Kaijun (Kevin) Zhu

Abstract
Under-ballast mats (UBMs) have become more popular recently in railroad track engineering. Benefits of introducing an under-ballast mat layer(s) into the track include, but are not limited to: increasing track resilience, reducing ballast breakage, decreasing noise and vibration, and protecting bridge decks. One of the most essential parameters used to evaluate the performance of UBMs is the bedding modulus. Currently, the German Deutsches Institut für Normung (DIN) 45673 is the… Show more

Abstract
Under-ballast mats (UBMs) have become more popular recently in railroad track engineering. Benefits of introducing an under-ballast mat layer(s) into the track include, but are not limited to: increasing track resilience, reducing ballast breakage, decreasing noise and vibration, and protecting bridge decks. One of the most essential parameters used to evaluate the performance of UBMs is the bedding modulus. Currently, the German Deutsches Institut für Normung (DIN) 45673 is the only test standard specifying procedures to quantify UBM bedding modulus by placing the UBM between two steel plates. However, steel plates might not be an ideal representation of the actual track loading environment. Thus, other types of support conditions have been used to test the UBM bedding modulus, including concrete block and the geometric ballast plate (GBP) specified by European Standard (EN) 16730. How these different support conditions affect the performance of an UBM has not been fully investigated. To better quantify the effects of varying support conditions on UBM bedding modulus, testing was performed in the Research and Innovation Laboratory (RAIL) in University of Illinois at Urbana-Champaign (UIUC). It was found that for a specific type of UBM, the tested bedding modulus values were similar when supported by the steel plate or concrete block, while the value was considerably lower when the mat was supported by the GBP. Finite element simulations were performed to further study the stress distributions under these various support conditions. The results from this study can help practitioners better represent the application environment during the UBM bedding modulus tests by suggesting the appropriate support condition. Show less

Abstract
Reducing the allowable operating speed or imposing temporary speed restrictions are common practices to prevent further damage to rail track when defects are detected related to certain track components. However, the speeds chosen for restricted operation are typically based on past experience without considering the magnitude of the impact load around the rail joints. Due to the discontinuity of geometry and track stiffness at the bolted rail joints, an impact load always exists… Show more

Abstract
Reducing the allowable operating speed or imposing temporary speed restrictions are common practices to prevent further damage to rail track when defects are detected related to certain track components. However, the speeds chosen for restricted operation are typically based on past experience without considering the magnitude of the impact load around the rail joints. Due to the discontinuity of geometry and track stiffness at the bolted rail joints, an impact load always exists. Thus, slower speeds may not necessarily reduce the stresses at the critical locations around the rail joint area to a safe level. Previously, the relationship between speed and the impact load around the rail joints has not been thoroughly investigated. Recent research performed at the University of Illinois at Urbana-Champaign (UIUC) has focused on investigating the rail response to load at the joint area. A finite element model (FEM) with the capability of simulating a moving wheel load has been developed to better understand the stress propagation at the joint area under different loading scenarios and track structures. This study investigated the relationship between train speed and impact load and corresponding stress propagation around the rail joints to better understand the effectiveness of speed restrictions for bolted joint track. Preliminary results from this study indicate that the contact force at the wheel–rail interface would not change monotonically with the changing train speed. In other words, when train speed is reduced, the maximum contact force at the wheel–rail interface may not necessarily reduce commensurately. Show less

Abstract
A rail joint typically is one of the weakest elements of a track superstructure, primarily because of discontinuities in its geometric and mechanical properties and the high-impact loads induced by these discontinuities. The development of continuously welded rail has significantly reduced the number of rail joints, but many bolted joints remain installed in rail transit systems. Because of the unique loading environment of a rail transit system (especially high-frequency… Show more

