Tread Braking Research Articles

Failure analysis for power car wheels based on contact positions and tread slope

Abstract During service operation, the thermal cracks and shelling at power car wheel are initiated before a regular profiling of railway wheel. The damaged railway wheel is reprofiled prior to regular inspection and the maintenance cost for the railway wheel is sharply increased. Therefore, it is necessary to analyze and understand the damage mechanism of power car wheel in order to reduce maintenance cost. In the present paper, to analyze the damage cause and reduction for the power car wheel with tread brakes, the observations of by replication test was performed and fracture characteristics depend on contact zones are carried out. Contact stress is analyzed with respect to the wheel tread slope and contact positions. The result shows the zone between tread center and wheel rim demonstrated white layer by higher hardness and lower fracture toughness and faster crack growth rate in view point of fracture mechanic characteristics. The 1/20 of wheel tread slope can effectively mitigates surface damage of power car wheel.

Influence of Temperature Rising of Tread Braking on Wheel Wear for Heavy Haul Freight Car

Influence of Temperature Rising of Tread Braking on Wheel Wear for Heavy Haul Freight Car

Wheel tread profile evolution for combined block braking and wheel–rail contact: Results from dynamometer experiments

Wheel treads are subject to different types of damage such as wear, rolling contact fatigue (RCF), thermal cracks, plastic deformation and flats caused by wheel sliding. These types of damage cause changes in the tread profile of the wheel, which necessitates frequent wheel reprofiling in order to maintain the ride comfort of the vehicle.In this study, a series of full-scale tread braking experiments, including wheel–rail rolling contact, were conducted to clarify the factors influencing the wheel tread profile evolution. The experiments focused on plastic deformation and the wear caused by the rolling contact and tread braking.The results showed that the maximum tread indentation was 0.20mm at the rolling contact center when the stop braking action was repeated 40 times. This was caused by the plastic deformation of the wheel tread, which, in turn, was the result of high contact pressure and material softening from high temperatures caused by tread braking. The results were supported by the observed tread protrusions near the rolling contact area and also by the difference in the rolling contact area hardness and that of the other wheel tread areas.

Effect of elevated temperature on shelling property of railway wheel steel

A type of tread damage called thermal mechanical shelling (TMS) is one of the main causes for heavy haul wheel removals in North America. Wheels are considered to have a shorter service life for TMS than other causes of removals as rolling contact fatigue (RCF) strength decreases because of elevated temperature from tread brakes. In this study, to evaluate the relationship between RCF life and elevated temperature, RCF tests and FE analyses were conducted. Twin disc type RCF tests with induction heating were applied to evaluate RCF life under thermal cycling simulating tread brakes. Test results show that RCF life was shorter with the higher maximum temperature and the longer holding time at elevated temperature. Softening of the material because of microstructure change was observed for the test specimens tested at the maximum 500°C. Then, crack tip opening behaviour was evaluated by elasto-plastic FE analysis. FE analysis results showed that crack tip opening displacement (CTOD) was larger with the higher temperature and the longer holding time at elevated temperature. Moreover, CTOD increased with a decrease of the material strength. These results indicate that wheel life of TMS decreases by acceleration of fatigue crack propagation with elevated temperature.

The influence of snow on the tread braking performance of a train: A pin-on-disc simulation performed in a climate chamber

In trains with tread brakes, the coefficient of friction between the brake block and the railway wheel determines the stopping distance. The blocks have traditionally been manufactured from cast iron. Although these blocks have good braking capacity, their use is often restricted due to the squealing noise they emit. Tests of alternative composite block materials have been successful under summer conditions; however, in regions with snowy winters the use of such materials has been limited due to problems with braking capacity under snowy conditions. This research aims to develop a laboratory-scale test methodology for evaluating the braking capacity of tread brake materials under winter and snowy conditions. A pin-on-disc machine placed in a climate chamber was used for testing, and a block of standard cast iron was compared with blocks of standard composite materials. The results indicated that the blocks of standard composite materials generate a much smoother surface on the counter wheel and a significantly lower friction coefficient under snowy conditions. A second test series evaluated blocks of alternative composite materials, and a candidate material with low noise and a sufficiently high sliding friction coefficient was selected for further study. A third test series examining geometrical changes in the contact surface in terms of milled parallel tracks was performed; it revealed that the braking capacity under winter conditions can be increased by milling actions if the parallel tracks are properly oriented – in this case, at an angle of 45° to the sliding direction.

