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Wheel tread profile evolution for combined block braking and wheel–rail contact: Results from dynamometer experiments
Author’s Accepted Manuscript Wheel Tread Profile Evolution for Combined Block Braking and Wheel–Rail Contact: Results from Dynamometer Experiments Katsuyoshi Ikeuchi, Kazuyuki Handa, Roger Lundén, Tore Vernersson www.elsevier.com/locate/wear
PII: DOI: Reference:
S0043-1648(16)30145-4 http://dx.doi.org/10.1016/j.wear.2016.07.004 WEA101738
To appear in: Wear Received date: 5 October 2015 Revised date: 6 July 2016 Accepted date: 7 July 2016 Cite this article as: Katsuyoshi Ikeuchi, Kazuyuki Handa, Roger Lundén and Tore Vernersson, Wheel Tread Profile Evolution for Combined Block Braking and Wheel–Rail Contact: Results from Dynamometer Experiments, Wear, http://dx.doi.org/10.1016/j.wear.2016.07.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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
#
* [email protected]
ABSTRACT 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 0.20 mm 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.
1. BACKGROUND Wheel treads are subject to different types of damages such as wear, rolling contact fatigue (RCF), thermal cracks, plastic deformation, and flats caused by wheel sliding. [1] These types of damage result in a change in the tread profile, which aggravates wheel–rail contact forces, both vertical by the wheel out-of roundness and lateral by the impaired vehicle dynamics. It is possible that the forces are increasing or the ride comfort is compromised by changes of the tread profile. This might also accelerate deterioration of track and vehicle components and cause vibration [2]. An understanding of the tread damage mechanisms is essential to reduce the costs of wheel repair and maintenance. Several factors such as speed, axle load, wheel–rail adhesion, wheel material, and braking conditions affect the wheel tread damage [3]-[7]. This study is focused on the evolution of the wheel tread profiles subject to block braking and wheel-rail contact. The conditions of conventional block-braked trains were investigated. Full-scale dynamometer experiments [1] were conducted with tread braking to reproduce the wheel tread wear. These experiments involved using sintered brake blocks and the wheel–rail contact, which was accomplished by a rail–wheel. Stop braking was performed repeatedly. The profile and hardness of the tread were measured and evaluated, and the temperatures and crack development of the tread were monitored. The wheel wearing mechanisms were clarified. Thus, an understanding of the mechanisms directly affects the selection of the materials for wheel and brake blocks. In addition, it enables the development of a more efficient wheel maintenance procedure. This study is focused on an experimental approach for observing wheel wear. In a parallel paper [8], thermally induced wheel tread cracking at the brake test stand was studied quantitatively.
2. EXPERIMENTAL CONDITIONS The full-scale brake dynamometer employed in this study is shown in Figure 1. In the experiments, the blockbraked wheel was in rolling contact with the rail-wheel (wheel having a rail-head profile) connected to flywheels and an electric motor. This arrangement allowed simultaneous tread braking with realistic vertical and longitudinal wheel–rail contact forces.
Fig. 1. The experimental setup in the dynamometer Table 1 The testing conditions Initial speed of braking Wheel material Diameter of wheel Brake block Number of tread braking cycles Wheel load Moment of inertia Wheel–rail contact position Initial wheel temp. before braking Wheel speed in cooling operation Brake block pressing force Railwheel material Friction condition Wheel and rail profile Shape of brake block
130 km/h
160 km/h
ER7 855 mm K-blocks 40
ER7 855 mm K-blocks 40
10 tons = 98 kN 2550 kgm2 (corresponds to 20 [t] axle load) Stable All thermocouple value marked under 60 °C 50 km/h 30 kN Japanese wheel steel (0.65% carbon steel) Dry Shown in Figures 2 and 3 Shown in Figure 4
Fig. 2 A schematic of rail and wheel profile. (Rail: 60kg Japanese rail, Wheel: DSB97-1)
730 790 855
Fig. 3. A schematic of wheel.
90mm
Wheel rotating direction
Thermocouple Depth: 40mm
150mm
10mm
90mm
These values of size are approximately.
Fig. 4. A schematic of brake block. Thermo camera
Thermo couple
Thermo couple Wheel (f855)
Railtrack Wheel (f1000mm)
Brake block
Fig. 5. A schematic of the experimental settings .
