near-threshold fatigue of adhesive joints

224 Pages·2010·8.33 MB·English

NEAR-THRESHOLD FATIGUE OF ADHESIVE JOINTS:
EFFECT OF MODE RATIO, BOND STRENGTH
AND BONDLINE THICKNESS

by:

Shahrokh Azari

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 Shahrokh Azari, 2010
Abstract

Near-Threshold Fatigue of Adhesive Joints: Effect of Mode Ratio,
Bond Strength and Bondline Thickness

Shahrokh Azari, Doctor of Philosophy
Mechanical and Industrial Engineering Department
University of Toronto, 2010

The main objective of the project was to establish a fracture-mechanics energy-based
approach for the design of structural adhesive joints under cyclic loading. This required
understanding how an adhesive system behaved near its fatigue threshold, and how the key
factors affected this behavior in a fresh undegraded joint. The investigated factors were mode
ratio (phase angle), substrate material, surface treatment and surface roughness (both affecting
the bond strength), bondline thickness and load ratio.
It was first required to understand how the adhesive system behaved under quasi-static
loading by examining a fracture mechanics-based design approach for adhesive systems with
different substrate materials and geometries. Experiments were initially performed to
characterize the strength of aluminum and steel adhesive systems based on the fracture envelope,
critical strain energy release rate as a function of the mode ratio. Ultimate failure loads of
aluminum and steel adhesive joints, having different overlap end conditions and different
geometries were then experimentally measured. These values were compared with the failure
loads extracted from the fracture envelope. Considering the toughening behavior of the adhesive
in the fracture mechanics analyses, a very good agreement (average of 6%) was achieved
between the predictions and experiments for all types of overlap end conditions and geometries.
Different fatigue threshold testing approaches, which are commonly used in the literature
or suggested by the ASTM standard, were evaluated for the cracked and intact fillet joints.
Based on the experimental and analytical studies, the most appropriate technique for fatigue
testing and characterization of adhesive systems was suggested.
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Comparing the mixed-mode near-threshold behavior of different adhesive systems with
the fracture behavior and fatigue mode-I and mixed-mode high crack growth rates showed the
high sensitivity of the mixed-mode near-threshold fatigue to the subtle changes in the interfacial
bond strength.
In order to make a baseline for the design of adhesive joints under cyclic loading, similar
to the previous fracture tests and following the energy-based approach, fatigue behavior was
characterized as a function of the loading mode ratio for aluminum and steel adhesive joints.
The effect of substrate material, surface treatment, bondline thickness, surface roughness
and fatigue testing load ratio on the near-threshold fatigue behavior of adhesives joints was
evaluated experimentally. The experimental observations were then explained using finite
element modeling.
To generalize the conclusions, the majority of experiments and studies covered a broad
range of crack growth rates, as low as fatigue threshold and as high as 10-2 mm/cycle. Having
understood the significant testing and design parameters, an adhesive system can be designed
based on a safe cyclic load that produces an insignificant (for automotive industry) or reasonably

low but known crack growth rate (for aerospace industry).
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Acknowledgements

I would like to express my sincere gratitude and appreciation to my supervisors,
Professor Jan K. Spelt and Professor Marcello Papini for providing me with the continuous
guidance, enthusiasm and encouragement to assist me in conducting a successful research. Their
visions, insights and suggestions not only fueled enormously my research but also will have a
lasting influence in my future career. I feel extremely honored and fortunate to be their student.
I would also like to offer my thanks to Dr. Allan Hull at Engineering Materials Research
for patiently supporting and helping me in setting up and performing the fatigue experiments.
Our discussions and his excellent suggestions were valuable sources in guiding me towards
success in the project.
I would like to thank my Ph.D. committee, Professor Kortschot and Professor Naguib for
the valuable comments and suggestions offered during my annual exams. The thesis benefited
greatly from your input.
I would like to acknowledge the Mechanical and Industrial Engineering Department of
the University of Toronto for accepting me and giving such a wonderful opportunity to pursue
my Ph.D. I am also grateful of the financial support from General Motors of Canada, Ontario
Centres of Excellence and Natural Sciences and Engineering Research Council of Canada.
Regular communications and progress meetings with the members of General Motors
Research and Development and Planning was a valuable source of technical information and
guidance. My special thanks at GM R&D and Planning goes to Dr. Jessica Schroeder, Dr.
Douglas Faulkner, Dr. Justin Gammage (GM Canada), Dr. Blair Carlson and Dr. John Ulicny.
I would like to extend my acknowledge to the other members of the Materials and
Process Mechanics Laboratory, Maciej Jastrzebski, Amin Ghobeity, Dwayne Shirely, Amir
Ameli, Amirhossein Mohajerani, Naresh Datla, Siva Nadimpalli, David Ciampini, Oleg
Belashov and Navdeep Dadhiala for not only their technical assistance but also for creating a
very good atmosphere to work and share ideas. My thanks also go to Andrew Covato, a summer
student who assisted me in establishing the remote control set-up for fatigue tests, and Dr.
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Mojtaba Eskandarian, from NRC Aluminum Technology Center, who collaborated with our
group during the fracture studies.
I would like to thank my parents for their help and support.
Last in this list but first in my heart is my fiancée, Morvarid. I want to really thank you
for giving me endless support, love, caring and happiness, which gave me the strength and
motivation to remain persistent through these years, and also for sacrificing so many things in
your life, including your time, so that I can work intensely on my research. This thesis is
dedicated to you Morvarid, an incalculable treasure I found in my life.
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Table of Contents