Thermal capacity of tread-braked railway wheels. Part 1: Modelling

Tread brakes are still a common frictional braking system used on metro and suburban trains. Here the wheels are safety-related components and there is a need to develop design specifications and guidelines to ensure that the wheels perform properly under the service conditions to which they are exposed. In the present paper, a model is proposed and developed that represents typical conditions in metro and suburban operations, in particular during sequential stop braking. The analysis also considers drag braking, mechanical loading, residual stresses and wheel–axle interference fit. Finite element modelling, with an advanced temperature-dependent material model, together with a fatigue analysis is employed to quantify the wheel’s performance. An application example demonstrates the method for a typical metro wheel. In a companion paper, further applications are presented that demonstrate important aspects of the thermal capacity of tread-braked railway wheels.

Thermal capacity of tread-braked railway wheels. Part 2: Applications

Tread braking is a common friction-based braking system that finds use on metro and suburban trains. Here the wheels are safety-related components and there is a need to develop design specifications and guidelines to ensure that the wheels perform properly under the service conditions to which they are exposed. In the present paper, examples of applications are given that employ a modelling framework that was developed in a companion paper. The examples represent typical conditions in metro and suburban operations, in particular during sequential stop braking. Also results for drag braking, mechanical loading, residual stresses and wheel–axle interference fit are given. Parametric studies are performed to demonstrate the influence of load levels and other factors on the fatigue life of the wheels. The results should be useful for establishing design rules that consider the thermal capacity of tread-braked railway wheels.

Performance analyses of brake blocks used by Indian Railways

Friction characteristics of cast iron, two “L-type” and two “K-type” composite brake blocks, used by Indian Railways for tread braking, were obtained experimentally under dry conditions using full scale stop braking experiments. Apparent friction coefficient of brake blocks was seen to depend on brake load, sliding speed, as well as sliding distance. Brake block friction characteristics along with traction-slip and running resistance characteristics for wagons and locomotives were used to estimate frictional heat generation rates at brake block-wheel and rail-wheel interfaces as a function of brake block type, brake load, number of wagons, tonnage per axle, nature of terrain (flat, uphill, downhill, etc.), train speed and time lag in braking. Braking performance was more consistent for cast iron brake blocks than that for composite brake blocks. Further, thermal loads on locomotive wheels were also lower for cast iron brake blocks, particularly with time lag in braking.

Wear of brake blocks for in-service conditions—Influence of the level of modelling

Wear of brake blocks at tread braking of railway wheels is one of the key factors that control the time period between scheduled train maintenance and hence has a substantial impact on the life cycle cost of the vehicle. Wear simulations using two different numerical models, one thermal model and one thermoelastic model, are presented implementing wear rates for some brake block materials exploiting results from previously performed temperature-controlled pin-on-disc tests. It is found that the thermal model and the thermoelastic model give the same wear for cases when thermoelastic instability effects are small. However, for cases when the thermoelastic effects increase the local contact pressures and surface temperatures, large differences are found.

