David Y. Jeong

ESTIMATION OF THE FATIGUE LIFE OF RAILROAD JOINT BARS

This paper investigates the influence of physical track conditions in the vicinity of a rail joint on the fatigue life of the joint bars. Recent derailments due to broken joint bars, such as the Minot, ND accident in January 2002, have highlighted the need for better understanding of the effects of joint conditions on premature joint bar failure. Fatigue life estimates can be used to guide the selection of inspection intervals for joint bars in service. Engineering approximations are used to infer the dynamic load factor at a rail joint due to joint characteristics including: • rail end gap; • joint efficiency (looseness); • track stiffness (vertical foundation modulus). A three-dimensional finite element analysis of a rail joint is conducted and the dynamic load is applied to develop an estimate of the live (bending) stresses at the joint due to passing wheels. These stresses are then used to estimate the fatigue life of the joint bars. The methodology is demonstrated for 132RE rail with companion joint bars. The effect of thermal expansion (or the temperature difference below the rail neutral temperature) is investigated. Typical wheel loads and railcar speeds are considered and results are presented for a baseline a joint condition.

EFFECT OF WHEEL/RAIL LOADS ON CONCRETE TIE STRESSES AND RAIL ROLLOVER

As a result of vertical and lateral wheel/rail forces, high contact stresses can develop at the interface between the rail base and tie. Under certain conditions, these stresses can exceed the strength of the concrete tie and result in deterioration of the tie and ultimately derailment due to rail rollover. This failure mode has been determined to be the probable cause of at least two derailments where the ties were found to have a triangular wear pattern. Following these derailments, a field investigation revealed this pattern of failure present in an appreciable portion of concrete ties [1]. Closedform analyses have been conducted to examine combinations of wheel/rail loads and contact conditions that produce concrete tie rail seat deterioration or rail rollover. These results indicate that under certain circumstances truck-side L/V permitted by the Federal Railroad Administration (FRA) Safety Criterion on Wheel/Rail Loads can result in stresses above the AREMA specified minimum design compressive strength of concrete used in concrete ties. Furthermore the analysis indicated that under certain circumstances truck-side L/V permitted by the FRA Safety Criterion can result in rail rollover. The analyses show that rail rollover can be a problem for new concrete ties, but is more of a problem in the presence of rail seat deterioration described above. This work is sponsored by FRA Office of Research and Development under the track research program.

Bond between Smooth Prestressing Wires and Concrete: Finite Element Model and Transfer Length Analysis for Pretensioned Concrete Crossties

Pretensioned concrete ties are increasingly employed in railroad high speed and heavy haul applications. The bond between prestressing wires or strands and concrete plays an important role in determining the transfer length of pretensioned concrete members, but little research was done to characterize the transfer length in terms of steel reinforcement and concrete factors for railroad concrete ties. The Federal Railroad Administration is sponsoring a comprehensive test program at Kansas State University (KSU) aimed at quantitatively correlating prestressing steel and concrete variables with the transfer length of pretensioned concrete crossties. The Volpe Center has been applying the data obtained in the KSU test program to develop bond models that can be used in transfer length prediction and failure analysis of concrete ties. This paper describes finite element (FE) model development related to the smooth prestressing wire whose dominant bonding mechanisms with concrete are chemical adhesion and friction. The commercial FE software, Abaqus, is employed, and the steel-concrete interface is discretized with cohesive elements. A user bond model is developed within the elastoplastic framework and implemented for axisymmetric and 3D cohesive elements. The bond model defines constitutive relations in terms of normal and shear stresses vs. interfacial dilation and slips. The bond behavior is initially linear elastic, followed by adhesion and friction that are governed by a yield function and a plastic flow rule specific for the smooth wire-concrete interface. The main bond material parameters are normal and shear elastic stiffness, initial adhesive strength, plastic slip at which adhesion first breaks completely, and coefficient of friction. Except for the coefficient of friction, which is determined with reference to the open literature, the bond parameters are calibrated from untensioned pullout tests and pretensioned prism tests conducted at KSU. The calibrated bond parameters exhibit a dependence on the nominal compressive strength of concrete at the time of pretension release. Because considerable concrete creeping has been observed in the periods between pretension release and concrete strain measurement in the test program, an additional concrete material parameter, basic creep compliance, can be calculated and applied to adjust the concrete surface strain data. The user bond model is then validated with transfer length data measured on actual concrete crossties made with a smooth prestressing wire in a tie manufacturing plant.

