Analysis of Fatigue Crack Propagation in Welded Steels

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Analysis of Fatigue Crack Propagation in Welded Steels

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Marquette University
[email protected]
Master's Theses (2009 -)

Dissertations, Theses, and Professional Projects

Analysis of Fatigue Crack Propagation in Welded Steels
Roberto Angelo DeMarte
Marquette University

Recommended Citation
DeMarte, Roberto Angelo, "Analysis of Fatigue Crack Propagation in Welded Steels" (2016). Master's Theses (2009 -). 388. http://epublications.marquette.edu/theses_open/388

ANALYSIS OF FATIGUE CRACK PROPAGATION IN WELDED STEELS
By Roberto A. DeMarte, B.S.M.E.
A Thesis submitted to the Faculty of the Graduate School, Marquette University,
In Partial Fulfillment of the Requirements for the Degree of Master of Science
Milwaukee, Wisconsin December 2016

ABSTRACT ANALYSIS OF FATIGUE CRACK PROPAGATION IN WELDED STEELS
Roberto A. DeMarte, B.S.M.E.
Marquette University, 2016
This thesis presents the study of fatigue crack propagation in a low carbon steel (ASTM A36) and two different weld metals (AWS A5.18 and AWS A5.28). Fatigue crack propagation data for each weld wire is of interest because of its use for predicting and analyzing service failures. Fatigue crack growth test specimens were developed and fabricated for the low carbon steel base metal and for each weld wire. Weld specimens were stress relieved prior to fatigue testing. Specimens were tested on a closed-loop servo hydraulic test machine at two different load ratios. Fatigue test data was collected to characterize both Region I and Region II crack propagation for each material. Test materials were characterized and fracture surfaces were analyzed. Experimental test results were compared to fatigue striation measurements taken using a scanning electron microscope (SEM).
Region II fatigue crack propagation data for ASTM A36 was found to be in agreement with existing R=0.05 and R=0.6 data for ferritic-pearlitic steels. Region II fatigue crack propagation data for weld metal was generally the same as ASTM A36 and within the limits of other weld metals. Scanning electron microscopy of the Region II fracture surfaces showed that they all exhibited similar fracture features (striations), indicating that the crack propagation mechanism was the same in all cases.
Region I fatigue crack propagation data resulted in higher βˆ†πΎπ‘‘β„Žvalues for AWS A5.18 as compared to AWS A5.28. βˆ†πΎπ‘‘β„Žvalues for ASTM A36 were in agreement with published values for mild steel. βˆ†πΎπ‘‘β„Žvalues were greater for load ratios R=0.05 as compared to R=0.6. The greater βˆ†πΎπ‘‘β„Ž values for R=0.05 are thought to be caused by crack closure. βˆ†πΎπ‘‘β„Ž values for ASTM A36 and AWS A5.18 were greater than those of AWS A5.28. The grain structure of AWS A5.28 was found to be finer than those of ASTM A36 and AWS A5.18 and is thought to be the cause of the lower βˆ†πΎπ‘‘β„Ž values.

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ACKNOWLEDGEMENTS
Roberto A. DeMarte, B.S.M.E.
I express my gratitude to the many people who lent their support and encouragement in completing the requirements for the master’s program, especially:
Dr. Raymond Fournelle, my advisor and thesis director, who provided guidance and served as a mentor throughout the course of my graduate studies.
Dr. Matthew Schaefer and Dr. James Rice for taking the time to assist me with this undertaking and serving on my thesis committee.
The many Deere & Co. employees, especially my supervisor Serena Darling, who granted the time and personal support to see this thesis to completion.
My family, Faye, Katrina, and Sarah, who always provided encouragement and sacrificed time together over the course of this academic endeavor. Their faith in me gave me focus and confidence to make this project a success.

