Papers by Author: Seok Jin Kwon

Abstract: The railway wheel in long-term running had experienced the wheel damage due to fatigue crack and shelling. The damaged wheel in railway vehicle would cause a poor ride comfort, a rise in the maintenance cost and even fracture of the wheel, which then leads to a tremendous social and economical cost. It is necessary to evaluate long-term damage of railway wheel in order to ensure the safety of wheel. To evaluate the damage for railway wheels, the measurements for the replication of wheel surface and residual stress of railway wheel using x-ray diffraction system were carried out. The result shows that the residual stress of wheel is depend on the running distance and thermal gradient during brake application also that the replication test can be applied in new evaluation method of wheel damage.

Abstract: Wheels of the railway vehicle play the important role for driving train through wheel-rail interaction. Especially wheel profile is one of the most important design factors to rule the running stability and safety of train. Accordingly, the control of rolling contact fatigue-related defects is an ongoing concern for both safety and cost reasons. This process is referred to as ratcheting. Wear of wheel and rail surfaces occur due to a mixture of adhesive, abrasive and corrosive processes. In wheel/rail systems with little wear, such failure is manifested by the appearance of closely spaced micro-cracks. In the present paper, a evaluation of surface defects of wheel and rail for Korean high-speed railway. The main research application is the wheel-rail maintenance of Korea high-speed train.

Abstract: Upon investigation of the damaged wheels it was determined that the cracking was caused by thermal fatigue during on-tread friction braking. The thermal cracks appear as short cracks oriented axially on the wheel tread. Severe heating of the wheel tread during braking was believed to be a contributing the variation of residual stress which is related to wheel failure. It is necessary to evaluate the residual stress due to deterioration of wheel tread in order to ensure the safety of wheel. In the present paper, the residual stress of railway wheel for deterioration using x-ray diffraction system is evaluated. The result shows that the residual stress of wheel is depend on the running distance and the residual stress needs to be inspected between the wheel diameter of 800 and 780mm.

Abstract: Rolling Contact Fatigue (RCF) damage on the surface of rails such a head check, squats is a growing problem. Since rail fractures can cause derailment with loss of life and property, the understanding of rail fracture mechanism is important for reducing damages on the rail surface. In this study, we have investigated RCF damage, fatigue growth and fracture surface morphology on the surface of broken rail using failure analysis and finite element (FE) analysis. The investigation indicates that the crack grows at about 20° to the depth of 8mm from the surface and branches into two cracks. One crack propagates downward at about 47°, the other propagates upward. Since the crack growth rate of the downward crack was faster than that of upward crack, rail eventually was broken. Since the downward branches lead to fracture of the rail, they are more dangerous to the integrity of rails. It has been observed that White Etching Layer (WEL) occurs within the surface of broken rail. It was found that the fatigue crack initiation and propagation was accelerated by WEL.

Abstract: The objective of this study is to clarify the effect of hub contact shape on contact pressure and fatigue life with regard to the selection of a suitable taper design near the end of the fit. A numerical asymmetric-axisymmetric finite element model was developed in order to determine the contact stress state of press-fitted shaft by using four types of tapered contact surfaces on the hub. The variations of fatigue crack initiation life according to the change of tapered contact surfaces on the hub were evaluated by using the Smith-Watson-Topper (SWT) multiaxial fatigue criterion. As the result, comparing with the contact pressure and the fatigue crack initiation life, maximum decrease of contact pressure and maximum increase of fatigue crack initiation life were obtained for the 1/400 m/m tapered hub subjected to a bending load near the fretting fatigue limit. Furthermore, as the change of bending load, the optimal amout of taper in hub which fatigue life gets into maximum is varied. Therefore, we suggest that the best performance, in terms of pressure distribution and fatigue life of press fit, can be obtained by using a proper taper values for the hub element.