Abstract………………………………………………………………..…………………………ii
Acknowledgements…………………………………………………….………………………..iv
Table of Contents………………………………………………………..….…………………...vi
List of Figures…………………………………………………………………………………....xi
List of Tables…………………………………………………………………….……………...xx
List of Appendices………………………………………………………………..……….…..xxii
List of Nomenclatures………………………………………………………………………...xxiv
Chapter 1: Introduction…………………………………………………………….…………..1
1. Motivation………………………………………………..……………………………..……..1
2. Objectives…………………………………………………………….……………...………..2
3. Thesis Outline………………………...……………………………….………………………2
Chapter 2: Fracture Load Predictions and Measurements………………………………...…6
1. Introduction……………………………………………………………………………………6
2. Specimen Preparation…………………………………………………………………………7
3. Fracture Envelope………………………………………………………………………..…....9
3.1. Experimental Approach……………………………………………………………..……9
3.2. DCB Data Analysis, G Calculation and Mode Partitioning……………………….……12
3.2.1. Beam Theory……………………………………………………………………..12
3.2.2. Beam-on-Elastic-Foundation Model………………………………………..……13
3.3. Experimental Results for DCB Specimens………………………………………..…….13
4. Quasi-Static Fracture Tests on CLS and SLS Joints……………………………...………….18
4.1. Data Analysis, G Calculation and Mode Partitioning of CLS and SLS Joints………….20
4.2. Experimental Results and Predictions……………………………………….………….22
4.2.1. Aluminum Adhesive System………………………………………………….....22
4.2.2. Steel Adhesive System…………………………………………………………...29
5. Conclusions………………………………………………………………………...………...30
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Appendix 2.A. Beam Deflection Analysis for Fracture Specimens – CLS Joints…….…………33
Appendix 2.B. Beam Deflection Analysis for Fracture Specimens – ALS Joints……………….34
6. References………………………………………………………………………………..…..36
Chapter 3: The Effect of Mode Ratio and Bond Interface on the Fatigue Behavior….........39
1. Introduction…………………………………………………………………………………..39
2. Experimental Approach………………………………………………..…………………….40
2.1. Specimen Preparation…………………………………………………………………...40
2.1.1. Aluminum System…………………………………………………………...…..42
2.1.2. Degreased Steel System………………………………………………………….42
2.1.3. Zn-Phosphated Steel System……………………………………………………..42
2.2. Fatigue Testing……………………………………………………………..……...……43
2.3. Crack Length Measurement and Data Reduction…………………………………….…43
2.3.1. Fracture Experiments…………………………………………………….…..…..43
2.3.2. Fatigue Experiments………………………………………………………..……44
3. Experimental Results and Discussions……………………………………………….….…..47
3.1. Effect of Loading Phase Angle on Fatigue and Fracture Behavior………………….….47
3.1.1. Aluminum Adhesive System……………………………………………….……47
3.1.2. Degreased Steel Adhesive System…………………………………….……..…..57
3.1.3. Zn-Phosphated Steel Adhesive System……………………………………...…..59
3.2. Effect of Substrate Stiffness on Fatigue Behavior…………………………………....…64
4. Conclusions……………………………………………………………………………….….64
Appendix 3.A. Accuracy of Beam-on-Elastic-Foundation Model……………………………...67
Appendix 3.B. Beam Deflection Analysis for CLS Joints………………………………………69
5. References…………………………………………………………………………………....70
Chapter 4: Fatigue threshold behavior of adhesive joints………………………………..….73
1. Introduction……………………………………………………………………………….…73
2. Experimental Approach……………………………………………………………………..77
2.1. Effect of Fillet on G ……………………………………………………………..…….77
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2.2. Effect of Testing Approach – Force vs. Displacement Control…………………….…..80
2.3. Sensitivity of Fatigue Crack Growth to Interfacial Bond Strength………………..……82
3. Results and Discussion………………………………………………………………....……83
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3.1. Effect of fillet on G …………………………………………………………………….83
th
3.2. Effect of Testing Approach - Force Control vs. Displacement Control…………….…..88
3.2.1. Effect of dG/da…………………………………………………………………..91
3.2.2. Effect of Adhesive Stiffness……………………………………………………..95
3.3. Sensitivity of Fatigue Crack Growth to Interfacial Bond Strength……………………..97
3.3.1. Bond Strength Influenced by Surface Roughness……………………………….97
