Fatigue Behavior in Hygrothermally Degraded toughened epoxy Adhesives

200 Pages·2011·3.15 MB·English

FATIGUE BEHAVIOR IN HYGROTHERMALLY
DEGRADED TOUGHENED EPOXY ADHESIVES
by
Naresh Varma Datla
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Department of Mechanical and Industrial Engineering
University of Toronto
© Copyright by Naresh Varma Datla, 2011
Fatigue Behavior in Hygrothermally Degraded Toughened Epoxy
Adhesives
Naresh Varma Datla
Doctor of Philosophy
Department of Mechanical and Industrial Engineering
University of Toronto
2011
Abstract
A method to measure the mixed-mode fatigue behavior of environmentally degraded
adhesive joints was developed. Firstly, the absorption and desorption of water in two different
rubber-toughened epoxy adhesives was measured gravimetrically. The water absorption in both
adhesives showed anomalous behavior that was fitted to a new “sequential dual Fickian” (SDF)
model. The water desorption in both adhesives was modelled accurately using Fick’s law, and
there was a significant difference in the amount of retained water after drying in the two
adhesives.
The effects of long-term aging were studied using open-faced specimens made with two
different rubber-toughened epoxy adhesives. The contrasting results illustrated the effects of
environmental degradation on the matrix and toughener. Furthermore, the differences in the
degradation behavior of both adhesives, combined with gravimetric and dynamic mechanical
thermal analysis (DMTA) results, were used to illustrate the role of retained water in degrading
the toughening mechanisms. The measured fatigue results invalidated the environmental index
(EI) hypothesis for fatigue behavior, at least for the relatively short aging times studied here.
Compared with aging under constant humidity, the fatigue performance of joints was found to be
ii
superior after aging in a cyclic salt-spray environment due to the lower water concentrations in
the adhesive.
The effects of test environment humidity and temperature on the fatigue behavior were
also studied using closed, un-aged specimens. Both individual and combined effects of
temperature and humidity on fatigue behavior were studied. In elevated temperature and
humidity environment, joint performance at higher crack growth rates was degraded solely due
to the effect of the increased temperature, whereas fatigue performance at low crack growth rates
degraded predominantly because of elevated moisture.
Finally, to generalise the techniques developed to automotive aluminum sheets, a
reinforced specimen was developed that avoids yielding of thin aluminum sheet adherends while
loading. Fatigue testing with these reinforced specimens revealed that the fatigue behavior was
sensitive to the loading phase angle and the orientation of rolling lines on the sheet. These
reinforced specimens were also used to study the effects of long-term aging and the effects of
test environment.

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Acknowledgments
First of all, I would like to express my sincere gratitude to my doctoral supervisors, Prof.
Jan K. Spelt and Prof. Marcello Papini, for providing the opportunity to pursue my doctoral
dissertation and for their continuous guidance and encouragement during this research. I am
fortunate to have had spend the past few years learning from them about science and life.
I would like to thank Dr. Allan Hull at Engineering Materials Research for patiently
helping me setting up and performing fatigue experiments. The fruitful discussion with him and
his valuable suggestions have helped my work.
I would also like to extend my gratitude to my committee members Prof. Chul B. Park,
Prof. F. Ben Amara, Prof. Craig A. Steeves for their valuable comments and suggestions.
I am grateful for the financial support from General Motors of Canada, Centres of
Excellence and Natural Sciences and Engineering Research Council. My special thanks to
researchers at General Motors Dr. Jessica Schroeder, Dr. Douglas Faulkner, Dr. Blair Carlson
and Dr. John Ulicny for providing technical information and valuable suggestions during the
progress meetings and regular communications.
I would like to thank my lab mates for always being helpful and creating an enjoyable
work place. My special thanks are to Amir Ameli and Shahrokh Azari for their collaboration in
setting up the experiments.
I am indebted to my parents, Rama Raju and Vijaya Lakshmi, and my brother, Suresh, for
their constant source of motivation and encouragement in fulfilling my aspirations. I dedicate
my thesis to my parents.
Most of all, I would like to thank my wife, Kanthi. Her endless support, love, and
happiness gave me the strength to successfully pursue my research. I owe my future success to
you.
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Table of Contents

