Cord-rubber composites play a critical role in various components, such as air spring bellows used in secondary suspension systems for rail vehicles. The reliable performance of these components requires a thorough understanding of their material behavior and fatigue characteristics. This research addresses the existing knowledge gap by developing a representative specimen design and methodology to analyze and optimize fatigue performance under varying conditions.
A flat specimen geometry has been identified as suitable for investigating the behaviour of the base material under different loading scenarios. Finite element analysis and optical strain measurements were used to design and validate the specimen geometry to ensure its applicability in experimental testing. Fracture patterns were examined using radiography and micro-computed tomography, which revealed delamination as the primary damage type. This novel specimen design provides a significant advance in the evaluation of fatigue behavior and accelerates the development of air springs and cord rubber layups.
A systematic study of fatigue strength was conducted using biaxially loaded flat specimens with varying fiber angles of ±15°, ±25° und ±35°. A failure criterion based on 20% elongation in the axial direction was defined to ensure the applicability of the specimen tests. The results showed that larger fiber angles reduced fatigue strength, with a 10° increase correlating to a 15% reduction in tolerable lateral displacement amplitude at 50 000 load cycles. A master S/N curve was established using the maximum fiber strain, unifying the fatigue test data into a comprehensive design model for cord-rubber composites.
Further analysis assessed the transferability from the specimen level findings to full-scale air spring bellows. Tests focused on two distinct loading conditions, pure pulsating tensile stress with a strain ratio of 0 and pulsating tensile stress with a strain ratio ranging from 0.2 to 0.3, which induced significant delamination. Comparison of two series of specimens with different strain ratios revealed that higher average strains prolonged service life. Under the loading conditions analyzed, the maximum tolerable fiber strain increased from 0.08 to 0.11 at 50 000 load cycles, highlighting a strong correlation between model predictions and experimental component results.
This work establishes a robust methodology for time and cost efficient fatigue design of cord-rubber composites under various loading conditions. The developed specimen testing approach and analytical models provide valuable insights into the material’s response to complex loading scenarios, paving the way for improved durability and optimized design of air spring systems.
Deep rolling is a mechanical post-treatment process employed to enhance the surface layer properties of railway axles. This technique involves the controlled plastic deformation of the near-surface layer, leading to a reduction in surface roughness, work hardening of the treated area, and the introduction of compressive residual stresses. These effects are particularly beneficial for high-strength steels, such as the 34CrNiMo6 steel used for railway axles, which is the focus of the current investigation. The deep rolling process is utilised to increase fatigue strength, improve resistance to crack initiation, and reduce crack propagation. Furthermore, existing defects, such as those caused by ballast impacts or corrosion, are inhibited from propagating into cracks. This process enables the design of lighter railway axles with equivalent service life. The reduction in material usage during production and the enhancement in energy efficiency during operation positively impact the component's carbon footprint. Additionally, the reduction in unsprung masses decreases wheel and rail wear by lowering dynamic contact forces, thereby contributing to reduced life cycle costs of the railway system, as demonstrated by the favorable influence on track access charge calculations.
A series of experimental tests is conducted on a railway axle. These include tensile tests and cyclic tests to characterise the material properties, as well as tests to investigate the modifications induced by the deep rolling process. Microstructural analysis, hardness measurements, surface roughness assessments, residual stress measurements, and work hardening investigations are performed and compared between the non-deep rolled and deep rolled regions of the railway axle. Conventional residual stress measurement techniques, such as X-ray diffraction and hole drilling method, exhibit limitations when measuring at greater depths and are incapable of determining the range of balancing tensile residual stresses, which is critical for fatigue strength evaluation. Consequently, a numerical simulation model of the deep rolling process is developed to reliably ascertain these conditions. The elastic-plastic material behaviour is represented using a Chaboche material model, parameterised based on low cycle fatigue test results, and the simulation outcomes are validated against the residual stress measurements. Thus, a numerical tool is developed to determine the residual stresses introduced by deep rolling and to explore various deep rolling parameters and optimisations. The study investigates and discusses the most influential deep rolling parameters, such as deep rolling force and feed rate, as well as the effects of prestresses induced by the manufacturing process and optimisations involving repeated deep rolling.
