Papers by Author: Jae Heon Lee

Abstract: The concept of light weight design will be very helpful for the tilting train to travel around a curve at a faster speed because the light weight car body makes the center of gravity lower. The impact characteristics of composite materials should be investigated because the impact property of composites is very important. This paper investigates the low-velocity impact characteristics and residual strength conducted on honeycomb sandwich panels. Low-velocity impact tests were carried out at different energy levels on honeycomb sandwich panels using a drop-weight apparatus. The impact-induced damages such as fiber breakage, matrix crack and delamination are examined with an optical microscope apparatus. Residual strength was evaluated by applying three point bending load to honeycomb sandwich panels. Experimental results show that the residual strength is greatly reduced as the impact energy increases. From the microscopic observation data, matrix crack, fiber breakage and delamination are the main impact damages of honeycomb sandwich panels.

Abstract: Recently, the use of composite material to railway vehicles is gradually increasing due to its light weight. The concept of light weight design will be very helpful for the tilting train to travel around a curve at a faster speed because the light weight car body makes the center of gravity lower. The impact characteristics of composite materials should be investigated because the impact property of composites is very important for the design and repair of a tilting train. This paper investigates the low-velocity impact and damage characteristics conducted on woven carbon/epoxy laminates for a tilting train. Low-velocity impact tests were carried out at different energy levels on woven type of laminates using a drop-weight apparatus. The impact-induced delaminations are examined by ultrasonic technique such as C-Scan. Those delaminations are also examined by laser technique such as ESPI (Electronic Speckle Pattern Interferometry). Experimental results show that the type of damage is dependent on the impact energy level and the delamination area becomes larger as the impact energy increases.

Abstract: In this paper the fatigue life of spur gear was investigated by changing the shot peening condition. From bending fatigue test depending on various shot peening intensity, fatigue characteristics were investigated. The causes of reduction in fatigue life were analyzed by observing the surface of gear with Scanning Electron Microscope(SEM), and impact of residual stress to fatigue characteristics was identified by measuring compressive residual stress depending peening intensity by depth. The results show that the optimum shot ball velocity is 65 m/s and optimum peening time is 8 minutes. From SEM image, the micro-crack was observed at the surface in case of over peening. This seems to be the factor which reduces fatigue life by decreasing compressive residual stress of surface.

Abstract: In this paper the life extension of automobile drive plates will be investigated. The material of specimen is a high carbon steel treated by shot peening, which is most important in the manufacturing process of drive plates. The optimum shot peening condition was investigated by changing the feeding speed and exposure time. The fatigue crack was observed at the fracturing surface of specimens by using scanning electron microscopy (SEM). The distribution of compressive residual stress induced by shot peening process was investigated by using X-ray diffraction. The number of cycles to failure increases at the beginning of exposure. The life cycle decreases after passing some interval of exposure, which will be called as optimum peening zone. Experimental results show that the residual stress distribution and the number of cycles to failure of a drive plate are greatly affected by peening process.