1 th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems (CM215) Colorado Springs, Colorado, USA WHEEL TREAD PROFILE EVOLUTION FOR COMBINED BLOCK BRAKING AND WHEEL-RAIL CONTACT RESULTS FROM DYNAMOMETER EXPERIMENTS Katsuyoshi Ikeuchi *, Kazuyuki Handa *, Roger Lundén #, Tore Vernersson #$ * Railway Technical Research Institute, Tokyo, Japan # CHARMEC / Department of Applied Mechanics, Chalmers University of Technology, Gothenburg, Sweden $ also ÅF Industry, Gothenburg, Sweden * ikeuchi.katsuyoshi.95@rtri.or.jp ABSTRACT Wheel treads are subject to different types of damage such as wear, rolling contact fatigue (RCF), thermal cracks, plastic deformation and also flats caused by wheel sliding. Some of these phenomena is followed by a change in tread profile which results in frequent wheel reprofiling to keep rich comfort of the vehicle. In this study, a series of full-scale tread braking experiments, including wheel-rail rolling contact, were conducted in order to clarify the influencing factors of evolution of wheel tread profile. The experiments focused on plastic deformation and wear caused by rolling contact and tread braking. The presented results show that the maximum tread depression is.2 mm at the rolling contact center after 4 times stop braking actions. This is considered to be caused by plastic deformation of the wheel tread induced by high contact pressure and material softening due to high temperatures from tread braking. This result is supported by the observed protrusion of the tread near the rolling contact area and also by a difference of hardness between the rolling contact area and other tread area. 1. BACKGROUND Wheel treads are subject to different types of damage such as wear, rolling contact fatigue (RCF), thermal cracks, [1] plastic deformation and also flats caused by wheel sliding. These phenomena are followed by a change in tread profile which induce wheel-rail contact forces, both vertically by wheel out-of roundness and laterally by impaired vehicle dynamics. This will accelerate deterioration of track and vehicle components and cause vibration and discomfort for passengers[2]. For reducing cost of wheel repair and maintenance, an understanding of the mechanisms of tread damage is essential. However, several factors affect wheel tread damage such as speed, axle load, wheel-rail adhesion, wheel material, braking conditions, etc[3]-[7]. In the present study, the focus is on the evolution of the tread profiles of wheels subject to block braking and wheel-rail contact. Conditions of conventional block braked trains are investigated. In order to reproduce wheel tread wear, full-scale dynamometer experiments [1] are carried out with tread braking using sintered brake blocks and the wheel-rail contact accomplished by a rail-wheel. Repeated stop braking is performed. The profile and hardness of the tread are measured and evaluated. Also temperatures and crack development are observed. In the parallel paper [8] thermally induced cracking of the wheel treads at the brake test stand tests is studied experimentally and numerically. 2. EXPERIMENTAL CONDITIONS The full-scale brake dynamometer employed in this study is shown in Figure 1. In the experiments, the block braked wheel is in rolling contact with a rail-
reduced and cooling of the braked wheel and the rail-wheel is performed at low speed. Prior to testing, the brake blocks are bedded (to obtain a large contact area between block and tread) by running some 3 stop braking actions at low speed and low block-wheel contact force. Fig. 1. Experimental setup in dynamometer Table 1 Testing conditions Wheel material ER7 9 Diameter of wheel 855 mm Brake block Sintered block Wheel load 1 tons = 98 kn Moment of inertia 255 kgm 2 (corresponds to 2 tons axle load) Wheel-rail contact Stable position Initial wheel 6 o C temperature of braking Initial speed of braking 16 km/h 13km/h Wheel speed in cooling 5 km/h operation Brake block pressing force 3 kn Number of tread 2x4 times braking cycles wheel (wheel having a rail-head profile) connected to flywheels and an electric motor. This arrangement makes it possible to achieve simultaneous tread braking and realistic vertical and longitudinal wheel-rail contact forces. The tests are performed with a wheel made from material ER7[9] and K-blocks[1]. Axle load 2 tons and braking initial speeds 13 and 16 km/h are tested. Further information on the testing conditions is given in Table 1. At testing the