 | Lobel T. The hybrid bondline: a novel disbond-stopping design for adhesively bonded composite joints: Diss. ... Dr.-Ing. / Deutsches Zentrum für Luft- und Raumfahrt, Institut für Faserverbundleichtbau und Adaptronik, Braunschweig. – Köln: DLR, 2016. - IX, 205 p.: ill. - (Forschungsbericht; 2016-68). - Res. also Germ. - Bibliogr.: p.185-202.
- ISSN 1434-8454
Шифр: (Pr 1120/2016-68) 02
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Abbreviations ................................................... V
List of Symbols ............................................... VII
1 Introduction ............................................... 1
1.1 Initial Situation and Problem Statement .................... 1
1.2 Thesis Outline ............................................. 2
2 Bonding of Composite Aircraft Structures ................... 5
2.1 Advantages of Bonded Composite Joints ...................... 5
2.2 Examples for Bonded Aircraft Structures .................... 7
2.3 Classification of Bonded Aircraft Structures ............... 9
2.4 Chapter Summary ........................................... 11
3 Limitations of Bonded Composite Joints .................... 13
3.1 Strength Uncertainties .................................... 13
3.1.1 Determination of Adhesive Material Data ............ 13
3.1.2 Structural Analysis Methods of Bonded Joints ....... 14
3.2 Fatigue of Bonded Joints .................................. 19
3.3 Process and Surface Sensitivity ........................... 20
3.4 Failure Characteristics of Bonded Joints .................. 21
3.4.1 Failure Classification ............................. 21
3.4.2 In Service Accidents ............................... 22
3.5 Certification Challenges .................................. 23
3.5.1 Legal Aspects ...................................... 23
3.5.2 Approach for Attaining Compliance .................. 25
3.6 Chapter Summary ........................................... 27
4 Crack-Stopping Concepts ................................... 29
4.1 State-of-the-Art .......................................... 29
4.1.1 Bonded and Bolted Joints ........................... 29
4.1.2 Adhesive Toughening ................................ 30
4.2 Current Research Approaches ............................... 31
4.2.1 General Considerations ............................. 31
4.2.2 Bonded and Bolted Joints ........................... 32
4.2.3 Z-Pinning .......................................... 33
4.2.4 RHEA Concept ....................................... 34
4.2.5 Adhesive Toughening ................................ 34
4.2.6 Surface Modifications .............................. 35
4.2.7 Self-Healing ....................................... 36
4.2.8 Multi-Zone Bondline ................................ 37
4.3 Chapter Summary .......................................... 40
5 A Hybrid Bondline Approach ................................ 41
5.1 Requirements of a Novel Disbond-Stopping Concept .......... 41
5.2 The Novel Disbond-Stopping Concept ........................ 42
5.3 Research Hypotheses ....................................... 43
5.3.1 Hypothesis I: Crack-Arresting by a Sharp Change
of Materials ....................................... 43
5.3.2 Hypothesis II: Maintaining Mechanical Strength ..... 44
5.3.3 Hypothesis III: Obtaining Strength Benefits after
Crack Arrest ....................................... 45
5.4 Requirements of the Crack-Stopping Material ............... 46
5.4.1 Mechanical Requirements ............................ 46
5.4.2 Chemical and Physical Requirements ................. 47
5.5 Technical Approach and Methods ............................ 47
5.6 Chapter Summary ........................................... 48
6 Material Selection and Characterization ................... 49
6.1 Structure and Approach .................................... 49
6.2 Composite Parts ........................................... 50
6.3 Thermoplastic Material .................................... 52
6.3.1 Selection of the Thermoplastic Material ............ 52
6.3.2 Chemical and Thermal Analytical Characterization
of Thermoplast ..................................... 54
6.3.3 Mechanical Characterization of Thermoplast ......... 59
6.4 Epoxy Adhesive ............................................ 65
6.4.1 Selection of Epoxy Adhesive ........................ 65
6.4.2 Chemical and Thermal Analysis Characterization of
Epoxy Adhesive ..................................... 66
6.4.3 Mechanical Characterization of Epoxy Adhesive ...... 68
6.5 Property Comparison ....................................... 71
6.5.1 Physical Properties ................................ 71