Position (Depth from rim)
Flange
15mm
10mm below from tread
45mm 75mm
Fig. 6. A schematic of the thermocouple position in the wheel. Two speed-type tests were performed using a wheel made from material ER7 [9] and K-blocks [10]. Specimens were used for 40 cycles with 160 km/h and other specimens were used for the other cycles with 130 km/h. An axle load of 196 kN and initial braking speeds of 130 km/h and 160 km/h were tested. Further information on the testing conditions is presented in Table 1. During the tests, the rail-wheel was pressed toward the block-braked wheel with a constant force as shown in Figure 2. The system was accelerated to the specified initial braking speed. After the speed was attained, the motor was disconnected, and braking was imposed by pressing the brake block toward the wheel tread with a constant force. After each braking, the normal force between the braked wheel and the rail-wheel was reduced, and cooling by convection with the surrounding air for the braked wheel and the rail-wheel was performed at a low speed until the wheel temperature measured by the three thermocouples fell below 60C. Prior to the testing, the brake blocks were bedded (to obtain a large contact area between the block and the tread) by performing 30 stop braking actions at low speeds and low block–wheel contact forces. In the experiment, lateral forces were not involved, and spin creepage was almost zero. Longitudinal creepage was not measured because of the low data accuracy. The brake test stand was instrumented to measure speed, brake torque, brake (normal) force, and the vertical (i.e., radial) contact force between the rail-wheel and the braked wheel. Further, temperatures in the wheel and the brake block were measured by three thermocouples in the wheel and two thermocouples in the block, all located at 10 mm below the contact surfaces, as shown in Figures 4 and 6. The thermocouples in the brake block were used only for checking the temperature in the case of abnormally elevated temperatures. For this reason, the effect of brake blocks is not further discussed in this study. The tread temperature distributions were detected by a thermocamera. After every 5 to 10 stop braking actions, the tread profile was measured at four locations around the wheel circumference by using the MiniProf equipment (Greenwood Engineering) [11]. In addition, dye penetrant examination of the wheel tread was performed to inspect for surface cracking [7], [12]. After a completed series of 40 stop braking actions, microhardness tests were conducted by using a Rebound-Type Portable Hardness Tester (Mitutoyo HARDMATIC HH411) [13]. The obtained hardness values were converted to Vickers hardness (HV) using conversion tables. 3. RESULTS Typical results of the braking data are given in Table 2. The table shows the maximum power (which is the friction power between the brake block and the wheel), the braking energy, and the braking time. Examples of the braking power and speed vs. the elapsed time for 130 km/h and 160 km/h are shown in Figures 7 and 8, respectively. Table 2 The testing results Braking speed Maximum braking power Braking energy Braking time
130km/h
160km/h
260 kW
330 kW
6.5 MJ 50 s
10 MJ 60 s
400
200
300
150
200
100
100
50
0
Wheel speed [km/h]
Power [kW]
Power Braking power Wheelspeed speed Wheel
0 0
20
40
60
Time [s]
Fig. 7. Example of braking power and speed vs. elapsed time for braking starting at 130 km/h 400
200 Wheel speed
150
Power [kW]
300
200
100
100
50
0
Wheel speed [km/h]
Power
0 0
20
40
60
Time [s]
Temperature [℃], Speed [km/h]
Fig. 8. Example of braking power and speed vs. elapsed time for braking starting at 160 km/h 400
45mm 15mm
350
75mm Speed
300 250 200
150 100 50 0 0
20
40 Time [s]
60
80
100
Fig. 9. Example of wheel temperature from thermocouple and speed vs. elapsed time for braking at 160 km/h, as measured using thermocouples. Furthermore, Figure 9 illustrates an example of the wheel temperature from the thermocouple versus elapsed time for the 160km/h test, along with the variation of speed (same as in Figure 8). The reason for the differences detected in the wheel temperatures could be the banding of the block–wheel contact caused by frictionally induced thermoelastic instabilities. Details on this can be obtained from previous research [8]. It was observed that the wheel temperatures at 45 mm reached approximately 220°C and 250 °C during the stop braking cycles for 130 km/h and 160 km/h, respectively. Thermal cracks and material adhesion to the wheel tread were observed after 40 stop braking actions. It seems that some material of the brake block was transferred to the wheel tread. This material continues to be removed and transferred. For detailed information on the observed thermal cracking, see reference [8]. Figure 10 shows the evolution of the wheel tread profile at a position around the contact area between the braked wheel and the rail-wheel. The Y coordinate is from the flange back face and the Z coordinate is relative to the point at the horizontal coordinate of 65 mm. Tread indentation was observed at Position A, which is at the center of the rolling contact area. In contrast, tread protrusion was observed at Position B, which is just outside the rolling
