Fatigue Analysis of a Bicycle Fork A Major Qualifying Project Submitted to the Faculty Of the WORCESTER POLYTECHNIC INSTITUTE In Partial Fulfillment of the Requirements for the Degree of Bachelor of Science By _____________________________ Nathaniel A. Jannetti _____________________________ Bradford L. Lynch April 29, 2010 _____________________________ Professor Diana A. Lados _____________________________ Professor James D. Van de Ven Table of Contents Table of Figures ........................................................................................................................................... iii Table of Tables ............................................................................................................................................ iv Abstract ......................................................................................................................................................... v 1 Introduction ........................................................................................................................................... 1 1.1 Problem Statement and Motivation ............................................................................................... 1 1.2 Objectives ..................................................................................................................................... 1 1.3 Approach and Methodology.......................................................................................................... 2 2 Materials and Processing Characterization: .......................................................................................... 4 2.1 Definitions of Processing Terms ................................................................................................... 4 2.2 Fabrication Process ....................................................................................................................... 5 2.3 Testing Procedure ......................................................................................................................... 7 2.4 Fork Blades ................................................................................................................................... 8 2.5 Steerer Tube ................................................................................................................................ 11 2.6 Crown Race ................................................................................................................................. 13 2.7 Welding Wire .............................................................................................................................. 15 2.8 Brazing Filler Metal .................................................................................................................... 16 2.9 Dropout and Disc Brake Caliper Mount ..................................................................................... 17 2.10 Analysis of the Weld and Heat Affected Zone ........................................................................... 18 2.10.1 The Regions of the HAZ ..................................................................................................... 19 3 Geometry Characterization ................................................................................................................. 23 3.1 Preliminary Model ...................................................................................................................... 23 3.1.1 Fork Blade Modeling Procedure ......................................................................................... 23 3.1.2 Steerer Tube Modeling Procedure ...................................................................................... 28 3.1.3 Crown Modeling Procedure ................................................................................................ 28 3.1.4 Disc Brake Mount Modeling Procedure ............................................................................. 29 3.1.5 Dropout Modeling Procedure .............................................................................................. 30 4 Fork Analysis, Testing, and Optimization .......................................................................................... 34 4.1 Fatigue Testing ............................................................................................................................ 34 4.1.1 ASTM F2273-03 Fatigue Test ............................................................................................ 34 4.2 Finite Element Analysis .............................................................................................................. 36 4.2.1 First Analysis of Fatigue Test ............................................................................................. 36 i 4.2.2 Final Analysis of Fatigue Test ............................................................................................ 41 4.3 FEA Model Validation ................................................................................................................ 47 4.3.1 Custom Fatigue Testing Solution - Design and Fabrication ............................................... 47 4.3.2 Custom Fatigue Testing Machine - Setup and Operation ................................................... 55 4.3.3 Location of Fatigue Failure ................................................................................................. 56 4.3.4 Evaluation of Fatigue Crack Initiation and Propagation ..................................................... 58 4.4 Fork Optimization ....................................................................................................................... 62 4.4.1 Optimization Goals ............................................................................................................. 62 4.4.2 Fork Geometry Optimization .............................................................................................. 62 4.4.3 Testing of Optimized Fork Geometry ................................................................................. 65 4.5 Material Solutions to Improve Fatigue Life ................................................................................ 65 5 Conclusions and Future Work ............................................................................................................. 66 6 Acknowledgements ............................................................................................................................. 68 7 References ........................................................................................................................................... 69 Appendix A: ASTM Standard Test Methods for Bicycle Forks ................................................................. 71 Appendix B: Materials Samples and Test Results ...................................................................................... 79 Appendix C: Fractographic Analysis of the Fatigue Fracture .................................................................... 90 Appendix D: Chemical Analysis of the Materials ...................................................................................... 92 Appendix E: X-Ray Diffraction Results ..................................................................................................... 95 Appendix F: Fe-C Phase Diagram .............................................................................................................. 96 Appendix G: Temperature Evaluation of HAZ ........................................................................................... 97 Appendix H: Fatigue Properties for FEA ................................................................................................... 99 Appendix I: Calculations for Flow Through Pneumatic Cylinder ............................................................ 101 ii Table of Figures Figure 1-1: Location of the Bicycle Fork, an Isolated Fork, and the Fatigue Crack Location. .................... 2 Figure 1-2: The Parts of a Fork Blade. .......................................................................................................... 3 Figure 2-1: Illustration of TIG Welding Technique. ..................................................................................... 5 Figure 2-2: Two Metals Being Connected Through Brazing. ....................................................................... 5 Figure 2-3 : Fabrication Process. .................................................................................................................. 6 Figure 2-4: Process Flow Chart. ................................................................................................................... 7 Figure 2-5: Spectrograph of the Fork Blade. ................................................................................................ 9 Figure 2-6: Optical (left) and SEM (right) Micrographs of the Fork Blade Material. .................................. 9 Figure 2-7: Numbered Regions of the Fork Blade. ..................................................................................... 10 Figure 2-8: Graph of Tensile Strength across the Fork Blade. ................................................................... 11 Figure 2-9: Spectrograph Results for the Steerer Tube. .............................................................................. 12 Figure 2-10: Optical (left) and SEM (right) Micrographs of the Steerer Tube Material. ........................... 12 Figure 2-11: Optical