimental results. The microstructure changes caused by residual strain will affect the magnetostriction coefficient and the DM curve. Therefore, the DM curves of low carbon steel specimens with different levels of residual strains are measured by the optimized transducer to characterize the residual strain. The normalized results demonstrate that the slope of the DM curve obviously changes with the residual strain. This leads to the conclusion that the DM measurement method can be used to characterize the residual strain of the low carbon steels.

Reconstruction of complex shaped crack from ECT signals based on a fast forward solver using an advanced multi-media element

Abstract: In this paper, the conventional database type fast forward solver for efficient simulation of eddy current testing (ECT) signals is upgraded by using an advanced multi-media finite element (MME) at the crack edge for treating inversion of complex shaped crack. Because the analysis domain is limited at the crack region, the fast forward solver can significantly improve the numerical accuracy and efficiency once the coefficient matrices of the MME can be properly calculated. Instead of the Gauss point classification, a new scheme to calculate the coefficient matrix of the MME is proposed and implemented to upgrade the ECT fast forward solver. To verify its efficiency and the feasibility for reconstruction of complex shaped crack, several cracks were reconstructed through inverse analysis using the new MME scheme. The numerical results proved that the upgraded fast forward solver can give better accuracy for simulating ECT signals, and consequently gives better crack profile reconstruction.

Stochastic fluid dynamics simulations of the velocity distribution in protoplasmic streaming

Abstract: Protoplasmic streaming in plant cells is directly visible in the cases of Chara corallina and Nitella flexilis, and this streaming is understood to play a role in the transport of biological materials. For this reason, related studies have focused on molecular transportation from a fluid mechanics viewpoint. However, the experimentally observed distribution of the velocity along the flow direction x, which exhibits two peaks at Vx = 0 and at a finite Vx(≠0), remains to be studied. In this paper, we numerically study whether this behavior of the flow field can be simulated by a 2D stochastic Navier–Stokes (NS) equation for Couette flow in which a random Brownian force is assumed. We present the first numerical evidence that these peaks are reproduced by the stochastic NS equation, which implies that the Brownian motion of the fluid particles plays an essential role in the emergence of these peaks in the velocity distribution. We also find that the position of the peak at Vx(≠0) moves with the variation in the strength D of the random Brownian force, which also changes depending on physical parameters such as the kinematic viscosity, boundary velocity, and diameter of the plant cells.

Application of back-propagation neural networks to defect characterization using eddy current testing

Abstract: Eddy current testing is widely used for the automatic detection of defects in conductive materials. However, this method is strongly affected by probe scanning conditions and requires signal analysis to be carried out by experienced inspectors. In this study, back-propagation neural networks were used to predict the depth and length of unknown slits by analyzing eddy current signals in the presence of noise caused by probe lift-off and tilting. The constructed neural networks were shown to predict the depth and length of defects with relative errors of 4.6% and 6.2%, respectively.

A study on influence of strong magnetic field on fracture of interface crack considering magneto-elastic coupling effects

Abstract: The influence of strong magnetic field on stress intensity factor of an interface crack is studied in this paper. The nonlinear piezomagnetic property and magnetostriction effect have been taken into consideration in the theoretical analyses. This multi-field coupled problem is solved through a sequential coupling approach. The perturbed magnetization caused by the deformation around the crack is solved under magnetic boundary conditions. After modified by the perturbed magnetization, the initial loads are updated with magnetic forces for iterative calculation. With this strategy, the distributions of the stress and displacement at the crack region approach to the real solution gradually. Numerical results show that the influence of the external magnetic field on fracture behaviors is not ignorable. For structures with interface crack serving in a strong magnetic field, e.g., the multi-layer welded structures in the Tokamak device, the magneto-elastic coupling effects have to be considered to deal with its fracture problem.

Stochastic Fluid Dynamics Simulations of the Velocity Distribution in Protoplasmic Streaming

Abstract: Protoplasmic streaming in plant cells is directly visible in the cases of \textit{Chara corallina} and \textit{Nitella flexilis}, and this streaming is understood to play a role in the transport of biological materials. For this reason, related studies have focused on molecular transportation from a fluid mechanics viewpoint. However, the experimentally observed distribution of the velocity along the flow direction $x$, which exhibits two peaks at $V_x\!=\!0$ and at a finite $V_x( ot=\!0)$, remains to be studied. In this paper, we numerically study whether this behavior of the flow field can be simulated by a 2D stochastic Navier-Stokes (NS) equation for Couette flow, in which random Brownian force is assumed. We present the first numerical evidence that these peaks are reproduced by the stochastic NS equation, which implies that the Brownian motion of the fluid particles plays an essential role in the emergence of these peaks in the velocity distribution. We also find that the position of the peak at $V_x( ot=\!0)$ moves with the variation in the strength $D$ of the random Brownian force, which also changes depending on physical parameters such as the kinematic viscosity, boundary velocity and diameter of the plant cells.

Stochastic Fluid Dynamics Simulations for the Velocity Distribution of Protoplasmic Streaming

Abstract: Protoplasmic streaming in plant cells is driven by the myosin molecule, which is called the molecular motor The molecular motor also activates flows on/inside animal cell membranes; therefore, protoplasmic streaming has attracted considerable interest and has been extensively studied However, the experimentally observed velocity distribution, which exhibits two peaks at $V_x\!=\!0$ and finite $V_x( ot=\!0)$ along the flow direction $x$, remains to be studied In this paper, we numerically study whether the behaviour of the flow field can be simulated by a 2D stochastic Navier-Stokes (NS) equation in which Brownian random force is assumed We present the first numerical evidence that these peaks are reproduced by the stochastic NS equation, which implies that the Brownian motion of fluid particles plays an essential role in the presence of peaks in the velocity distribution We also discuss the dependence of the flow field on the strength $D$ of the Brownian random force in detail

A Stable FEM-BEM Hybrid Method for the Numerical Simulation of Magnetomechanical Coupled Problem With Both Inductive and Conductive Current Excitations Aiming to Application to Tokamak In-Vessel Structures

Abstract: The in-vessel structures of tokamak devices sustain large electromagnetic force due to both induced eddy current and halo current. The coupling effect between the electromagnetic field and the mechanical vibration of the structures has a significant influence on the structural dynamic response. To assess the coupled mechanical behavior of in-vessel structures, a numerical method was proposed in this article based on the hybrid finite-element method and boundary-element method. The plasma current and halo current were modeled as a series of current filaments and a pair of current source–sink, respectively. To deal with the nonlinearity due to the coupling term of the magnetic flux density and the velocity, the block Gauss–Seidel iterative algorithm was adopted in the numerical method. The proposed numerical method was first validated against the experimental data of the TEAM 16 benchmark problem and then applied to the dynamic analysis of a simplified halo current problem of typical tokamak structures. The numerical method was proved both effective and numerically stable for the analysis of magnetomechanical coupled problem based on the reasonable simulation results.