contact area. The tread indentation was extremely small at both Position C (at the brake block contact area) and Position D (outside of the contact area). The observed deviation from the initial tread profile was a result of a combination of wear (i.e., removal of material) and plastic deformation that moved the material laterally on the wheel tread. The indentation/protrusion was larger at the higher speed. A more detailed analysis of the observed evolution of the tread profile is presented in Figures 11 and 12. The figures illustrate the initial tread profile indentation as a function of the axial position on the tread and accumulated braking energy for the four positions on the wheel tread indicated in Figure 10. The ratio of indentation to braking −6 energy was approximately 500 × 10 mm/MJ at Position A (center of rolling contact area) and almost zero at Position 3 C (which is for contact with the block only). The indentation volume was approximately 3000–6000 mm after braking for 40 times. Hardness testing of the freshly retrued wheel tread, where 2 mm was removed after a stop braking test, revealed a baseline hardness of 220HV for the wheel tread. Figure 13 shows that the hardness of the rolling contact area Position A was higher than that, while at Position C, where there was only contact with the brake block, the hardness was lower than the baseline value. The hardness values on the tread ranged between the values at Positions A and C. 4. DISCUSSION It was observed that the wheel tread profile evolution at severe stop braking was primarily affected by the plastic deformation of the wheel material caused by rolling contact at an elevated temperature. The wheel temperature increased because of tread braking that softened the wheel material. This resulted in increased deformation by the rolling contact. This may be explained by the material properties for the ER7 material given in reference [8], where tread temperatures and thermal cracking of the tread are investigated for the same set of experiments. The yield stress is reduced by about 50% from 20ºC to 450ºC and is further reduced at even higher temperatures. Results from the thermocamera indicate that the tread reaches temperatures in the order of 500ºC during braking, thus generating substantial softening of the material. Of course, wheel wear would have also occurred by the wheel-rail contact only even if no tread brake would have been applied. However, experience from the field indicates that this amount of wear is quite low [14]. According to typical wear laws, wear occurs because of some slip or sliding in the wheel/rail interface. However, the wear is negligible in comparison to the effect of the rolling contact pressure at high temperature as introduced by tread braking for the present experimental settings. Under these conditions, a characteristic evolution of the steel material microstructure was found, which indicated high-strain-rate deformation at intermediate temperatures [15]. This could also cause the evolution of the mechanical properties of the wheel material along with the microstructural evolution [16]. However, some wear (removal of material) also occurred in brake block contact band. Outside the rolling contact area, the plastic flow of the wheel material from the rolling contact caused a protrusion of the tread. This protrusion could be modified by the wear caused by the brake block. In particular, tread wear could only be detected after the 160 km/h braking cycles, where the protrusion near the contact was smaller than the protrusion that could result in a state of constant volume. The pressure from the brake block (1 MPa) was much lower than the rolling contact pressure (1 GPa). This could be disregarded with respect to the plastic deformation. In addition, the results indicated that the frictional wear of the wheel tread by contact with the brake block was small at severe stop braking when compared with the plastic phenomena. A clear difference between the rolling contact area and the tread area (which is only in contact with the brake block) is observed in the hardness distribution from Figure 13. This is likely due to work/strain hardening associated with plastic flow at the rolling contact. Both areas were heated by the block and were thus subject to the annealing and softening of the material. However, the rolling contact area was subject to plastic deformation and work hardening, which increased its hardness. Similar results were obtained in a study of wheels from revenue traffic [16]. The detailed mechanism of the strength evolution could be understood through the investigation of microstructural characteristics [17]. The detailed results and discussions on microstructure-property relationship on the tread surface for the similar experiments is referred to [18].
0times
10times
20times
30times
35times
40times
0times
5times
11times
15times
25times
30times
35times
40times
20times
1.0
1.0
Initial speed of braking 160km/h
Initial speed of braking 130km/h Position A
0.0
0.0
Position A Position B
-1.0 Position C Position D
z [mm]
z [mm]
Position B Position C
-1.0
Position D
-2.0
-2.0
Wheel / rail contact band
Wheel / rail contact band Brake block contact band
Brake block contact band -3.0
-3.0 70
80
90
100 y [mm]
110
120
70
130
80
90
100
110
120
130
y [mm]
surface indentation [mm]
surface indentation [mm]
Fig. 10. The measured evolution of the tread profiles
y [mm]
y [mm]
Fig. 11. The indentation relative to the initial tread profile as a function of the axial position of the tread and of accumulated braking energy for braking speeds of 130 km/h and 160 km/h A
B
C
D
B
100
200
C
D
0.2
0.1
0 0
100
200
300
400
500
Surface indentation [mm]
Surface indentation [mm]
0.2
A
0.1 0 0
300
400
500
-0.1
-0.1
-0.2
-0.2
-0.3
-0.3 Braking energy [MJ]
Braking energy [MJ]
Fig. 12. Average indentation relative to the initial tread profile as a function of the axial position of the tread and the accumulated braking energy for braking speeds of 130 km/h and 160 km/h
130km/h
● Measured value
250 200 150
100 50
160km/h
300 220HV
Hardness [HV]
Hardness [HV]
300
○ Average
250
220HV
200 150 100 50
0
0 Position A Position C Position A Position C Fig. 13. Hardness results. The dashed yellow line is baseline hardness of wheel tread.
