development of failure criteria and experimental teseting for composite adhesively bonded scarf ...

134 Pages·2014·2.67 MB·English

by  Tammy Ritchey

DEVELOPMENT OF FAILURE CRITERIA AND EXPERIMENTAL TESETING FOR
COMPOSITE ADHESIVELY BONDED SCARF REPAIRS UTILIZING

STRUCTURAL PA STE ADHESIVES

by

Tammy Nicole Ritchey

A thesis submitted i n partial fulfillment
of the requirem ents for the degree

o f

Master o f Science

in

Mechanical Engineering

MONTANA STATE UNIVERSITY

Bozeman, Montana

May 2014
©COPYRIGHT
by
Tammy Nicole Ritchey
2014
All Rights Reserved
ii

ACKNOWLDEGMENTS
A special thank you to my committee members, Dr. Douglas Cairns, Dr. David
Miller, and Dr. Jared Nelson for their guidance and mentorship throughout the course of
this research. Daniel Samborsky was instrumental in the experimental portion. Finally,
thank you to the Montana State University composites group for the hard work dedicated
to the completion of this research.
iii

TABLE OF CONTENTS

1. INTRODUCTION ........................................................................................................ 1
2. BACKGROUND .......................................................................................................... 4
Composite Damage and Analysis ................................................................................. 4
Composite Repair Methods........................................................................................... 5
Understanding Joints as a Simplified Repair ................................................................ 7
Bonded Joint Configurations and Stress States .................................................. 7
Failure Modes ..................................................................................................... 9
Repair Design Parameters ........................................................................................... 10
Film and Paste Adhesives ................................................................................. 11
Bondline Thickness .......................................................................................... 13
Adherend Thickness ......................................................................................... 14
Cure Conditions ................................................................................................ 15
Repair Analysis Criterion ........................................................................................... 16
Certification Standards ..................................................................................... 16
Application of Standards to Composite Laminates .......................................... 17
Failure Criteria ................................................................................................. 18
Tresca Yield Criterion. ......................................................................... 18
Von Mises Hencky Theory. .................................................................. 18
Maximum Normal Stress Theory. ........................................................ 19
Strain-Energy Release Rate. ................................................................. 19
Literiature Review ...................................................................................................... 20
Development and Testing of ............................................................................ 20
Adhesively Bonded Repair Specimen .............................................................. 20
Adhesively Bonded Composite Scarf Joint Testing ......................................... 22
Composite Repair Finite Element Modeling .................................................... 22
Testing and Modeling of Repairs in ................................................................. 23
Composite Sandwich Structures ....................................................................... 23
Scope of Immediate Research ..................................................................................... 25
3. MANUFACTURING AND TESTING ...................................................................... 26
Development of Experimental Joint Specimens ......................................................... 26
Joint Coupon Configurations ............................................................................ 26
Layup Background and Motivation. ..................................................... 27
Scarf Angle Motivation. ....................................................................... 28
Manufacturing Technique for ........................................................................... 28
Scarfed Parent Composite ................................................................................ 28
Bonding of Joint Specimens ............................................................................. 30
Adhesive Selection. .............................................................................. 30
iv

TABLE OF CONTENTS - CONTINUED

Bondline Thickness. ............................................................................. 31
Surface Preparation. ............................................................................. 32
Adhesive Application and Curing. ....................................................... 32
Finishing and Verification. ................................................................... 33
Test Setup and Data Acquisition ...................................................................... 35
Development of Experimental Non ............................................................................ 38
Through Thickness Repair Specimen ......................................................................... 38
Repair Coupon Configurations ......................................................................... 38
Through Thickness and Scarf Angles. ................................................. 38
Adhesive Selection. .............................................................................. 39
Manufacturing Technique for ........................................................................... 41
Scarfed Parent Composite ................................................................................ 41
Repair Patch Bonding Method ......................................................................... 42
Material Preparation. ............................................................................ 42
Finishing and Validation. ..................................................................... 45
Test Setup and Data Acquisition ...................................................................... 45
4. FINITE ELEMENT MODEL DEVELOPMENT ...................................................... 48
Background and Previous Work ................................................................................. 48
MATLAB PDE Model Development ......................................................................... 49
Geometry and Boundary Conditions ................................................................ 49
Mechanical Properties ...................................................................................... 50
Governing Equations: Plane Strain ................................................................. 51
Mesh Generation .............................................................................................. 53
Implementation of Failure Criteria ................................................................... 54
Model Runs ...................................................................................................... 57
Algorithm Application ..................................................................................... 57
5. EXPERIMENTAL TEST RESULTS ......................................................................... 60
Joint Specimen Results ............................................................................................... 60
Strength Results and Failure Modes ................................................................. 60
Strain Results .................................................................................................... 63
Stress vs Far Field Strain. ..................................................................... 64
Von Mises Strain vs Far Field Strain. .................................................. 65
Adherend Comparison. ......................................................................... 66
Non Through Thickness Repair Specimen Results .................................................... 68
Damage Progression ......................................................................................... 68
Strength Results ................................................................................................ 70
Strain Data Collection ...................................................................................... 71
ARAMIS Strain Validation .............................................................................. 72
v

