Prediction of Fracture Toughness and Durability of Adhesively Bonded Composite Joints with

182 Pages·2017·6.66 MB·English

Florida International University
FIU Digital Commons
FIU Electronic Theses and Dissertations University Graduate School
11-2-2015
Prediction of Fracture Toughness and Durability of
Adhesively Bonded Composite Joints with
Undesirable Bonding Conditions
Vishal Musaramthota
Florida International University, [email protected]
DOI:10.25148/etd.FIDC000242
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Recommended Citation
Musaramthota, Vishal, "Prediction of Fracture Toughness and Durability of Adhesively Bonded Composite Joints with Undesirable
Bonding Conditions" (2015).FIU Electronic Theses and Dissertations. 2513.
https://digitalcommons.fiu.edu/etd/2513
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FLORIDA INTERNATIONAL UNIVERSITY
Miami, Florida

PREDICTION OF FRACTURE TOUGHNESS AND DURABILITY OF ADHESIVELY
BONDED COMPOSITE JOINTS WITH UNDESIRABLE BONDING CONDITIONS

A dissertation submitted in partial fulfillment of the
requirements for the degree of
DOCTOR OF PHILOSOPHY
in
MATERIAL SCIENCE AND ENGINEERING
by
Vishal Musaramthota

2016
To: Interim Dean Ranu Jung
College of Engineering and Computing

This dissertation, written by Vishal Musaramthota, and entitled as Prediction of Fracture Toughness
and Durability of Adhesively Bonded Composite Joints with Undesirable Bonding Conditions,
having been approved in respect to style and intellectual content, is referred to you for judgment.

We have read this dissertation and recommend that it be approved.

_______________________________________
Dwayne McDaniel

_______________________________________
Jose R. Almirall

_______________________________________
Xiangyang Zhou

_______________________________________
Benjamin Boesl, Co-Major Professor

_______________________________________
Norman D.H. Munroe, Co-Major Professor

Date of Defense: November 02, 2015

The dissertation of Vishal Musaramthota is approved.

_______________________________________
Interim Dean Ranu Jung
College of Engineering and Computing

_______________________________________
Andrés G. Gil
Vice President for Research and Economic Development
and Dean of the University Graduate School

Florida International University, 2016
ii
© Copyright 2016 by Vishal Musaramthota

All rights reserved.
iii
DEDICATION
This dissertation is dedicated to my ever loving parents for their invaluable sacrifices and
encouragements. Their words of support and encouragement during the challenges of graduate
school and life will always ring in my ears. I am truly thankful for having them in my life. This
work is also dedicated to my sisters, for their everlasting love and providing me with the moral
support I needed without which I would have not been able to accomplish my doctoral dissertation
successfully. My appreciation extends to my all my friends for their love and support.
A special feeling of gratitude and gratefulness to my dear friend mentioned below who supported
me through thick and thin of my PhD career.
Keerthi Pattem
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ACKNOWLEDGMENTS
I take this opportunity to express my sincere gratitude towards Dr. Dwayne McDaniel
(Committee Member and PI of this research project) for his support, motivation and valuable
discussions during this entire research work. His continuous guidance helped me immensely in the
research. I thank him for long discussion hours he dedicated just to clear the knots in this research.
His ideas always gave a new dimension to the research, and proposed many different aisles of
solution. He taught me of doing things efficiently and to achieve maximum output. He has taught
me to challenge myself and stretch my limitations. I am thankful for his efforts, time and dedication
for completion of my dissertation and developing my career.
My sincere thanks go to Dr. Norman Munroe (Major Professor) for his continuous
guidance in developing me as a researcher. A simple thank you is not enough to express my
gratitude towards him. His untiring support and encouragement throughout my Ph.D. has made the
path very clear and easy. My deepest gratitude goes to him for providing opportunity, direction and
motivation in the various research domains that he allowed me to work for. His ever-enthusiastic
spirit, critical perspicacity, and energetic approach were the sole motivation of my persistence.
My gratitude also extends to Dr. Benjamin Boesl (Co-Major Professor) for the genuine
interest he expressed in my research work along with serving as a co-major professor in my
dissertation committee. His experience shared during my proposal defense has helped me to
undertake more research strategies. He has always been very supportive and encouraging and
provided a big help with his research facilities. Much learning was gained through attending his
insightful discussions and his valuable suggestions.
Special thank you to Dr. Xiangyang Zhou (Committee member) for letting me utilize his
research facilities at University of Miami throughout this research. He has always been very
supportive and was always available to guide me through my research. I thank Dr. Jose Almirall
(Committee member) for his support throughout this research and providing me with his valuable
v
insights on characterization techniques that can be adopted to solve issues. This dissertation
benefited greatly from his input. He always encouraged me on employing critical thinking in my
analyses.
I would like to acknowledge Applied Research Center (ARC), Mechanical and Materials
Engineering Department (MME), Advanced Materials Engineering Research Institute (AMERI),
Florida Center for Analytical Electron Microscopy (FCAEM) and University of Miami (UM) for
laboratory support. I thank the University Graduate School for extending a Doctoral Evidence
Acquisition (DEA) and Dissertation Year Fellowship (DYF) fellowships to support my research. I
also acknowledge the financial support from Federal Aviation Administration Cooperative
Agreement 12-C-AM-FIU.
A special thank you is extended to a good friend and colleague Tomas Pribanic, research
analyst (Applied Research Center), for generously imparting his technical knowledge and brilliant
ideas and for his moral support throughout this research. By mimicking his personality, of
constructive criticism and a tireless work ethic, I was able to conduct my research in a more efficient
and systematic manner. Additionally, I would like to extend my thanks to Anthony Abrarho and
Dr. Aparna Aravelli for their help and support during this research period.
I need to make a separate paragraph for my friends here. Puneet, Sushma and Chandan who
have been very helpful in my early years of Ph.D. and continuing till date. Special adoration to
Keerthi and admiration to Aishwharya, Sudheer, Raghavendra and Bala who are always willing to
talk to me endlessly. The gratitude extends further to my dear friend Kavitha, who you cannot stop
loving. She supported me through the time I’ve known her and an epicenter for all my food
cravings. Thanks to Neha and Mouna for always being there for me and frustrating me throughout.
All you guys have been amazing and very special, which I treasure always. I cannot imagine my
university years or indeed the years to follow without your friendship.
vi
ABSTRACT OF THE DISSERTATION
PREDICTION OF FRACTURE TOUGHNESS AND DURABILITY OF ADHESIVELY
BONDED COMPOSITE JOINTS WITH UNDESIRABLE BONDING CONDITIONS
by
Vishal Musaramthota
Florida International University, 2016
Miami, Florida
Professor Norman Munroe, Co-Major Professor
Professor Benjamin Boesl, Co-Major Professor