Another aspect involves the evolution of the contact patch between the wheel and the rail-wheel. This was examined by the use of pressure-sensitive paper. The shape was found to change from an ellipse with dimensions of approximately 14 mm × 12 mm to an ellipse with dimensions of 25 mm × 11 mm after 40 stop braking actions at a speed of 160 km/h. Thus, the wheel tread deformation was found to increase the contact patch area. This, in turn, lowered the contact pressure that reduced further plastic deformation. The contact patch in the experiments did not move laterally on the wheel, which was a deviation from the operating conditions. 5. CONCLUSION In the 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 the plastic deformation and wear caused by the rolling contact and tread braking. The wheel tread evolution at severe stop braking was found to be affected primarily by plastic deformation. The results indicated that the maximum tread indentation was 0.20 mm at the rolling contact center after 40 stop braking actions. This was caused by the plastic deformation of the wheel tread induced by high contact pressure and material softening from high tread braking temperatures. The protrusion of the tread near the rolling contact area and the observed difference of hardness between the rolling contact area and the other tread area supported this result. Future investigations could include different wheel materials, brake block materials, speeds, braking loads, and contact loads. Numerical modeling and simulation could result in further insights. ACKNOWLEDGMENT This paper stands on a research collaboration carried out between Chalmers Railway Mechanics (CHARMEC) at Chalmers University of Technology in Sweden and Railway Technical Research Institute. 6. REFERENCES [1] K. Handa, Y. Kimiura, Y. Mishima, Surface cracks initiation on carbon steel railway wheels under concurrent load of continuous rolling contact and cyclic frictional heat, Wear 268 (2010) 50-58. [2] A. Johansson, J C O. Nielsen, Out-of-round railway wheels—wheel-rail contact forces and track response derived from field tests and numerical simulations, Proc. Instn Mech. Engrs, Part F: J Rail Rapid Transit, 217 (2003) 135-146.
[3] A. Bevan, P. Molyneux-Berry, B. Eickhoff, M. Burstow, Development and validation of a wheel wear and rolling contact fatigue damage model, Wear 307 (2013) 100-111. [4] A. Ekberg, E. Kabo, Fatigue of railway wheels and rails under rolling contact and thermal loading -- an overview, Wear 258 (258) 1288-1300. [5] K. Cvetkovski, J. Ahlstrom, Characterisation of plastic deformation and thermal softening of the surface layer of railway passenger wheel treads, Wear 300 (2013) 200-204. [6] T. Vernersson, S. Caprioli, E. Kabo, H. Hansson, A. Ekberg, Wheel Tread Damage: A Numerical Study of Railway Wheel Tread Plasticity under Thermomechanical Loading, Proc. Instn Mech. Engrs, Part F: J Rail Rapid Transit 224 (2010) 435-443. [7] K. Handa, F. Morimoto, Influence of wheel/rail tangential traction force on thermal cracking of railway wheels, Wear 289 (2012) 112-118. [8] A. Esmaeili, S. Caprioli, M. Ekh, A. Ekberg, R. Lundén, T. Vernersson, K. Handa, K. Ikeuchi, T. Miyauchi, Thermomechanical Cracking of Railway Wheel Tread: A Combined Experimental and Numerical Approach, Proc. of 10th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems (CM2015), 2015. [9] Railway applications–Wheelsets and bogies–Wheels–Product requirements, CEN, EN 13262:2004+A2, European Standard, Brussels, 2011, pp 48. [10] UIC code 541-4, Brakes–Brakes with composite brake blocks: General conditions for certification of composite brake blocks, International Union of Railways (UIC), 3rd edition, 2007. [11] Miniprof Wheel product information (Browsed 2016.6.3, ftp://ftp.greenwood.dk/miniprof/pdf/MiniProfWheel.pdf) [12] Product guide of Super-Check (Browsed 2016.6.3, http://www.marktec.co.jp/Portals/0/resources/e/product/pdf/eSuper-Check.pdf) [13] Catalog of HH-411 impact type hardness testing unit (Browsed 2016.6.3, http://www.mitutoyo.co.jp/eng/support/service/catalog/06/E4299_810.pdf) [14] K. Tanifuji, T. Sakuyama, The Characteristics of Wheel Wear in Shinkansen Electric Cars and Its Effect on the Running Vibration: In Case of Conical-Shaped Wheels with a Conicity of 1/40, JSME international journal Series 3, 31-2 (1988) 457-464. [15] N. Köppen, B. Karlsson, Tensile deformation behavior of near fully pearlitic steels at various temperatures and strain rates, Chalmers University of Technology, Gothenburg, Sweden 2006 (contained in N. Köppen´s licentiate thesis), pp. 17. [16] K. Handa, Y. Kimiura, Y. Mishima, Ferrite and Spheroidized Cementite Ultrafine Microstructure Formation in an Fe-0.67 Pct C Steel for Railway Wheels under Simulated Service Conditions, Metallurgical and Materials Transactions A 40 (2009) 2901-2908. [17] K. Handa, Y. Kimiura, Y. Yasumoto, T. Kamioka, Y. Mishima, Effect of deformation and annealing temperatures on ultrafine microstructure development and yield strength of pearlitic steel through continuous recrystallization, Materials Science and Engineering A 527 (2010) 1926-1932. [18] K. Handa, K. Ikeuchi, R. Lunden, T. Vernersson, Influence of braking conditions and wheel material on microstructure evolution and hardness variation of the running surface of tread-braked railway wheels, Proc. of International Wheelset Congress 2016, In process, (2016).
