Experimental and Numerical Investigation of Adhesively Bonded Composite Patch Repair of an Inclined Center Cracked Aluminium Panel under Static and Fatigue Load R. Srilakshmi A Dissertation Submitted to Indian Institute of Technology Hyderabad In Partial Fulfilment of the Requirements for The Doctor of Philosophy Department of Mechanical and Aerospace Engineering September 2014 Declaration I declare that this written submission represents my ideas in my own words, and where others’ ideas or words have been included, I have adequately cited and referenced the original sources. I also declare that I have adhered to all principles of academic honesty and integrity and have not misrepresented or fabricated or falsified any idea/data/fact/source in my submission. I understand that any violation of the above will be a cause for disciplinary action by the Institute and can also evoke penal action from the sources that have thus not been properly cited, or from whom proper permission has not been taken when needed. _________________________ (Signature) R. Srilakshmi ME10P001 i Approval Sheet , ii Acknowledgements First of all, I’d like to give my sincere thanks to my honorific supervisor, Dr M. Ramji, who accepted me as his Ph.D. student without any hesitation. Thereafter, he offered me valuable advice, patiently supervising my work, and always guiding me in the right direction. I’ve learned a lot from him, without his help I could not have finished my dissertation successfully. I also appreciate the advice of the Doctoral committee members, Prof. Dr. K.V.L. Subramaniam, Dr. R. Prashant Kumar and Dr. Ashok Kumar Pandey for their critical comments, which enabled me to notice the weaknesses of my dissertation and make the necessary improvements according to their comments. I take this opportunity to sincerely acknowledge Dr. Vinayak Eswaran, Head, Department of Mechanical and Aerospace Engineering for providing necessary infrastructure and resources to accomplish my research work. I gratefully acknowledge all the faculty members of the Department of Mechanical and Aerospace Engineering. My sincere acknowledgement goes to Prof. U. B. Desai, the Director of Indian Institute of Technology Hyderabad for providing best environment for doing research. I am also thankful to Prof. F. A. Khan for providing all academic needs. I am indebted to Mr Raja Guru Prasath for his generous help doing experiments. I am very much grateful to Dr. C.Viswanath and Dr. Prakash Vyasarayani for their substantial help in my research work. I would like to thank Mr. N Jagannathan Scientist-C, NAL, Bangalore for valuable suggestions in conducting fatigue crack growth experiments. I would like to extend huge, warm thanks to my group mates (past and present), Amith kulkarni, Bhanu prakash, Mohammed Kashfuddoja, Veerkar V. Vikranth, Rahul Pai, Jabir Ubaid, Viswajeeth Bise, Saranath, Naresh, Pratap, Sourab khedhar, Lokesh, Yogesh wagh, Harilal, Yagnik, Milind and Karthikeyan for providing supportive and friendly environment. I would like to thank all workshop staff for helping me in fabricating specimens. iii I am grateful to some of my friends Vandana, Anupama Bhol, Saritha and Swetha and some of research scholars Somashekar, Rajesh and Mr Murali krishna, for their valuable suggestions during my work. I thank my parents, Mr. Gopalakrishna Murthy and Mrs. Devasena, for striving hard to provide a good education to me. I especially thank my sister, Hemamalini and my brother Sanjay in taking care my son during my thesis work. Last but not least, I am greatly indebted to my husband Dr. M. Ravikanth and my son Pranav. iv Dedications To my Mother & my Professor v Abstract Composite patch repair is gaining importance in extending the fatigue life of an aging aircraft. An aircraft, during its service life, it is subjected to severe structural and aerodynamic loads which results from repeated landings and take off, fatigue, ground handling, bird strikes and environmental degradation such as stress corrosion. However due to limited budgets and escalated procurement costs in replacing the aircraft, aircrafts service life need to be extended beyond their design life. Hence, a reinforcement or repair of damaged aircraft is essential to improve its service life. Among various available repair techniques bonded composite repair is mostly preferred. There is lot of research carried out in the safety and life prediction of composite patch repair applied on straight center cracked panels under in plane tensile load. In field, always cracks that appear on structures are of mixed mode and therefore, it is necessary to study the behavior of composite patch repair applied to inclined center cracked panels under in plane tensile static and fatigue loads. In the present work, a three dimensional finite element analysis (FEA) is carried out to study and compare the performance of single and double sided patch bonded over an cracked aluminum panel (2014-T6) having an inclined crack at 45˚. Carbon fiber reinforced polymer (CFRP) is the patch material chosen as part of this work. From FEA based study, it has been found that in case of single sided repair the stress intensity factor (SIF) at the unpatched surface tends to be higher than that of the unrepaired panel SIF. This is due to additional bending load arising due to shift in neutral axis after repair. Further, there are different parameters such as patch lay-up, patch thickness and patch shape and dimensions which affect the performance of the repaired panel. Out of them, patch shape plays a major role on SIF reduction. A detailed finite element based study has been carried out to arrive at the effective patch shape. Later, a genetic algorithm (GA) based optimization technique is employed in conjunction with FEA to arrive at an optimum patch dimensions resulting in higher reduction in SIF near the crack-tip. Further, to predict the whole field strain over the patch surface and also the shear strain distribution over the thickness of adhesive layer, an experimental investigation has been carried out using digital image correlation (DIC) technique. Lastly, three dimensional fatigue analysis using FEA has been conducted to study the crack growth in repaired and unrepaired panel. DIC is also effectively used to monitor the crack growth during the fatigue loading. The obtained experimental results have been compared