TABLE OF CONTENTS - CONTINUED

Mechanics of Materials. ....................................................................... 72
Strain Gauge. ........................................................................................ 73
Strain Results .................................................................................................... 75
6. ANALYSIS OF RESULTS ........................................................................................ 79
Adhesive Comparison ................................................................................................. 79
Failure Modes ................................................................................................... 79
Stress and Strain Comparison ........................................................................... 80
Joint to Repair Comparison ........................................................................................ 82
Finite Element to Experimental Comparison .............................................................. 84
Maximum Von Mises Strain: Repairs .............................................................. 85
Far Field Strain vs Max .................................................................................... 90
Von Mises Strain: Repairs ................................................................................ 90
FEA Adhesive Comparison .............................................................................. 92
FEA Joint Comparison ..................................................................................... 93
7. DISCUSSION ............................................................................................................. 99
Industry Application ................................................................................................... 99
Manufacturing Flaws ........................................................................................ 99
2D Joint Mechanics Methods ......................................................................... 100
To Repair or Not to Repair ............................................................................. 102
Failure Criteria for Certification Standards .................................................... 102
Geometric and Material General Conclusions ............................................... 103
Geometry. ........................................................................................... 103
Adherend Material. ............................................................................. 104
Adhesive Material. ............................................................................. 104
Repair Design Optimization Process .............................................................. 104
8. SUMMARY AND CONCLUSIONS ....................................................................... 106
Objective I ................................................................................................................. 106
Objective II ............................................................................................................... 109
Objective III .............................................................................................................. 111
Future Work .............................................................................................................. 111
Extended Research ......................................................................................... 111
Joint and Repair Coupon ................................................................................ 112
Manufacturing Processes ................................................................................ 112
Additional Testing .......................................................................................... 113
Augmenting Current Results. ............................................................. 113
Adhesive Investigation. ...................................................................... 114
vi

TABLE OF CONTENTS - CONTINUED

Subscale Testing. ................................................................................ 114
Finite Element Modeling ................................................................................ 114
Concluding Remarks ................................................................................................. 115
REFERENCES ............................................................................................................... 117
vii

LIST OF TABLES
Table Page
1. Composite Repair Design Factors……………………………………...….…11

2. G-L Required Scarf Angles for Composite Test Specimens……….…....……17

3. Summary of Failure Criteria Characteristic Equations…………….………....20

4. Experimental Joint Configurations……………………………….…………..27

5. Fiber and Resin Specifications for Composite Substrates and
Previously Tested Data for Undamaged/Unflawed Specimens………….…...28

6. Specimen Length Dimensions…………………………………………….….29

7. Mechanical Properties for 2-Part Paste Epoxy Adhesive ……………………..……...31

8. Scarf Joint Specimen Test Matrix…………………………….………………37

9. Comparison of Structural Paste Adhesive Mechanical Properties……..……..39

10. Repair Specimen Configurations…………………………………….……….40

11. Repair Ply Lengths for All Specimen Configuration…………..……………..42

12. Test Matrix for Repair Specimens Including Ultimate Failure
Load and Adhesive Failure Mode………………………………....…………47

13. Mechanical Properties Used in FEA Model…………………….……………51

14. Mesh Dependence Test Iterations for 5 Degree, 50% Through
Thickness Repair Model……………………………………………….……..54

15. Summary of Finite Element Analysis Model Runs……………………..…….57

16. Summary of Failure Strength and Modes for Joint Specimens……….……...61

17. Strain Results of Scarf Joint Specimens at Failure…………………..………..63

18. Strength Results for Carbon Uni/Epoxy Adhesive
Repair Specimens……………………………………………….…………...70

19. Carbon Uni/Epoxy Adhesive Repair Data at Adhesive Failure…………....…75
viii

LIST OF TABLES – CONTINUED
Table Page
20. Maximum Von Mises Strain in Epoxy Adhesive for
Repair Specimens…………………………………………………………….77

21. Adhesive Comparison in 15 Degree, 50% Repair Specimen
at Adhesive Failure………………………………………………….………..81

22. Joint vs Repair Comparison at the Adhesive Failure Stage
with Carbon Uni Adherend and Epoxy Adhesive…………………….………82

23. Experimental to FEA Comparison at Adhesive Failure for
Carbon Uni Adherend Joint and Repair Configurations……………….…..…85

24. Strength Knockdowns for Manufacturing Flaws in 4-Ply
Composite Layups…………………………………………………….…...…99

25. Current Design Analysis of Scarf Repairs of
Varying Angles………………………………………………………..……101
ix

LIST OF FIGURES
Figure Page
1. Wind Turbine Blade Damages………………………………………………….4
2. External Patch Composite Repair Scheme……………………………………...6
3. Scarfed Patch Composite Repair Scheme……………………………………....6
4. Images of Composite Repair Procedure………………………………………...7
5. Basic Joint Types……………………………………………………………….8
6. Basic Joint Types……………………………………………………………….8
7. Adhesive Failure Mode………………………………………………………....9

8. Cohesive Failure Mode…………………………………………………...…...10

9. Substrate Failure Mode………………………………………….…………….10

10. Effect of Bondline Thickness on Failure Stress in
Single Lap Joints……………………………………………………………..13

11. Influence of Bondline Thickness on Shear and Tensile
Strength in Butt Joints………………………………………………………..14

12. Micro-voids Observed in a Lap Joint…………………………………………15

13. Scarf Repair Test Specimen Scheme for CSIR-National
Aerospace Laboratories Research on Development of
Damage Tolerant Adhesive Bonded Repair……………………………….…21

14. Repair Coupon Configuration for Varying Scarf
Overlap in Composite Sandwich Structures………………………………….24

15. Elements in a Scarf Joint……………………………………………………...26
16. Schematic of VARTM………………………………………………………..29

17. Milling Process……………………………………………………………....30