Acknowledgments .......................................................................................................................... iv
Table of Contents ............................................................................................................................ v
List of Tables ................................................................................................................................. ix
List of Figures ............................................................................................................................... xii
List of Appendices ...................................................................................................................... xxii
Chapter 1 Introduction .................................................................................................................... 1
1.1 Background and motivation ................................................................................................ 1
1.2 Objectives ........................................................................................................................... 2
1.3 Overview of thesis .............................................................................................................. 2
Chapter 2 Modified DCB and CLS Specimens for Mixed-mode Fatigue Testing of
Adhesively Bonded Thin Sheets ................................................................................................ 6
2.1 Introduction .......................................................................................................................... 6
2.2 Experiments .......................................................................................................................... 7
2.2.1 Reinforced sheet specimen ........................................................................................ 7
2.2.2 Fatigue tests ............................................................................................................. 10
2.3 Results and discussion........................................................................................................ 12
2.3.1 Effect of reinforcing adhesive .................................................................................. 12
2.3.2 Effect of phase angle and sheet rolling-line orientation .......................................... 17
2.3.3 Effect of test environment ........................................................................................ 23
2.4 Conclusions ........................................................................................................................ 25
Appendix 2A Adhesive stresses for unequal adherends [7] .................................................... 26
Appendix 2B Comparison of G calculated using FE and analytical model ............................ 30
2.5 References .......................................................................................................................... 32
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Chapter 3 Hygrothermal properties of highly toughened epoxy adhesives .................................. 34
3.1 Introduction ........................................................................................................................ 34
3.2 Mathematics of diffusion models ....................................................................................... 35
3.2.1. Dual Fickian model ................................................................................................. 36
3.2.2 Langmuir model ....................................................................................................... 39
3.2.3 Fickian model in desorption ..................................................................................... 40
3.3 Experimental procedure ..................................................................................................... 41
3.4 Results and discussion ................................................................................................ 42
3.4.1 Moisture absorption ................................................................................................. 42
3.4.2 Moisture desorption ................................................................................................. 60
3.4.3 XPS analysis ............................................................................................................ 65
3.5 Conclusions ........................................................................................................................ 66
3.6 References .......................................................................................................................... 68
Chapter 4 The Effects of Test Temperature and Humidity on the Mixed-mode Fatigue
Behavior of a Toughened Adhesive Aluminum Joint .............................................................. 70
4.1 Introduction ........................................................................................................................ 70
4.2 Experiments ........................................................................................................................ 71
4.2.1 Specimen preparation ............................................................................................... 71
4.2.2. Fatigue tests ............................................................................................................ 72
4.3 Results and Discussion ....................................................................................................... 75
4.3.1. Effect of temperature .............................................................................................. 75
4.3.2 Effect of humidity level ........................................................................................... 87
4.3.3 Combined effect of higher temperature and humidity ............................................. 96
4.4 Conclusions ........................................................................................................................ 99
4.5 References ........................................................................................................................ 101
Chapter 5 Mixed-mode fatigue behavior of degraded toughened epoxy adhesive joints ........... 104
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5.1 Introduction ...................................................................................................................... 104
5.2 Experiments ...................................................................................................................... 105
5.2.1 Specimen preparation ............................................................................................. 105
5.2.2 Aging conditions .................................................................................................... 108
5.2.3 Gravimetric measurements .................................................................................... 109
5.2.4 Fatigue tests ........................................................................................................... 109
5.2.5 Measurement of residual adhesive thickness ......................................................... 110
5.3 Results and discussion...................................................................................................... 110
5.3.1 Moisture diffusion .................................................................................................. 110
5.3.2 Fresh open-faced specimens .................................................................................. 113
5.3.3 Aging of joints in constant humidity environments ............................................... 115
5.3.4 Aging of joints in cyclic environment ................................................................... 123
5.4 Conclusions ...................................................................................................................... 127
Appendix 5A Moisture diffusion ........................................................................................... 129
5.5 References ........................................................................................................................ 131
Chapter 6 Effects of aging on the fatigue behavior of two toughened epoxy adhesives ............ 133
6.1 Introduction ...................................................................................................................... 133
6.2 Experimental .................................................................................................................... 134
6.2.1 Open-faced specimen preparation .......................................................................... 134
6.2.2 Aging and test conditions ....................................................................................... 137
6.2.3 Fatigue testing procedures and environment ......................................................... 137
6.2.4 Adhesive rubber tougheners .................................................................................. 138
6.2.5 DMTA .................................................................................................................... 140
6.3 Results and Discussion ..................................................................................................... 140
6.3.1 Gravimetric analysis .............................................................................................. 140
6.3.2 DMTA .................................................................................................................... 142
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6.3.3 Fatigue behavior of joints with adhesive 1 ............................................................ 148
6.3.4 Fatigue behavior of joints with adhesive 2 ............................................................ 166
6.4 Conclusions ...................................................................................................................... 169
6.5 References ........................................................................................................................ 171
Chapter 7 Conclusions and Recommendations ........................................................................... 173
7.1 Conclusions ...................................................................................................................... 173
7.1.1 Fresh adhesive joints .............................................................................................. 173
7.1.2 Water diffusion in toughened epoxy adhesives ..................................................... 173
7.1.3 Effects of test environment .................................................................................... 174
7.1.4 Effects of long-term aging environments .............................................................. 175
7.2 Future work ...................................................................................................................... 177