Temperature and thermoelastic instability at tread braking using cast iron friction material

Braking events in railway traffic often induce high frictional heating and thermoelastic instability (TEI) at the interfacing surfaces. In the present paper, two approaches are adopted to analyse the thermomechanical interaction in a pin-on-disc experimental study of railway braking materials. In a first part, the thermal problem is studied to find the heat partitioning between pin and disc motivated by the fact that wear mechanisms can be explained with a better understanding of the prevailing thermal conditions. The numerical model is calibrated using the experimental results. In a second part, the frictionally induced thermoelastic instabilities at the pin–disc contact are studied using a numerical method and comparing them with the phenomena observed in the experiments. The effects of temperature on material properties and on material wear are considered. It is found from the thermal analysis that the pin temperature and the heat flux to the pin increase with increasing disc temperatures up to a transition stage. This agrees with the behaviour found in the experiments. Furthermore, the thermoelastic analysis displays calculated pressure and the temperature distributions at the contact interface that are in agreement with the hot spot behaviour observed in the experiments.

Prediction of the Initial Residual Stresses in Railway Wheels Induced by Manufacturing

This article presents a prediction of the residual stresses that are introduced by the heat treatment in the manufacture of railway wheels. It is well known that this treatment induces the formation of a layer of residual circumferential (hoop) compressive stresses on the tread surface. This stress field may be reversed during service loading, especially tread braking. Tensile stresses at the wheel tread that arise from repeated braking may lead to the initiation of surface cracks. The work presented here defines a methodology for determining the as-manufactured residual stress field prior to service loading. A thermal and mechanical finite element model was used to study the residual stress evolution during the heat treatment. The heat transfer coefficient during quenching was determined by means of a dedicated experimental setup. The results show that the numerically computed residual stresses correlate strongly with the experimental residual stresses determined by ultrasound when the model material includes time-dependent effects such as creep or viscoplasticity.

Tread braking of railway wheels – temperatures generated by a metro train

Tread braking of railway wheels results in the kinetic energy of the train being dissipated into the wheel and blocks in the form of heat. This heat is further conducted into adjacent structures, notably the cold rail, and also transferred into the surroundings by convection and radiation. Heat partitioning between wheel and block is, for short time periods, controlled by local thermal interactions at the contact point and by the conductive properties of the bodies. However, for a metro train that performs longer periods of intermittent braking (or for drag braking) convective and radiation cooling properties of the components come into play. In the present study, results from brake rig tests and from in-field testing of a metro train are presented and used to calibrate a simulation model. It is found that the cooling level of the wheels of the metro train is substantially lower than for the wheels of a freight wagon. Moreover, it is found that the first axle on the metro train is exposed to higher cooling levels than the remaining axles. In a numerical example, temperatures of tread braked wheels are calculated using the new findings for a metro train, and the results obtained are compared with wheel temperatures as calculated assuming freight wagon conditions.

Modelling of temperatures during railway tread braking: Influence of contact conditions and rail cooling effect

The temperature rise of wheels and blocks due to frictional heating during railway tread braking along with the transfer of heat through the wheel–rail contact is studied in this paper. In particular, heat partitioning between block, wheel and rail for stop braking cycles is considered. The wheels are of interest because they are a limiting factor for railway tread braking systems. Two types of thermal models are employed to investigate the maximum temperatures over the wheel tread. In a circumferential (plane) model of wheel, block and rail, the heat transfer problem is studied by use of a finite element formulation of the two-dimensional time-dependent convection–diffusion equation. The hot spot phenomenon is simulated by introducing a prescribed wheel-fixed contact pressure distribution between wheel and block. In an axisymmetric (axial) model of wheel, block and rail, the lateral movements of the wheel–rail contact are studied. A general result is that the cooling effect provided by the rail is important when local temperatures on the tread are considered, but not when studying bulk temperatures created in a single stop braking event. Furthermore, it is found from the lateral movements of the wheel–rail contact that slow oscillations result in maximum temperatures over the wheel tread that are somewhat lower than for travelling on straight track (rolling at the rolling circle).

