Ali Esmaeili*, S. Caprioli, A. Ekberg, M. Ekh, R. Lundén, T. Vernersson, K. Handa, K. Ikeuchi, T. Miyauchi, J. Ahlström

Transcription

1 THERMOMECHANICAL CRACKING OF RAILWAY WHEEL TREADS: A COMBINED EXPERIMENTAL AND NUMERICAL APPROACH Ali Esmaeili*, S. Caprioli, A. Ekberg, M. Ekh, R. Lundén, T. Vernersson, K. Handa, K. Ikeuchi, T. Miyauchi, J. Ahlström Chalmers University of Technology, CHARMEC, Sweden Railway Technical Research Institute,Tokyo, Japan

2 Outline Background & Goals Brake rig experiments Strain controlled isothermal experiments Material model FEM analysis Comparison of experiment and simulation Conclusions Ongoing & Future works 2

3 Background Wheel material ER7 Experiments viscous effects in ER7 material already at 300 C Heat from braking and rolling contact load crack initiation and propagation Wheel with radial crack from rim to hub due to fatigue crack initiation and propagation 3

4 Goals Investigate thermal cracking of railway wheel treads simulations and experiments Brake rig experiments (at RTRI) temperatures, crack widths, validation of simulation models for RCF damage Study mechanical behavior of near pearlitic wheel steel (ER7) at elevated temperatures by laboratory experiments o o Cyclic hardening/softening Viscous behavior Improve the modeling and simulation tools based on observed behaviour in experiments. 4

5 Brake rig experiments Repeated stop braking, a full-scale brake dynamometer Inertia on axle of railwheel (contact traction) Acceleration and braking with prescribed wheel-railwheel contact force, followed by cooling at low speed and low contact force Set-up of RTRI brake rig employed in the test. 5

6 Brake rig experiments, cont d Test conditions: Axle load 20 tonnes (brake rig inertia and contact load) Diameter of braked wheel 855 mm Diameter of railwheel 1000 mm One brake block: width 80 mm and length 320 mm Brake force 30 kn Case S160 S130 C160 Brake block type sinter sinter composite Initial speed (km/h)

7 Brake rig experiments, cont d Thermal Banding Wheel tread temperatures during the final 15 brake cycles Sinter blocks: banded contact, high local temperatures Organic composite blocks: uniform contact, low temperatures 7

8 Brake rig experiments, cont d Measured crack width on the wheel tread. No cracks were found for case C160 (organic composite material) during 40 cycles! 8

9 Brake rig experiments, cont d Measured crack width on the wheel tread. No cracks were found for case C160 (organic composite material) during 40 cycles! Cracks on the wheel tread (S130): Inspection of crack width on the wheel trea3d after braking cycles. 9

10 Strain controlled isothermal experiments with and without hold time Low strain rate Hold Time Material: ER7 Specimens from wheel rim Temperatures: Strain amplitudes: 0.6% and 1% Strain rate up to s -1 Stress response for 400 C, 1% strain amplitude Note: Strain rate dependence and substantial relaxation during hold time viscous material behavior in material model necessary! 10

11 Material model Hardening to capture cyclic behaviour: Calibration: m i X i Xi X C X X M i i i i i X i i 1, Norton law to capture viscous behaviour: 1 t The material parameters are identified - minimizing difference between experimental results and model predictions * f c n Temperature dependent material parameters for ER7 rim material 11

12 Material model Plastic (rate-independent) material model obtained from viscoplastic model by: New calibration needed Temperature dependent material parameters for ER7 rim material 12

13 Material model, cont d Model (viscoplasticity) calibration: Comparison experiment (blue) and simulation (red). 325 C strain amplitude 1% During hold time: Stress drops from 680 MPa to 430 MPa. Reduction by 37% Cyclic behaviour Relaxation behaviour 13

14 Material model, cont d Model (viscoplasticity) calibration: Comparison experiment (blue) and simulation (red). 500 C strain amplitude 1% During hold time: Stress drops from 390 MPa to 100 MPa. Reduction by 75% Cyclic behaviour Relaxation behaviour 14

15 FEM analysis 30 sector of the wheel FE mesh features hexahedral 20-node elements Banding of block-wheel contact is simulated Geometry of the S-shaped railway wheel sector 15

16 FEM analysis, cont d Case S160 (a) One 50 mm wide band Max Temp 538 o C (b) One 80 mm wide band Max Temp 396 o C (c) Two 25 mm wide bands Max Temp 513 o C 16

17 FEM analysis, cont d Viscoplastic material model Simulations with viscoplastic material model results in unrealistic stress and strain response. At higher temperatures, almost elastic response was predicted for rolling contact passages over tread caused by strain rates in the order of 10 3 s -1. Improved viscoplastic material model required that is suitable for high strain rates! 17

18 FEM analysis, cont d Plastic material model eff [%] (3) (3) 450 (4) (4) T [ o C] 250 (2) (3) (4) 1 band 80 mm 2 bands 2x25 mm 1 band 50 mm (1) (1) (2) (2) No. of overrollings Evolution of effective strain S160, 1 band 50 mm (1) Time [s] Case S160. Temperature variation for the uppermost node at the centre of the contact 18

19 Comparison experiment and simulation Case S160: Kapoor empirical model predict fatigue crack initiation: N r = ε c ε r N r Number cycles to initiation of ratcheting induced fatigue ε c Critical value of plastic strain 10 ε r Represents the ratcheting strain increment N r Predicted by simulation 4 braking cycles N r Experimental observation 7 cycles 19

20 Conclusions Brake rig testing with repeated stop braking cycles have been performed Pronounced banding, with high local temperatures, found for sinter material blocks Less clear banding and lower temperatures for organic composite blocks Thermal crack growth detected only for sinter blocks Low cycle fatigue tests with hold time of the ER7 wheel material have been performed. Viscous effects already at 325 o C Experimental validation of FEM simulation High strain rate failure of the viscoplastic material model Plastic material model RCF life prediction rather good agreement of the test rig results and simulations 20

21 Ongoing & future work High strain rates & different strain rates improve viscoplastic material model Continue work on analysing thermal cracking of treads 21

22 Thanks for your attention!