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The Joint Advanced Materials and Structures Center of Excellence
Damage Tolerance and Damage Tolerance and Durability of Adhesively Durability of Adhesively
Bonded Composite StructuresBonded Composite StructuresThomas SiegmundThomas Siegmund
CT SunCT Sun
2The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Adhesively Bonded Composite Structures
• Motivation and Key Issues – Contribute to the development of reliability of
adhesively bonded structures• Objective
– Develop experimental and numerical methods to design and analyze design
• Approach– Nonlinear fracture mechanics methods (CTOA and
cohesive zone models)– Develop related educational and training material
3The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
FAA Sponsored Project Information
• Principal Investigators & Researchers– Thomas Siegmund (PI, School of Mechanical
Engineering)– CT Sun (Co-PI, School of Aeronautics and
Astronautics)• FAA Technical Monitor
– Curt Davies• Other FAA Personnel Involved
– No current• Industry Participation
– No current
4The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Adhesively Bonded Composite Structures – CZM Approach
PI: Thomas Siegmund, Professor, School of Mechanical Engineering, [email protected]
Objective: Demonstrate the use of the cohesive zone model approach in the analysis of adhesively bonded structures
Approach: Employ previously developed data for Hysol EA9394 to typical structural joints
5The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
0
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100
0 0.02 0.04 0.06
0.508mm1.524mm3.048mmcz law
Δn [mm]
T n [M
Pa]
0
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90
100
0 0.02 0.04 0.06
0.508mm1.524mm3.048mmcz law
Δn [mm]
T n [M
Pa]
Cohesive Zone Model
Δ
T
T
Local Parameters:• Traction (T) – Separation (Δ)• H2O Concentration C(H2O)
C(H2O)• Load, Displacement• Environment• Time• Cycles
• Traction-Separation
• Damage
2
Adherent
Adhesive
CZ Elements
Finite element model withCohesive elementsF
F
Global Parameters:
COD
6The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
L-Joint
Feiha et al., International Journal of Adhesion and Adhesives, 2005, 47-59
Model entirely with continuum elementsAdhesive with
continuum elements & cohesive elements
7The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
L-Joint
Global Stresses
Stresses in Adhesive
Tractions in CZ
8The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Joint Strength & Bondline Thickness
Structural (beam) elementsAdhesive with continuum elements & cohesive elements
Group A - Beam Elements
Group B – Cohesive Elements
Group C
Interface
Precrack / Cohesive Crack
CAD to FEM strategy:Define parts connected by
tie constraints to form assembly
Mesh parts independently
9The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
6.40E+03
6.60E+03
6.80E+03
7.00E+03
7.20E+03
7.40E+03
7.60E+03
7.80E+03
8.00E+03
8.20E+03
0 0.5 1 1.5 2 2.5 3 3.5
Hysol Thickness (mm)
Failu
re L
oad
(N)
Adhesive Cohesive --> Adhesive Cohesive Peel (adhesive)
Joint Strength & Bondline Thickness
10The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Skin – Stiffener Joint: Effect of Flange Thickness
Structural (shell ) elements & Cohesive elements
J. F. Mandell et al., J. Solar Energy Engineering, 125, 2003, 522-530.
11The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Skin – Stiffener Joint: Effect of Flange Thickness
tflange =4.5 mm tflange =1.0 mm
Delamination growth from flange free end
Delamination growth from flange center
12The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Sandwich Panel Lap Joint
Solid elements for sandwich & Cohesive elements
Viscous Regularization of Damage in Cohesive Zone becomes essential in complex problems:
( ) ( ) 01 1v v vD D D T D Tμ
= − ⇒ = −&
13The Joint Advanced Materials and Structures Center of Excellence
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Sandwich Panel Lap Joint
1st CZLast CZ
Computation w/o regularization
Computation with regularization may well predict complete joint failure in a modelConvenience of obtaining a numerically converging solution must not be considered as a real failure load,and can only be considered as an upper bound.
0.05
0.01
0.0005
0.047
0.0050.0001
μ=0.0001 μ varying
14The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Education & Training
Course: “Computational Fracture Mechanics”(1) Review of classical fracture mechanics concepts for elastic materials;
(K, J)(2) Computational methods for classical fracture mechanics for elastic
materials; (singular elements etc.)(3) Computational methods of crack growth in elastic solids (including
modeling with cohesive zone models, model generation, analysis, convergence criteria, fracture and fatigue);
(4) Review of classical fracture mechanics concepts for nonlinear material; (J, deformation theory vs. plasticity)
(5) Computational methods for nonlinear fracture mechanics (cohesive zone model, R curves);
(6) Continuum damage mechanics concepts and computational aspects (void growth models, localization, ductile fracture).
Numerical examples (ABAQUS CAE, ABAQUS Standard) for all chapters.