The fatigue behaviour of the railway axles is examined through both experimental and numerical simulation results, providing a comprehensive overview of the deep rolling application. The concept of local fatigue strength is employed to analyse the influence of deep rolling on fatigue performance and, consequently, on the design of railway axles. The impact of various deep rolling parameters and optimisations is considered and compared. A commercial crack propagation software is utilised to assess the influence of residual stresses induced by deep rolling on crack propagation behaviour in the presence of an existing crack, and the effects of different deep rolling parameters are investigated. The beneficial properties of deep rolling are achieved through controlled plastic deformation of the material. However, this process alters the material and may result in undesirable surface damage and subsequent deterioration of fatigue properties if the parameters are not selected appropriately. To address the potential limitations of the process, a method for assessing pre-damage caused by deep rolling process itself is developed and validated through tests involving multiple deep rolling passes over the same area. This strain-based damage calculation enables the evaluation of pre-damage induced by deep rolling for different parameters and optimisations.
The increasing significance of sustainable mobility and environmentally friendly transportation systems has led to a surge in research and development in this area. The railway transportation, as an environmentally friendly and sustainable mode of transportation, plays a crucial role in providing affordable solutions for public passenger transport in urban, suburban and regional areas. The development of innovative technologies aims to meet the needs of operators and passengers while minimizing environmental impact by addressing the major challenges of quality, reliability, cost-effectiveness, energy efficiency and emissions. Currently, metro bogies often exhibit one axle per wheelset, resulting in heavy brake actuators and high mass brake discs. These unsprung brake disc masses have a negative impact on track damage, emission and energy consumption including noise and pollutants. Furthermore, as particulate emissions from transport vehicles become a growing concern for human health and the environment, non-exhaust emissions from wear and turbulence gain significance. The brake system is a major source of wear-related particulate emissions and is therefore a focus of research to reduce such emissions. An innovative braking concept using sintered friction materials and advanced components, along with a transmission stage, should reduce chassis mass and enhance the tribological behaviour of the braking mechanism. Furthermore, this system aims to diminish noise and particulate emissions in line with lightweight construction requirements.
In the pursuit of sustainable transportation solutions, this dissertation focuses on the development of an innovative braking system for railway transportation which plays a vital role in fostering environmentally friendly mobility. By utilizing a gear stage, the system eliminates the necessity of the brake discs on unsprung railway axles, thereby significantly reducing wheel, track and superstructure damage and subsequently lowering the total life cycle costs. In addition, minor braking forces enable the use of lightweight brake components, contributing to an overall weight reduction. Nevertheless, increased sliding speeds at the friction partners related to the gear ratio pose a challenge that will be scientifically investigated throughout the project. On the flipside, fundamental tribological investigations will be carried out to evaluate friction partners with novel, environmentally friendly material compositions. The subsequent analyses using selected test specimens on an innovative brake test rig for rail vehicles will offer valuable insights into the friction value stability, wear behaviour and temperature load. The simulation-based analyses will complement the experimental work. Therefore, the scientific output will provide a deeper understanding of global and local system characteristics and facilitating targeted investigations of braking concept. The innovative braking system is expected to make a significant contribution environmental protection and sustainable mobility.
The increasing demands on contemporary mechanical engineering and the need to reduce resource consumption necessitates the use of complex designs and high-strength materials. This enables basic functions, decreases component weight, and mitigates expenses associated with materials and resources.
The objective of this doctoral thesis is to analyze the fatigue behavior of a welded material in consideration of a subsequent treatment process. The investigation concentrates on the grinding of the seam transition to acquire new findings regarding the material's service life during cyclic loading. To produce welding samples from S355 and S960 sheets for this purpose, a specific welding method mainly utilized for intricate component structures will be employed. Subsequently, the fatigue behavior of the samples will be investigated and evaluated through simulations, measurements, and analysis of fracture surfaces. A focal point here is the impact of misalignment and angular distortion on service life.
The test methodology aims to establish fundamental principles that will be integrated into forthcoming service life assessments. Furthermore, the acquired knowledge will be utilized to authenticate a practical implementation on further high-strength shaped tubes.
The project's research activities and resulting scientific findings aim not only to reduce costs and save time in manufacturing intricate welded structures, but also to establish a framework for designing welded and post-treated welded joints using high-strength steel.