Advanced composite materials have enabled the conventional aircraft structures to reduce
weight, improve fuel efficiency and offer superior mechanical properties. In the past, materials such
as aluminum, steel or titanium have been used to manufacture aircraft structures for support of
heavy loads. Within the last decade or so, demand for advanced composite materials have been
emerging that offer significant advantages over the traditional metallic materials. Of particular
interest in the recent years, there has been an upsurge in scientific significance in the usage of
adhesively bonded composite joints (ABCJ’s). ABCJ’s negate the introduction of stress risers that
are associated with riveting or other classical techniques. In today’s aircraft transportation market,
there is a push to increase structural efficiency by promoting adhesive bonding to primary joining
of aircraft structures. This research is focused on the issues associated with the durability and
related failures in bonded composite joints that continue to be a critical hindrance to the universal
acceptance of ABCJ’s. Of particular interest are the short term strength, contamination and long
term durability of ABCJ’s.
One of the factors that influence bond performance is contamination and in this study the
influence of contamination on composite-adhesive bond quality was investigated through the
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development of a repeatable and scalable surface contamination procedure. Results showed an
increase in the contaminant coverage area decreases the overall bond strength significantly. A direct
correlation between the contaminant coverage area and the fracture toughness of the bonded joint
was established. Another factor that influences bond performance during an aircraft’s service life
is its long term strength upon exposure to harsh environmental conditions or when subjected to
severe mechanical loading. A test procedure was successfully developed in order to evaluate
durability of ABCJ’s comprising severe environmental conditioning, fatiguing in ambient air and
a combination of both. The bonds produced were durable enough to sustain the tests cases
mentioned above when conditioned for 8 weeks and did not experience any loss in strength.
Specimens that were aged for 80 weeks showed a degradation of 10% in their fracture toughness
when compared to their baseline datasets. The effect of various exposure times needs to be further
evaluated to establish the relationship of durability that is associated with the fracture toughness of
ABCJ’s.
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TABLE OF CONTENTS
CHAPTER PAGE

INTRODUCTION ..................................................................................................................... 1
1.1 Introduction ....................................................................................................................... 1
1.2 Composites and Adhesives................................................................................................ 2
1.2.1 Composites ............................................................................................................ 2
1.2.1.1 Classification of Composites .......................................................................................... 4
1.2.1.2 Economic aspects of Composites and their commercial products: .................... 6
1.2.2 Adhesives ............................................................................................................... 8
1.2.2.1 Classification of Adhesives ............................................................................................. 9
1.2.2.2 Economic aspects of Adhesives and Commercial Products .............................. 10
1.3 Background on Adhesive bonding .................................................................................. 11
1.3.1 Adhesive Bond Strength ...................................................................................... 12
1.3.2 Mechanisms of Adhesive Bonding ...................................................................... 15
1.3.2.1 Mechanical Interlocking ............................................................................................... 15
1.3.2.2 Electrostatic Theory ....................................................................................................... 15
1.3.2.3 Diffusion Theory ............................................................................................................ 16
1.3.2.4 Wetting Theory ............................................................................................................... 16
1.4 Manufacturing of bonded composite joints ..................................................................... 17
1.5 Overview of this study .................................................................................................... 19
1.6 Contributions ................................................................................................................... 20
1.7 References ....................................................................................................................... 21

LITERATURE REVIEW ........................................................................................................ 27
2.1 Structural adhesives and their chemistry ......................................................................... 27
2.2 Static Strength Quantification of Adhesive Joints .......................................................... 31
2.3 Factors affecting composite bond integrity ..................................................................... 32
2.3.1 Surface Preparation .............................................................................................. 32
2.3.2 Contamination and its effects on durability ......................................................... 33
2.4 Quality Control (QC) ...................................................................................................... 35
2.4.1 Bondline control .................................................................................................. 36
2.4.2 Surface preparation .............................................................................................. 37
2.5 Test Methods ................................................................................................................... 37
2.5.1 Wedge Crack Tests .............................................................................................. 38
2.5.2 Shear Tests ........................................................................................................... 40
2.5.2.1 Interlaminar shear strength (ILSS) Tests ............................................................... 41
2.5.3 Double Cantilever Beam Test .............................................................................. 42
2.5.4 Overview of Test Methods .................................................................................. 43
2.6 Surface Characterization ................................................................................................. 44
2.6.1 Surface Analytical Tools ..................................................................................... 44
2.6.1.1 FTIR ............................................................................................................. 44
2.6.1.2 Contact Angle .............................................................................................. 45
2.7 Moisture Uptake .............................................................................................................. 46
2.8 Durability ........................................................................................................................ 47
2.9 Summary ......................................................................................................................... 48
2.10 References ....................................................................................................................... 50
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