6.5.2 Mechanical Properties .............................. 73
6.6 Critical Remarks .......................................... 74
6.7 Chapter Summary ........................................... 76
7 Manufacturing Approaches for the Disbond-Stopping
Feature Concept ........................................... 77
7.1 Overall Manufacturing Principle ........................... 77
7.2 Preliminary Investigations ................................ 79
7.2.1 Co-curing of Thermoplast ........................... 79
7.2.2 Simultaneous Bonding and Welding Trials ............ 81
7.3 Simultaneous Curing and Welding ........................... 82
7.4 Sequenced Curing and Welding .............................. 84
7.4.1 Thermoplastic Welding Technologies ................. 84
7.4.2 An Introduction to Ultrasonic Welding .............. 86
7.4.3 Feasibility Study of Ultrasonic Welding for the
Hybrid Bondline Concept ............................ 87
7.5 Comparison of Autoclave Processing and Ultrasonic Weld
Healing ................................................... 91
7.6 Chapter Summary ........................................... 93
8 Material Transition on Different Scales ................... 95
8.1 Transition between PVDF and Matrix Resin .................. 95
8.1.1 Microscopic Investigations of the Co-Curing
Interface .......................................... 95
8.1.2 Energy-Dispersive X-Ray Analysis ................... 98
8.2 Transition between PVDF and Epoxy Adhesive ............... 100
8.3 Chapter Summary .......................................... 101
9 Surface Pretreatment of Epoxy-Based Composites for
Bonding .................................................. 103
9.1 State-of-the-Art Surface Preparation and Research
Trends ................................................... 103
9.2 Atmospheric Pressure Plasma Study ........................ 107
9.2.1 Parameter Selection ............................... 107
9.2.2 Surface Analysis .................................. 112
9.2.3 Mechanical Tests .................................. 115
9.2.4 Critical Remarks .................................. 117
9.3 Chapter Summary .......................................... 118
10 Crack-Stopping Capability of the Hybrid Bondline ......... 119
10.1 DCB Tests ................................................ 119
10.1.1 Samples Produced in Autoclave Process ............. 119
10.1.2 Ultrasonic Welding Samples ........................ 123
10.1.3 DCB Performance with Respect to the
Manufacturing Technique ........................... 125
10.2 Fatigue Test Setup ....................................... 125
10.3 Fatigue Reference Test Campaign .......................... 130
10.4 CLS Tests of the Hybrid Bondline ......................... 136
10.4.1 Fatigue Tests ..................................... 136
10.4.2 Residual Strength Tests ........................... 141
10.5 Critical Remarks ......................................... 143
10.6 Chapter Summary .......................................... 145
11 Stress Distribution and Design Considerations ............ 147
11.1 Structure and Approach ................................... 147
11.2 Methods .................................................. 148
11.2.1 Finite Element Model .............................. 148
11.2.2 Shear Strain Measurement by Use of Digital Image
Correlation ....................................... 153
11.3 Stress Distribution within the Hybrid Bondline ........... 156
11.3.1 Distinction of Cases and Static Test Campaign ..... 156
11.3.2 Joints Without a Crack ............................ 158
11.3.3 Joints with an Approaching Crack .................. 161
11.3.4 Joints with an Arrested Crack ..................... 163
11.4 Derived Design Guidelines ................................ 167
11.4.1 Recommendations Regarding the DSF Width and
Pitch ............................................. 167
11.4.2 PVDF-Epoxy Transition ............................. 169
11.5 Critical Remarks ......................................... 171
11.6 Chapter Summary .......................................... 173
12 Conclusions and Future Prospects ......................... 175
12.1 Concept Limitations ...................................... 175
12.2 Upscaling of Manufacturing Process for Industrial
Processes ................................................ 176
12.3 Selection of Alternative Materials ....................... 178
12.4 Application Scenarios .................................... 179
12.5 Conclusions and Outlook .................................. 182
Bibliography .................................................. 184
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