Air Force Institute of Technology AFIT Scholar Theses and Dissertations Student Graduate Works 3-2020 Laser Shock Peening Pressure Impulse Determination via Empirical Data-Matching with Optimization Software Colin C. Engebretsen Follow this and additional works at: https://scholar.afit.edu/etd Part of the Plasma and Beam Physics Commons, and the Structures and Materials Commons Recommended Citation Engebretsen, Colin C., "Laser Shock Peening Pressure Impulse Determination via Empirical Data-Matching with Optimization Software" (2020). Theses and Dissertations. 3209. https://scholar.afit.edu/etd/3209 This Thesis is brought to you for free and open access by the Student Graduate Works at AFIT Scholar. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of AFIT Scholar. For more information, please contact [email protected]. LASER SHOCK PEENING PRESSURE IMPULSE DETERMINATION VIA EMPIRICAL DATA-MATCHING WITH OPTIMIZATION SOFTWARE DISSERTATION Colin C. Engebretsen, Major, USAF AFIT-ENY-DS-20-M-260 DEPARTMENT OF THE AIR FORCE AIR UNIVERSITY AIR FORCE INSTITUTE OF TECHNOLOGY Wright-Patterson Air Force Base, Ohio DISTRIBUTION STATEMENT A. APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED. APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED. The views expressed in this thesis are those of the author and do not reflect the official policy or position of the United States Air Force, Department of Defense, or the United States Government. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. AFIT-ENY-DS-20-M-260 LASER SHOCK PEENING PRESSURE IMPULSE DETERMINATION VIA EMPIRICAL DATA-MATCHING WITH OPTIMIZATION SOFTWARE DISSERTATION Presented to the Faculty Department of Aeronautics and Astronautics Graduate School of Engineering and Management Air Force Institute of Technology Air University Air Education and Training Command In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Colin C. Engebretsen, BS, MS Major, USAF September 2019 DISTRIBUTION STATEMENT A. APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED. APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED. AFIT-ENY-DS-20-M-260 Abstract Laser shock peening (LSP) is a form of work hardening by means of laser induced pressure impulse. LSP imparts compressive residual stresses which can improve fatigue life of metallic alloys for structural use. The finite element modeling (FEM) of LSP is typically done by applying an assumed pressure impulse, as useful experimental measurement of this pressure impulse has not been adequately accomplished. This shortfall in the field is a current limitation to the accuracy of FE modeling, and was addressed in the current work. A novel method was tested to determine the pressure impulse shape in time and space by optimization driven data-matching. FE model development and material model verification was completed in Abaqus. A 2D and 3D model type study was conducted. A proof of concept data-matching optimization tool was developed and verified. This data-matching optimization tool, using the Hooke-Jeeves optimization algorithm, was then applied to match experimentally collected residual stress measurements from single LSP treated spots in 2024-T351 aluminum specimens. Validation of this “best-fit” pressure impulse was attempted in a 6Al-4V titanium material model for the same LSP treatment process. A combination Johnson-Cook viscoplasticity and Mie-Grüneisen equation of state (EOS) material model was shown to be amply sufficient for modeling the highly dynamic LSP event. A 2D axisymmetric FE model was shown to adequately represent a square LSP treatment process, in terms of residual stress field results with the use of a linear adjustment factor. The Hooke-Jeeves optimization algorithm proved highly successful at working through a FE model “black box” to match a target residual stress outcome. Further, this method was successful in matching the residual stress field of experimentally collected data. The validation of the best-fit pressure impulse in titanium was not a perfect match, but exhibited enough accuracy to be useful to design engineers in certain cases, and further shows potential for improvement and implementation toward this impulse matching goal. 1 Ineffable Creator, You who are the true source of light, and wisdom. Pour forth upon my dense intellect a ray of your splendor that may take away the darkness of sin and ignorance. Endow me with a keenness of understanding, a retentive memory, measure and ease of learning, and discernment of what I read. Grant me rich grace with words, and the ability to express myself in thoroughness and charm. Guide the beginning of my work, direct its progress, and bring it to successful completion. This I ask through my Lord and Savior Jesus Christ, true God and true Man, who lives and reigns with you and the Holy Spirit. Amen - Saint Thomas Aquinas, A student’s prayer 2 Acknowledgments This work would not be possible without the funding of the Air Force Research Laboratories, and the support of my research committee. A special thanks to my sponsor, Dr. Langer for her guidance, and to my research advisor, Dr. Palazotto for his continued patience and willingness to weather my brainstorms and give tough direction. Other special thanks to the AFIT lab and model shop staff for their professionalism and willingness to help in any way possible, especially in a pinch. To pursue a PhD is a prodigious exploit which fluctuates between labor of love and labor of loathing, is punctuated by moments of consternation and self-doubt, and culminates with as much self-discovery as self-improvement. The affair demands a great deal of backing and encouragement from friends and family; these are the real heroes of this endeavor. To the men of our biweekly bible study group (and home-improvement cohort), whose fellowship I highly value; to my parents and siblings, whose undue pride inspires me to be my best self; to my beautiful wife and children, my most cherished cheerleaders, and through whom the title doctor is made lowly to those of husband and father; but especially to my wife, who willingly sacrificed more than I would have asked of her to ensure my success. To these I bestow indelible gratitude. Colin 3 Table of Contents Page Table of Contents ............................................................................................................................. 4 List of Figures .................................................................................................................................. 7 List of Tables .................................................................................................................................. 11 List of Nomenclature & Acronyms ................................................................................................ 12 I. Introduction & Background ....................................................................................................... 13 Objective ................................................................................................................................ 13 Overview ................................................................................................................................ 14 Motivation .............................................................................................................................. 14 Background ............................................................................................................................ 15 Laser Systems ......................................................................................................................... 17 Literature Review ................................................................................................................... 17 Material Improvement from LSP ........................................................................................... 18 Computer Modeling of LSP ................................................................................................... 28 Measurement of LSP Artifacts ............................................................................................... 49 Summary ................................................................................................................................ 58 II. Theoretical Foundations ............................................................................................................ 61 Chapter Overview .................................................................................................................. 61 Finite Difference, Finite Element Modeling, and Material Properties ................................... 61 Pressure Impulse Formation ................................................................................................... 70 Hooke-Jeeves Optimization Algorithm .................................................................................. 74 Hole-drilling Residual Stress Measurement ........................................................................... 76 Surface Displacement Measurement ...................................................................................... 80 III. Modeling and Experimental Methods ...................................................................................... 82 Chapter Overview .................................................................................................................