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TABLE OF CONTENTS
ACKNOWLEDGEMENTS ............................................................................................................... i TABLE OF CONTENTS................................................................................................................... ii LIST OF FIGURES........................................................................................................................ iv LIST OF TABLES.........................................................................................................................viii I. INTRODUCTION....................................................................................................................... 1 II. LITERATURE REVIEW.............................................................................................................. 3
2.1. Review of Fatigue ......................................................................................................... 3 2.2. Fatigue Crack Growth in Steel ....................................................................................... 6 III. EXPERIMENTAL SETUP........................................................................................................ 13 3.1. Specimen Materials .................................................................................................... 13 3.2. Manufacture of ASTM E647 Standard Compact C(T) Tension Specimen for Fatigue Crack
Growth Rate Testing .................................................................................................. 15 3.3. Test Procedures.......................................................................................................... 20
3.3.1.Fatigue Crack Growth Measurements ................................................................ 20 3.3.2.Tensile Testing ................................................................................................... 25 3.3.3.Hardness Testing................................................................................................ 26 3.4. Characterization of Fracture Surfaces ......................................................................... 27 3.5. Characterization of Microstructures ........................................................................... 27 IV. RESULTS & DISCUSSION...................................................................................................... 28 4.1. Chemical Composition of Base and Weld Metals ........................................................ 28 4.2. Metallography............................................................................................................ 29

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4.3. Mechanical Properties................................................................................................ 33 4.4. Fatigue Test Results and Fractography........................................................................ 35
4.4.1.Region II Fatigue Crack Growth .......................................................................... 35 4.4.2.Region I Fatigue Crack Propagation and Fatigue Crack Threshold (βˆ†πΎπ‘‘β„Ž) ........... 45 4.4.3.Fractography...................................................................................................... 53 V. SUMMARY AND CONCLUSION ............................................................................................. 58 VI. RECOMMENDATIONS FOR FUTURE WORK ......................................................................... 62 VII. BIBLIOGRAPHY AND REFERENCES...................................................................................... 63 VIII. APPENDICES ..................................................................................................................... 65 Appendix A: Tensile Specimen Dimensions and Manufacture ...................................... 66 Appendix B: Instron Model 5500R Test Machine Set-up for Tensile Tests .................... 67 Appendix C: Tensile Load-Elongation Curves ............................................................... 71 Appendix D: Metallography ........................................................................................ 76 Appendix E: Rockwell B Hardness Measurements ....................................................... 78 Appendix F: Set-up, Start and Operation of 20,000 lbf MTS Test Machine for the Fatigue Crack Growth Tests ..................................................................................................... 82 Appendix G: Instructions for Measuring Crack Length with DinoLite Camera............... 96 Appendix H: Fatigue Crack Growth Test Results ........................................................ 107 Appendix I: Test Machine Information ...................................................................... 150 IX. THESIS SIGNATURE PAGE.................................................................................................. 151 X. THESIS APPROVAL FORM ................................................................................................... 152

iv LIST OF FIGURES

Figure 2.1.
Figure 2.2. Figure 2.3.
Figure 2.4.
Figure 3.1. Figure 3.2. Figure 3.3. Figure 3.4. Figure 3.5. Figure 3.6. Figure 3.7. Figure 3.8. Figure 3.9. Figure 4.1.
Figure 4.2.
Figure 4.3.
Figure 4.4. Figure 4.5.

