FRACTURE AND FAILURE ASSESSMENT OF FATIGUE-CRACKED CIRCULAR HOLLOW SECTION X-JOINTS OU ZHIYONG (B. Eng. Hons.), NUS A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2013 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. ________________________ OU ZHIYONG 25 July 2013 ACKNOWLEDGEMENTS This thesis reports the research work carried out at the Department of Civil and Environmental Engineering, National University of Singapore. Firstly, I would like to express my gratitude to my supervisors Professor Somsak Swaddiwudhipong and Assistant Professor Qian Xudong for their continuous guidance and support throughout my PhD study, for their patience, motivation, enthusiasm, and for their great personality. The knowledge and inspirations from them would benefit my whole life. My sincere thanks also goes to Professor Peter William Marshall for his valuable inputs and insightful suggestions to the research project. I thank my colleagues and friends in the Department of Civil and Environmental Engineering: Zhang Sufen, Wu Jun, Yang Wuchao, Chen Jian, Li Ya, Nguyen Chien Thang, Yuthdanai Petchdemaneengam, Kittikun Jitpairod, and other friends, for the meaningful discussions, friendship, and encouragements. My appreciation also goes to the lab staff in the Structural Engineering Laboratory, Koh Yian Kheng, Ang Beng Oon, Li Wei, Tan Annie, for the kind assistance in the experimental work. I would like to acknowledge research scholarship provided by the National University of Singapore and the sponsorship from the Maritime and Port Authority of Singapore and American Bureau of Shipping, Singapore. Above all, my family, especially my wife, have given me unending support and love, for which my mere expressions of thanks would never suffice. i TABLE OF CONTENTS ACKNOWLEDGEMENTS . i TABLE OF CONTENTS ii SUMMARY . vii LIST OF TABLES . ix LIST OF FIGURES . xi LIST OF SYMBOLS AND ABBREVIATIONS .xx Chapter Introduction . 1.1 Background and motivations . 1.2 Objectives and scopes of research 1.3 Key contributions 1.4 Outline of the thesis . Chapter Literature Review 2.1 Introduction . 2.2 Fracture mechanics fundamentals 2.2.1 Introduction . 2.2.2 Fracture mechanics theories . 2.2.2.1 Linear-elastic fracture toughness . 2.2.2.2 T-stress .13 2.2.2.3 Validity limits of linear fracture mechanics and stress intensity factor 14 2.2.2.4 Elastic-plastic fracture toughness 16 2.2.2.4.1 Non-linear elastic theory .16 2.2.2.4.2 J-integral 17 2.2.2.4.3 Crack tip opening displacement 17 2.2.2.4.4 Relationship between K, J, and CTOD 19 2.2.3 Fracture toughness test .20 2.2.3.1 Introduction 20 2.2.3.2 Fracture toughness testing standards .20 2.2.3.3 Common fracture mechanics specimens .21 2.3 Failure assessment diagram methods 23 2.3.1 Introduction 23 2.3.2 First version of the failure assessment diagram approach 24 2.3.3 Modified failure assessment diagram approach .29 2.3.4 Failure assessment hierarchy in BS7910 .32 ii 2.3.4.1 Introduction . 32 2.3.4.2 Level — simplified assessment . 32 2.3.4.3 Level — normal assessment 33 2.3.4.3.1 Level 2B: material-specific FAD . 33 2.3.4.3.2 Level 2A: generalized FAD . 38 2.3.4.4 Level — ductile tearing assessment . 39 2.3.5 Constraint effects on fracture . 41 2.3.6 Recent major updates in structural integrity assessment . 44 2.4 Tubular joints 45 2.4.1 Introduction . 45 2.4.2 Joint classification . 45 2.4.3 Basic issues regarding tubular joints 45 2.4.4 Definition of ultimate strength . 46 2.4.5 Research on tubular joints with cracks and fracture of tubular joints 48 2.5 Metallurgy of carbon steel . 50 2.5.1 Introduction . 50 2.5.2 Steel and the Fe-C phase diagram 50 2.5.3 Heat treating practices 52 2.6 Lamellar tearing . 53 2.6.1 Introduction . 53 2.6.2 Mechanism of lamellar tearing . 