Analysis of Minimum Rail Size in Heavy Axle Load Environment

The effects of increasing axle loads on rail integrity are examined in this paper. In the present context, rail integrity refers to the prevention and control of rail failures. Rail failures usually occur because cracks or defects develop and grow from cyclic forces caused by the repeated passage of wheel loads over the rails, i.e. metal fatigue. Once a crack or defect has formed, it may grow to a critical size and cause a sudden fracture of the rail. Moreover, a broken rail may cause a train to derail. Rail integrity evaluations are performed in this paper by applying a framework developed previously to estimate track capacity. The framework is exercised using two different criteria while varying axle loads: (1) allowable rail deflections and bending stresses, and (2) metal fatigue characterized in terms of propagation life (also referred to as slow crack-growth life). The engineering analyses based on these criteria are described. Results from these analyses are used to provide the rational basis for estimating the minimum rail size under heavy axle loads.

Improved tank car design development : ongoing studies on sandwich structures

The Government and industry have a common interest in improving the safety performance of railroad tank cars carrying hazardous materials. Research is ongoing to develop strategies to maintain the structural integrity of railroad tank cars carrying hazardous materials (hazmat) during collisions. This paper describes engineering studies on improved tank car concepts. The process used to formulate these concepts is based on a traditional mechanical engineering design approach. This approach includes initially defining the desired performance, developing strategies that are effective in meeting this performance, and developing the tactics for implementing the strategies. The tactics are embodied in the concept. The tactics and concept evolve through engineering design studies, until a design satisfying all of the design requirements is developed. Design requirements include service, manufacturing, maintenance, repair, and inspection requirements, as well as crashworthiness performance requirements. One of the concepts under development encases the pressurized commodity-carrying tank in a separate carbody. Moreover, this improved tank car concept treats the pressurized commodity-carrying tank as a protected entity. Welded steel sandwich structures are examined as a means to offer protection of the commodity tank against penetrations from impacting objects in the event of a collision. Sandwich structures can provide greater strength than solid plates of equal weight. Protection of the tank is realized through blunting of the impacting object and absorption of the collision energy. Blunting distributes impact loads over a larger area of the tank. Energy absorption reduces the demands on the commodity tank in the event of an impact. In addition, the exterior carbody structure made from sandwich panels is designed to take all of the in-service loads, removing the commodity tank from the load path during normal operations. Design studies described in this paper focus on the protection aspect of using sandwich structures. Studies are conducted to investigate the influence of different parameters, such as sandwich height and core geometry, on the forcedeformation behavior of sandwich structures. Calculations are carried out numerically using nonlinear finite element analysis. These analyses are used to examine the crashworthiness performance of the conceptual design under generalized impact scenarios.

Finite Element Bond Models for Seven-Wire Prestressing Strands in Concrete Crossties

Seven-wire strands are commonly used in pretensioned concrete ties, but its bonding mechanism with concrete needs further examination to provide a better understanding of some concrete tie failure modes. As a key component in the finite element (FE) analysis of concrete crossties, macro-scale or phenomenological FE bond models are developed for seven-wire strands in this paper. The strand-concrete interfaces are homogenized with a thin layer of cohesive elements applied between the strand and concrete elements. Further, the cohesive elements are assigned traction-displacement constitutive or bond relations that are defined in terms of normal and shear stresses versus interfacial dilation and slip. The bond relations are developed within an elasto-plastic framework that characterizes the adhesive, frictional and/or dilatational bonding mechanisms in the interface. The yield functions and plastic flow rules specific for the seven-wire strands are presented. The bond parameters are calibrated from untensioned pullout tests and pretensioned prism tests conducted on concrete specimens. The bond models are then verified with (1) the surface strain data measured on actual concrete crossties made at a tie manufacturing plant, and (2) the force-displacement relation obtained in a center negative moment test conducted also on concrete crossties.