Innovative materials, such as wood composites and high-strength steels, play a pivotal role in improving the efficiency of various structural applications. In the mobility sector, there is a growing emphasis on lightweight technologies aimed at reducing energy consumption and emissions. In rail transport, significant efforts are underway to integrate these materials to enhance environmental performance and economic efficiency. Wood composites are particularly notable for their sustainability, while high-strength steels offer exceptional strength-to-weight ratios. These advancements drive the development of innovative vehicle designs that are both high-performing and eco-friendly. However, the use of these innovative materials introduces new challenges. Unlike conventional structural materials, their properties are often not well-documented or entirely unknown. Reliable data must be obtained by experimental testing to achieve accurate input parameters for design with special focus on the fatigue assessment. This is critical in order to ensure a safe and reliable performance of such structures in final operation considering mechanical as well as environmental effects. Therefore, the research work investigates the fatigue performance of such innovative materials and their joining techniques with focus on wood composites as well as steel structures considering their potential applications in rail vehicle design. Alongside evaluating the structural durability, the study examines key factors such as sustainability, weight efficiency, and the effects of environmental conditions like humidity and temperature. Experimental testing utilizing specially designed test setups enables a fundamental characterization of the basic material and joint properties. These findings are further processed applying advanced numerical simulation methods, offering a detailed basic understanding of the local mechanical behaviour and resilience of these materials under diverse operating conditions. By combining experimental and simulation approaches, the research offers scientific insights into the suitability of wood composites and steel materials and demonstrates how they can meet the functional demands of traditional structural solutions while providing advantages in sustainability and efficiency with special focus on rail vehicle design.
Railway systems around the world suffer damage to vehicles and tracks during operation. To control this damage, it must be checked and cleaned regularly. This is a time-consuming and expensive process. Researchers are still trying to understand why and how the different types of damage happen.
When rail vehicles pass through tight curves, there's a lot of stress on the wheel-rail contact. This causes wear on both the wheel and the rail. Many railways, like subways or narrow-gauge railways, have routes with a lot of curves with small radii. This makes them particularly affected by damage due to wear. Damage from wear and tear is a big problem for Swiss meter-gauge railways, especially slip waves. This waving, which occurs on the head of the inner rail of the curve, causes vibrations and noise when the train passes over it. It can affect ride comfort and the environment, and it can also damage other system components. On some Swiss narrow-gauge railways, the growth of slip waves is so intense that the tracks can no longer be used in some cases.
This thesis looks closely at a type of damage called "slip wave," which is mainly about understanding why the rails wear unevenly, and how that affects the tracks. This includes a detailed examination of the excited system resonances and the cause of their excitation. The interaction with wheel polygons and wheel and rail profiles will also be considered. To this end, new simulation models will be developed based on the latest technology. To make these models, we will do a lot of tests in the lab and out in the field. These tests will give us information about how slip waves start and grow. We'll use some of the test results to calibrate.
The bogie of a rail vehicle is a dynamic system and experiences stochastic excitations from the track. Until now, the chassis as an oscillating system has been insufficiently investigated. Currently, there is no stiffness optimization between the components with respect to vibration behavior and strength, and there are no structural dynamic investigations of the individual components. As the desired lightweight construction results in new, softer structures, it is increasingly common for components to start vibrating during operation. These poorly damped vibrations can quickly lead to fatigue cracking, which reduces safety and increases warranty and maintenance costs.
The aim of this work is to develop a stable, robust and validated method (from loads to stresses) for the optimization and design of vibration relevant structural components with respect to stiffness and strength. First, operational data measured on a vehicle are analyzed using statistical signal processing methods, taking into account the laws of structural dynamics and strength. Vehicle and track parameters are also used and their correlation to the measured data is investigated. Since the spectral methods promise time and cost savings in calculation and evaluation, the aim is to define the loads (substitute excitation for further calculation) in the frequency range. Scientific methods are being developed to create parameterized excitation functions from existing measurement data in order to gain a better understanding of the measurement data and to extract relevant components with respect to fatigue. The system response (stress on the structural component) is also given in the frequency domain and requires spectral evaluation methods. Within the scope of this activity, the scientific extension of spectral fatigue methods for transient excitations is worked on. The developed concepts are evaluated, compared and validated by tests and selected damage cases, which are calculated both transiently and spectrally. In addition, a coupling of a topology optimizer with the developed evaluation methodology is aimed at in order to be able to derive structural components with desired structural dynamic properties and sufficient strength already in the design process.
The scientific findings and the methodology developed in this doctoral thesis should contribute to an application-oriented and reliable design and optimization of vibrating structural components.