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List of Tables
Chapter 2
Table 2. 1 Mechanical properties of adhesive and adherends at room temperature. ..................... 8
Table 2. 2 Strain energy release rate and phase angle calculated using finite element model for
laminate adherend and equivalent stiffness adherend specimens. A 600 N load
applied to both adherends for ADCB specimen with crack length of 60 mm. .......... 16
Table 2. 3 G and Ψ obtained from finite element model by changing E of reinforcing adhesive.
A 500 N load was applied to both adherends of the ADCB specimen with crack
length of 60 mm. Sheet thickness was 2 mm and other properties as per Table 2.1. 16
Table 2. 4 G and Ψ obtained from finite element model for different sheet thicknesses. A 500 N
load was applied to both adherends of the ADCB specimen with crack length of 60
mm. ............................................................................................................................ 16
Table 2. 5 Elemental composition (atomic %) of failure surface compared with bare pretreated
sheet and bulk adhesive using XPS analysis. ............................................................. 19
Table 2. 6 Average surface roughness, R , measured on the sheet side of the fracture surface
a
along and across the length of the specimen. Average and standard deviation of
readings from 3 different locations in the threshold region. ...................................... 20
Chapter 3
Table 3. 1 Mechanical and physical properties of adhesives 1 and 2 at room temperature as
supplied by the manufacturers. .................................................................................. 42
Table 3. 2 Different exposure conditions for adhesives 1 and 2 and saturated salt solutions used
to achieve different levels of RH. .............................................................................. 42
Table 3. 3 SDF model parameters obtained by curve fitting the experimental gravimetric results
at different combinations of temperature and RH for adhesive 1. M values obtained
1∞
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from PDF model are also given. Each data point is given as an average of three
values obtained from the repetitions. SD shows the standard deviation. .................. 47
Table 3. 4 Langmuir model parameters obtained by curve fitting to the experimental gravimetric
results at different combinations of temperature and RH for adhesive system 1. Each
data point is given as an average of three values obtained from the repetitions. SD
shows the standard deviation. .................................................................................... 48
Table 3. 5 SDF model parameters obtained by curve fitting to the experimental gravimetric
results at different combinations of temperature and RH for adhesive 2. M values
1∞
obtained from PDF model are also given. Each data point is an average of three
values obtained from the repetitions. SD shows the standard deviation. .................. 51
Table 3. 6 Langmuir model parameters obtained by curve fitting to the experimental gravimetric
results at different combinations of temperature and RH for adhesive 2. Each data
point is an average of three values obtained from the repetitions. SD shows the
standard deviation. ..................................................................................................... 51
Table 3. 7 Percentage of oxygen atoms associated with different chemical bonds with their
binding energy for fresh, saturated wet and dried samples of adhesives 1 and 2. Each
data point is an average of three repetitions. .............................................................. 66
Chapter 4
Table 4. 1 Temperature and humidity conditions used in fatigue experiments. Number of
thresholds reached and ADCB specimens tested. ...................................................... 73
Table 4. 2 Mechanical properties of adhesive at room temperature as provided by the
manufacturer and of adherends taken from ref. [17,18]............................................. 75
Chapter 5
Table 5. 1 Stages of the cyclic aging environment. Salt spray was applied in the ambient stage
four times for 30 s each. ........................................................................................... 107
x