No abstract

No abstract

Rolling contact fatigue initiated defects such as surface corrugation, head check, squat, are one of growing problems in high speed railway line. A squat is generally developed below the rail surface and grows parallel to surface until it turns down into rail. Estimation of critical crack size and crack growth rate is an essential to prevent rail from failure and develop cost effective railway maintenance strategy. In this study, we predict crack growth rate of a rail with a squat defect. For this purpose, a rail model with a squat defect is developed. Timoshenko’s beam theory is applied to calculate the global bending stress at the crack tip and Hertzian contact model is applied to calculate the local contact stresses at the surface of rail by simulating rolling over a railway wheel on a rail. Stress intensity factors are calculated from the total stress at the crack tip. Fatigue crack growth curve of 60kg rail steel is applied to calculated crack growth rate. Software to predict crack growth life through whole life cycle is developed. We expect that we can make a more cost effective rail maintenance strategy by considering the crack growth analysis for a defective rail.

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.

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.