Abstract: This paper presents the results of an experimental investigation of fretting wear characteristics on the contact surface of press-fitted shaft subjected to a cyclic bending load. A series of interrupted fretting wear tests with press-fitted specimens were carried out by using a rotating bending fatigue test machine. The evolution of contact surface profile of press-fitted shaft due to fretting wear were measured with a profilometer. The local wear coefficient during the running-in period is discussed from experimental results and FE analysis. It is found that the maximum depth of fretting wear by repeated slip between shaft and boss occurred at the close of contact edge at the early stage of fatigue life and the regions of worn surface are expanded to the inner side of contact edge as increasing number of fatigue cycles. The initial fretting wear rate at the early stage of fatigue life increased rapidly at all loading condition. After steep increasing, the increase of wear rate is nearly constant in the low bending load condition. The local wear coefficient in running-in period decrease dramatically at the early stage of fretting wear.

Abstract: In this paper the fretting wear simulation technique with the press-fitted specimens have been developed, which can consider the running-in period of total wear process by adopting nonlinear wear coefficient. The amount of microslip and contact variable at press-fitted and at bending loaded condition of press-fitted shaft was analysed by applying finite element method. With the finite element analysis result, a numerical approach was applied to predict fretting wear based on modified Archard's equation and updating the change of contact pressure caused by local wear with influence function method. The predicted wear profiles of press-fitted specimens at the contact edge were compared with the experimental results obtained by rotating bending fatigue tests. It is shown that the predicted wear profiles considering the running-in period with nonlinear wear coefficent is consistent with experimental results than that with constant wear coefficient. Therefore, the fretting wear simulation technique proposed is feasible and efficient for numerical simulation of fretting wear on press fits at the initial stage of fatigue life.

Abstract: The defect initiation and crack propagation in wheel may result in the damage of the railway vehicle or derailment. Therefore, it is important to evaluate the characteristics of the wheel tread. In the present paper, the characteristics of wheel tread based on contact positions, running distance and brake pattern are evaluated. To evaluate the damage for railway wheels, the measurement for the replication of wheel surface is carried out. The result shows that the damaged wheel tread is remarkably depended on the contact positions between wheel and rail. It should be noted that the replication test can be applied in new evaluation method of wheel damage.

Abstract: White etching layer(WEL) is a phenomenon that occurs on the surface of rail due to wheel/rail interactions such as excessive braking and acceleration . Rolling contact fatigue(RCF) cracks on the surface of rail have been found to be associated with the WEL. In this study, we have investigated RCF damages of white etching layer in the laboratory using twin disc testing. These tests consist of wheel flat tests and rolling contact fatigue tests. The WEL has been simulated by wheel flat test. It has been founded that the WEL with a bright featureless contrast is formed on the surface of specimen by etching. Rolling contact fatigue test was conducted by using flat specimens with the WEL generated by the wheel flat test. It has been observed that two types of cracks occur within the specimen, the first initiated at the interface between the WEL and the undeformed area, the second initiated at the center of the WEL.

Abstract: Fretting damage is a critical problem to prevent failure of press-fitted shaft such as the rotor of a steam turbine, railway axles or coupling. To clarify the characteristics of surface damage due to fretting in press-fitted shaft, experimental methods were applied to small-scale specimen with different bending load conditions. Fatigue tests and interrupted fatigue tests of press-fitted specimen were carried out by using a rotate bending fatigue test machine. Macroscopic and microscopic characteristics were examined using scanning electron microscope (SEM), optical microscope or profilometer. It is found that small fatigue cracks are nucleated early in life regardless of bending stress, and thus the most portion of fatigue life on press fits can be considered to be crack propagation process. Most of surface cracks are initiated near the contact edge, and multiple cracks are nucleated and interconnected. Furthermore, the fretting wear rates at the contact edge increase rapidly at the initial stage of total fatigue life. It is thus suggested that the fatigue crack nucleation and propagation process is strongly related to the evolution of surface profile by fretting wear in press fits.