Universit`
a degli Studi di Pisa
FACOLT `
A DI INGEGNERIA
Corso di Laurea Magistrale in Ingegneria Aerospaziale
Controlled Delamination in Carbon-Epoxy
Composite Laminate Specimens for Bolted Joint
Tests: Numerical Analysis and Experimental
Activity
Relatori:
Prof.ssa Ing. Roberta Lazzeri
Prof. Ing. Giorgio Cavallini
Prof. Satchi Venkataraman
Candidato: Nicola Giorgi
“Secondo alcuni autorevoli testi di tecnica aeronautica, il calabrone non pu`o volare a causa della forma e del peso del proprio corpo, in rapporto alla superficie alare. Ma il calabrone non lo sa e vola lo stesso.” - Igor’ Ivanoviˇc Sikorskij
Abstract
Controlled Delamination in
Carbon-Epoxy Composite Laminate Specimens for Bolted Joint Tests: Numerical Analysis and Experimental Activity
by Nicola Giorgi
Master of Science in Aerospace Engineering University of Pisa, 2015
Bolted joints are widely used in the aerospace industry but often represent a critical area of damage initiation, both on composite and metallic parts. End-life inspec-tions performed on several aircraft, found multiple instances of delamination dam-age initiation around the bolt holes of pin-loaded joints on carbon/epoxy composite structural panels. To investigate the severity of such damage and the conditions in which it can bring to dangerous failure, a research program will perform single and double-lap bolted joints tests using special composite specimen in which con-trolled and consistent delamination has been introduced around the hole location. At present time, however, there is no standard procedure to introduce such damage in a composite specimen. This thesis presents a research program with the aim of defining the best method to introduce a controlled and consistent delamination damage on a carbon/epoxy composite specimen with a countersunk hole. Several methods have been proposed and analyzed in detail using Finite Element simulations and experimental activity.
Contents
List of Figures iv
List of Tables viii
1 Introduction 1 1.1 Composite materials . . . 1 1.2 Manufacturing of composites . . . 8 1.3 Damage in composites . . . 12 1.4 Research goals . . . 15 2 Literature review 17 2.1 Bolted joints on composites . . . 17
2.2 Analytical and numerical modeling . . . 25
2.3 Damage characterization . . . 29
3 Proposed methods 32 3.1 “Shaped Indenter Damage Induction” . . . 32
3.2 “Two-Step Drill Indentation” . . . 35
4 Finite Element Analysis 37 4.1 Benchmarks . . . 37
4.1.2 Delamination . . . 44
4.1.3 Inter-laminar stress and free edge effect . . . 48
4.2 Finite Element Analysis . . . 52
4.2.1 Stress analysis . . . 52
4.2.2 Material damage prediction . . . 56
4.2.3 Delamination prediction . . . 58
5 Experimental activity 60 5.1 Fixture . . . 61
5.2 First test: practice session . . . 63
5.2.1 Specimens . . . 63 5.2.2 Parameters . . . 67 5.3 Second test . . . 71 5.3.1 Specimens . . . 71 5.3.2 Parameters . . . 74 6 Results 76 6.1 FEA results . . . 76 6.1.1 Stress Analysis . . . 77 6.1.2 Damage prediction . . . 84 6.1.3 Delamination prediction . . . 85 6.2 Test results . . . 88
6.2.1 First test: practice session . . . 88
6.2.2 Second test . . . 101
7 Conclusions 108 7.1 Future works . . . 109
Bibliography 111
List of Figures
1.1 Bell-Boeing V-22 Osprey . . . 2
1.2 Boeing 787 Dreamliner . . . 3
1.3 Carbon-Carbon parts in a Space shuttle . . . 4
1.4 Sandwich structure . . . 5
1.5 Examples of parts made in carbon fibers . . . 6
1.6 Manufacturing of composite . . . 9
1.7 Application of a vacuum bag . . . 10
1.8 Concept of stacking sequence . . . 11
1.9 Different stages of damage in composites . . . 12
1.10 Evolution of damage in a [0◦/90◦]xS composite layup, [7] . . . 13
1.11 Interlaminar stress in a cross ply laminate . . . 14
1.12 Free edge effect visual explanation . . . 15
2.1 Failure modes of pin-loaded joints . . . 18
2.2 ASTM standard bearing failure test specimen . . . 20
2.3 ASTM standard single-lap bearing failure test fixture . . . 21
2.4 Bending behavior of a bolted joint during a single-lap test . . . 21
2.5 ASTM standard double-lap bearing failure test fixture . . . 22
2.6 Opening modes of cracks . . . 24
2.7 Traction-Separation bi-linear response of cohesive elements in Abaqus 28 2.8 Ultrasonic inspection visual explanation . . . 30
2.9 Example of microscopy on a damaged composite specimen. . . 31
3.1 Scheme of the “Shaped Indenter Damage Induction” method . . . 32
3.2 Different types of indenter . . . 33
3.3 Desired contact points . . . 34
3.4 Scheme of the constraint . . . 34
3.5 Scheme of the 2-Step Indentation method . . . 35
4.1 Representation of a three-points flexure test . . . 38
4.2 Three-points flexure test specimen . . . 38
4.3 Instron testing machine . . . 40
4.4 Three-points flexure test support and indenter . . . 41
4.5 Modeled part of the specimen . . . 42
4.6 Finite Element Model . . . 43
4.7 Three-point bending test results . . . 43
4.8 Scheme of the Alfano delamination problem . . . 44
4.9 Alfano delamination problem FE model . . . 44
4.10 Cohesive elements benchmark - Static analysis . . . 46
