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Proquest Dissertations INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy subm itted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materiais (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6” x 9” black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. Bell & Howell Information and Learning 300 North Zeeb Road, Ann ArtXJr, Ml 48106-1346 USA 800-521-0600 UMI' MECHANISM-BASED MODELS OF FATIGUE CRACK GROWTH DISSERTATION Presented in Partial Fulfillment of the Requirements For the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Sassan Steven Shademan, B.S, M.S ***** The Ohio State University 2000 Dissertation Committee: Professor W.O. Soboyejo, Advisor Approved by Professor A.B.O. Soboyejo, Co-Advisor Professor Hamish Fraser Professor Glenn Daehn Advisor Professor Susan Olisek (Graduate School Rep) Department of Material Science and Engineering UMI Number: 9994937 Copyright 2001 by Shademan, Sassan Steven All rights reserved. UMi UMI Microform 9994937 Copyright 2001 by Bell & Howell Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. Bell & Howell Information and Leaming Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 ABSTRACT In recent years, investigators in the area of fatigue have realized the importance of alternative empirical equations to the widely accepted Paris equation. However, most of the existing crack growth equations are still restricted to cases where fatigue crack growth is dependent on only one or two variables. As a result, most of the efforts to develop life prediction methodologies have been limited by the lack of fatigue laws that can account for more than one or two variables. In this work, a single multiparameter equation is introduced that can asses the combined effects of mechanical, physical and microstructural variabilities on fatigue crack growth rate of TI-6AI-4V alloys. The multiparameter equation showed that, for mill annealed Ti-6A1-4V microstructures, specimen thickness and test frequency have minimal effects on fatigue crack growth rate, while AK and Kmax are the most important contributors to the crack growth rate. For Widmanstatten microstructures, the model shows that the coarser the colony size, the lower the fatigue crack growth rate. Also, the existing empirical fatigue crack growth equations are only applicable over parametric ranges of crack driving force that are within the bounds where the expressions provide good fits to measured data. However, It is well known that the mechanisms of fatigue crack growth may vary significantly over the parametric ranges of crack driving force. There is, therefore, a need for a simple physically-based model that considers the essential features of fatigue crack growth. In this work, A physically-based model is presented for the prediction of fatigue crack growth in Ti-6AI-4V alloys. The model assumes that the crack extension per cycle is directly proportional to the change in the crack-tip opening displacement, CTOD, during cyclic loading between the maximum and minimum stress intensity factor. A simple power law equation is then derived for the prediction of fatigue crack growth as a function of the stress intensity factor range. The model is validated for fatigue crack growth in the near­ threshold, Paris and high-AK regimes, for both mill annealed and Widmanstatten Ti-6A1-4V microstructures. Anomalous short crack growth behavior is a problem in Ti-6AI-4V. However, the basic causes of anomalous short crack growth in Ti-6AI-4V are not well understood. This lack of understanding may lead to a non-conservative estimates of the total fatigue lives of engineering structures and components fabricated from Ti-6AI-4V. It may also lead to unpredicted or unpredictable failures. In this work, microstructurally short fatigue cracks are shown to grow at stress intensity factors below the long crack fatigue threshold in Widmanstatten Ti-6AI-4V microstructures. Anomalously high fatigue crack growth rates and crack growth retardation is also shown to occur in the short crack regime at stress ratio of 0.1. The fast crack growth rates are associated with partially developed crack wakes that result in lower levels of plasticity-induced closure. These result in higher effective stress intensity factor ranges and faster fatigue growth rates in the short crack regime. The work also shows that cracks are retarded when they interact with prior p grain boundaries in fine and intermediate Widmanstatten microstructures. Similar crack growth retardation phenomena are observed at colony boundaries in coarse Widmanstatten microstructure. The slower crack growth rates are attributed to the retarding effects of the boundaries and the crack deflection that is generally associated with crack growth beyond the retarding boundaries. Ill Dedicated to my wife IV ACKNOWLEDGMENTS I would like to thank my, Dr. Wole Soboyejo and my co-advisor, Dr. Alfred Soboyejo for intellectual support, encouragement, and enthusiasm throughout the course of this research program. Without the efforts on their part, this dissertation would not have been completed. I would also like to thank my dissertation committee members. Dr. Hamish Fraser and Dr. Glenn Daehn. The courses taught by them during my graduate study at Ohio State University were also useful in this research. I thank my colleagues. Dr. Chris Mercer and Dr. Vikas Sinha for helping me get started with my experiments. Useful technical discussions with them are also acknowledged. I also with to thank the technical staff members, Mr. Lloyd Barnhart, Mr. Ken Kushner, Mr. Steve Bright, Mr. Gary Dodge and Mr. Cameron Begg at Ohio State for providing assistance with the experimental work. VITA October 18, 1959 Bom-Shahpoor, Iran 1988 B.S. Welding Engineering The Ohio State University, Columbus, OH 1991 M.S. Welding Engineering The Ohio State University, Columbus, OH 1994 M.S. Materials Science and Engineering The Ohio State University, Columbus, OH 1998-present Graduate Research Associate The Ohio State University, Columbus, OH PUBLICATIONS 1. A. B. 0. Soboyejo, S. Shademan, M. Foster, N. Katsube and W. O. Soboyejo, “A multiparameter Approach to the Prediction of Fatigue Crack Growth in Metallic Materials”, submitted. 2. S. Shademan, A. B. O. Soboyejo, J. F. Knott and W. O. Soboyejo, “A Physically-Based Model for the Prediction of Long Fatigue Crack Growth in Ti-6AI-4V”, submitted. 3. V. Sinha, S. Shademan and W. O. Soboyejo, “Effect of Colony Size on Long Fatigue Crack Growth Behavior in Ti-6AI-4V”, To be submitted. 4. A. B. 0. Soboyejo, S. Shademan, V. Sinha and W. O. Soboyejo, “Statistical Modeling of Microstructural Effects on Fatigue Behavior of cx/p Titanium Alloys”, To be submitted. 5. J. C. Lipplod, S. Shadem an and W. A. Baeslack in, “The Effect of Specimen Strength and Thickness on Cracking Susceptibility During the Sigmajig Weldability Test", The Welding Journal, March 1996, pp. 81-92. VI FIELD OF STUDY Major Field: Materials Science and Engineering vu TABLE OF CONTENTS Abstract. Dedication. Acknowledgments. Vita................................................................................................................................................. vi List of Tables ............................................................................................................................... xiii List of Figures .............................................................................................................................. xv Chapters: 1. introduction .......................................................................................................................... 1 1.1 Background .................................................................................................................. 1 1.2 R eferences.................................................................................................................... 4 2. Literature Review.............................................................................................................. 6 2.1 Fundamentals of Fracture Mechanics ................................................................. 6 2.1.1 Introduction ....................................................................................................... 6 2.1.2 Griffith Fracture Theory ................................................................................... 8 2.1.3 Strain Energy Release Rate (G) .................................................................
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