The Joint Advanced Materials and Structures Center of Excellence Damage Tolerance and Durability of Damage Tolerance and Durability of Adhesively Bonded Composite Adhesively Bonded Composite Structures Structures Hyonny Kim, Associate Professor, Dept. Structural Engineering, U Hyonny Kim, Associate Professor, Dept. Structural Engineering, U C San Diego C San Diego C.T. Sun, Professor, School of Aeronautics & Astronautics C.T. Sun, Professor, School of Aeronautics & Astronautics Thomas Thomas Siegmund Siegmund , Associate Professor, School of Mechanical Engineering , Associate Professor, School of Mechanical Engineering
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Damage Tolerance and Durability of Adhesively Bonded Composite Structures
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The Joint Advanced Materials and Structures Center of Excellence
Damage Tolerance and Durability of Damage Tolerance and Durability of Adhesively Bonded Composite Adhesively Bonded Composite
StructuresStructuresHyonny Kim, Associate Professor, Dept. Structural Engineering, UHyonny Kim, Associate Professor, Dept. Structural Engineering, UC San DiegoC San Diego
C.T. Sun, Professor, School of Aeronautics & AstronauticsC.T. Sun, Professor, School of Aeronautics & AstronauticsThomas Thomas SiegmundSiegmund, Associate Professor, School of Mechanical Engineering, Associate Professor, School of Mechanical Engineering
2Purdue University – Joint Advanced Materials and Structures Center of Excellence
Damage Tolerance and Durability of Adhesively Bonded Composite Structures
• Motivation and Key Issues– failure prediction of composite adhesive joints remains a difficult problem
• multiple failure modes and complex failure processes• damage initiation and growth influenced by geometry, loading, and environmental
factors such as moisture, temperature, etc.– damage in joints is difficult to detect – must design structures to be tolerant to
reasonably-sized flaws• accurate models are needed to predict failure and assess damage tolerance
• Objectives– investigate physical phenomena and processes leading to failure in adhesively
bonded joints– account for bondline thickness and environmental conditions– develop models describing these phenomena
• Approach:– combined experimental/analytical investigations supporting development of
models
3Purdue University – Joint Advanced Materials and Structures Center of Excellence
FAA Sponsored Project Information
• Principle Investigators & Researchers– Hyonny Kim (now at UCSD)– C. T. Sun– Thomas Siegmund– Post-Doc: Steffen Brinkmann– Students: Haiyang Qian, Nicholas Girder, Matt Wan
• former students: Jibin Han (Dec 2005), J. Lee (May 2006), T.T. Khoo (Dec. 2006), Hee Seok Roh
• FAA Technical Monitor– Curt Davies
• Industry Participation– ABAQUS
4Purdue University – Joint Advanced Materials and Structures Center of Excellence
Focus Areas Towards Achieving Objectives:
– Adhesive constitutive behavior for use in bonded joint analyses
– Effect of adhesive thickness on mixed mode fracture of joints
– Effect of bondline thickness on strength of adhesively bonded joints – CTOA approach
– Influence of moisture, cyclic loading and time dependence on joint fracture – Cohesive zone model approach
5Purdue University – Joint Advanced Materials and Structures Center of Excellence
Hyonny Kim, Associate Professor, UC San Diego, [email protected]: Jungmin Lee (PhD May 2006), Richard Khoo (MS Dec 2006), Hee Seok Roh
Project I. Adhesive Constitutive Behavior Measurement and Bondline Thickness Dependent Mixed Mode Fracture
Objective:– support analysis tools used for design and damage tolerance– use of nonlinear FEA and fracture mechanics based analyses has
become more routine• VCCT and cohesive-zone incorporated into commercial FEA codes
Approach– Accurately measure material property data as crucial ingredients to
increasingly capable and available modeling tools– defining improved methods for constitutive curve measurement– investigate bondline thickness dependent mixed mode fracture
– premature failure – not measuring entire constitutive curve
• voids always present due to manufacturing method (casting)
• initiation of failure leads to immediate cross-width fracture and thus can not develop significant plastic deformation
– does not include effects of:• adherend constraint on adhesive
layer• possible material micro-structural
differences between thin adhesive layer vs. thick bulk
Gage Section
8Purdue University – Joint Advanced Materials and Structures Center of Excellence
Bondline Thickness Dependent Mixed Mode Fracture
• motivation:– fracture mechanics is capable tool for dam. tolerance
analysis– need mixed mode strain energy release rate (SERR) data
• approach:– SERR measured for range of bondline thickness to
establish mixed mode fracture envelope database– observed processes occurring at crack tip– use nonlinear FEA to understand bondline effect in
measured data– establish fracture criteria in joints that accounts for
bondline thickness dependent GIC and GIIC
Mode Mix(% mode II)
ta = 0.008 in.