rail-wheel is pressed towards the block braked wheel with a constant force. The system is accelerated to the specified braking initial speed, after which the motor is electrically disconnected and braking is imposed by pressing the brake block towards the wheel tread with a constant force. After braking to stop the force between the braked wheel and the rail-wheel is The brake test stand is instrumented to measure speed, brake torque, brake (normal) force and the vertical (i e radial) contact force between the railwheel and the braked wheel. Further, temperatures in wheel and brake block are measured by three thermocouples in the wheel and two thermocouples in the block, all at depth 1 mm below the contact surfaces. Tread temperatures are measured using a thermocamera. After every 5 to 1 stop braking actions, the tread profile is measured at four locations around the wheel circumference using the MiniProf equipment (Greenwood Engineering). Also penetrant examination of the tread is performed [7], [8]. After a completed series of 4 stop braking actions, microhardness tests are carried out using Rebound Type Portable Hardness Tester (Mitutoyo HARDMATIC HH411). The obtained hardness values are converted to Vickers hardness (HV) using conversion tables. 3. RESULTS Some typical results of braking data are given in Table 2 with maximum power, braking energy and braking time. Examples of histories of speed and power are shown in Figure 2 for 13 km/h and in Figure 3 for 16 km/h. Further, in Figure 4 an example of temperature histories are provided for 16km/h which indicates the scatter observed in the tests. The reason for the detected differences in the tread temperatures can be banding of the blockwheel contact, see details in the parallel paper[8], caused by frictionally induced thermoelastic instabilities. It is found that wheel temperature at a depth of 1 mm below the tread reaches about 22 o C during the stop braking cycle for 13 km/h and about 25 o C during the stop braking cycle for 16 km/h. After 4 stop braking actions thermal cracks and material transfer to the wheel tread were observed. Table 2 Testing results Braking speed 13km/h 16km/h Maximum power 26 kw 33 kw Braking energy 6.5 MJ 1 MJ Braking time 5 sec. 6 sec.
Vertical [mm] Temperature [ ] Wheel speed [km/h] Vertical [mm] Power [kw] Wheel speed [km/h] Power [kw] Wheel speed [km/h] 4 3 2 1 Power Wheel speed 2 4 6 Time [s] 16 14 12 1 Fig. 2. Example of speed and brake power histories for braking starting at 13 km/h 4 3 Power Wheel speed 8 6 4 2 2 15 at the brake block contact area, and Position D, which is at the outside of the contact area, depression of the tread is extremely small. The observed deviation from the initial tread profile is a result of a combination of wear (i e removal of material) and of plastic deformation which moves material laterally on the wheel tread. The wear can be caused both by the block and the rail-wheel contacts while the plastic deformation is caused by the contact with the rail-wheel at elevated temperature in the wheel. The tendency for the experiment at 13 km/h initial braking speed is same as at 16 km/h although the depression / protrusion is larger at the higher speed. times 1times 2times 3times 35times 4times 1. Initial speed of braking 13km/h 2 1. Position A Position B 1 2 4 6 8 Time [s] Fig. 3. Example of speed and brake power histories for braking starting at 16 km/h 4 3 2 Wheel-1 Wheel-3 Wheel-2 Speed 5 2 15 1-1. -2. -3. 7 8 9 1 11 12 13 1.. Railtrack wheel contact area Brake block contact area Horizontal [mm] Position C Position D times 5times 11times 15times 2times 25times 3times 35times 4times Position A Initial speed of braking 16km/h 1 5 Position B 2 4 6 8 1 12 Time [s] Fig. 4. Example of temperature histories for braking at 16 km/h as measured using thermocouples. Figure 5 shows the evolution of the wheel tread profile around the contact area between the braked wheel and the rail-wheel. At Position A, which is at the center of rolling contact area, depression of the tread is observed. On the other hand, at Position B, which is just outside the rolling contact area, the tread is found to protrude. At Position C, which is -1. -2. Railtrack wheel contact area Brake block contact area -3. 7 8 9 1 11 12 13 Horizontal [mm] Position C Position D Fig. 5. Measured evolution of tread profiles. Horizontal coordinate is from flange back face and vertical coordinate is relative to point at horizontal coordinate 65 mm.