54 2.6.3 Factors contributing to lamellar tearing 54 Chapter Material Properties Tests 56 3.1 Introduction . 56 3.2 Tensile coupon tests for PJP+ X-joints . 57 3.2.1 Tensile coupon tests for J1-1F 57 3.2.2 Tensile coupon tests for J1X-F . 64 3.2.2.1 Tensile coupon tests in the rolling direction . 64 3.2.2.2 Tensile coupon tests in the through- thickness direction of the chord . 68 3.2.3 Tensile coupon tests for J1-2F 75 3.3 Fracture toughness tests for PJP+ X-joints . 78 3.3.1 Fracture toughness test for J1-1F 78 3.3.2 Fracture toughness test for J1X-F . 86 3.3.2.1 Fracture toughness test in the rolling direction . 86 3.3.2.2 Fracture toughness test in the through-thickness direction 89 iii 3.4 Tensile coupon tests for XN1 93 3.5 Fracture toughness tests for XN1 96 3.6 Conclusions 100 Chapter Improved Crack Length Expressions for the DC(T) and the M(T) Specimens . 101 4.1 Introduction 101 4.2 Improved Crack Length Expressions for the DC(T) specimens . 102 4.2.1 Introduction 102 4.2.2 ASTM E399 implementation 104 4.2.3 Finite element program . 106 4.2.4 FE Results 110 4.2.4.1 Mesh convergence 110 4.2.4.2 Crack length expression for B/W = 0.5 and υ = 0.3 for compliance measured at the crack mouth 111 4.2.4.3 Effect of Poisson’s ratio on crack mouth compliance 116 4.2.4.4 Effect of specimen thickness on crack mouth compliance . 120 4.2.4.5 Crack length expression based on load-line compliance 122 4.2.5 Summary 128 4.3 Improved Crack Length Expressions for the M(T) specimens . 129 4.3.1 Introduction 129 4.3.2 ASTM E647 implementation 130 4.3.3 Finite element program . 132 4.3.4 Results 136 4.3.4.1 Uniform stress versus uniform displacement loadings . 136 4.3.4.2 The compliance unifying parameter x for the M(T) specimen . 137 4.3.4.3 Effect of notch geometry 142 4.3.4.4 Crack length expression with B/W = 0.02 and h/W = 0.05 143 4.3.4.5 Effect of specimen thickness . 146 4.3.4.6 Effect of Poisson’s ratio 148 4.3.5 Summary 151 4.4 Conclusions 152 Chapter Residual Strength Tests of Cracked X-joints under In-plane Bending 153 5.1 Introduction 153 5.2 Residual strength test of cracked PJP+ X-joints 157 5.2.1 Test Methodologies 157 5.2.2 Instrumentations . 159 iv 5.2.3 Residual strength test of J1-1F . 162 5.2.4 Residual strength test of J1X-F 172 5.2.5 Residual strength test of J1-2F . 173 5.2.6 Residual strength test of J2-1GF 178 5.3 Residual strength test of cracked high-strength thick-walled X-joint XN1 183 5.3.1 Setup for fatigue-cracking 184 5.3.2 Fatigue-cracking procedures 186 5.3.3 Setup and instrumentation for the residual strength test of cracked XN1 187 5.3.4 XN1 residual strength test procedures 188 5.3.5 Results and discussions 189 5.3.5.1 Fatigue-cracking 189 5.3.5.2 Residual strength test . 190 5.4 Discussions 196 5.5 Conclusions . 201 Chapter Lamellar Splitting in Tubular Joints 203 6.1 Introduction . 203 6.2 Experimental program . 207 6.2.1 Geometry . 207 6.2.2 Loading . 207 6.2.3 Existing fatigue cracks . 207 6.3 Material Properties in the Rolling Direction . 209 6.3.1 Tensile Test . 209 6.3.2 Fracture Toughness 210 6.4 Lamellar Splitting Failure 213 6.5 Investigation into Lamellar Splitting 218 6.5.1 Through-Thickness Tensile Test 218 6.5.2 Through-Thickness Fracture Test . 218 6.5.3 Macroetching . 219 6.5.4 Chemical Composition . 221 6.5.5 Microscopic Examination 222 6.6 Finite Element Simulation 224 6.7 Discussions 228 6.8 Conclusions and Recommendations . 231 Chapter Failure Assessment of X-Joints 233 7.1 Introduction . 233 v 7.2 Failure assessment of a reference T-joint 235 7.2.1 Introduction 235 7.2.2 Failure assessment curve modified by crack-front constraints . 235 7.2.3 The experiment of a cracked T-joint . 237 7.2.4 Finite element modelling of T-joints . 