Thermal cracking of a railway wheel tread due to tread braking–critical crack sizes and in fluence of repeated thermal cycles

A numerical study of tread cracking due to thermal loading induced by tread braking is carried out. The analysis features a computationally efficient approach combining two-dimensional finite-element simulations with an analytical evaluation of resulting stress intensity factors. The analysis identifies critical sizes for when existing surface cracks are prone to propagate under thermal loading and resulting crack lengths after propagation. The results imply that fully functional brake systems are not likely to induce thermal crack propagation under normal stop braking, but that with pre-existing defects, a severe drag braking due to malfunctioning brakes may cause very deep cracking. Furthermore, the analysis concludes the cracking to be a static phenomenon related to the most severe brake cycle, i.e. later brake cycles of similar or lower severity will not cause any significant propagation of existing cracks. Additional three-dimensional finite-element simulations are used to validate the model, and they indicate two-dimensional results to be on the conservative side. This paper originated from the 2011 IHHA Conference.

Influence of wheel/rail tangential traction force on thermal cracking of railway wheels

In terms of the durability of carbon steel railway wheels, so-called tread thermal cracking, one of severe damages on the tread surface, was investigated to understand the dominating factor and the countermeasures against thereto. Based on the temperature conditions predicted by material investigation and numerical analysis, the residual stress state during tread braking and after cooling was estimated. On the basis of the analysis on the distribution morphology of the cracks, we have focused on the wheel/rail tangential traction force as a source of the sequential external force imposed on all circumferences of wheel tread. A series of full-scale experiments were carried out to clarify the influence of residual stress and tangential force on tread thermal cracking. The wheel/rail tangential force and the localized tensile residual stress caused by localized heating, rather than wheel weight, largely affect the thermal cracking.

Thermal Stress Analysis of the Heavy-haul Freight Car's Wheel Web Plate under the Wheel Tread Braking

As an effective and efficient solution to enhance the capacity of the freight car transportation,the increase of axle loads may lead the wheel enduring larger heat loads and suffering more severe damage.In order to offer valuable technical supports for the operation,especially braking working condition,it is necessary and meaningful to compare the thermal stress of the wheel web plates for different axle load freight cars and reveal how the changes of the axle load affect the distribution of the temperature and stress.Basing on the thermal elasto-plastic theory,a new idea is put forward,which firstly studys the residual stress generated by the heat treatment process,and then simulates the distribution of the temperature and thermal stress for the wheels with residual strain using the time history of the heat braking power calculated from the omnidistance working conditions of the Datong-Qinhuangdao railway as a theraml load.It has been proved the residual stress generated by the heat treatment process cannot be ignored and the web plates will bear greater thermal and residual stress along with the increase of axle loads,by means of comparing the ones of the 21 t,25 t and 30 t axle load wheel web plate.

컨테이너 철도차륜의 안전성 평가에 관한 연구

철도차량의 고속화가 가속화되면서 화물을 운송하던 컨테이너 차량이 차륜의 파손에 의하여 탈선하는 사고가 발생하여 많은 물적 피해가 발생하고 있으며, 이러한 철도차량의 사고는 많은 인명 피해와 물적 피해를 가져오는 대형 사고로 발전할 수 있다. 따라서 이에 대한 재발 방지를 위한 차륜의 파손 해석에 대한 연구가 필요한 실정이다. 철도차량의 차륜은 기계적 하중과 열하중를 동시에 받으며, 기계적 하중으로는 철도차량의 무게에 의한 수직하중과 곡선 구간을 운행할 때 차륜과 레일의 접촉부에 수평하중이 발생하며, 철도차량의 제동시 답면제동에 의한 반복적인 열하중을 받는다. 이러한 차륜에 발생하는 기계적 하중과 열하중은 차륜의 균열과 잔류응력 등을 발생시키는 것으로 알려져 있다. 따라서, 본 연구에서는 차량 주행 시의 브레이크 이력과 하중 조건을 고려한 열 구조 연성해석을 수행하여 차륜에 부하되는 최대응력을 추정하였으며, 이 값을 파괴역학 파라미터인 응력확대계수에 적용하여 차륜의 안전성을 평가하였다. Recently, high speed of container freight cars is causing fatigue damage of wheel. Sudden failure accidents cause a lot of physical and human damages. Therefore, damage analysis for wheel prevents failure accident of container freight car. Wheel receives mechanical and thermal loads at the same time while rolling stocks are run. The mechanical loads applied to wheel are classified by the horizontal load from contact of wheel and rail in curve line section and by the vertical force from rolling stocks weight. Also, braking and deceleration of rolling stocks cause repeated thermal load by wheel tread braking. Specially, braking of rolling stocks is frictional braking method that brake shoe is contacted in wheel tread by high breaking pressure. Frictional heat energy occurs on the contact surface between wheel tread and brake shoe. This braking converts kinetic energy of rolling stocks into heat energy by friction. This raises temperature rapidly and generates thermal loads in wheel and brake shoe. There mechanical and thermal loads generate crack and residual stress in wheel. Wetenkamp estimated temperature distribution of brake shoe experimentally. Donzella proposed fatigue life using thermal stress and residual stress. However, the load applied to wheel in aforementioned most researches considered thermal load and mechanical vertical load. Exact horizontal load is not considered as the load applied to wheel. Therefore, above-mentioned loading methods could not be applied to estimate actual stress applied to wheel. Therefore, this study proposed safety estimation on wheel of freight car using heat-structural coupled analysis on the basis of loading condition and stress intensity factor.