Hardness [HV] Hardness [HV] Surface depression [mm] Surface depression [mm].3 A B C D.3 A B C D.2.2.1.1 1 2 3 4 5 1 2 3 4 5 -.1 -.1 -.2 Braking energy [MJ] -.2 Braking energy [MJ] Fig. 6. Depression relative to initial tread profile as function of axial position on tread and of accumulated braking energy for braking speed 13 km/h Fig. 7. Depression relative to initial tread profile as function of axial position on tread and of accumulated braking energy for braking speed 16 km/h 13km/h Measured value Average 16km/h 3 3 25 25 2 2 15 15 1 1 5 5 A C Flange Fig. 8. Hardness results. A C Flange
A more detailed analysis of the observed evolution of the tread profile is presented in Figures 6 and 7 where the depression of the initial tread profile as function of axial position on the tread and of accumulated braking energy are shown for the four positions on the wheel tread indicated in Figure 5. The ratio depression to braking energy was found to be about 5 1-6 mm / MJ in position A (center of rolling contact area) and almost zero in position C which is for contact with the block only. The hardness of the rolling contact area (Position A) is higher than the area which is only in contact with brake block (Position C), see Figure 8. The hardness at the flange means the original hardness of wheel. The value is intermediate between the values at Position A and Position C. 4. DISCUSSION It is found that the wheel tread profile evolution at severe stop braking is affected mainly by plastic deformation of wheel material caused by rolling contact at elevated temperature. The wheel temperature increases due to tread braking which softens the wheel material, resulting in increased deformation by the rolling contact. At these conditions a characteristic evolution of microstructure of the steel material takes place, which indicates occurrence of high-strain rate deformation at intermediate temperatures[11]. This will also cause evolution of mechanical properties of the wheel material along with the evolution of microstructure[12]. However, also some wear (removal of material) occurs in this area. Outside the rolling contact area, the plastic flow of wheel material from rolling contact will cause a protrusion of the tread, a protrusion that can be modified by wear caused by the brake block. In fact, wear on the tread can only be detected after the 16km/h braking cycles, where the protrusion near the contact is smaller than what can be presumed from a consideration of volume constancy. It should be noted that the pressure from the brake block ( 1 MPa) is much lower than the rolling contact pressure ( 1 GPa) and can be disregarded with respect to plastic deformation. Moreover, the results indicate that frictional wear of the wheel tread by contact with the brake block is small at severe stop braking, as compared to plastic phenomena. which is only in contact with the brake block. Both areas are heated by the block and are thus subject to annealing and softening of the material. However, the rolling contact area is subject to plastic deformation and work hardening which increases its hardness. Similar results are found from a study of wheels from revenue traffic[12]. The detailed mechanism of the strength evolution will be understood through the investigation of microstructure characteristics[13]. Another aspect is the evolution of the contact patch between the wheel and the rail-wheel. Information on this was obtained by use of pressure-sensitive paper. The shape is found to change from an ellipse having dimensions about 14 mm 12 mm into a 25 mm 11 mm ellipse after 4 stop braking actions from speed 16 km/h. Thus, it is found that wheel tread deformation increase the area of the contact patch which, in turn will lower the contact pressure which will hence reduce further plastic deformation. It should also be mentioned that in the present experiments the contact patch does not move laterally on the wheel, which is a deviation from the operation conditions. 5. CONCLUSION In the present study, a series of full-scale tread braking experiments, including wheel-rail rolling contact, were conducted in order to clarify the influencing factors of evolution of wheel tread profile. The experiments focused on plastic deformation and wear caused by rolling contact and tread braking. It is found that the wheel tread evolution at severe stop braking is affected mainly by plastic deformation. The presented results show that the maximum tread depression is.2 mm at the rolling contact center after 4 times stop braking actions. This is caused by plastic deformation of the wheel tread induced by high contact pressure and material softening due to high temperatures from tread braking. This result is supported by protrusion of the tread near the rolling contact area and also by the observed difference of hardness between the rolling contact area and other tread area. Future investigations could include different wheel materials, brake block materials, speeds, braking loads, contact loads, etc. Further insight should be gained by numerical modeling and simulation. The hardness distribution shows a clear difference between the rolling contact area and the tread area
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