An investigation into the formation of squats in rails: modelling, characterization and testing

Rolling contact fatigue (RCF) is an important form of damage in wheels and rails that typically has surface and subsurface cracks. Squats are one of the major RCF defects that occur in the running band of rails and can create high dynamic forces and cause rail fracture if they are not detected and treated in time. In the current research, three advanced methods are developed in order to obtain a better understanding of the formation mechanism of RCF defects and, especially, squats in rails: 1) A new thermomechanical tool for numerically modelling the wheel–rail contact, 2) A new experimental setup for physically simulating the wheel–rail interaction and 3) A new computed tomography (CT) procedure for characterizing the wheel–rail defects. The first part presents a coupled thermomechanical modelling procedure for the wheel–rail contact problem and computes the flash–temperature and stress–strain responses when thermal effects are included. The contact temperature and thermal stresses could be driving factors for squats initiation. A three–dimensional (3D) elasto–plastic finite element model is built considering the wheel–track interaction. When the wheel is running on the rail, frictional energy is generated in the contact interface. The model is able to convert this energy into heat by using a coupled thermomechanical approach. The numerical models calculate the flash–temperature and thermomechanical stresses in the wheel and rail. In the second part, a new downscale test setup is designed and built for investigating the interaction between wheel and rail, especially under impact–like loading conditions, which are supposed to be often associated with rail squats. The test rig is intended to remedy the lack of dynamic similarity between the actual railway and the existing laboratory testing capability, by considering the factors that contribute to high–frequency dynamics of the wheel–track system. This part of the thesis further presents the results of some experiments carried out using the newly–built setup to verify the ideas behind its development. The third part presents the development of a computed tomographic (CT) scanning technique to reconstruct the 3D geometry of the RCF cracks in the railhead. Squat defects are associated with complex crack networks at the subsurface. Sample rails having squats of different severities are taken from the Dutch railway network. Various specimens of different sizes are prepared and investigated with the CT scanner. A detailed procedure of the CT experiment and post–processing is described. The proposed 3D visualization method, together with the necessary geometric definitions, is then used for enabling effective measurement and characterization of the squat cracks. Based on this research, the main new insights into the formation of rail squats are as follows: i) the WEL formation via martensitic phase transformation turns out to be possible; this is confirmed through the thermomechanical wheel–rail contact modelling; ii) the impact–like loading conditions and high–frequency dynamic characteristics of the wheel–track system appear to be essential for the squat formation; this is confirmed through the vehicle–track testing using the new test rig; and iii) the occurrence of different crack orientations followed by the primary and secondary V–shaped cracks turns out to be important in the squat formation; this is confirmed through the CT scanning and metallographic observations.Railway Engineerin

A new small-scale test rig for the wheel-rail contact studies

Railway Engineerin

Computation of stress intensity factors in an initiating RCF crack using a 3D modelling approach

Railway Engineerin

Nucleation of squat cracks in rail, calculation of crack initiation angles in three dimensions

A numerical model of wheel-track system is developed for nucleation of squat-type fatigue cracks in rail material. The model is used for estimating the angles of squat cracks in three dimensions. Contact mechanics and multi-axial fatigue analysis are combined to study the crack initiation mechanism in rails. Nonlinear material properties, actual wheel-rail geometries and realistic loading conditions are considered in the modelling process. Using a 3D explicit finite element analysis the transient rolling contact behaviour of wheel on rail is simulated. Employing the critical plane concept, the material points with the largest possibility of crack initiation are determined; based on which, the 3D orientations/angles of the possible squat cracks are estimated. Numerical estimations are compared with sample results of experimental observations on a rail specimen with squat from the site. The findings suggest a proper agreement between results of modelling and experiment. It is observed that squat cracks initiate at an in-plane angle around 13°-22° relative to the rail surface. The initiation angle seen on surface plane is calculated around 29°-48°, while the crack tend to initiate in angles around 25°-31° in the rail cross-section.Structural EngineeringCivil Engineering and Geoscience

Scaling strategy of a new experimental rig for wheel-rail contact

A new small–scale test rig developed for rolling contact fatigue (RCF) investigations in wheel–rail material. This paper presents the scaling strategy of the rig based on dimensional analysis and mechanical modelling. The new experimental rig is indeed a spinning frame structure with multiple wheel components over a fixed rail-track ring, capable of simulating continuous wheelrail contact in a laboratory scale. This paper describes the dimensional design of the rig, to derive its overall scaling strategy and to determine the key elements’ specifications. Finite element (FE) modelling is used to simulate the mechanical behavior of the rig with two sample scale factors of 1/5 and 1/7. The results of FE models are compared with the actual railway system to observe the effectiveness of the chosen scales. The mechanical properties of the components and variables of the system are finally determined through the design process.Structural EngineeringCivil Engineering and Geoscience

Determining the angles of squat cracks via CT scanning and metallographic observations

This study investigates the angles θ1, θ2, and θ3 that squat crack faces form with respect to three orthogonal planes: the rail top, the longitudinal-vertical cross-section and the lateral-vertical cross-section. Rail samples with squats of various severities are obtained from the field. Their three-dimensional crack networks are reconstructed using CT (computed tomography) scanning and serial cutting. A 3D visualization method, together with the necessary geometric definitions, is developed for enabling effective measurement and characterization of the squat cracks. It is found that the cracks can be characterized by four orientations (T1 – T4). The variation ranges of the crack angles are determined for each orientation that satisfies 132° ≤θ1 ≤ 150°, 6° ≤θ2 ≤ 36° and 67° ≤θ3 ≤ 81°. By investigating the occurrence frequency of the orientations, it is found that T4 and T1 together form the primary V-shaped cracks of the squats, and T2 and T3 together form the secondary V-shaped cracks. A finite element modelling of the wheel-track system, in combination with contact mechanics and multi-axial fatigue analysis, successfully relates the stress state to the RCF cracks.Railway Engineerin