Temperatures at railway tread braking. Part 3: wheel and block temperatures and the influence of rail chill

Tread braking generates high temperatures in railway wheels and brake blocks as the kinetic energy of the running train is transformed into heat. The temperatures induced in the components are here analysed with particular focus on the cooling influence from the rolling contact between the hot wheel and a cold rail. Controlled brake rig tests are reported, where the rolling contact is studied using a so-called rail-wheel in contact with the braked wheel, along with results from field tests. The data from these experimental studies are used for calibration of a simulation tool for calculation of wheel and block temperatures. The calibrated model analyses heat partitioning between block, wheel and rail and finds the resulting temperatures at braking. The rail chill is found to have a considerable influence on the wheel temperatures for long drag braking cycles. A successful calibration of the model using data from field tests is also reported.

Temperatures at railway tread braking. Part 2: calibration and numerical examples

Temperatures in wheels and brake blocks at railway tread braking are studied under brake rig conditions. Results from rig experiments are reported for drag braking at constant brake power for brake blocks made of cast iron and of sinter and composite materials. The influence of block configuration, brake power, and brake speed is studied. A thermal model of railway tread braking, developed in a companion paper, is calibrated using the experimental data. This model analyses wheel and block temperatures and also the partitioning of heat between wheel and block in the brake tests. Numerical examples indicate the influence of brake block type on temperatures and on wheel-block heat partitioning. The influence of rail chill is studied in a complementary paper.

Temperatures at railway tread braking. Part 1: Modelling

A thermal model of railway tread braking is developed for use in routine calculations of wheel and brake block temperatures. Two-dimensional finite-element models of block(s) and wheel are coupled via a contact interface that controls the heat generation and also the heat partitioning between block and wheel through thermal contact resistances. The surface temperature variations around the wheel circumference as caused by frictional heating and intermediate cooling are accounted for in a mean sense, assuming high-speed sliding conditions. The thermal power generated at the block-wheel interface during braking is determined from train braking data. A model for heat transfer from the rolling wheel into the rail is developed where a film with thermal contact resistance is placed at the wheel-rail contact interface. The present model can be used to efficiently design tread braking systems for both freight and passenger trains. It can handle stop braking, drag braking at constant brake power, and also intermediate periods of cooling. The temperature history during a full train route can be calculated. The inclusion of heat transfer from wheel to rail means that the model is useful for comparing brake rig tests, where normally the chilling influence from the rail is not included, with in-field tests. Two companion papers with experimental results supplement the present numerical modelling. A brief numerical example demonstrates the heat partitioning and the influence of rail chill (about 30 per cent) for two braking configurations.