Papers by Author: Jin Yi Lee

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Authors: Jin Yi Lee, Ji Seoung Hwang, Kyung Chul Lee, Se Ho Choi
Abstract: It is important to estimate a magnetic field’ intensity distribution for the application of to industrial nondestructive evaluations. A magnetic camera provides the same spatial resolution and a distribution of a quantitative magnetic field with homogeneous lift-off. The method for arraying sensors is important on the magnetic camera because it must obtain a high spatial resolution. This work proposes an equation to evaluate the limitation of spatial resolution on the condition of the decided size of a magnetic sensor package. The possibility of obtaining the high spatial resolution in spite of the above-mentioned limitation would also be verified.
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Authors: Ji Seong Hwang, Jong Woo Jun, Se Ho Choi, Cheol Woong Kim, Kazuhiro Ogawa, Jin Yi Lee
Abstract: Nondestructive testing using magnetic field is useful for detection of a crack on ferromagnetic material. The magnetic field distribution has to be obtained for quantitative evaluation of crack direction, size, and shape. Also, a crack can be evaluated by using the inverse problem analysis. However, an analysis method using a dipole model can be used to analyze the magnetic field distribution around a crack at a higher speed than the finite element method (FEM). Therefore, a dipole model simulation can provide useful information which can be used for the inverse problem analysis. However, the magnetic charge per unit area, m, and the permeability, μ, has been treated as constants. Therefore, analyzed results have been different from experimental results in most cases. This paper proposes the improved dipole model simulation method, which assumes that the magnetic charges per unit area exist at the section areas, edge lines and summits of a crack. Also, the magnetic charges per unit area were assumed to depend on the square of the crack depth. The improved method is validated by comparing its results with the experiment results obtained with the use of the magnetic camera.
2371
Authors: Jin Yi Lee, Ji Seong Hwang
Abstract: The magnetic camera using magnetic lens is proposed to satisfy the demands such as obtaining the distribution of the magnetic flux leakage (DMFL) on the high lift-off, and improving sensitivity. However, DMFL is strongly affected by the magnetization direction. Therefore, the DMFL has low intensity when the crack is not perpendicular to the magnetizing direction. Also, quantitative nondestructive evaluation would be difficult. This paper proposes an improved magnetization method to evaluate a crack quantitatively, regardless of the crack direction. The secondary magnetic source, which is perpendicular to the primary magnetic source, is introduced in the scan type magnetic camera. The intensity of two values of ∂B/∂x (1st differential to the magnetization direction) of the DMFL can be used to evaluate the crack volume. The experimental and the numerical analysis results are considered to verify this phenomenon.
1447
Authors: Jin Yi Lee, Ji Seoung Hwang, Se Ho Choi, Jae Kyoo Lim
Abstract: It is important to estimate the intensity distribution of a magnetic field as a possible magnetic method in an industrial nondestructive evaluation application. A magnetic camera provides the distribution of a quantitative magnetic field with homogeneous lift-off and same spatial resolution. Magnetic flux leakage near a crack on the specimen can be amplified by using a 3- dimensional magnetic fluid, that is to say a magnetic lens. This study introduces the experimental consideration of the effects of magnetic lenses for concentrating of magnetic flux. The experimental results showed that the magnetic fluid has a sufficient lens effect for magnetic camera and effect of improvement in probability of detection.
241
Authors: Cheol Woong Kim, Sang Hoon Lee, Jin Yi Lee, Kee Joo Kim
Abstract: The fixation of the vertebral column using transpedicular screws is a well-establish technique. Multi-axial pedicle screw designs allow deviation of the screw away from the perpendicular to the longitudinal rod, which facilitates application of a screw-rod system such as Cervical Attachment System (CAS) into the curved spine. This study was performed a static (compression bending and torsion) and dynamic (compression bending fatigue) empirical analysis of CAS component such as pedicle screw, rod and set screw based on the guideline of ASTM F136- 98 using Ultra-high Molecular Weight Polyethylene (UHMWPE) blocks as test vertebral bodies.