Abstract
A rail joint typically is one of the weakest elements of a track superstructure, primarily because of discontinuities in its geometric and mechanical properties and the high-impact loads induced by these discontinuities. The development of continuously welded rail has significantly reduced the number of rail joints, but many bolted joints remain installed in rail transit systems. Because of the unique loading environment of a rail transit system (especially high-frequency, high-repetition loads), defects related to bolted rail joints (e.g., joint bar failures, bolt hole cracks, and cracks in the upper fillet) continue to cause service failures and can pose derailment risks. Recent research in the Rail Transportation and Engineering Center at the University of Illinois at Urbana–Champaign has focused on investigating crack initiation in the bolt hole and fillet areas of bolted rail joints. Stress distribution was investigated at the rail-end bolt hole and upper fillet areas of standard, longer, and thicker joint bars under static loading conditions. Numerical simulations were organized into a comprehensive parametric analysis performed with finite element modeling. Preliminary results indicated that the longer joint bar performed similarly to the standard joint bar but the thicker joint bar reduced rail vertical displacement and rail upper fillet stresses compared with the standard joint bar. However, the thicker joint bar also may generate higher stresses at the rail-end bolt hole. Additionally, joint bar performance was dependent on the rail profile and bolt hole location. Show less

Abstract
As one of the weakest locations in the track superstructure, the rail joint encounters different types of defects and failures, including rail bolt-hole cracking, rail head-web cracking or separation, broken or missing bolts, and joint bar cracking. The defects and failures are mainly initiated by the discontinuities of both geometric and mechanical properties due to the rail joint, and the high impact loads induced by the discontinuities. Continuous welded rail (CWR) overcomes most… Show more

Abstract
As one of the weakest locations in the track superstructure, the rail joint encounters different types of defects and failures, including rail bolt-hole cracking, rail head-web cracking or separation, broken or missing bolts, and joint bar cracking. The defects and failures are mainly initiated by the discontinuities of both geometric and mechanical properties due to the rail joint, and the high impact loads induced by the discontinuities. Continuous welded rail (CWR) overcomes most disadvantages of the rail joints. However, a large number of rail joints still exist in North American Railroads for a variety of reasons, and bolted joints are especially prevalent in early-built rail transit systems. Cracks are often found to initiate in the area of the first bolt-hole and rail-head-to-web fillet (upper fillet) at the rail end among bolted rail joints, which might cause further defects, such as rail breaks or loss of rail running surface. Previous research conducted at the University of Illinois at Urbana-Champaign (UIUC) has established an elastic static Finite Element (FE) model to study the stress distribution of the bolted rail joint with particular emphasis on rail end bolt-hole and upper fillet areas. Based on the stress calculated from the FE models, this paper focuses on the fatigue performance of upper fillet under different impact wheel load factors and crosstie support configurations. Preliminary results show that the estimated fatigue life of rail end upper fillet decreases as impact factor increases, and that a supported joint performs better than a suspended joint on upper fillet fatigue life. Show less

Abstract
The rail joint is typically considered to be one of the weakest locations in the track superstructure. Defects and failures—including bolt hole cracks, head–web cracking or separation, broken or missing bolts, and joint bar cracking—have been found to start at rail joints and the area surrounding these joints. The initiation and propagation of these defects are primarily attributable to the discontinuities of both geometric and mechanical properties in the rail joint area and the… Show more

Abstract
The rail joint is typically considered to be one of the weakest locations in the track superstructure. Defects and failures—including bolt hole cracks, head–web cracking or separation, broken or missing bolts, and joint bar cracking—have been found to start at rail joints and the area surrounding these joints. The initiation and propagation of these defects are primarily attributable to the discontinuities of both geometric and mechanical properties in the rail joint area and the high-impact loads induced by the discontinuities. Loosened or missing rail joint bolts can decrease the overall stiffness at the joint. A loosened rail joint can also accelerate certain types of defects, such as cracking around the bolt hole and the head–web fillet area (the upper fillet area) close to the rail end. These cracks present both economical and safety concerns as they can significantly reduce the service life of the rail or joint bar and even lead to breaks in the rail. However, the effect of bolt condition on stress propagation around bolted rail joints is not thoroughly understood. This study investigated the effects of bolt loading and missing-bolt configurations on the stress distribution at the bolt hole and the upper fillet area under static loading conditions. A comprehensive parametric analysis was performed with finite element modeling. Preliminary results showed that when bolt loading increased, the rail vertical displacement and stresses on the rail upper fillet decreased, but the stresses on bolt holes increased. The two center bolts, which were closest to the rail end, were the most sensitive bolts in terms of variation in stresses in response to changes in bolting and torqueing. Show less