Papers by Author: Seong Kyun Cheong

Abstract: Shot peening generate compressive residual stress, which reduces repeatedly assessed tension stress and increase fatigue life. In this paper, the fatigue characteristics are evaluated according to shot peening condition for the real differential gear. The specimen is a straight bevel gear which transmits rotation of engine to running wheel and it is caburized. The bending fatigue test was done by the jig of our own making. The peening time was changed to find the best condition for the long fatigue life time. Fractography of specimen was analyzed by SEM to detect the location of initial crack. The experimental results show that the optimum peening condition is at 65m/s of shot ball speed and 8min of shot peening time. From the SEM image, the location of initial defect and direction of crack propagation were found

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: Functionally Graded Materials (FGMs) are composite materials that have continuous material variation along with geometry. This paper introduces a method for FEA-based design and layered manufacturing (LM) of FGMs. An FGM solid model is first created by referring to the libraries of primary materials and composition functions. The model is then discretized into an object model onto which appropriate material properties are mapped. Next, the object model is adaptively meshed and converted into an FE model. FEA using ANSYS is finally performed to estimate stress levels. This FEA-based design cycle is repeated until a satisfactory solution is obtained. The object model is then fed to the fabrication system where a process planning is performed to create instructions for LM machines. As a laser-based LM method, Direct Metal Deposition (DMD) at the University of Michigan is briefly described. A specific example (FGM pressure vessel) is shown to illustrate the entire FEA-based design and DMD fabrication cycle.

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.