ta = 0.020 in.
ta = 0.040 in.
ta = 0.060 in.
0 4 5 6 450 3 3 3 5
75 3 3 3 3
100 4 7 4 6
Matrix of Completed Tests (all tests at RT ambient):
test specimen details:adherends: 2024-T4 Al alloy, 0.25 x 1.0 x 6.0 in.adhesive: PTM&W ES6292 epoxy paste adhesivebondline thickness range: 0.008 to 0.060 in.
Laser ExtensometerLong-Distance
MicroscopeTest
Specimen
9Purdue University – Joint Advanced Materials and Structures Center of Excellence
Results – Mixed Mode GC Envelope
0.00
5.00
10.00
15.00
20.00
25.00
0 20 40 60 80 100
Mixed-Mode Ratio (%)
Gc (
lb/in
)
8mil 20mil 40mil 60mil
MMR 0%
MMR 100%
MMR 50%
Large Shear Strain Visible
Small Shear Strain Visible
No Shear Strain Visible
FEA Plastic Zone Predictions – Mode I
8 mil. – PZ Highly Constrained
40 mil. – PZ Moderately Constrained
60 mil. – PZ Unconstrained
Adherend
Adherend
Adhesive
Plastic Zone (PZ) Contours at Growth Initiation:
GC highest for 40 mil joints of 0% to 75% mixed mode ratio
Crack Growth Process Observation by LD Microscope
10Purdue University – Joint Advanced Materials and Structures Center of Excellence
Summary: Comparison of Shear Strength Test and Fracture Properties
• Fracture properties and shear strength test properties show opposite trend over bondline thickness range 0.008 to 0.06 in.
• Fracture Tests: – GIC and GC at 50% Mode II
optimum for ta = 0.04 in.– GC at 75% Mode II relatively
insensitive to ta– GIIC increasing (could plateau and
go down for higher ta than investigated)
– optimal constraint of plastic zone gives highest GC
for higher ta– shear failure strain decreasing for
higher ta– related to localization of plastic
and failure process zone for higher ta
0 0.01 0.02 0.03 0.04 0.05 0.06Bondline Thickness ta (in.)
0
1
2
3
4
Para
met
er N
orm
aliz
ed b
y Va
lue
at t a
= 0
.008
in.
τYield
γFail
GIC
GC 50% Mode IIGC 75% Mode IIGIIC
Fracture Properties
Strength-Test Properties
11Purdue University – Joint Advanced Materials and Structures Center of Excellence
Project I: Conclusions to Date &Benefits to Aviation Industry
• Tools and Protocols:– modified shear strength tests: localized damage/fracture
develops for thick bonds – this should be accounted for in data processing and analyses
– dogbone test for constitutive curve partially successful– new specimen is being designed that is easy to test like dogbone
but accounts for confinement of adhesive layer• Data
– strong bondline thickness effect observed for fracture and shearstrength tests
– fracture properties and strength test properties show opposing trends over range of bondline thickness
• Analysis– plastic zone confinement shown via FEA to affect critical SERR
dependency on bondline thickness
12Purdue University – Joint Advanced Materials and Structures Center of Excellence
Project II: Modeling Thickness Effect on Strength of Adhesive Lap Joint Using CTOA
C.T. Sun, Professor [email protected], School of Aeronautics & Astronautics, Purdue University
Haiyang Qian, Ph.D. Student
Objective – Develop a CTOA fracture criterion to model adhesive thickness-dependent lap joint strength
Approach – Conduct fracture experiments using DCB specimens with various adhesive thicknesses to validate the proposed CTOA approach and to determine the limitation on its applicability with finite element analyses of the experiments
• Joint strength increases as the bondlinethickness decreases up to 0.25 mm
14Purdue University – Joint Advanced Materials and Structures Center of Excellence
Fracture Initiation is Mode I Dominant in Lap Joints
Thin Layer
Thin Layer
Thick Layer
Thick Layer
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.1 0.2 0.3 0.4 0.5 0.6
Adhesive thickness (mm)
Ener
gy R
elea
se R
ate
(J/m
)
GIGII
Stress Decreasing
A
B