Failure analysis of railroad concrete crossties in the center negative flexural mode using finite element method

Finite element models are developed for the railroad concrete crossties and are employed to analyze their center negative flexural responses to two center binding conditions: a center negative moment test condition and a hypothetical deteriorated ballast support condition. These conditions can lead to center negative flexural cracks and, eventually, sudden, catastrophic failure of the concrete ties when the loads reach critical magnitudes. When the concrete ties fail completely and consecutively in track, the gage can be sufficiently widened to cause derailment. The finite element results are first validated with the available test data, and the validated finite element models are then employed to obtain and evaluate the cracking/failure patterns and force–displacement characteristics of the concrete ties in the center negative flexural mode in static and dynamic analyses. The finite element analyses predict the critical wheel loads above which catastrophic tie failure is likely to occur, and the dynamic critical failure loads are shown to depend on the center binding ballast support conditions as well as the worn concrete tie conditions. The worn tie conditions that include bottom abrasion and prestress loss can significantly reduce the dynamic critical failure loads and, thereby, adversely affect the center negative flexural performance of the concrete ties.

On Railroad Tank Car Puncture Performance: Part II - Estimating Metrics

This paper is the second in a two-part series on the puncture performance of railroad tank cars carrying hazardous materials in the event of an accident. Various metrics are often mentioned in the open literature to characterize the structural performance of tank cars under accident loading conditions. One of the consequences in terms of structural damage to the tank during accidents is puncture. This two-part series of papers focuses on four metrics to quantify the performance of tank cars against the threat of puncture: (1) speed, (2) force, (3) energy, and (4) conditional probability of release.In Part I, generalized tank car impact scenarios were illustrated. Particular focus is given to the generalized shell impact scenario because performance-based requirements for shell puncture resistance are being considered by the regulatory agencies in United States and Canada. Definitions for the four performance metrics were given. Physical and mathematical relationships among these metrics were outlined. Strengths and limitations of these performance metrics were discussed.In this paper (Part II), the multi-disciplinary approach to develop engineering tools to estimate the performance metrics is described. The complementary connection between testing and modeling is emphasized. Puncture performance metrics, which were estimated from other sources, are compared for different tank car designs. These comparisons are presented to interpret the metrics from a probabilistic point of view. In addition, sensitivity of the metrics to the operational and design factors is examined qualitatively.

On Railroad Tank Car Puncture Performance: Part I — Considering Metrics

This paper is the first in a two-part series on the puncture performance of railroad tank cars carrying hazardous materials in the event of an accident. Various metrics are often mentioned in the open literature to characterize the structural performance of tank cars under accident loading conditions. One of the consequences in terms of structural damage to the tank during accidents is puncture. This two-part series of papers focuses on four metrics to quantify the performance of tank cars against the threat of puncture: (1) speed, (2) force, (3) energy, and (4) conditional probability of release. In this paper (Part I), generalized tank car impact scenarios are illustrated. Particular focus is given to the generalized shell impact scenario because performance-based requirements for shell puncture resistance are being considered by the regulatory agencies in United States and Canada. Definitions for the four performance metrics are given. Physical and mathematical relationships among these metrics are outlined. Strengths and limitations of these performance metrics are discussed. In Part II, the multi-disciplinary approach to develop engineering tools to estimate the performance metrics will be described. The complementary connection between testing and modeling will be emphasized. Puncture performance metrics, which were estimated from other sources, will be compared for different tank car designs. These comparisons will be presented to interpret the metrics from a probabilistic point of view. In addition, sensitivity of the metrics to the operational and design factors will be examined qualitatively.

Rail Integrity Experience on the Washington Metro System

The Washington Metropolitan Area Transit Authority (WMATA) provides passenger rail service to the nation’s capital. Although the rail system carries only passenger trains, the rail integrity issues that WMATA must manage are similar to those that freight railroads also face. These issues include occurrences of broken rail from internal rail head defects, detection of such defects, and repair of the rail to restore service. Another example is the development of damage on the running surface of the rail, called rolling contact fatigue (RCF). Such surface damage is known to adversely affect the detection of internal rail head defects beneath RCF conditions. While WMATA’s rail integrity issues may be similar to those that freight railroads also encounter, the management of such issues are different, which are also discussed in this paper.This paper describes the recent experience of broken rails on the WMATA rail system. In addition, results from engineering fracture mechanics analyses are presented to help understand how operational, environmental, design, and maintenance factors influence rail failure.Copyright