15The Joint Advanced Materials and Structures Center of Excellence
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Project I: Conclusion to Date
• Developed tools to measure cohesive zone properties• Explore various levels of model complexity• Explore capabilities of commercial software (ABAQUS)• Develop CAD to FEM modeling strategies
• Examples:– Bondline thickness dependence of joint strength
Solid model– Fillet radius influence on L-joint strength
Structural – solid model– Shell – stiffener structure
Structural model– Sandwich panel joint
Convergence of solution
16The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Project I: A Look Forward
• Benefit to Aviation:– Response to increase in need for understanding
adhesive bonding processes and their reliability– Novel analysis approaches to aerospace structures– Address to fundamental issues in bonded structures– Provide related training and education material
• Future needs– Long term response of adhesively bonded structures:
fatigue & environment, variable amplitude loading– Further understanding of nonlinear failure processes
17The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Project II: Prediction of Adhesive Lap Joint Strength 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 predict thickness-dependent adhesive lap joint strength
Approach – Conduct fracture experiments using DCB and single lap specimens of various adhesive thicknesses to validate the proposed CTOA approach and to determine the limitation on its applicability with finite element analyses of the experiments
18The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Adhesive Thickness Effect on the Strength of Lap Joints
T
T
l
L
t
adhesvie
Adherend: Aluminum Alloy 7075
Adhesive: HYSOL EA9394
Surface Treatment: Semco Pasa-Jell 105 (etching method)
L=3in, l=1in, T=0.125int=0.008in, 0.01in, 0.02in, 0.06in
01
23
45
67
0 0.5 1 1.5
Bondline Thickness (mm)
Stre
ngth
of J
oint
s (k
N)
• Experimental result
• Joint strength increases as the bondlinethickness decreases up to 0.25 mm
19The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Size of K-Dominance Zone in DCB
gularSinNonyy
Singularyy
Singularyy
−+=Λ
σσ
σ
x2K I
yy πσ =
Degree of K-Dominance
+ 0 (1)
0
0.02
0.04
0.06
0.08
0.1
0.12
0 1 2 3 4Adhesive thickness (mm)
K-D
omin
ance
zon
e si
ze (m
m)
Steel Hinge
L
t
l
Wta
20The Joint Advanced Materials and Structures Center of Excellence
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Plastic Zone Size in DCB Hysol EA9394
0
10
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50
0 0.01 0.02 0.03Strain
Stre
ss (M
Pa)
Dog-Bone Specimens Test DataSandia National Lab DataLinear ElasticExtended CurveAdhesive Thickness: 0.8 mm
Plastic Zone Size: 0.78 mm90% K-Dominance: 0.04 mm
0.78 mm
• Small scale yielding assumption is violated
21The Joint Advanced Materials and Structures Center of Excellence
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Aluminum
114.3 mm (4.5 in)38.1 mm (1.5 in)
3.175 mm (0.125 in) Hysol EA-9394
Crack tip
P
DCB Test Result for Hysol EA9394 by LEFM
80
90
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140
0 1 2 3 4Adhesive thickness (mm)
Failu
re lo
ad (N
)
0
300
600
900
0 1 2 3 4Adhesive thickness (mm)
Ener
gy re
leas
e ra
te (J
/m^2
)
22The Joint Advanced Materials and Structures Center of Excellence
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0.2 mm
d α
r0
1
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7
8
0 0.5 1 1.5 2
Adhesive thickness (mm)
CTO
A (D
egre
e)CTOA as Fracture Criterion in
DCB
Measured CTOA
23The Joint Advanced Materials and Structures Center of Excellence
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Stress Decreasing
A
B
-100
0
100
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300
400
500
600
0 5 10 15 20 25
Overlap (mm)
Nor
mal
Stre
ss (M
Pa)
Fracture Path in Single Lap Joint
24The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue UniversityPurdue Univeristy
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.2 0.4 0.6
Adhesive thickness (mm)
Ener
gy R
elea
se R
ate
(J/m
)
GIGII
Embedded crack length: 0.3-0.7 mm
Location: 0.05 mm above the lower interface
Embedded Crack for Failure Prediction in Single Lap Joints
Mode I dominated
25The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
CTOA under Failure Load for Single Lap Joints
• CTOA is independent of adhesive thickness before failure mode change
0
1
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0 0.5 1 1.5 2
Adhesive Thickness (mm)
CTO
A (D
egre
e)
26The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Project II: Conclusions to Date
• LEFM is not suitable for predicting fracture in DCB adhesive specimens because of large plastic zone relative to the K-dominance zone size
• A single CTOA value can be used to predict fracture in DCB specimens with different adhesive thicknesses.
• Failure loads of lap joints can be predicted using the CTOA measured with DCB specimens in conjunction with an assumed crack embedded near the adhesive/adherend interface.
27The Joint Advanced Materials and Structures Center of Excellence
Purdue UniversityPurdue University
Project II: 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– 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 models to
predict general bonded joint strength– Extend the CTOA fracture criterion to include bonded plates or shells under general
loading conditions– Conduct experiments to verify the proposed fracture criterion