The Causes of “Shear Fracture” of Dual-Phase Steels Dissertation Presented in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in the Graduate School of The Ohio State University By Ji-Hyun Sung, M.S. Graduate Program in Mechanical Engineering The Ohio State University 2010 Dissertation Committee: Robert H. Wagoner, Advisor June Key Lee Sudarsanam Suresh Babu Rebecca B. Dupaix ⓒ Copyright by Ji-Hyun Sung 2010 ABSTRACT Dual Phase (DP) steels are a class of advanced high strength steel (AHSS) in increasing use for sheet formed automotive parts. In spite of attractive combinations of high strength, high ductility, and low cost, the widespread adoption of DP steels has been limited because practical die tryouts exhibit forming failures far earlier than predicted by standard industrial methods. These failures, often referred to as “shear fractures,” occur in regions of high curvature and with little apparent necking, in contrast to “normal” or tensile fractures. Conventional wisdom attributes shear fractures to a postulated damage mechanism related to the special microstructure of DP steels. In order to reproduce, characterize, and analyze such fractures in a laboratory setting and to understand their origin of the inability to predict them, a novel draw-bend formability (DBF) test was devised based on displacement control. DP steels from several suppliers with tensile strengths ranging from 590 to 980 MPa were tested over a range of rates and bend ratios (R/t, inner bend radius / sheet thickness). The new DBF test reliably reproduced three kinds of fractures identified as Type I, II, and III, corresponding to tensile fracture, transitional fracture, and shear fracture, respectively. These tests revealed a surprising result: the occurrence of shear fractures increased at higher deformation rates. This degradation of formability was shown to be principally a result of deformation-induced heating, which is greatly accentuated for AHSS because of their ii high plastic energy absorption and commensurate high temperature increases, up to 100 degrees C. In order to understand and quantify the role of deformation-induced heating on plastic localization, temperatures were measured and simulated using a novel new empirical plasticity constitutive form describing the flow stress as a function of strain, strain-rate, and temperature. Designated the “H/V model”, the new constitutive model consists of three multiplicative functions describing (a) strain hardening and its temperature sensitivity, (b) strain-rate sensitivity, and (c) temperature sensitivity. This form allows a natural transition from unbounded strain hardening at low temperatures toward saturation behavior at higher temperatures, consistent with many observations. Thermo-mechanical finite-element simulations using the H/V model confirmed its accuracy and the magnitude of the role on shear fracture. Failure types were predicted, as well as quantitative. For most of the DP steels tested, heating induced by deformation was identified as the dominant effect in producing unpredicted fractures. This is a result of standard industrial techniques that do not take non-isothermal effect into account, in particular constructing forming limit diagrams from low-speed / isothermal testing, and use of isothermal finite element modeling to analyze industrial sheet forming operations. Microstructural damage can also contribute to shear fracture, but it was a secondary factor for all but one of the alloys tested, in one test direction. iii DEDICATION To my beloved wife, Sang Hee, and my lovely daughter, Aria (Yubin). iv ACKNOWLEDGMENTS I would like to express my sincere appreciation to my academic advisor, Dr. Robert H. Wagoner, for his ongoing mentoring, continuous support and encouragement. His teaching and advice assisted and guided me not only throughout the completion of my dissertation but also during the entirety of my studies at The Ohio State University. My sincere appreciation is extended to my dissertation committee members: Dr. J.K. Lee, Dr. S. Babu, and Dr. R. Dupaix for their devotion of time and their valuable advice. Sincere thanks are further extended to all the students and post doc. in the Wagoner Research Group and all members of Auto/Steel Partnership (A/SP) team, for their participations and assistances in various aspects of this research work. Special thanks go to Dr. Ji Hoon Kim, Mr. Hojun Lim, Mr. A. Madeshia, Dr. J. Fekete in GM, and Dr. Hyunok Kim in EWI. I would also like to extend my greatest thanks to my father and mother for bringing me into this world and for protecting me and providing me with continuous motivation and enormous emotional support. Without their support, the completion of this dissertation would not have been possible. Lastly and most