Fatigue in wheel and rail materials

Transcription

1 Fatigue in wheel and rail materials Johan Ahlström Dept of Materials and Manufacturing Technology Chalmers Univ Tech, Sweden CHARMEC Chalmers Railway Mechanics

2 Fatigue in wheel and rail materials Outline LCF basic studies Wheels and rails Typical R&D topics Cracks Charmec Materials related projects Fatigue related examples from each project Future work Conclusions

3 Low cycle fatigue (LCF) experiments uniaxial cyclic deformation Typical geometry of LCF test bars: R Well-defined surface state in gauge section, and perfect alignment of load string are critical factors for good repeatability! 15 Strain control: R ε =-1, δε/δt = 0.01 s -1 ; Triangular wave form Total strain amplitudes of 0.4, 0.6 and 1.0 %. Stress control: R σ =-1, same frequencies as in strain controlled tests; Triangular wave form; Stress amplitude levels matching the stabilised stress levels (at N f /2) in the strain controlled experiments. Ratchetting tests: Stress controlled with small positive mean stress.

4 Cyclic vs monotonic strain hardening 650 N f values for each test: σ/2 at N f /2 [MPa] Strain control data Fit to strain data Stress control data Fit to stress data Monotonic σ-ε curve Very low spread in fatigue life, N f, between stress and strain controlled tests => the choice of control parameter cannot be critical! Modern rail production => very high quality material! ε t /2 at N f /2 [%] Ahlström, Johan; Karlsson, Birger: Fatigue behaviour of rail steel - a comparison between strain and stress controlled loading. Wear, 258 (7-8) pp

5 Elongation due to ratchetting Elongation [%] σ mean =56.5 MPa σ/2=617.5 MPa σ/2=560.5 MPa σ/2=465.5 MPa Cycle number Stress-strain cycles used to calibrate material model of cyclic plasticity Johansson, Göran; Ahlström, Johan; Ekh, Magnus: Parameter identification and modeling of large ratcheting strains in carbon steel. Computers and Structures, 84 pp

6 Results on grain refinement and fatigue behaviour Reference 100 µm ASTM 6.5 Mean intercept length, l = 33.6 µm Modified µm ASTM 8 Mean intercept length, l = 20.0 µm Ahlström, Johan; Karlsson, Birger: Modified Railway Wheel Steels: Production and Evaluation of Mechanical Properties with Emphasis on Low-Cycle Fatigue Behavior. Metallurgical and Materials Transactions A, 40A pp

7 Low cycle fatigue behaviour of alternative steels for highly stressed rail components Switches and crossings Monotonic and cyclic stress-strain behaviour True stress [MPa] CrV4 900A Mn True strain [%] σ/2 at N f /2 [MPa] CrV4 900A Mn ε t /2 at N f /2 [%] Martensitic 51CrV4 strong cyclic softening Pearlitic 900A slight cyclic softening Austenitic Mn13 cyclic hardening

8 Wheel-Rail system development in general Examples of R&D : Higher strength wheel and rail materials Contact forces and profile distributions Wear and friction control Damage monitoring; maintenance strategies Traction and braking control Initiation and propagation of cracks New rail Rail run with friction modifier (constant friction c., ca 0.35) Rail run dry, high friction c. (> 0.6) Pictures from voestalpine Schienen GmbH

9 Squats general appearance 1 4 Sections 7 Section 4

10 From Krste Cvetkovski s PhD work RCF cluster damage in a wheel Section perpendicular to running surface Crack cluster with a main crack ~4-6mm below the surface. Substantial crack branching observed ~250mm long, centered in the middle of the tread 2mm Section along rolling direction, approximately 50mm from rim face

11 Third body material From Krste Cvetkovski s PhD work 8.0µm Heavily deformed fragments in cracks

12 From Krste Cvetkovski s PhD work Composition of residuals

13 From Krste Cvetkovski s PhD work Crack growth theory Mixed mode crack propagation Rubbing of crack faces cause shearing of material into the crack Additional material is sheared out and starts to disintegrate with concurrent oxidation.