3.3.2. Bond Strength Influenced by Batch-to-Batch Variability……………………….97
3.3.3. Bond Strength Influenced by Surface Treatment…………………………….....102
3.3.4. Discussion……………………………………………………………….….…..105
4. Conclusions……………………………………………………………………….…….…..105
Appendix 4. Effect of Displacement and Load Ratio on Fatigue Threshold……….……….....107
A.1. Introduction………………………………………………………….…..……...107
A.2. Results and Discussion…………………………………………………………108
5. References……………………………………………………………………….………….110
Chapter 5: Hypotheses for the Effect of Crack Speed on Crack Path Selection.…………114
1. Introduction………………………………………………………………….………….….114
2. Change in the Local Phase Angle………………………………………….……………….114
3. Local Toughening and the Effect of Damage Zone…………………………….…………..118
4. References……………………………………………………………………………….....121
Chapter 6: Effect of Adhesive Thickness - Part I: Experiments…………………………...123
1. Introduction…………………………………………………………………………………123
2. Experimental Approach………………………………………………………………..…...124
2.1. Specimen Preparation………………………………………………………………….124
2.2. Fatigue Testing………………………………………………………….……….…….125
2.3. Fracture Testing………………………………………………………………………..126
2.4. Strain Energy Release Rate Calculation…………………………………………….…126
3. Experimental Results and Discussion………………………………………………….…...127
3.1. Fatigue Tests…………………………………………………………………………...127
3.2. Fracture Tests…………………………………………………………………………..133
4. Conclusions……………………………………………………………………..…………..136
5. References………………………………………………………………………….……….138
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Chapter 7: Effect of Bondline Thickness – Part II. Analysis and Finite Element
Modeling…………………………………………………………………………………….....140
1. Introduction…………………………………………………………………………………140
2. Finite Element Modeling…………………..…………………………………………….…142
3. Effect of Bondline Thickness on Fatigue Behavior……………………………….………..144
3.1. Effect of Bondline Thickness on Mode-I Fatigue Behavior…………….……..………144
3.2. Effect of Bondline Thickness on Mixed-Mode Fatigue Behavior……………………..154
4. Effect of Bondline Thickness on Fracture Behavior……………………………….……….158
4.1. Effect of Bondline Thickness on Crack Initiation…………………………………......158
4.2. Effect of Bondline Thickness on the Steady-State G ……………………………..…..160
c
5. Conclusions…………………………………………………………………………………162
6. References…………………………………………………………………………………..164
Chapter 8: Effect of Surface Roughness…………………………………………………….166
1. Introduction……………………………………………………………………………..…..166
2. Experimental Approach………………………………………………………………...…..167
2.1. Specimen Preparation……………………………………………………………….…167
2.2. Fatigue Testing………………………………………………………………….……..169
2.3. Fracture Testing………………………………………………………………………..172
2.4. Strain Energy Release Rate Calculation……………………………………….…..…..174
3. Effect of Surface Roughness on Fatigue Behavior…………………………………………174
3.1. Mixed-Mode, ADCB Specimens……………………………………………….….…..174
3.1.1. Bonding Area and Fracture Surface Area………………………………….…...175
3.1.2. Crack Path…………………………………………………………….……...…177
3.1.3. Wettability and Void Formation………………………………………………..179
3.2. Mode I, DCB Specimens………………………………………………………….…...180
4. Effect of Surface Roughness on Fracture Behavior of ADCB Specimens……………...….182
5. Conclusions…………………………………………………………………………………183
6. References…………………………………………………………………………………..187
Chapter 9: Conclusions and Future Work………………………………………………….189
1. Summary and Conclusions…………………………………………………………………189
1.1. Fracture Studies………………………………………………………………………..189
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1.2. Fatigue Threshold Behavior…………………………………………………………...191
1.3. Effect of Mode Ratio on Fatigue……………………………………………….…...…193
1.4. Effect of Bond Strength………………………………………………………………..194
1.4.1. Effect of Surface Treatment on Fatigue……………………………….…….….194
1.4.2. Effect of Surface Roughness on Fatigue and Fracture………………………….194
1.5. Effect of Bondline Thickness on Fatigue and Fracture………………………………..196
2. Future Work………………………………………………………………………………...197
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