importantly, I wish to express greatest thanks to my wife who has continuously supported me with her patience and commitment, giving me wholehearted encouragement in every step that I have taken. Her enduring strength and love have been the source of my energy that allows me to move forward in life. Words can hardly describe my gratitude and thank to her for being always there with me. I would like to acknowledge my wife, Sang Hee Park, for her endless love in sharing my life. v VITA April 23, 1975 .......................................Born - Daegu, South Korea 1999.......................................................B.S. Mechanical Engineering, Kyungpook National University, Daegu, Korea 2001.......................................................M.S. Mechanical Engineering, Kyungpook National University, Daegu, Korea 2001 - 2004 ...........................................Chief Engineering Officer, PKM-328, The 2nd Fleet of Republic of Korea Navy, PyungTaek, Korea 2004 - 2005 ...........................................Researcher, Body Engineering Team, R&D Center, Hyundai Motors, Hwasung, Korea 2006 - 2007 ...........................................Graduate Research Associate, Engineering Research Center for Net Shape Manufacturing (ERC/NSM), The Ohio State University, Columbus, Ohio 2007 - 2010 ...........................................Graduate Research Associate, Wagoner Research Group The Ohio State University, Columbus, Ohio vi PUBLICATIONS 1. Kim, H., Ji Hyun Sung, Sivakumar, R., and Altan, T., “Evaluation of Stamping Lubricants using the Deep Drawing Test”, International Journal of Machine Tools and Manufacturer, Vol. 47/14, pp. 2120-2132, 2007. 2. Kim, H., Ji Hyun Sung, and Altan, T., “Investigation of Galling in Forming Galvanized AHSS by Using the Twist Compression Test (TCT)”, accepted for the publication to the Journal of Materials Processing Technology, November 2007. 3. H.Lim, M. G. Lee, Ji Hyun Sung and R. H. Wagoner, “Time-dependent Springback” , International Journal of Material Forming, Apr. 3, 2008. 4. R. Padmanabhan, Ji Hyun Sung, H. Lim, M. C. Oliveira, L. F. Menezes and R. H. Wagoner, “Influence of Draw Restraining Force on the Springback in Advanced High Strength Steels”, International Journal of Material Forming, Vol. 10, pp. 1-4, 2008. 5. Wagoner, R.H, Kim, J.H. and Ji Hyun Sung, "Formability of advanced high strength steels.", International Journal of Material Forming 2, pp.359-362, 2009. 6. Ji Hyun Sung, J.H. Kim, and R.H. Wagoner, "A plastic constitutive equation incorporating strain, strain-rate, and temperature", accepted to International Journal of Plasticity, Mar. 2009. 7. Ji Hyun Sung, Ji Hoon Kim and R.H. Wagoner, “Constitutive Equation of AHSS”, presented in NADDRG 2008, May 14-15, 2008, Windsor, CANADA. 8. R.H. Wagoner, A. Madeshia, Ji Hyun Sung, J. H. Kim, “Plasticity and Failure of Advanced High Strength Steel”, presented in Plasticity 2009, Jan. 3 – 9, St. Thomas, USA. 9. H. Kim, A. Bandar, Y. Yang, Ji Hyun Sung and R.H. Wagoner, “Failure analysis of advanced high strength steels(AHSS) during draw bending”, presented in IDDRG 2009 proceedings, pp.449-460, June 1-3, Golden, CO, USA. 10. J. H. Kim, Ji Hyun Sung, R.H. Wagoner, “Thermo-mechanical FEM of draw- bend formability tests”, presented in IDDRG 2009 proceedings, pp. 503-512, June 1-3, Golden, CO, USA. 11. Ji Hyun Sung, J.H. Kim, and R.H. Wagoner, “Accurate constitutive eq. for dual- phase steels”, presented in IDDRG 2009 proceedings, pp. 165-176, June 1-3, Golden, CO, USA. 12. Ji Hyun Sung, J.H. Kim, and R.H. Wagoner, “Sheet formability of advanced high strength steels”, presented in NSF CMMI GRANTEE CONFERENCE 2009, June 22-25, Honolulu, HI, USA. 13. H. Wagoner, Ji Hyun Sung, J. H. Kim: "The Formability of Dual-Phase Steels", Proc. 2009 International Symposium on Automobile Steel (ISAS09), Dalian, China, September 5-8, 2009. vii 14. Ji Hyun Sung, J.H. Kim and R.H. Wagoner, "Draw-bend fracture of dual-phase steels", (in preparation). 15. J. H. Kim, Ji Hyun Sung and R.H. Wagoner, "Finite element simulation of shear failure of advanced high strength steels", (In preparation) FIELDS OF STUDY Major Field: Mechanical Engineering Studies in: Material Characterization, Metal Forming Technology, and Design viii TABLE OF CONTENTS ABSTRACT....................................................................................................................... ii DEDICATION.................................................................................................................. iv ACKNOWLEDGMENTS .................................................................................................v VITA.................................................................................................................................