239 7.2.5 Results 241 7.2.5.1 Assessment using the original FAD 242 7.2.5.2 Assessment using constraint-modified FAD . 246 7.2.5.3 Level 3C FAC 250 7.2.6 Discussions 250 7.2.7 Summary 254 7.3 Failure assessment of PJP+ and high-strength X-joints . 255 7.3.1 Introduction 255 7.3.2 Failure assessment for J1-1F . 255 7.3.2.1 Effect of Crack-front profile . 256 7.3.2.2 Level 2A and 2B failure assessment of J1-1F 258 7.3.3 Failure assessment for J1-XF 265 7.3.4 Failure assessment of J1-2F 270 7.3.5 Failure assessment of thick-walled joint XN1 . 272 7.3.6 Discussions and conclusions . 275 7.4 Conclusions 280 Chapter Conclusions and Future Work . 282 8.1 Introduction 282 8.2 Summary of main findings and conclusions 284 8.3 Proposed future work 288 8.3.1 Residual strength of fatigue cracked concrete-filled tubular joint 288 8.3.2 Fatigue induced lamellar splitting . 289 Appendix A: List of Publications . 290 References . 292 vi Appendix A: List of Publications Appendix A: List of Publications Journal Papers Qian, X., Li, Y., Ou, Z. Ductile tearing assessment of high-strength steel X-joints under in-plane bending. Engineering Failure Analysis, 28, 2012, p.176-191. Qian, X., Swaddiwudhipong, S., Nguyen, C.T., Petchdemaneengam, Y., Marshall, P., Ou, Z. Overload effect on the fatigue crack propagation in large-scale tubular joints. Fatigue and Fracture of Engineering Materials and Structures, 2012, Article in press. Ou Z., Qian X. Swaddiwudhipong S. Residual strength and failure assessment of fatigue cracked large-scale X-joints under in-plane bending. Preparation in progress. Ou Z., Swaddiwudhipong S., Qian X. Improved crack length expressions for the DC(T) and the M(T) specimens. Preparation in progress. Qian X., Ou Z., Swaddiwudhipong S., Marshall P. (2012) Brittle failure caused by lamellar splitting in a large-scale tubular joint with fatigue cracks. Marine Structures. Submitted for publication. Qian X., Petchdemaneengam Y., Swaddiwudhipong S., Marshall P., Ou Z., Nguyen C. T. (2013) Fatigue performance of tubular X-joints with PJP+ welds: I – experimental study. Journal of Constructional Steel Research. Accepted for publication as at July 2013. Qian X., Nguyen C. T., Petchdemaneengam Y., Ou Z., Swaddiwudhipong S., Marshall P. (2012) Fatigue performance of tubular X-joints with PJP+ Welds: II – numerical investigation. Journal of Constructional Steel Research. Accepted for publication as at July 2013. 290 Appendix A: List of Publications Conference papers Qian X., Ou Z., Swaddiwudhipong S. (2010) Failure assessment of a cracked circular hollow section T-joint including the effect of crack-front constraints. Proceedings of the 13th International Symposium on Tubular Structures, Hong Kong, pp. 507 – 514. Okuda S., Ou Z. (2010) Biodegradable vacuum-formed modularized shelter. New Frontiers: Proceedings of the 15th International Conference on Computer-Aided Architectural Design Research in Asia CAADRIA, Hong Kong, pp. 565 – 657. 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Houston, TX, USA: Offshore Technol Conf, 1996. 305 [...]... 7.7: Key fracture toughness values in the two directions of J1-XF chord 266 Table 7.8: J1-XF level 2 and level 3C failure assessment results 269 Table 7.9: J1-2F level 2 failure assessment results 272 Table 7.10: XN1 failure assessment results 274 Table 7.11: Comparison of failure assessment results for fractured X- joints by level 2A, 2B, and 3C curves with J = Jmax ... residual strength test of cracked XN1 188 Figure 5.45: Fatigue crack initiation at the notch front of XN1 189 xvi Figure 5.46: illustration of fatigue crack profile in XN1 189 Figure 5.47: Load-displacement behaviour of XN1 192 Figure 5.48: (a) the amount of crack extension and the crack-front profile before and after ductile crack extension; (b) a