4.11 Cohesive elements benchmark - Dynamic analysis . . . 47
4.12 Inter-laminar stress benchmark FE model and mesh . . . 48
4.13 Interlaminar Shear stress τ23 at the 0◦/90◦ interface, comparison be-tween different elements . . . 50
4.14 Interlaminar Stress σ3 at the +45◦/ − 45◦ interface, comparison be-tween different elements . . . 50
4.15 Interlaminar Shear stress τ23 at the 0◦/90◦ interface, comparison be-tween different mesh density . . . 51
4.16 Interlaminar Stress σ3 at the +45◦/ − 45◦ interface, comparison be-tween different mesh density . . . 51
4.17 Modeled area . . . 53 v
4.18 The symmetry condition problem . . . 54
4.19 Stress analysis model and mesh . . . 55
4.20 Damage prediction model and mesh . . . 57
4.21 Delamination prediction FE model . . . 59
5.1 Detail of the aligner . . . 61
5.2 Assembly of the fixture . . . 62
5.3 Comparison between standard and modified specimen . . . 63
5.4 Specimen with countersunk holes . . . 65
5.5 Specimen with counterbore holes . . . 65
5.6 Two specimens used in the practice session . . . 66
5.7 A specimen positioned inside the fixture, ready to be tested . . . 67
5.8 Detail of the indentation in progress . . . 68
5.9 Specimen inside the testing machine during the test . . . 69
5.10 Calibration tests . . . 70
5.11 The fixture overlap problem . . . 72
5.12 Specimen used in the second session of tests . . . 73
5.13 Calibration tests . . . 75
6.1 Medium spherical indenter results - τ13 . . . 78
6.2 Medium spherical indenter results - σ3 . . . 78
6.3 Large spherical indenter results - τ13 . . . 79
6.4 Large spherical indenter results - σ3 . . . 79
6.5 Medium cylindrical indenter results - τ13 . . . 81
6.6 Medium cylindrical indenter results - σ3 . . . 81
6.7 Conical indenter results - τ13 . . . 83
6.8 Conical indenter results - σ3 . . . 83
6.9 Damage prediction results . . . 84
6.11 Delamination prediction results - Reaction force Vs Displacement plot 86
6.12 Delamination prediction results - Advancing front . . . 87
6.13 Spherical indenter - Test results . . . 88
6.14 Spherical indenter - C-Scan results . . . 90
6.15 Superposition of the fixture’s diameter on the C-Scan results . . . 91
6.16 Backside view of calibration hole D after the indentation procedure . 91 6.17 Conical indenter - Test results . . . 92
6.18 Conical indenter - C-Scan results . . . 93
6.19 Two-Step Drill Indentation - Test results . . . 94
6.20 Detailed A-scan of hole C . . . 95
6.21 Two-Step Drill Indentation method - C-Scan results . . . 96
6.22 Cross sections used for the microscopy . . . 97
6.23 Spherical Indenter Damage Induction - Hole C - 45◦ cross section . . 99
6.24 Spherical Indenter Damage Induction - Hole C - 90◦ cross section . . 99
6.25 Two-Step Drill Indentation - Hole C - 0◦ cross section . . . 100
6.26 Two-Step Drill Indentation - Hole C - 90◦ cross section . . . 100
6.27 Two-Step Drill Indentation - LOC4 stacking sequence - Test results . 101 6.28 Two-Step Drill Indentation - LOC4 stacking sequence - C-Scan results 102 6.29 Superposition of the fixture’s diameter on the C-Scan results . . . 103
6.30 Two-Step Drill Indentation - SV-1 stacking sequence - Test results . . 104
6.31 Two-Step Drill Indentation - SV-1 stacking sequence - C-Scan results 105 6.32 LOC 4 stacking sequence microscopy - 45◦ cross section . . . 107
6.33 LOC 4 stacking sequence microscopy - 90◦ cross section . . . 107
List of Tables
4.1 AS4/3501-6 Material properties [38] . . . 39
4.2 LOC 4 stacking sequence . . . 39
4.3 Testing machine specifications . . . 40
4.4 Quasi-isotropic stacking sequence . . . 52
4.5 IM7/8552 Material properties [47] . . . 56
4.6 IM7/8552 Hashin damage model properties . . . 56
4.7 IM7/8552 Cohesive properties [47] . . . 59
5.1 Modified bearing specimen specifications . . . 64
5.2 Tested specimen specifications . . . 66
5.3 First test parameters . . . 68
5.4 LOC 4 & SV-1 stacking sequence . . . 72
5.5 Modified tested specimen specifications . . . 73
LIST OF SYMBOLS AND ABBREVIATIONS σn with n = 1, 2, 3 Normal stress components
τnm with n, m = 1, 2, 3 Shear stress components
un with n = 1, 2, 3 Displacement components
n with n = 1, 2, 3 Strain components
SnT with n = 1, 2, 3 Tensile strength
SnC with n = 1, 2, 3 Compressive strength
Snm with n, m = 1, 2, 3 Shear strength
Enn with n = 1, 2, 3 Young modulus in the 3 principal directions
νnm with n, m = 1, 2, 3 Poisson’s ratio
Gnm with n, m = 1, 2, 3 Shear modulus
GIC Critical strain energy release rate for Mode I
GIIC Critical strain energy release rate for Mode II
GIIIC Critical strain energy release rate for Mode III
SI, SII, SIII Strength values for the 3 crack opening modes
FE Finite Element
FRP Fiber Reinforced Plastic
CFRP Carbon Fiber Reinforced Plastic
ASTM American Society for Testing and Materials
ACKNOWLEDGMENT
Very special thanks to Prof. Roberta Lazzeri and Prof. Satchi Venkataraman who gave me the possibility to have this wonderful experience abroad.
Very thanks also to everyone I met during my stay in San Diego, if this experience is been so great, is only thanks to you.
