Papers by Author: Jin Yi Lee

Abstract: Operation of the corresponding large machine is not allowed during the regular NDT period. Therefore, reducing the test period and extending the test frequency, i.e., lowering the machine stoppage time due to an inspection is a very important technique for efficient energy usage. For this reason, IT-based real-time non-destructive testing (IT-NDT) technology has been being developed. This presentation reports on a magnetic camera, which is one IT-NDT technology, including the up-to-date experiences.

Abstract: Single sensor scanning (hereafter SSS) used to inspect cracks on paramagnetic materials can measure the distribution of the root mean squared value (RMS) of the magnetic field around crack tips quantitatively when sheet type current is induced on the specimen. The vertical direction magnetic field alternates to the surface of the crack tips because the sheet type induced current on the specimen is distorted by the existence of the crack in an SSS system. The RMS distribution of the magnetic field, which can be measured by using SSS, depends on the crack size and shape, so it can be used to evaluate a crack size quantitatively. An algorithm of quantitative nondestructive testing and evaluation of cracks of various shapes and sizes on the aluminum alloy, Al7075 is proposed in this paper.

Abstract: Wheels are essential parts of an express train, as they support the train’s weight, but because of the limited time available for testing and repair in the maintenance factory, the 1000 wheels of each high-speed train must be tested within 1 h. Therefore it is essential to develop a new NDT system that is able to (a) detect cracks quickly and (b) provide high crack-detection ability regardless of the state of the materials. In this paper we propose a scan-type magnetic camera to satisfy this need. We use a linearly integrated Hall sensor array (LIHaS) as a magnetic sensor to make high-speed testing possible, and we use a small, yoke-type magnetizer as the magnetic source for the magnetic camera to assure high crack-detection ability. We tested the method by examining the cracks on a sample wheel traveling at 27.5km/h.

Abstract: Two important factors in damage tolerance engineering are the damage size that can be tolerated and the use of applied nondestructive testing (NDT) technique for detection of defects. Generally, NDT of large mechanical structures, such as vessels and pipes in nuclear power plants, is carried out according to national codes, and the damaged mechanical parts detected during periodic NDT are repaired or replaced after the evaluation. However, unexpected accidents can occur if cracks in the structure grow and exceed the tolerance limit during operation. Therefore, there is a need to develop NDT techniques that can detect micro-damage in the vessels and pipes. In this paper, we propose a cylinder-type magnetic camera to fill this need. We use a cylindrically integrated Hall sensor array (CIHaS) as a magnetic sensor to create magnetic images, and we use a solenoid coil as the source of the magnetic field detected by the magnetic camera in order to achieve high crack-detection ability. The proposed method is tested with an examination of the cracks on a prepared pipe sample.

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.

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.

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.

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.

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.

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.