Highlights
> A series of full-scale brake block-wheel-rail contact experiment were conducted. > The maximum tread depression is 0.20 mm at the rolling contact center in the experiment. > This is caused by plastic deformation of the wheel tread. > It is induced by high contact pressure and high temperature from treads braking. > This result is supported by protrusion of the tread and difference of hardness.
PII: DOI: Reference:
S0043-1648(16)30145-4 http://dx.doi.org/10.1016/j.wear.2016.07.004 WEA101738
To appear in: Wear Received date: 5 October 2015 Revised date: 6 July 2016 Accepted date: 7 July 2016 Cite this article as: Katsuyoshi Ikeuchi, Kazuyuki Handa, Roger Lundén and Tore Vernersson, Wheel Tread Profile Evolution for Combined Block Braking and Wheel–Rail Contact: Results from Dynamometer Experiments, Wear, http://dx.doi.org/10.1016/j.wear.2016.07.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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
#
* [email protected]
ABSTRACT 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 0.20 mm 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.
1. BACKGROUND Wheel treads are subject to different types of damages such as wear, rolling contact fatigue (RCF), thermal cracks, plastic deformation, and flats caused by wheel sliding. [1] These types of damage result in a change in the tread profile, which aggravates wheel–rail contact forces, both vertical by the wheel out-of roundness and lateral by the impaired vehicle dynamics. It is possible that the forces are increasing or the ride comfort is compromised by changes of the tread profile. This might also accelerate deterioration of track and vehicle components and cause vibration [2]. An understanding of the tread damage mechanisms is essential to reduce the costs of wheel repair and maintenance. Several factors such as speed, axle load, wheel–rail adhesion, wheel material, and braking conditions affect the wheel tread damage [3]-[7]. This study is focused on the evolution of the wheel tread profiles subject to block braking and wheel-rail contact. The conditions of conventional block-braked trains were investigated. Full-scale dynamometer experiments [1] were conducted with tread braking to reproduce the wheel tread wear. These experiments involved using sintered brake blocks and the wheel–rail contact, which was accomplished by a rail–wheel. Stop braking was performed repeatedly. The profile and hardness of the tread were measured and evaluated, and the temperatures and crack development of the tread were monitored. The wheel wearing mechanisms were clarified. Thus, an understanding of the mechanisms directly affects the selection of the materials for wheel and brake blocks. In addition, it enables the development of a more efficient wheel maintenance procedure. This study is focused on an experimental approach for observing wheel wear. In a parallel paper [8], thermally induced wheel tread cracking at the brake test stand was studied quantitatively.
2. EXPERIMENTAL CONDITIONS The full-scale brake dynamometer employed in this study is shown in Figure 1. In the experiments, the blockbraked wheel was in rolling contact with the rail-wheel (wheel having a rail-head profile) connected to flywheels and an electric motor. This arrangement allowed simultaneous tread braking with realistic vertical and longitudinal wheel–rail contact forces.
Fig. 1. The experimental setup in the dynamometer Table 1 The testing conditions Initial speed of braking Wheel material Diameter of wheel Brake block Number of tread braking cycles Wheel load Moment of inertia Wheel–rail contact position Initial wheel temp. before braking Wheel speed in cooling operation Brake block pressing force Railwheel material Friction condition Wheel and rail profile Shape of brake block
130 km/h
160 km/h
ER7 855 mm K-blocks 40
ER7 855 mm K-blocks 40
10 tons = 98 kN 2550 kgm2 (corresponds to 20 [t] axle load) Stable All thermocouple value marked under 60 °C 50 km/h 30 kN Japanese wheel steel (0.65% carbon steel) Dry Shown in Figures 2 and 3 Shown in Figure 4
Fig. 2 A schematic of rail and wheel profile. (Rail: 60kg Japanese rail, Wheel: DSB97-1)
730 790 855
Fig. 3. A schematic of wheel.
90mm
Wheel rotating direction
Thermocouple Depth: 40mm
150mm
10mm
90mm
These values of size are approximately.