Initial crack mode mixity
•Stress concentration near the joint edge and near the interface• Initial flaw (crack) is under mode I loading• Crack growth is along the interface (red line)
15Purdue University – Joint Advanced Materials and Structures Center of Excellence
DCB Test Resultsfailure modes transition from mode I fracture to interfacial
failure as adhesive thickness decreases below a certain level
50
60
70
80
90
100
110
120
130
140
0 0.5 1 1.5 2 2.5 3 3.5
Bondline thickness (mm)
Failu
re L
oad
(N)
0
20
40
60
80
100
120
140
0 2 4 6 8 10
Opening End Displacement (mm)
Load
(N)
0.95mm
1.3mm
3.3mm
0
20
40
60
80
100
120
140
160
0 5 10 15
Opening End Displacement (mm)
Load
(N)
0.4mm0.44mm0.85mm
Sudden Failure
Load vs Adhesive Thickness
16Purdue University – Joint Advanced Materials and Structures Center of Excellence
Effect of Adhesive Thickness on Failure Mode
• Mode I crack propagates in thicker adhesive
•Transition of failure mode in thinneradhesive
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120Applied Load (N)
Inte
rfaci
al N
orm
al S
tress
es (M
Pa) s22-6mil
s22-20mils22-60mil
Maximum Normal Stress
Normal stress at the interface
17Purdue University – Joint Advanced Materials and Structures Center of Excellence
CTOA Criterion for Hysol EA9394
•CTOA is independent of adhesive thickness before failure mode change
0
1
2
3
4
5
6
7
8
0 0.5 1 1.5 2
Adhesive Thickness (mm)
CTO
A (D
egre
e)
18Purdue University – Joint Advanced Materials and Structures Center of Excellence
Project II: Conclusions to Date &Benefits to Aviation Industry
• Tools and Protocols:– Critical CTOA concept: CTOA is a fracture criterion that
is independent of adhesive thickness if failure mode remains mode I. This is the case for thicker bondlines
• Data– Critical CTOA data determined in dependence of bond
line thickness • Analysis
– FEA analysis predictions using critical initial CTOA and failure mode transition due to high interfacial stress between adherend and adhesive layer
19Purdue University – Joint Advanced Materials and Structures Center of Excellence
Project III: Influence of Bondline Thickness, Moisture, Load History
environmental degradation– Models: cohesive zone models in 3D, monotonic, fatigue, coupled for
moisture/load interaction– Image analysis: Digital image correlation for strain fields, quantitative fracture
surface analysis and fracture reconstruction
20Purdue University – Joint Advanced Materials and Structures Center of Excellence
Crack Growth Resistance Environmental Degradation
Displacements andStrain fields
Force –Displacement
Record
Finite Element Method with
Cohesive Zone
Force –Displacement
Record
SpeckleImages
Displacements andStrain fields
Force –Displacement
Record
Finite Element Method with
Cohesive Zone
Force –Displacement
Record
SpeckleImages
Stereo-FractographyDigital Image CorrelationMeX
a0 Δa 1 mm
0.021
-0.0035
εyy
a0 Δa 1 mma0 Δa 1 mm
0.021
-0.0035
εyy
0.021
-0.0035
εyy
Experimental Facilities
21Purdue University – Joint Advanced Materials and Structures Center of Excellence
Computational Modeling
• The Cohesive Zone Model:– Describes local energy dissipation during fracture and fatigue– Is conveniently coupled to other fields (plasticity, moisture, heat,
electrical…)
F
F
Global Parameters:• Force (F) – Displacement (COD)• Environment (H2O)
COD
H2O
Δ
T
T
Local Parameters:• Traction (T) – Separation (Δ)• H2O Concentration C(H2O)
C(H2O)
Finite element model withcohesive elements & H2O transport
Adherent
Adhesive
CZ ElementsDiffusion Elements
• Load, Displacement• Environment• Time• Cycles
• Traction-Separation• Concentration• Damage
Finite element model withCohesive elements, moisture transport, and cyclic damage