Developing A Priority-Based Decision Making Mod To Evaluate Geometric Configuration Of Urban Interchanges

The present article involves in evaluation and engineering judgment of various geometric configurations for highway interchanges by considering substantial parameters over the discretion process. The geometric, economical and architectural criteria as the fundamental indicators are divided into related sub-indicators and the total combinations of such sub-elements from the general criterion for establishment of decision making process. Hence, this article deals with geometric configuration analysis of interchanges as a complex decision problem by the use of analytic hierarchy process (AHP) which is a structured technique to analyze complicated engineering systems. By considering an interchange as a case study in north of Tehran, the capital of Iran, the performance of the proposed method has been examined in order to select the most suitable type of interchange by forming the evaluation process of AHP and taking into account the given design and construction data. a wide range of notorious criteria and desired prerequisites are available. Owing to established the AHP model and perform the decision-making method, the Expert Choice analytical software has been utilized. The evaluation results are determined in terms of priorities for various options and their decision weights in the case study. However the presented model is able to be applied for other cases and different alternatives. As a tentative finding, using directional pattern for the case example of current work has been the optimum variant rather than parallel alternatives i.e. semi-directional and loop schemes.Structural EngineeringCivil Engineering and Geoscience

Reliability analysis of idealized tunnel support system using probability-based methods with case studies

In order to determine the overall safety of a tunnel support lining, a reliability-based approach is presented in this paper. Support elements in jointed rock tunnels are provided to control the ground movement caused by stress redistribution during the tunnel drive. Main support elements contribute to stability of the tunnel structure are recognized owing to identify various aspects of reliability and sustainability in the system. The selection of efficient support methods for rock tunneling is a key factor in order to reduce the number of problems during construction and maintain the project cost and time within the limited budget and planned schedule. This paper introduces a smart approach by which decision-makers will be able to find the overall reliability of tunnel support system before selecting the final scheme of the lining system. Due to this research focus, engineering reliability which is a branch of statistics and probability is being appropriately applied to the field and much effort has been made to use it in tunneling while investigating the reliability of the lining support system for the tunnel structure. Therefore, reliability analysis for evaluating the tunnel support performance is the main idea used in this research. Decomposition approaches are used for producing system block diagram and determining the failure probability of the whole system. Effectiveness of the proposed reliability model of tunnel lining together with the recommended approaches is examined using several case studies and the final value of reliability obtained for different designing scenarios. Considering the idea of linear correlation between safety factors and reliability parameters, the values of isolated reliabilities determined for different structural components of tunnel support system. In order to determine individual safety factors, finite element modeling is employed for different structural subsystems and the results of numerical analyses are obtained in different design scenarios. Finally, the reliability index values are obtained for the entire support structure in different design scenarios. The results of the work demonstrates that proposed reliability evaluation method of tunnel support system is effective not only for investigating the reliability of individual elements in the structure, but also for building an overall estimation about reliability performance of the entire tunnel structure.Structural EngineeringCivil Engineering and Geoscience

Residual fatigue life evaluation of rail at squats seeds using 3D explicit finite element analysis

A modeling procedure to predict the residual fatigue life of rail at squats seeds is developed in this article. Two models are involved: a 3D explicit Finite Element (FE) model to compute the stress and strain at squats in rail, and the J-S fatigue damage model to determine the residual fatigue life on the basis of the computed stress and strain. In the FE model dynamic effects of wheel-rail system under rolling contact is taken into account. Bilinear isotropic elastic-plastic material properties are adopted to represent the hardening of wheel and rail. Squats are subject to multiple loading cycles. The geometry of the squat is varied in the simulation corresponding to a growing squat at different ages. It is found that small squats lead to fatigue failure while severe ones lead to ratcheting failure.Structural EngineeringCivil Engineering and Geoscience

Development of a New Downscale Setup for Wheel-Rail Contact Experiments under Impact Loading Conditions

A new downscale test rig is developed for investigating the contact between the wheel and rail under impact-like loading conditions. This paper presents the development process of the setup, including review and synthesis of the potential experimental techniques, followed by scalability, mechanical and operational analysis of the new setup. The new test rig intends to remedy the lack of dynamic similarity between the actual railway and the existing laboratory testing capability, by taking into account the factors that contribute to high-frequency dynamics of the wheel-track system. The paper first reviews the functionalities of the existing test techniques in the literature. Based on this survey, the category of the scaled wheel on the rail track ring is chosen. Afterwards, three potential alternatives are identified under the chosen category and the optimum mechanism is achieved through finite element modelling and analysis of the structures. A downscale test rig, consisting of multiple wheel components running over a horizontal rail track ring, effectively fulfilled the requirements needed for analogical testing of the wheel-rail contact behaviour. The new test rig is a unique experimental setup due to the involvement of high-frequency dynamic vibrations in the wheel-track system and analogy of the incorporated elements and loading to those of the real-life system. This paper further presents the results of some real experiments carried out using the newly-built setup to support substantial ideas behind its development.Railway Engineering(OLD) MSE-