Fig. 4. A schematic of brake block. Thermo camera
Thermo couple
Thermo couple Wheel (f855)
Railtrack Wheel (f1000mm)
Brake block
Fig. 5. A schematic of the experimental settings .
Position (Depth from rim)
Flange
15mm
10mm below from tread
45mm 75mm
Fig. 6. A schematic of the thermocouple position in the wheel. Two speed-type tests were performed using a wheel made from material ER7 [9] and K-blocks [10]. Specimens were used for 40 cycles with 160 km/h and other specimens were used for the other cycles with 130 km/h. An axle load of 196 kN and initial braking speeds of 130 km/h and 160 km/h were tested. Further information on the testing conditions is presented in Table 1. During the tests, the rail-wheel was pressed toward the block-braked wheel with a constant force as shown in Figure 2. The system was accelerated to the specified initial braking speed. After the speed was attained, the motor was disconnected, and braking was imposed by pressing the brake block toward the wheel tread with a constant force. After each braking, the normal force between the braked wheel and the rail-wheel was reduced, and cooling by convection with the surrounding air for the braked wheel and the rail-wheel was performed at a low speed until the wheel temperature measured by the three thermocouples fell below 60C. Prior to the testing, the brake blocks were bedded (to obtain a large contact area between the block and the tread) by performing 30 stop braking actions at low speeds and low block–wheel contact forces. In the experiment, lateral forces were not involved, and spin creepage was almost zero. Longitudinal creepage was not measured because of the low data accuracy. The brake test stand was instrumented to measure speed, brake torque, brake (normal) force, and the vertical (i.e., radial) contact force between the rail-wheel and the braked wheel. Further, temperatures in the wheel and the brake block were measured by three thermocouples in the wheel and two thermocouples in the block, all located at 10 mm below the contact surfaces, as shown in Figures 4 and 6. The thermocouples in the brake block were used only for checking the temperature in the case of abnormally elevated temperatures. For this reason, the effect of brake blocks is not further discussed in this study. The tread temperature distributions were detected by a thermocamera. After every 5 to 10 stop braking actions, the tread profile was measured at four locations around the wheel circumference by using the MiniProf equipment (Greenwood Engineering) [11]. In addition, dye penetrant examination of the wheel tread was performed to inspect for surface cracking [7], [12]. After a completed series of 40 stop braking actions, microhardness tests were conducted by using a Rebound-Type Portable Hardness Tester (Mitutoyo HARDMATIC HH411) [13]. The obtained hardness values were converted to Vickers hardness (HV) using conversion tables. 3. RESULTS Typical results of the braking data are given in Table 2. The table shows the maximum power (which is the friction power between the brake block and the wheel), the braking energy, and the braking time. Examples of the braking power and speed vs. the elapsed time for 130 km/h and 160 km/h are shown in Figures 7 and 8, respectively. Table 2 The testing results Braking speed Maximum braking power Braking energy Braking time
130km/h
160km/h
260 kW
330 kW
6.5 MJ 50 s
10 MJ 60 s
400
200
300
150
200
100
100
50
0
Wheel speed [km/h]
Power [kW]
Power Braking power Wheelspeed speed Wheel
0 0
20
40
60
Time [s]
Fig. 7. Example of braking power and speed vs. elapsed time for braking starting at 130 km/h 400
200 Wheel speed
150
Power [kW]
300
200
100
100
50
0
Wheel speed [km/h]
Power
0 0
20
40
60
Time [s]
Temperature [℃], Speed [km/h]
Fig. 8. Example of braking power and speed vs. elapsed time for braking starting at 160 km/h 400
45mm 15mm
350
75mm Speed
300 250 200
150 100 50 0 0
20
40 Time [s]
60
80
100
Fig. 9. Example of wheel temperature from thermocouple and speed vs. elapsed time for braking at 160 km/h, as measured using thermocouples. Furthermore, Figure 9 illustrates an example of the wheel temperature from the thermocouple versus elapsed time for the 160km/h test, along with the variation of speed (same as in Figure 8). The reason for the differences detected in the wheel temperatures could be the banding of the block–wheel contact caused by frictionally induced thermoelastic instabilities. Details on this can be obtained from previous research [8]. It was observed that the wheel temperatures at 45 mm reached approximately 220°C and 250 °C during the stop braking cycles for 130 km/h and 160 km/h, respectively. Thermal cracks and material adhesion to the wheel tread were observed after 40 stop braking actions. It seems that some material of the brake block was transferred to the wheel tread. This material continues to be removed and transferred. For detailed information on the observed thermal cracking, see reference [8]. Figure 10 shows the evolution of the wheel tread profile at a position around the contact area between the braked wheel and the rail-wheel. The Y coordinate is from the flange back face and the Z coordinate is relative to the point at the horizontal coordinate of 65 mm. Tread indentation was observed at Position A, which is at the center of the rolling contact area. In contrast, tread protrusion was observed at Position B, which is just outside the rolling