22Purdue University – Joint Advanced Materials and Structures Center of Excellence
Monotonic Loading
0
10
20
30
40
50
60
70
80
90
100
0 0.02 0.04 0.06
0.508mm1.524mm3.048mmcz law
Δn [mm]
T n [M
Pa]
0
10
20
30
40
50
60
70
80
90
100
0 0.02 0.04 0.06
0.508mm1.524mm3.048mmcz law
Δn [mm]
T n [M
Pa]
F
F
G( )( )nTCTOD∗
∂Δ =
∂
-200
-100
0
100
200
300
400
0 100 200 300 400 500 600 700 800 900
Point Number
z-va
lue
(µm
)
-300
-250
-200
-150
-100
-50
0
50
100
150
200
250 350 450 550 650 750 850 950
Stereo Images PairLeft -- Right
Digital Elevation Maps
Fracture Profiles Fracture Profiles
Some plasticity
max , ,σ δ Γ
23Purdue University – Joint Advanced Materials and Structures Center of Excellence
Fatigue Loading
1
10
100
1000
10000
100000
0.01 0.1 1 10 100 1000 10000
Strain Energy Release Rate Range, ΔG [J/m2]
Fatig
ue C
rack
Pro
poga
tion
Rat
e, d
a/dN
(μ
m/c
ycle
)
max CG G=
( )0.85245da GdN
= Δ
-200
-150
-100
-50
0
50
100
150
200
0 500 1000 1500 2000 2500
distance along path (µm)
z-va
lue
(µm
)
max max,0 (1 ), ,Dσ σ δ= − Γ
FE-CZMExperiment
Fractography
24Purdue University – Joint Advanced Materials and Structures Center of Excellence
Time Dependence
Wedge test with constant loading
0
500
1000
1500
2000
2500
0 2 4 6 8 10 12
Time at Load (min)
Cra
ck E
xten
sion
(mic
rom
eter
s)
Precrack Stable Unstable
25Purdue University – Joint Advanced Materials and Structures Center of Excellence
Moisture Effects on Joint Fracture
Experiment Simulation
Adhesive
Cohesive
26Purdue University – Joint Advanced Materials and Structures Center of Excellence
Project III: Conclusions to Date &Benefits to Aviation Industry
– In-situ crack growth– Digital image correlation applied to adhesives– Quantitative fractography– Environmentally assisted crack growth with wedge test– Time dependent crack growth with wedge test
• Data– Preliminary data on fatigue crack growth resistance and moisture
assisted crack growth
27Purdue University – Joint Advanced Materials and Structures Center of Excellence
A Look Forward
• Benefit to Aviation– in response to increasing use of adhesive bonding
– Analysis Tools: supports sophisticated computation-based design• failure process prediction, including adhesive plasticity• CTOA, VCCT, Cohesive Zone model • now available in commercial codes• simulation tools can reduce time to conduct extensive environmental
degradation tests– Data: addressing important issues of bondline thickness
• definition and use of local failure criteria that are not bondline thickness dependent
– Protocols: test methods to obtain fracture and constitutive data• seeking to define simpler tests and remove necessity to collect data as
function of bond thickness• Fractography
28Purdue University – Joint Advanced Materials and Structures Center of Excellence
A Look Forward
• Future Needs– results to date concentrated on adhesive using metal adherends – future work
needed to investigate other adherend (namely composite) and adhesive types and failure modes: interfacial (a.k.a. adhesion) and mixed interfacial/cohesive failure + composite failure
– investigate combined loading (simultaneous effects of temperature, humidity, cyclic loading) for range of bondline thickness and mode mix ratio
– establish mixed mode fracture criteria that accounts for bondline thickness– integrate aspects of individual crack growth models into cohesive zone approach– development of improved test specimen for constitutive curve measurement– account for localized failure evolution in modeling of shear tests – demonstrate
transferability to joints of generic configuration– use the developed fracture models to find optimized adhesive thicknesses for
different adhesives– develop a embedded crack concept in conjunction with the developed fracture