T Modelling & quantifying the influence of water on wheel/rail adhesion levels

Transcription

1 T Modelling & quantifying the influence of water on wheel/rail adhesion levels University of Sheffield VIRTUAL VEHICLE Research Center 1

2 Project Aims Develop a creep force model that predicts the effects of water on adhesion in the wheelrail contact Phase 1: Review & Definition Phase 2: Experiment, Modelling and Validation Review of water research and modelling; definition of model inputs and GB operating scenarios Decision point Smallscale tribological tests Model developmen t Physical adhesio n modellin g Model parameterisatio n Full-scale tribologica l tests Model validatio n 2

3 TribologicalOutline testing Adhesion modelling Full-scale testing Model development & Validation Summary & Future Outlook 3

4 Tribological testing High Pressure Torsion (HPT) testing selected Generates data prior to sliding and in full sliding Representative contact conditions Specimens cut from wheel & rail materials Parameters investigated Load, run-in, water & Iron oxide - amounts/mass percentages & type (naturally oxidised, <5 micron,<50nm) 4

5 Test Issues Oxide + water at high percentage difficult to evenly apply Applied 3 rd body layer and layer during test will vary Time delay from application to loading Mixing due to consequence of rotation Oxide percentages uneven across contact? Upon movement layer is sheared. No entrainment of new 3 rd body layer Lots of steel on steel contact dominating Test progression changes the third body layer water amounts will reduce layer thickness reduced roughness increase 5

6 Adhesion modelling Squeeze flow theory Used to investigate the viscous properties of fluids by parallel-plate plastometer Contact of rough surfaces with iron oxide + water layer Modelling of normal load sharing between solid-solid (asperity) contact and iron oxide + water layer N 2a Rough surface approximated by zigzag surface for modelling Surface area of solid-solid contact Surface area of Fe 2 O 3 + water mixture h h Mean roughness 2 R a N h a 3N Fe 2 O 3 + Water mixture 6

7 Adhesion modelling HPT test conditions Rough surfaces Thin layer of iron oxide + water Reduction of film thickness by squeezing material out of the contact due to relative motion of surfaces Thick layer of iron oxide + water Layer too thin a = 2.5 mm, τ 0 = 200 MPa, p 0 = 600 MPa, 2R a = 20 µm, µ B = 0.25, µ O = 0.35, S = 37.5 GN/m, N = 11.8 kn Iron oxide + water layer separates surfaces, if sufficient material is in contact Low shear strength of the iron oxide + water layer causes low adhesion in the model! 7

8 Full-scale test rig Tram wheel-rail test rig at University of Pardubice Dry conditions (baseline) Wet conditions Bulk and constant water Variable amounts of water Drop-wise water application 8

9 Full-scale test results 1 drop 60 µl dry characteristics wet characteristics Change of traction characteristics with water rate Lower adhesion maximum at low water rates Low water rate and high creepage T/N 0,05 9

10 Surface appearance Adhesion model Paste-like layer disappears during drying 10

11 WILAC model development WILAC model (Water-Induced Low Adhesion Creep force model) Purpose: Simulation of adhesion curves for GB scenarios Structure: Multiple linear regression models Implementation of Polach's approach Blending of contact conditions 11

12 WILAC GUI and validation Locomotive tests: Dry Locomotive tests: Wet K. Six, A. Meierhofer, G. Müller & P. Dietmaier Physical processes in wheel-rail contact and its implications on vehicle-track interaction Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility, 2015, 53,

13 Understanding Summary of low & Future adhesion Outlook mechanisms improved through experimental work and low adhesion modelling Key parameters: oxide/water mixture % and roughness of surfaces Full-scale tests showed similar friction/water amount relationship to HPT, creep curves used as input to creep force model Water-Induced Low Adhesion Creep force (WILAC) model developed creep curve output related to wheel/rail contact conditions and water level Future outlook Project initiated to integrate WILAC with LABRADOR braking model Proposal for updating WILAC to take account of other low adhesion mechanisms Proposal for damage modelling incorporating WILAC 13