Residual Stress in Wheels: Comparison of Neutron Diffraction and Ultrasonic Methods, with Trends in RCF

The critical damage mechanism on many GB passenger train wheels is Rolling Contact Fatigue (RCF) cracking in the rim. Evidence from field observations suggests that RCF damage occurs much more quickly as the wheelsets near the end of their life. Wheel manufacturing processes induce a compressive hoop stress in the wheel rim; variations in residual stress through the life of a wheel may influence the observed RCF damage rates. This paper describes experiments to measure residual stresses in new and used wheel rims to identify whether this could be a significant factor, and compares the findings from neutron diffraction and ultrasonic birefringence methods. The scope goes beyond previous applications of neutron diffraction to railway wheels and identifies key considerations for future testing. Assuming that the as-manufactured stress distribution was similar for all three wheels tested, it is found that the stresses are redistributed within the wheel rim during its life as material is removed and plastic flow occurs. However, the hoop stress near the running surface remains compressive and may not have a large influence on the RCF damage rates

Study of the damage induced by thermomechanical load in ER7 tread braked railway wheels

Abstract This work aims to better understand the complex damage phenomena taking place at the wheel/brake block interface due to the thermomechanical load. An experimental procedure, articulated in three series of tests carried out with a bi-disc machine, was designed in order to experimentally simulate in controlled laboratory conditions the thermomechanical history of the real wheel during stop braking. The first series of tests was performed on ER7 wheel steel discs paired with cast iron shoe material discs, setting the sliding speed and the contact load in such a way to generate the heat flux needed to reproduce the typical tread temperature of a real wheel in stop braking. The second series was carried out by repeating the tests in the conditions of the first series and subsequently subjecting the tested wheel specimens to rolling/sliding contact with discs of 350HT rail steel. The third series was carried out by repeating the two phases of the second series and subsequently adding water to the contact interface of the wheel-rail specimens. Measurements of friction coefficient, surface temperature and weight changes were carried out during the tests. At the end, cross-sections of the specimens were observed with an optical microscope. The hardness along the depth was measured. It was observed that during the braking phase parts of the wheel specimen surface are coated by a discontinuous layer of cast iron that is transferred from the brake block specimens. During the braking phase and the subsequent phase of dry contact with the rail specimen, the transferred material is removed, promoting the nucleation of surface cracks; in addition, surface cracks are generated also by ratcheting due to high friction. During the subsequent wet contact phase, these cracks propagate in the wheel disc due to the pressurization of the fluid entrapped inside the cracks. The propagation of surface cracks in wet contact was assessed by a fracture mechanics approach, including the Finite Element simulation of a surface crack with entrapped fluid. The stress intensity factor range during a load pass was calculated and compared with the propagation threshold of the ER7 steel, determining this way the critical depth of surface cracks. This study is a step towards a damage tolerant approach for the designing and maintaining tread-braked wheels

Shattered Rim and Shelling of High-Speed Railway Wheels in The Very-High-Cycle Fatigue Regime Under Rolling Contact Loading

Due to the improvement of the wear property, rolling contact fatigue including shattered rim and shelling are the main failure causes of the high-speed railway wheels. In this paper, shattered rim and shelling occurred on the service wheels of the China Railway High-speed (CRH) trains were systematically investigated. The recorded data of the last ten years CRH operation indicated that all shattered rims and shelling were detected with serving \u3e106 km (corresponding to the fatigue life 107–109 cycles) which is very-high-cycle fatigue (VHCF). The crack initiationregion of shattered rim located at the depth of 10–25 mm from the tread, while that of shelling located at the depthsurfaces, i.e., similar VHCF features in uniaxial loading including the defect, fish-eye, and crack propagation region and unique VHCF features of the three dimensional crack surface feature, beach bands uniformly distributed in the crack propagation region, absence of fine granular area (FGA). The VHCF model considering the stress distribution, defect size and hardness were applied to discuss the failure mechanism of the shattered rim and shelling

EPFM Analysis of Subsurface Crack Beneath a Wheel Flat Using Dynamic Condition

AbstractDuring running due to frequent braking and suddenly jamming of brakes the Railway wheel skids over rails. This frequent skidding removes large amount of metal from the surface known as Wheel-flat defect. In this paper the wheel-flat and a subsurface crack in the beneath is studied using FEA. If the wheel-flat is not detected early the subsurface crack can originate in the beneath due to inclusion, may leads to fatal accidents. In this study wheel material is taken Elastic-plastic and J-Integral factor has been obtained. The wheel–rail vehicle is modelled as a mass–spring–damper syste

Crack Propagation and Microstructural Transformation on The Friction Surface of a High-Speed Railway Brake Disc

While brake disc wear represents a significant problem in high-speed rail systems, the progressive development of fatigue cracks during successive braking cycles also plays a great role in braking integrity. The modified microstructure consisting of a white etching layer (WEL) containing nanosized ferrite was observed on the friction surface of worn brake discs. In order to analyze how sequential thermal and mechanical stress affected crack propagation and microstructure evolution in brake discs, successive braking cycles were simulated on a full-scale braking bench test rig. Crack initiation and propagation mechanisms were proposed based on the experimental results, i.e., (i) occurrence of heat checking caused by heating and cooling transients during braking; (ii) heat checking increasing the roughness of the friction surface which in turn caused a local stress concentration and (iii) localized friction stress and thermal stress driving the heat checking to propagate and coalesce with the radial main crack. Analysis of the thermal-mechanical conditions that exist at the friction surface during braking indicates that WEL formation can be attributed to severe plastic deformation caused by the repeated friction between the disc and pads. Mechanical testing also indicated that WEL formation is not detrimental to brake disc integrity