contact area. The tread indentation was extremely small at both Position C (at the brake block contact area) and Position D (outside of the contact area). The observed deviation from the initial tread profile was a result of a combination of wear (i.e., removal of material) and plastic deformation that moved the material laterally on the wheel tread. The indentation/protrusion was larger at the higher speed. A more detailed analysis of the observed evolution of the tread profile is presented in Figures 11 and 12. The figures illustrate the initial tread profile indentation as a function of the axial position on the tread and accumulated braking energy for the four positions on the wheel tread indicated in Figure 10. The ratio of indentation to braking −6 energy was approximately 500 × 10 mm/MJ at Position A (center of rolling contact area) and almost zero at Position 3 C (which is for contact with the block only). The indentation volume was approximately 3000–6000 mm after braking for 40 times. Hardness testing of the freshly retrued wheel tread, where 2 mm was removed after a stop braking test, revealed a baseline hardness of 220HV for the wheel tread. Figure 13 shows that the hardness of the rolling contact area Position A was higher than that, while at Position C, where there was only contact with the brake block, the hardness was lower than the baseline value. The hardness values on the tread ranged between the values at Positions A and C. 4. DISCUSSION It was observed that the wheel tread profile evolution at severe stop braking was primarily affected by the plastic deformation of the wheel material caused by rolling contact at an elevated temperature. The wheel temperature increased because of tread braking that softened the wheel material. This resulted in increased deformation by the rolling contact. This may be explained by the material properties for the ER7 material given in reference [8], where tread temperatures and thermal cracking of the tread are investigated for the same set of experiments. The yield stress is reduced by about 50% from 20ºC to 450ºC and is further reduced at even higher temperatures. Results from the thermocamera indicate that the tread reaches temperatures in the order of 500ºC during braking, thus generating substantial softening of the material. Of course, wheel wear would have also occurred by the wheel-rail contact only even if no tread brake would have been applied. However, experience from the field indicates that this amount of wear is quite low [14]. According to typical wear laws, wear occurs because of some slip or sliding in the wheel/rail interface. However, the wear is negligible in comparison to the effect of the rolling contact pressure at high temperature as introduced by tread braking for the present experimental settings. Under these conditions, a characteristic evolution of the steel material microstructure was found, which indicated high-strain-rate deformation at intermediate temperatures [15]. This could also cause the evolution of the mechanical properties of the wheel material along with the microstructural evolution [16]. However, some wear (removal of material) also occurred in brake block contact band. Outside the rolling contact area, the plastic flow of the wheel material from the rolling contact caused a protrusion of the tread. This protrusion could be modified by the wear caused by the brake block. In particular, tread wear could only be detected after the 160 km/h braking cycles, where the protrusion near the contact was smaller than the protrusion that could result in a state of constant volume. The pressure from the brake block (1 MPa) was much lower than the rolling contact pressure (1 GPa). This could be disregarded with respect to the plastic deformation. In addition, the results indicated that the frictional wear of the wheel tread by contact with the brake block was small at severe stop braking when compared with the plastic phenomena. A clear difference between the rolling contact area and the tread area (which is only in contact with the brake block) is observed in the hardness distribution from Figure 13. This is likely due to work/strain hardening associated with plastic flow at the rolling contact. Both areas were heated by the block and were thus subject to the annealing and softening of the material. However, the rolling contact area was subject to plastic deformation and work hardening, which increased its hardness. Similar results were obtained in a study of wheels from revenue traffic [16]. The detailed mechanism of the strength evolution could be understood through the investigation of microstructural characteristics [17]. The detailed results and discussions on microstructure-property relationship on the tread surface for the similar experiments is referred to [18].
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Another aspect involves the evolution of the contact patch between the wheel and the rail-wheel. This was examined by the use of pressure-sensitive paper. The shape was found to change from an ellipse with dimensions of approximately 14 mm × 12 mm to an ellipse with dimensions of 25 mm × 11 mm after 40 stop braking actions at a speed of 160 km/h. Thus, the wheel tread deformation was found to increase the contact patch area. This, in turn, lowered the contact pressure that reduced further plastic deformation. The contact patch in the experiments did not move laterally on the wheel, which was a deviation from the operating conditions. 5. CONCLUSION In the 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 the plastic deformation and wear caused by the rolling contact and tread braking. The wheel tread evolution at severe stop braking was found to be affected primarily by plastic deformation. The results indicated that the maximum tread indentation was 0.20 mm at the rolling contact center after 40 stop braking actions. This was caused by the plastic deformation of the wheel tread induced by high contact pressure and material softening from high tread braking temperatures. The protrusion of the tread near the rolling contact area and the observed difference of hardness between the rolling contact area and the other tread area supported this result. Future investigations could include different wheel materials, brake block materials, speeds, braking loads, and contact loads. Numerical modeling and simulation could result in further insights. ACKNOWLEDGMENT This paper stands on a research collaboration carried out between Chalmers Railway Mechanics (CHARMEC) at Chalmers University of Technology in Sweden and Railway Technical Research Institute. 6. REFERENCES [1] K. Handa, Y. Kimiura, Y. Mishima, Surface cracks initiation on carbon steel railway wheels under concurrent load of continuous rolling contact and cyclic frictional heat, Wear 268 (2010) 50-58. [2] A. Johansson, J C O. Nielsen, Out-of-round railway wheels—wheel-rail contact forces and track response derived from field tests and numerical simulations, Proc. Instn Mech. Engrs, Part F: J Rail Rapid Transit, 217 (2003) 135-146.