14 Ideas for future studies, pictures from Stefano Beretta Crack growth under Mode II and III loading Combined compression and torsion loading, as in rolling contact fatigue Lower threshold level in Mode III than in Mode I. Fatigue crack propagation and threshold for shallow microcracks under out-of-phase multiaxial loading S. Beretta, S. Foletti, K. Valiullin

15 New axial-torsion test frame 100 kn axial force (push-pull, +/- 150 mm) 1100 Nm torque (+/- 45 degrees rotation) Induction heating for high temperature tests Small cabinet for tests C, with control of temperature and humidity (prototype set-up illustrated below) Wedge grips with cooling channels Conditioned air Re-circulation Cabinet with test object inside

16 Charmec Chalmers Railway Mechanics Programme areas: 1. Interaction of Train and Track 2. Vibrations and Noise 3. Materials and Maintenance 4. Systems for Monitoring and Operation 5. Parallel EU Projects 6. Parallel Special Projects A B C Annual budget ca 25 MSEK Started 1995, as competence center Nutek / Vinnova Trafikverket via EU project Shift2Rail + Industrial partners + Chalmers

17 Materials related Charmec projects 1(2) MU28 Mechanical performance of wheel and rail materials Bulk mechanical behaviour of wheel and rail materials at high, low and varying temperature including the influence of strain rate. Microstructural characterisation of anisotropy in the surface of wheels and rails and correlation to its effect on mechanical properties. MU29 Damage in wheel and rail materials Characterisation of field samples (X-ray-scan + sectioning) and experimental studies of squat crack propagation and branching. Examine how temperature and humidity affect crack face friction and crack topography. Damage initiation mechanisms experiments.

18 Materials related Charmec projects 2(2) MU30 Modelling of properties and damage in wheel and rail materials Damage initiation mechanisms modelling. Material properties to models in collaboration with other Charmec researchers. MU32 Modelling of thermomechanically loaded rail and wheel steels Material models for high temperatures Material models calibrated for biaxial loadings MU34 Influence of anisotropy on deterioration of rail materials Mechanical characterisation of anisotropy in rail surface. Material models to describe the mechanical behaviour

19 Charmec project MU28, Dimitris Nikas Damage processes due to moderate heating Heating Degree of spheroidization correlated with RT hardness loss Quantified degree of spheroidization

20 Charmec project MU30, Johan Ahlström Examples of results - Mechanical properties experiments and models Yield stress [MPa] Temperature [ o C] Stress (MPa) 250C06WR7T23, Stress relaxation during static hold thermal+mech strain mech strain -380 fit to thermal+mech Time (h)

21 Charmec project MU29, Casey Jessop Determination of crack network geometry by X-ray reconstruction and sectioning

22 Charmec project MU29, Casey Jessop Determination of crack network geometry by X-ray tomography

23 Charmec project MU29, Casey Jessop Examples of results - Thermal damage first experiments Picture by Casey Jessop

24 Uniaxial strain controlled isothermal experiments, implemented into viscoplastic material model (Ali, Magnus) Low strain rate Specimens from wheel rim, Temperatures: Strain amplitudes: 0.6% and 1% Strain rate up to s -1 First two cycles with strain rate s -1 Hold Time 600 C strain amplitude 1% Stress response for 400 C, 1% strain amplitude Div. Material & Computational Mechanics 24

25 Pre-straining using axial compression and torsion Knut Andreas

26 Deformation of rail surface approx same scale

27 Future work Biaxial test machine in place and running. Important issues: Test control and data sampling (all) Sensors and test system characterization (all, Knut Andreas) Data analysis (all, Johan) Tubular specimen design and tests (Knut Andreas) Bi-axial fatigue tests and High temperature tests (Dimitris) Test cabinet, controlled temperature and humidity (Casey) Influence of WEL on RCF crack initiation Collaboration with Robin; Mechanical loading on thermal damage - Project with DTU and BaneDanmark, INTELLISWITCH Somrita Dhar, experiments at Chalmers and in Syncrotron facility (residual stresses around squats, fatigue of Mn steels)