On fatigue life prediction of Al-alloy 2024 plates in riveted joints

The purpose of this paper is to numerically investigate the fatigue life and the fatigue crack growth path of 2024 aluminum plate riveted joints. For this purpose, according to field observations, the parameters affecting fatigue life are obtained. Relevant geometric parameters such as rivet shank length, hole diameter and dimensional tolerances, as well as the location pattern of the rivets and the material of the rivet joints are studied. In this study, modeling is performed to calculate the equivalent plastic strain using the finite element method. For this purpose, a three-dimensional elastoplastic model is used for simulation. The information obtained from the finite element method in this study made it possible to place the rivets in this type of joint for use in high safety structures such as the aerospace industry. Given the importance of the problem of crack growth in 2024 aluminum plates, having the geometrical and physical parameters of the problem, the goal is to achieve the exact path of crack growth and fatigue life of riveted joints. Fatigue crack growth simulation is performed on the samples using the boundary element method. The stress intensity factor for different loading modes is determined using the boundary element method. The results showed that the geometric parameters and the rivet material have a significant effect on fatigue cracking in aluminum plates

Numerical predictions of crack growth direction in a railhead under contact, bending and thermal loads

The effect of different operational loading scenarios on predicted crack growth direction for a propagating inclined railhead crack is assessed by 2D finite element simulations. Studied load scenarios include a moving Hertzian contact load, a temperature drop, rail bending due to a passing wheelset, and combinations thereof. The direction of the unbiased crack propagation is predicted using an accumulative vector crack tip displacement criterion. The numerical model is validated for the individual load scenarios. Restraints due to crack face locking are imposed by a threshold parameter, whose influence is also assessed. For combinations of thermal and contact loads, the predicted crack path is found to diverge gradually from transverse growth, corresponding to pure thermal loading, to shallow growth, corresponding to a pure contact load. For combined bending and contact loading, there is a discrete jump in the predicted crack direction as the contact load increased while the bending load is kept constant. These results are well aligned with empirical experience

Crack growth paths in rolling contact fatigue — Numerical predictions

Rolling contact fatigue (RCF) cracks in railway wheels and rails are costly and complex to deal with. Despite the extensive research efforts that have been put into understanding the mechanisms and developing appropriate predictive models for RCF crack growth, there are still a lot of open questions. This is the case regarding the direction and growth rate of RCF crack propagation under multiaxial wheel‒rail contact loading, which also interplays with rail bending and thermal loads during the operational life of a rail.In the first paper of this thesis (Paper A), a numerical procedure is developed to evaluate the effect of different operational loading scenarios on the predicted crack paths in rails. A 2D linear elastic finite element model of a rail part with an inclined surface-breaking crack has been implemented. The rail part is subjected to wheel‒rail contact load, rail bending, and temperature drop as isolated scenarios and in combinations. The effective crack propagation direction is predicted based on an accumulative Vector Crack Tip Displacement (VCTD) criterion that accounts for crack face locking effects through a reversed shear threshold parameter. It has been shown that the crack path for combined thermal and contact loads varies gradually between the pure load cases while the combination of bending and contact loading has an abrupt change in predicted crack paths. Furthermore, the dependency of the results on the reversed shear threshold parameter is investigated.The influence of crack face friction on the crack path is investigated in the second paper (Paper B). The numerical procedure developed in Paper A is utilised, and crack face friction is modelled by a Coulomb friction model.\ua0 Qualitative predictions are obtained for varying magnitude of the coefficient of friction, as well as for varying parameters of the crack growth criterion. It is observed that the frictional crack tends to go deeper into the rail under a pure contact load and for a combination of bending and contact loads, while the friction has a moderate influence on the crack path for combined thermal and contact loads. Furthermore, assessment of the ranges of crack face deformation indicates that friction reduces the crack growth rate

Application of a semianalytical strain assessment and multiaxial fatigue analysis to compare rolling contact fatigue in twin-disk and full-scale wheel/rail contact conditions

A semianalytical model is introduced to assess rolling contact fatigue problems in railway applications. The constitutive law is based on the nonlinear kinematic and isotropic hardening model of Chaboche–Lemaitre, which allows the cyclic elastoplastic strain under the contact surface to be evaluated. The much higher computational effectiveness in comparison with finite element (FE) analyses is quantified. The Dang Van multiaxial fatigue criterion is implemented to evaluate the rolling contact fatigue in the subsurface elastic region where cracking is relatively rare but more dangerous than surface cracks. The influence of the presence of sulfides in the wheel matrix in decreasing fatigue strength is also assessed by means of Murakami\u27s approach. The model is used to compare conditions under small-scale twin-disk experiments to full-scale wheel/rail contact conditions. It is found that, for the same Hertzian pressure, the small-scale contact is more conservative in that it causes a deeper plasticized layer as compared with the elliptical full-scale contact. In the investigated cases, crack initiation is also not expected according to Dang Van criterion in neither of the studied contact conditions