[3] A. Bevan, P. Molyneux-Berry, B. Eickhoff, M. Burstow, Development and validation of a wheel wear and rolling contact fatigue damage model, Wear 307 (2013) 100-111. [4] A. Ekberg, E. Kabo, Fatigue of railway wheels and rails under rolling contact and thermal loading -- an overview, Wear 258 (258) 1288-1300. [5] K. Cvetkovski, J. Ahlstrom, Characterisation of plastic deformation and thermal softening of the surface layer of railway passenger wheel treads, Wear 300 (2013) 200-204. [6] T. Vernersson, S. Caprioli, E. Kabo, H. Hansson, A. Ekberg, Wheel Tread Damage: A Numerical Study of Railway Wheel Tread Plasticity under Thermomechanical Loading, Proc. Instn Mech. Engrs, Part F: J Rail Rapid Transit 224 (2010) 435-443. [7] K. Handa, F. Morimoto, Influence of wheel/rail tangential traction force on thermal cracking of railway wheels, Wear 289 (2012) 112-118. [8] A. Esmaeili, S. Caprioli, M. Ekh, A. Ekberg, R. Lundén, T. Vernersson, K. Handa, K. Ikeuchi, T. Miyauchi, Thermomechanical Cracking of Railway Wheel Tread: A Combined Experimental and Numerical Approach, Proc. of 10th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems (CM2015), 2015. [9] Railway applications–Wheelsets and bogies–Wheels–Product requirements, CEN, EN 13262:2004+A2, European Standard, Brussels, 2011, pp 48. [10] UIC code 541-4, Brakes–Brakes with composite brake blocks: General conditions for certification of composite brake blocks, International Union of Railways (UIC), 3rd edition, 2007. [11] Miniprof Wheel product information (Browsed 2016.6.3, ftp://ftp.greenwood.dk/miniprof/pdf/MiniProfWheel.pdf) [12] Product guide of Super-Check (Browsed 2016.6.3, http://www.marktec.co.jp/Portals/0/resources/e/product/pdf/eSuper-Check.pdf) [13] Catalog of HH-411 impact type hardness testing unit (Browsed 2016.6.3, http://www.mitutoyo.co.jp/eng/support/service/catalog/06/E4299_810.pdf) [14] K. Tanifuji, T. Sakuyama, The Characteristics of Wheel Wear in Shinkansen Electric Cars and Its Effect on the Running Vibration: In Case of Conical-Shaped Wheels with a Conicity of 1/40, JSME international journal Series 3, 31-2 (1988) 457-464. [15] N. Köppen, B. Karlsson, Tensile deformation behavior of near fully pearlitic steels at various temperatures and strain rates, Chalmers University of Technology, Gothenburg, Sweden 2006 (contained in N. Köppen´s licentiate thesis), pp. 17. [16] K. Handa, Y. Kimiura, Y. Mishima, Ferrite and Spheroidized Cementite Ultrafine Microstructure Formation in an Fe-0.67 Pct C Steel for Railway Wheels under Simulated Service Conditions, Metallurgical and Materials Transactions A 40 (2009) 2901-2908. [17] K. Handa, Y. Kimiura, Y. Yasumoto, T. Kamioka, Y. Mishima, Effect of deformation and annealing temperatures on ultrafine microstructure development and yield strength of pearlitic steel through continuous recrystallization, Materials Science and Engineering A 527 (2010) 1926-1932. [18] K. Handa, K. Ikeuchi, R. Lunden, T. Vernersson, Influence of braking conditions and wheel material on microstructure evolution and hardness variation of the running surface of tread-braked railway wheels, Proc. of International Wheelset Congress 2016, In process, (2016).
Highlights
> A series of full-scale brake block-wheel-rail contact experiment were conducted. > The maximum tread depression is 0.20 mm at the rolling contact center in the experiment. > This is caused by plastic deformation of the wheel tread. > It is induced by high contact pressure and high temperature from treads braking. > This result is supported by protrusion of the tread and difference of hardness.