Modelling and Durability Assessment of Rubber Components in Rail Vehicles

Transcription

1 Modelling and Durability Assessment of Rubber Components in Rail Vehicles Peter Hansen, Salim Mirza and John Harris Modelling of Elastomeric Materials and Products, 14 October 2010, London

2 Objectives of the project Aims: To assess the durability of an elastomeric component used in rail vehicles using analytical and experimental methods. To use a fracture mechanics approach in combination with finite element analysis techniques to predict the fatigue crack growth in the component due to in-service loading conditions. Based on the analysis and testing results to recommend design changes to significantly increase the fatigue life of the component and to validate the changes London, 14 October 2010 MERL 2010 slide 2

3 Introduction Component: Spherical Bearing (Draft Head Coupler) for connecting train coaches Expected Design Life = 10 Years Rubber Layer (NR Compound) Inner Ring (Steel) Outer Bracket (Steel casting) London, 14 October 2010 MERL 2010 slide 3

4 Introduction Failure of spherical bearing after one to two years of service (or less in some instances), due to cracking of the rubber layer High costs of change over to new bearings on whole fleet London, 14 October 2010 MERL 2010 slide 4

5 Fracture Mechanics Approach Schematic description of the fracture mechanics approach T FRACTURE ANALYSIS MATERIAL FATIGUE DATA dc/dn To structural T i Crack growth failure c crack length T i T crack PREDICTION length c N Cycles N f London, 14 October 2010 MERL 2010 slide 5

6 Pure Shear Tests Material Fatigue Data Fatigue Crack Growth Model Crack growth rate (nm/c) Tearing Energy (kj/m 2 ) London, 14 October 2010 MERL 2010 slide 6

7 FE Model and Applied Loads Stress Analysis Longitudin al = ±200kN Pitch = ± 1.9 o Outer Bracket Rubber Roll = ± 0.8 o Inner Ring London, 14 October 2010 MERL 2010 slide 7

8 Stress Analysis Predicted strain distribution at extreme loads London, 14 October 2010 MERL 2010 slide 8

9 Crack Growth Modelling Idealised crack geometries and crack growth Directions Surface cracks Internal cracks Crack1- Circumferential surface crack and direction of growth Direction of crack growth Crack3- Internal Near Interface Crack4-Interla at Interface Crack2-Inward surface crack and direction of growth crack depth crack width London, 14 October 2010 MERL 2010 slide 9

10 Crack Growth Modelling Tearing Energy versus crack size using global energy difference method T U = A 25 Crack1-Surface Circumferential Tearing Energy (kj/m 2 ) Crack2-Surface Inward Crack3-Internal Near Interface Crack4-Internal at Interface crack depth(mm) London, 14 October 2010 MERL 2010 slide 10

11 Crack Growth Modelling Predicted tearing energy values for different applied loads Tearing Energy (kj/m2) Longitudinal and Combined Longitudinal combined Pitch Roll y(kj/m 2 ) Roll Tearing Energy Pitch and R crack depth(mm) 0 London, 14 October 2010 MERL 2010 slide 11

12 Crack Growth Modelling Predicted tearing energy at different longitudinal load levels Tearing Energy (kj/ /m2) kN 77kN 114kN 155kN 207kN crack depth (mm) London, 14 October 2010 MERL 2010 slide 12

13 Fatigue Crack Growth Calculations CASE 1: Fatigue crack growth predictions under monotonic loading of 200kN C0=0.05 C0=0.1 C0=0.5 crack size(mm m) no of cycles London, 14 October 2010 MERL 2010 slide 13

14 Fatigue test on new spherical bearing Fatigue Testing of Spherical Bearing Cyclic Loads = 200kN for 10,000 cycles 138mm 20mm Cracks after sectioning Predicted Crack Size ~40 mm after 10,000 cycles London, 14 October 2010 MERL 2010 slide 14

15 Fatigue Crack Growth Calculations CASE 2: Measure service fatigue loading for a 30 mile long track Tension only Compression only Load(KN) No of Cycles London, 14 October 2010 MERL 2010 slide 15

16 Fatigue Crack Growth Calculations CASE 2: Predicted fatigue crack growth after one year loading crack size(mm m) One Year-Ascending One Year-Descending 0 30 Mile Repeated Blocks no of cycles London, 14 October 2010 MERL 2010 slide 16

17 Design Modification Strains due to shrinkage from 170 o C to 23 o C during cooling after cure Shrinkage Relief through expansion of inner 20 % Reduction in rubber thick kness % Increase in internal diameter of Inner London, 14 October 2010 MERL 2010 slide 17

18 Design Modification Modified rubber compound with better fatigue resistance Original compound Crack growth rate (nm/c) Modified compound Tearing Energy (kj/m 2 ) London, 14 October 2010 MERL 2010 slide 18

19 The design changes proposed were: Prototype Spherical Bearing Minor geometry changes Expansion of the inner metal to reduce the shrinkage Modified rubber compound with better fatigue resistance London, 14 October 2010 MERL 2010 slide 19

20 Experimental Testing of Prototype Spherical Bearing Original Bearing After 10,000 cycles New Prototype Bearing After 200,000 cycles London, 14 October 2010 MERL 2010 slide 20

21 Conclusions Fracture mechanics approach in combination with finite element analysis has been employed to predict the fatigue crack growth in the elastomeric layer of a spherical bearing used to connect train coaches. The predicted crack growth under monotonic loading conditions was found to be in reasonable agreement with the experimental results. Good correlation was found in terms of both the crack location and crack size. Predicted fatigue crack growth for service loading condition was predicted to be around 80mm after one year of loading. Based on the results of the analysis and experiments design changes were proposed. Prototype bearings made incorporating the design changes were tested in fatigue at +/- 200kN longitudinal loads. The damage accumulated after 200,000 cycles with the prototype bearing was less than that obtained after 10,000 cycles with original bearing. London, 14 October 2010 MERL 2010 slide 21

22 Thank you Materials Engineering Research Laboratory Limited Wilbury Way, Hitchin, Hertfordshire, SG4 0TW, United Kingdom T: +44 (0) F: +44 (0) London, 14 October 2010 MERL 2010 slide 22