Project A6: Three-dimensional Microstructural Modelling Of Crack Initiation In Rail Steel

1. Project A6: Three-dimensional Microstructural Modelling Of Crack Initiation In Rail Steel John Garnham, Claire Davis - Metallurgy and Materials, Birmingham Francis Franklin, David Fletcher* - Mechanical Engineering, Newcastle Objectives • To characterise the 3-dimensional nature of crack initiation, early stage crack growth and microstructural deformation. • To develop the mechanics model to include 3-dimensional microstructural information. • To provide maintenance planning guidance for infrastructure managers. * Now Mechanical Engineering, Sheffield.

2. Background Compressive, directional, work-hardening of pearlitic rail microstructures Material response to cyclic loading. Above the plastic shakedown limit there will be permanent, incremental plastic deformation. Top image: R.Care [EIS / ARUP / TTCI / Network Rail, 2003] Bottom image: After Bower & Johnson, WEAR 144 / CM1990

3. Background Shakedown diagrams for rail microstructures Wheel-rail contact loads are such that stresses are in a zone near the plastic shakedown threshold (top image). Different contact geometries (wheel transverse location, wheel & rail profiles, etc.) plus dynamic forces can push contact above the threshold, as shown by the measured data (bottom image). The right shakedown limit calculation is needed! Improved calculations for strain hardening and layered materials have been made since the theory was introduced. Images: R.Care [EIS / ARUP / TTCI / Network Rail, 2003]

4. Background (Project A2) Microstructural observations, mechanisms and data • Fatigue crack initiation in rail principally occurs along highly strained pro-eutectoid ferrite bands, located at prior austenite grain boundaries, or along highly strainedpearlite nodule boundaries. • Microstructures containing less pro-eutectoid ferrite exhibit long lives until fatigue crack initiation, however a larger degree of scatter in life is observed. • Preferential straining of pro-eutectoid ferrite appears to be dependent on size and morphology. Surface of 220 grade rail SUROS test disc. Fatigue crack along strain flattened pro-eutectoid ferrite with crack branch to flattened ductile inclusion. (Section transverse to rail axis.)

5. Modelling Microstructure simulation for input into wear and crack initiation model: Initially a simple arrangement of hexagons was used to represent pearlite grains and pro-eutectoid (PE) ferrite. Further developments to use more realistic grain size variations and extensions to 3D capability have occurred. More realistic microstructure generated by cross-section through the 3D microstructure 3D microstructure generated by the Voronoi ‘1000 cell’ method Hexagonal representation.

6. Modelling • The wear - crack initiation model has been used to investigate the effect of changing microstructural parameters on the development of damage and hence cracking. • The model predictions mirror the experimental results in that microstructures with increased pro-eutectoid ferrite show increased damage and shorter lives until crack formation. Model results showing increased strain in thicker pro-eutectoid ferrite regions

7. 3D RCF development - very small cracks Observations of flaking on rail and twin disc surfaces, coupled to sections through the flakes, indicate that cracks form relatively quickly, grow to one prior austenite grain length then there is an incubation period before further growth. Surface flaking during twin disc rolling contact fatigue testing of rail steel, at 25% RCF life.

8. 3D RCF development - very small cracks Very small crack shape is linked to the prior austenite grain shape. Modelling indicates multiple single grain cracks that are similar to the multiple flakes and sectioned cracks observed during experimental studies. One grain length crack during twin disc rolling contact fatigue testing of rail steel, at 25% RCF life. Crack path map, which starts with surface-connected facets near-normal to the x-axis, growing to near-parallel neighbouring facets while shear deformation accumulates

9. 3D RCF development - small/medium cracks Serial sectioning of a 7 mm surface length RCF crack, taken from a 260 grade trafficked rail, has indicated that the crack has an approximate semi-circular shape, as predicted by contact mechanics. Superimposed images of one crack at eight polishing stages 3D crack shape data is being used to verify, and adjust, modelling for fatigue crack growth in rails.

10. 3D RCF modelling of small / medium cracks Crack data ( thick blue lines, right) has been used to re-construct the crack for modelling The data fitted a truncated semi-circle, i.e. a semi-circular crack with its top (open) edge worn away Most models have considered semi-circular or semi-elliptical cracks. Using this new (real) crack shape will improve crack growth rate predictions The models are not FE based, so accounting for non-idealised cracks is much more than simply re-meshing Some wheel contact pressure and crack location combinations considered in modelling crack growth rate

11. 3D RCF development - medium/large cracks Alternative 3D imaging of large cracks Longer cracks have been examined in 3-dimensions by comparative serial microscopy and X-ray micro-tomography Larger cracks become complex in shape (varying angles of propagation and non-symmetric nature). Information is required on the conditions for change from semi-elliptical to complex for modelling. X-ray computed micro-tomography 3D images of a section cut around a small RCF crack in a rail; complete section to left and section with corner slice digitally removed to right. Acknowledgement – assistance of Mayorkinos Papaelias (Univ. Birmingham) and TWI.

12. Additional observations - inclusions In both worn rail and rail test discs, near-contact surface, some of the distribution of strain-flattened, ductile inclusions had facilitated crack initiation and propagation. Flattened MnS based, ductile inclusions near test surface (view transverse to rail axis). Rail Disc

13. New and proposed research project New (since 2008) rail research projects • SAFERAIL – EU project, 4.5M Euro overall budget with 3M Euro EU Contribution, (Bham) • INTERAIL – EU project, 4.8M Euro overall budget with 3.28M Euro EU contribution, (Bham) • Network rail acoustic emission testing of cracking, ≈£80k, (Bham) • Network Rail 5 year research collaboration agreement with Engineering Faculty at Sheffield • Korean Rail Research Institute (KRRI) to base researcher at Sheffield to developed links with UK rail research Proposed rail research projects • EPSRC – “Bridging the gap between academic and industrial degradation models” (MMU / Bham / Newc / Sheff) • EPSRC - “Examining the role of ductile inclusions on crack initiation”