The dissertation project deals with the development of load-transmitting structural components of an excavator, namely the arm system and boom made out of steel. As measures for the weight reduction the use of high-strength steels and the associated effects on static and fatigue strength as well as the necessary manufacturing- and post-treatment processes are being investigated. This project aims to create the necessary scientific basis for the design and validation of welded lightweight constructions. As an application of this, a new innovative arm system is going to be developed and validated. The weight reduction and increased energy efficiency will make the battery-electric machine even more environmentally friendly and also reduce both noise and air pollution, which is particular an advantage when used in urban environments. A new methodology is being developed using numerical analysis of static load cases in conjunction with empirical values, which enables a safe and durable design of the aforementioned structural components in lightweight construction. Therefore the knowledge of the occurring operating loads is of great importance, which is why corresponding load collectives are evaluated by means of scenario tests as well as further long-term measurements with accompanying measurements of local and global stress and system parameters. From the analysis of the results of these measurements and the numerical analysis representative small-scale specimens are derived. They are mechanically tested and serve as a basis for the fatigue design of the high-strength steel structure. From the interaction of all working steps a validation methodology for lightweight structures is developed, ensuring enhanced system reliability. Finally, it is planned to carry out large-scale component tests with the newly developed lightweight components, whereby the developed design methodology and the lightweight design will be experimentally validated in order to prove the technical applicability of the scientific work.
This research focuses on the development of assessment methods for vibratory behavior of a railway disk brake system. The disk brake is a type of primary brake that is fundamental means for decelerating and stopping the railway vehicle. Hence, the stability and durability of the system are essential to ensure the safe operation. When the brake is applied, the pads are pressed on to the disk by the caliper assembly. The frictional force generated between the disk and the pads convert kinetic energy into heat energy. Here, the system is subject to self-excited vibrations generated by their frictional contact which can have hazardous impacts to the structure.
The self-excited vibrations of the brake systems are noticeable by their distinctive squeal and groan noise. The squealing noise is high-frequency noise that is often generated at low speeds. It is not always a critical safety issue, but it is a cause for discomfort and potential human health issues. On the other hand, groan noise results from low-frequency vibration, which may lead to structural damage, such as vibrational fatigue cracks and fractures. While it is relatively easy to suppress vibration and resonance by simply increasing the rigidity by increasing the thickness of the steel structures, this will lead to a heavier bogie frame. The heavier bogie will affect ride performance, track wear as well as other aspects of the railway vehicle operation and is not preferred. To design lightweight bogies, it is essential to develop a method to evaluate the vibratory characteristics of the structural design.
Finding the essential design parameters that influence the characteristics is important to model the vibration behavior of the brake systems. In the actual world, the system's vibration is affected by numerous factors. Therefore, it is difficult to determine the parameters that influence the characteristics from the actual operation measurement only. To eliminate the effect of the environmental conditions from the vibration measurement, reproductive experiments on a brake test rig will be performed under similar braking conditions to the actual operation. Based on both results, the research aims to develop a basic brake system simulation model. After establishing the basic model, the study seeks to expand it to a complex model, including the bogie structure. The simulation model will be validated by the experiments conducted under similar conditions in the brake test rig.
As a result of this dissertation project, there will be better quantitative methods to understand the overall vibration phenomenon of the brake system, including the bogie structure. Therefore, scientific outcome will be vital in helping to identify potential mechanical failures for future design and enhance the lightweight design of the bogies.
The interaction between the vehicle and track results in significant acquisition and maintenance expenses for the entire railway system. Therefore, an overall system approach is necessary to enhance the interaction between the vehicle and track and reduce these costs. It is crucial to comprehend the interaction between the running gear and track to ensure safe and cost-effective operations. The objective is to analyze the interaction between running gears and rails using elastic multi-body systems (MBS), while taking a holistic and systematic approach to the vehicle-track system. Precisely combining measurement data from vehicle and track measurements is crucial to achieve this goal.
The main objective of this work is to create a comprehensive calculation model that facilitates the efficient and secure design of railway vehicles at reduced acquisition and upkeep expenses. By deriving research findings from the ongoing work, it will be feasible to recommend optimal practices for operating the entire railway network. To establish the relevant loads that cause damage to the rail, a multi-body model of the vehicle is created and subjected to representative scenarios that reflect the intended use. Next, the influence of the numerous input parameters on the vehicle model will be investigated and whether these can be reduced for computational feasibility.
The analysis of vehicle and track measurements determines the critical input parameters for load simulation and provides a comprehensive understanding of the vehicle/track system. Assessment of various bogie and rail modeling concepts takes place simultaneously. The input variables extracted from the measurements are utilized in the resulting generalized model to realistically depict potential vehicle deployment scenarios.
The MBS models developed as part of this study will enable the acquisition of stress levels for additional fatigue strength testing of bogie fixtures vulnerable to vibrations. The models account for frequency-dependent energy components introduced into the vehicle from the track, which can cause damage to vibrating attachments. Based on these scientific findings, a more detailed understanding of the system network is to be achieved, which will enable a uniform further development of the entire railway system.