2207
Authors: Jin Yi Lee, Jong Woo Jun, Ji Seong Hwang, Se Hoon Lee
Abstract: The magnetic field distribution around a crack can be calculated more easily and quickly by using a dipole model than finite element method (FEM). This paper reports the development of numerical analysis software that uses an improved dipole model to analyze the magnetic field around cracks. The preprocessor in this software includes the crack formation software, which can distribute the magnetic charge per unit area, m, on the crack section area. Also the lift-off, measurement area and sensor interval, and magnetization direction can be considered in the preprocessor. Also, the postprocessor presents functions, such as the natural magnetic field distribution and ∂B/∂x, ∂B/∂y, as results. Also, the physical characteristics of the magnetic optical sensor and the Hall sensor are included in the postprocessor, and the magnetic field distribution can be changed to optical intensity and electrical signal distribution. The experiment results, which are obtained by using the magnetic camera on the crack, are compared with analysis results obtained by using the dipole model analysis software.
2383
Authors: Jong Woo Jun, Ji Seong Hwang, Kee Joo Kim, Kazuhiro Ogawa, Jin Yi Lee
Abstract: The yoke type magnetization coil and cross type magnetizer as a magnetic source, which induce a static magnetic field on a specimen, have been used to detect cracks on a ferromagnetic material. However, the cracks on paramagnetic materials such as aluminum alloy and stainless steel cannot be detected by using a static magnetic source. This paper proposes a magnetic source, which uses the induced sheet type current, for the detection of cracks on a paramagnetic material. The sheet type AC current can be induced by using a primary coil and core. And the copper film, which includes a sheet type current, is positioned on the specimen. Eddy currents are induced around a crack on the specimen because the secondary sheet type current can be induced on the specimen surface. The signal processing electrical circuits, which measure a Hall sensor peak output with the eddy current frequency, are developed and discussed. Also the validity of the proposed signal processing circuit is verified by detection of slit type cracks and a fatigue crack, which are introduced on aluminum alloy.
2379
Authors: Do Won Seo, Ho Chel Yoon, Jin Yi Lee, Jae Kyoo Lim
Abstract: Ceramics are significantly used in many industrial applications due to their excellent mechanical and thermal properties such as high temperature strength, low density, high hardness, low thermal expansion and good corrosion properties. To combine the specific advantages of ceramics with that of metals, they are often used together within one composite component. In this study, the effect of temperature on fracture characteristics of silicon nitride joined to 304 stainless steel brazed with Ti active alloy are investigated in room and high temperature regions. And analytical studies on the residual stress of dissimilar brazed joint are performed by the finite element method. Four-point bending strength and deflection of interlayer increase with increasing strain rate in room temperature. As the test temperature increases, the bending strength decreases, but the deflection of interlayer is almost constant. The residual stresses redistribute after cutting of joint and the maximum tensile stress occurs on the new free surface at the ceramic near the interface. The singularity of residual stresses at the ceramic near the interface is characterized by elastic-plastic properties of ceramic and inserted materials.­
881
Authors: Cheol Woong Kim, Bong Su Kang, Kee Joo Kim, Jin Yi Lee
Abstract: The research of the stress distributions and the structural deformation at the sliding core in artificial intervertebral disc under the dorsiflexion is becoming more significant. This research analyzes the finite element model of sliding core and evaluates the effect of radius of curvature and the friction coefficient at the sliding core on von-Mises stress and the contact pressure. New Models of the artificial intervertebral disc are suggested by the results of the sliding core is evaluated by the comparison of that of SB Charité III. Based on the above facts, the optimized radius of curvature of the sliding core is also suggested.
2211
Authors: Se Ho Choi, Ji Seong Hwang, Jong Woo Jun, Jin Yi Lee, Cheol Woong Kim
Abstract: Magnetic camera consists of magnetic source, arrayed small magnetic sensors, magnetic lens, analog-to-digital converter and interface, computer and monitor. The quantitative magnetic field around crack and its processed results could be obtained by using magnetic camera, and the crack could be inspected and evaluated quantitatively. In addition, the magnetic camera has to uphold with large lift-off to protect sensors from weak environment such as high temperature and mechanical vibration. However, the sensor sensitivity would be decreased when the lift-off was increased. Correspondingly, the improved techniques are necessary for increasing sensitivity of magnetic camera and probability of crack detection at the large lift-off. This paper proposes an image processing method, which separates a global full scale to the several regions and repeats shadings in each region, to increase a crack detection probability in the magnetic camera images such as ∂B/∂x and ∂2B/∂x∂y.
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