Papers by Author: 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.

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.

Abstract: To fabricate the aluminum alloys with good drawability, the textures evolution of the 3003 aluminum alloy sheets after rolling and subsequent annealing was studied. The measurement of the deformation textures was carried out for the sheets in which were cold rolled with high reduction ratio by using the symmetric roll. In addition, the change of the recrystallization texture was investigated after heat-treatments of the rolled sheets with various heat treatment conditions. Rolling without lubrication and subsequent annealing led to the formation of favorable rot-CND {001}<110> and γ-fiber ND//<111> textures in 3003 aluminum alloy sheets. From the results, the γ-fiber ND//<111> component well evolved during rolling at high reduction ratio (reduction over 90%, l/d parameter over 6.0). Among shear deformation textures, the γ-fiber ND//<111> was not rotated during recrystallization (350°C) in special condition.

Abstract: The diverse studies on Al/GFRP laminates with the circular holes, therefore, have been carried out recently. The recent studies just focused on the behavior of the fatigue crack propagation and the delamination when the shape and the size of the notches were changed. Therefore, this study evaluated the location effect of the defects in the vicinity of the circular notch of the high strength monolithic aluminum and Al/GFRP laminates on the initiation life (Ni) of the fatigue cracks, the relationship between the crack length (a) and the fatigue life (N). In addition, the fatigue crack behavior of Al/GFRP laminates was studied when the fatigue loading and the interlaminar delamination took place at the same time during crack propagation. In conclusions, (1) for the monolithic aluminum, 10% of the failure life at θ2=30° was more increased than that at θ3=60°. (2) The crack length and the fatigue life behavior of Al/GFRP laminates according to the location of the artificial defect were different from those of the monolithic aluminum. Namely, the fatigue life of θ1=0° and θ2=30° were remarkably shorter than those of θ3=60° and θ4=90°.

Abstract: If Fiber Reinforced Metal Laminates (FRMLs) were delaminated, the decrease of stiffness and fiber bridging effect would result in the sudden aggravation of fatigue characteristics. It was reported that the delamination of FRMLs resulted from the crack of Al alloy layers and that it depended on the crack growth. When FRMLs with circular holes was un-cracked but was delaminated, it was impossible to analyze them by conventional fracture parameters expressed as the function of cracks. Therefore, a new analytical model called Pseudo Crack Model (PCM) was suggested to compare the delaminations whether cracks were made or not. The relationship between the crack consumption rate (Ccrack) and the delamination consumption rate (Cdel) was discussed and it was also known that the effect of the Ccrack was larger than that of the Cdel.

Abstract: The research of 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 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 PRIMA OCT into the curved spine. This study was performed a static (compression bending and torsion) and dynamic (compression bending fatigue) empirical analysis of PRIMA OCT 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.

Abstract: The applicable bending moment equation of LIPCA is necessary even though the fiber layer ply orientation is changed. The aim of this research is to evaluate the relationship between the total effective moment (ME) and Bernoulli-Euler bending moment (M) when the ply orientations of unidirectional CFRP, which is one of the various laminate configurations in LIPCA, are changed. Since the related previous equation between the performance stroke range (h) and the radius of curvature (ρ) was just applicable to the CFRP ply orientation [0], it will be modified using these results. The related modified equation, which has nothing to do with the various CFRP ply orientation, is also suggested and compared with the previous equation.