with FEA estimates for their accuracy and they are in good coherence. It is found that the static strength and fatigue life of double sided repaired vi panel is higher than single sided repaired one. The utility of DIC as an accurate experimental technique for whole field strain prediction in repair applications is shown and turned out to be accurate when compared with FEA prediction thereby recommended for repair studies. vii Table of Contents Declaration ............................................................................................................................... i Approval Sheet ........................................................................................................................ ii Acknowledgements ................................................................................................................ iii Dedications .............................................................................................................................. v Abstract .................................................................................................................................. vi Table of contents .................................................................................................................. viii List of Figures ...................................................................................................................... xiii List of Tables ........................................................................................................................ xx Nomenclature ...................................................................................................................... xxii Abbrevations ...................................................................................................................... xxiii 1. Introduction and Literature review 1.1 Introduction to repair technology ................................................................................ 1 1.1.1 Bonded repair vs mechanical fasteners .............................................................. 2 1.1.2 Elements of fracture mechanics.......................................................................... 6 1.1.3 Composite materials ; an overview .................................................................... 8 1.1.4 Design considerations in adhesively bonded patch repair ................................. 9 1.1.5 Experimental techniques for strain measurements ........................................... 11 1.1.6 Introduction to fatigue loading and crack growth study ................................... 15 1.2 Motivation ................................................................................................................. 17 1.3 Literature review ....................................................................................................... 18 1.3.1 Mechanics based study of composite patch repair. .......................................... 18 1.3.2 Optimisation study of patch repair ................................................................... 19 1.3.3 Fatigue crack growth study .............................................................................. 21 1.3.4 Estimation of whole field strain distribution in repaired panel using viii digital image correlation..... .............................................................................. 23 1.4 Scope and objectives ................................................................................................. 23 1.5 Thesis layout ............................................................................................................. 25 2. Design and Performance Study of Repaired and Unrepaired Panel Using FEA 2.1 Introduction ............................................................................................................... 27 2.2 Geometry and material properties ........................................................................... 29 2.3 SIF evaluation using J –integral approach ................................................................ 30 2.4 Finite element modeling ............................................................................................ 32 2.4.1 Modeling of cracked panel ............................................................................... 32 2.4.2 Mesh convergence study................................................................................... 33 2.4.3 Comparison of analytical and numerically SIF of the cracked panel ............... 34 2.4.4 Finite modelling of repaired panel ................................................................... 35 2.4.5 Variation of SIF in unrepaired and repaired panel ........................................... 36 2.4.6 Variation of normal stress in unrepaired and repaired panel ............................ 37 2.4.7 Effect of patch lay-up configuration on repaired panel .................................... 37 2.4.8 Effect of patch thickness on repaired panel ............................ ......................... 38 2.4.9 Effect of adhesive thickness on repaired panel ............................ .................... 40 2.5 Effect of tapered patch on peel stress distribution......................... ........................... 41 2.7 Closure ........................................................................................................................ 41 3. Optimum Design of Patch Geometry and Dimension 3.1 Introduction ............................................................................................................... 43 3.2 Material properties .................................................................................................... 44 3.3 Finite element modelling of double sided patched panel with different patch shapes ....................................................................................................................... 44 3.3.1 Circular patch modeling ................................................................................... 45 3.3.2 Rectangular patch modeling ............................................................................. 46 3.3.3 Square patch modeling ..................................................................................... 47 3.3.4 Elliptical patch modeling .................................................................................. 47 ix