Schematic diagram of a middle tension test specimen, test data, and modeling process for generating fatigue crack growth data (π‘‘π‘Ž βˆ’ βˆ†πΎ) data. (a) Specimen and
𝑑𝑁
loading. (b) Measured data. (c) Rate data. [2] ......................................................... 7 Three modes of loading that can be applied to a crack. [8]...................................... 8 Logπ‘‘π‘Ž vs. Log βˆ†πΎ plot describing the three regions associated with crack growth
𝑑𝑁
rate. [5]................................................................................................................... 8 Comparison of load ratio (𝑅) effects on fatigue crack growth rate in JIS SS41 steel. Reprinted with Permission from SAE International. [12]........................................ 11 Specifications for machining compact specimen (units: mm)................................. 15 Specimen location and numbering on plasma cutter ............................................. 16 Welded specimen geometry after welding (units: mm) ......................................... 17 Vizient GMAW robot used for making the weld metal specimens.......................... 17 Specimen as welded (end view) ............................................................................ 19 Finished compact C(T) specimen (after machining) ............................................... 19 Compact C(T) specimen dimension used to calculate stress intensity range .......... 21 Crack measurement photo showing crack and calibration ruler in mm. ................. 25 Instron Tensile Test Machine ................................................................................ 26 ASTM A36 base metal microstructure consisting of proeutectoid ferrite and pearlite. ................................................................................................................ 29 Macroscopic view of a polished and etched section of the weld zone cut from an AWS A5.18 weld fatigue specimen parallel to the surface of the specimen showing the 15 mm weld zone through which a crack propagates. HAZ = heat affected zone...................................................................................................................... 30 AWS A5.28 test specimen base metal microstructure. Microstructure is identical to base metal microstructure as shown in Figure 4.1. ................................................ 31 AWS A5.18 microstructure consisting of acicular ferrite and carbides. .................. 31 Image of etched AWS A5.28 weld metal specimen at high magnification showing it to consist of fine acicular grains of ferrite with some fine carbides........................ 32

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Figure 4.6. AWS A5.28 microstructure consisting of a fine mixture of ferrite grains and carbides as well as a small mixture of acicular ferrite. ......................................................... 33
Figure 4.7. ASTM A36 fatigue crack propagation data for R=0.05............................................ 38 Figure 4.8. ASTM A36 fatigue crack propagation data for R=0.6.............................................. 39 Figure 4.9. AWS A5.18 fatigue crack propagation results for R=0.05. ...................................... 40 Figure 4.10. AWS A5.18 fatigue crack propagation results for R=0.6. ........................................ 41 Figure 4.11. AWS A5.18 fatigue crack propagation results for R=0.05. ...................................... 42 Figure 4.12. AWS A5.18 fatigue crack propagation results for R=0.6. ........................................ 43 Figure 4.13. Fracture surface of AWS A5.28 material test Specimen #55-66. Measurement units
are mm. ................................................................................................................ 45 Figure 4.14. βˆ†πΎπ‘‘β„Ž data for ASTM A36 at stress ratio R=0.05 with a test frequency of 25Hz. ...... 47 Figure 4.15. βˆ†πΎπ‘‘β„Ž data for ASTM A36 at stress ratio R=0.6 with a test frequency of 60Hz. ........ 48 Figure 4.16. βˆ†πΎπ‘‘β„Ž data for AWS A5.18 at stress ratio R=0.6 with a test frequency of 60Hz. ....... 49 Figure 4.17: βˆ†πΎπ‘‘β„Ž data for AWS A5.18 at stress ratio R=0.6 with a test frequency of 60Hz. ....... 50 Figure 4.18. βˆ†πΎπ‘‘β„Ž data for AWS A5.28 at stress ratio R=0.05 with a test frequency of 60Hz. ..... 51 Figure 4.19. βˆ†πΎπ‘‘β„Ž data for AWS A5.28 at stress ratio R=0.6 with a test frequency of 60Hz. ....... 52 Figure 4.20. High magnification image of fracture surface for Specimen #3 – ASTM A36. Image
taken at π‘Ž=23.6 mm and showing well defined fatigue striations and secondary cracks. Average striation spacing is 1.0 Β΅m............................................................ 55 Figure 4.21. High magnification image of fracture surface at for Specimen #13-0 - AWS A5.18 taken at π‘Ž=22.6 mm and showing well defined fatigue striations. Average striation spacing is 0.2 Β΅m................................................................................................... 55 Figure 4.22. High magnification image of fracture surface for Specimen #67-76 - AWS A5.28 taken at π‘Ž=22.5 mm and showing well defined fatigue striations. Average striation spacing is 0.18 Β΅m................................................................................................. 56 Figure 5.1. Summary of all fatigue crack propagation results for R=0.05. ................................ 60 Figure 5.2. Summary of all fatigue crack propagation results for R=0.6. βˆ†πΎπ‘‘β„Ž= 3.80 for both ASTM A36 and AWS A5.18. ................................................................................... 61 Figure A.1. Manufacturing specifications for tensile test specimen ......................................... 66 Figure B.1. Instron machine system controls .......................................................................... 67 Figure B.2. 10,000 lbf load cell identification........................................................................... 68 Figure B.3. Grip and gear shift lever identification .................................................................. 69