typical post-test sectioned piece showing... of fatigue cracks might reduce the strength and ductility of the joints and even lead to fracture failure There has been limited research in the residual strength of fatigue- cracked tubular joints with the experimental data still in scarcity In the present study, five fatigue cracked large-scale circular hollow section Xjoints with different surface crack profiles at the weld toe were tested under in-plane... BS7910 [1] failure assessment procedures for the assessment of fatigue- cracked CHS X- joints under in-plane bending through both experimental investigations and numerical simulations, and develop safe failure assessment procedures for CHS X- joints under in-plane bending The BS7910 [1] procedures integrate fracture toughness properties and tensile properties obtained from small-scale specimens to the assessment. .. Comparison of the fracture resistance curve in the X and Z direction 219 xvii Figure 6.11: (a) An etched section of a typical un-damaged segment in J 1X- F; (b) an etched section containing fatigue crack and lamellar splitting; (c) micro cavities at the mid-thickness of (b); (d) a magnified view of (c) 220 Figure 6.12: SEM images of J 1X- F chord material: (a) an un-etched piece in the transverse section. .. distinct appearance of the ductile tearing surface; (d) chord fatigue crack view from the cross section of the sectioned joint 168 Figure 5.14: Comparison of ACPD measurements against the actual fatigue crack profile 168 xv Figure 5.15: Chord fatigue crack profile at the right brace-to-chord intersection of J1-1F 169 Figure 5.16: Chord fatigue crack profile at the... 7.29: SIF along the chord crack of J1-2F 271 Figure 7.30: Level 2 assessment for J1-2F 271 Figure 7.31: FE model for XN1 273 Figure 7.32: SIF along the crack-front of chord crack of XN1 274 Figure 7.33: Failure assessment for XN1 274 Figure 7.34: Potential influence of the new fracture toughness limit in E1820 279 xix LIST OF SYMBOLS AND ABBREVIATIONS Symbol Definition... tubular joints is the lack of crack driving force solutions for complex structures like tubular joints The work presented in this thesis fills up the void in experimental data and applies the FAD concept in assessing large-scale fatigue crack X- joints through both experimental study and finite element simulations 1.2 Objectives and scopes of research The purpose of this research is to investigate the standard... extension; and (c) definition of the fatigue crack propagation angle θf and the ductile crack extension angle θc 194 Figure 5.54: New cracks developed on the chord at the weld toe of XN1 during residual strength test 195 Figure 5.55: Results of existing experimental tests on residual strength of cracked tubular joints 199 Figure 5.56: Load-displacement behaviour of. .. strain gauges reading of J1-1F 165 Figure 5.11: Comparison of load feedback from load cell and load derived from linear strain gauge with the assumption of elastic bending of the brace 166 Figure 5.12: Brace fatigue crack profile on the left brace of J1-1F and final crack profile before fracture 167 Figure 5.13: (a) sectioning of J1-1F; (b) measurement of brace crack depth . FRACTURE AND FAILURE ASSESSMENT OF FATIGUE-CRACKED CIRCULAR HOLLOW SECTION X-JOINTS OU ZHIYONG (B. Eng. Hons.), NUS A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF. Failure assessment of PJP+ and high-strength X-joints 255 7.3.1 Introduction 255 7.3.2 Failure assessment for J1-1F 255 7.3.2.1 Effect of Crack-front profile 256 7.3.2.2 Level 2A and 2B failure. failure assessment of J1-1F 258 7.3.3 Failure assessment for J1-XF 265 7.3.4 Failure assessment of J1-2F 270 7.3.5 Failure assessment of thick-walled joint XN1 272 7.3.6 Discussions and conclusions