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Figure C.1. Figure C.2. Figure C.3. Figure C.4. Figure D.5. Figure D.6. Figure D.7. Figure D.8. Figure E.1. Figure E.2. Figure H.1. Figure H.2. Figure H.3. Figure H.4. Figure H.5. Figure H.6. Figure H.7. Figure H.8.

Tensile test data from as fabricated tensile test specimens – ASTM A36 ............... 72 Tensile test data of stress relieved specimens – ASTM A36 ................................... 73 Tensile test data comparison for ASTM A36 base material .................................... 74 Tensile test data for weld metal AWS A5.18 and AWS A5.28 ................................. 75 AWS A5.28 metallographic specimen. Specimen was mounted in orientation for which the crack would grow perpendicular into the specimen. ............................. 76 AWS A5.28 metallographic specimen. Specimen was mounted in orientation for which the crack would grow in the direction of the arrow. .................................... 76 AWS A5.18 metallographic specimen. Specimen was mounted in orientation for which the crack would grow perpendicular into the specimen. ............................. 77 AWS A5.18 metallographic specimen. Specimen was mounted in orientation where the crack would grow in the direction of the arrow. .............................................. 77 Hardness gradient measurement profile on chemically etched test specimen Specimen #37-31 AWS A5.18. ............................................................................... 78 Hardness gradient measurement profile on chemically etched test specimen Specimen #52-90 AWS A5.28. ............................................................................... 80 Fatigue crack growth data for ASTM A36 at stress ratio R=0.05 with a test frequency of 25Hz and 10Hz. ............................................................................................... 137 Crack growth rate data and Paris Equation for ASTM A36 at stress ratio R=0.05 with a test frequency of 10 and 25Hz.......................................................................... 138 Fatigue crack growth data for ASTM A36 at stress ratio R=0.6 with a test frequency of 60Hz. .............................................................................................................. 139 Fatigue crack growth data and Paris Equation for ASTM A36 at stress ratio R=0.6 with a test frequency of 60Hz. ............................................................................ 140 Fatigue crack growth data for AWS A5.18 at stress ratio R=0.05 with a test frequency of 60Hz............................................................................................... 141 Crack growth rate data and Paris Equation for AWS A5.18 at stress ratio R=0.6 with a test frequency of 60Hz. .................................................................................... 142 Paris Equation for Specimen 13-0 AWS A5.18 at stress ratio R=0.6 with a test frequency of 60Hz............................................................................................... 143 Fatigue crack growth data for AWS A5.18 at stress ratio R=0.6 with a test frequency of 60Hz. .............................................................................................................. 144

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Figure H.9. Crack growth rate data and Paris Equation for AWS A5.18 at stress ratio R=0.6 with a test frequency of 60Hz. .................................................................................... 145
Figure H.10. Fatigue crack growth data for AWS A5.28 at stress ratio R=0.05 with a test frequency of 60Hz............................................................................................... 146
Figure H.11. Crack growth rate data and Paris Equation for AWS A5.28 at stress ratio R=0.05 with a test frequency of 60Hz. ............................................................................ 147
Figure H.12. Fatigue crack growth data for AWS A5.28 at stress ratio R=0.6 with a test frequency of 60Hz. .............................................................................................................. 148
Figure H.13. Crack growth rate data and Paris Equation for AWS A5.28 at stress ratio R=0.6 with a test frequency of 60Hz. .................................................................................... 149
AwsAstmSpecimenCrackFatigue Crack Propagation