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Use of flexible polymers
(adhesives) in the design
of aerospace applications
F KADIOGLU
University of Turkish Aeronautical Association
25-26/09/2014
Frame of the presentation

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Why polymer matrix composite
materials?
Some basic information about
adhesives & adhesive joints
Two examples
Why polymer matrix composite materials?
Polymer Matrix Composite Materials

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Composites can be very strong and stiff, yet very
light in weight, so ratios of strength-to-weight
and stiffness-to-weight are several times greater
than steel or aluminum
Fatigue properties are generally better than for
common engineering metals
Toughness is often greater too
Composites can be designed that do not corrode
like steel
Possible to achieve combinations of properties not
attainable with metals, ceramics, or polymers
alone
Disadvantages and Limitations of
Composite Materials
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Properties of many important composites are
anisotropic - the properties differ depending on
the direction in which they are measured – this
may be an advantage or a disadvantage
Many of the polymer-based composites are
subject to attack by chemicals or solvents,
temperature, humidity, just as the polymers
themselves are susceptible to attack
Composite materials are generally expensive
Manufacturing methods for shaping composite
materials are often slow and costly
Applications
Joining techniques of composites
Rivet joint
Adhesive
joint
Types of adhesive joints
(a) Single Lap
(b) Double Lap
Metal
Cfrp
Metal
(c) Butt Strap
(d) Step
(e) Bevel
(f) Scarf
Cfrp
Definition of an adhesive

An adhesive is defined as a
substance capable of holding
materials together by surface
attachment.

Structural Adhesives: usually reckoned to
be those with a high strength (50 MPa and
upwards) and (these days), a strain to failure
of at least 10% in tension, and which usually
have a tensile modulus of 2 GPa or so

Flexible Adhesives: significantly less stiff,
less strong, but much more ductile
Advantages
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Joining dissimilar materials (dissimilar
thin sheets and foils)
Improvement in the appearance of the
finished assembly by the elimination of
irregular surface
the fabrication of complex shapes
Relatively uniform distributions of stress
over the entire bonded area
Improving fatigue resistance and giving
good vibration damping
………
Disadvantages
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residual stresses in bonded joints
(different thermal expansion)
curing requirement
sensitivity to peel loading, creep
etc…
degradation of the adhesive caused
by heat, cold, chemical agents,
radiation, and bio-deterioration
not easily dismantled for repair or
salvage
assessment of bond quality
The most important parameter! Surface
pretreatment


Chemical solutions to clean the
surfaces
Surface modification methods
Loading types
Good
(shear)
Poor
(tension)
Worst
(cleavage)
Failure mechanisms

Adhesive failure


Cohesive failure

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Poor surface treatment (failure in
adhesive-adherend interlayer)
able to get mechanical performance of
an adhesive (failure in adhesive or in
adherend)
Adhesive-Cohesive failure

usually due to the adherend yielding
Design- Predictions
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Analytical Analysis
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Volkersen’s elastic adherend model
Goland & Reissner bending model
Numerical Analysis

FEM (the most effective tool)
Problems with analytical analysis
Volkersen’s elastic
adherend model


Goland & Reissner
bending model
P
P
x
ð
Shear
Stress
x
Transverse
(or Peel)
Stress
ð
Reality and three-dimensional stress
analysis
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differential straining of the adherends
causing a shear stress distribution in the
adhesive which is a maximum at the ends
of the overlap (Volkersen (1938))
offset loading of the lap joint which
causes the loaded adherend to bend
adjacent to the overlap region (Goland
and Reissner (1944))
end effects such as the adhesive free
surface, spew, and material and
geometric discontinuities which all affect
the stresses at the overlap ends (Crombe
and Adams (1981))
Poisson’s ratio effects
adherend plasticity
Some precautions to reduce the stress
concentrations at the ends
Metal
Cfrp
Metal
Cfrp
Examples
A
Structural Adhesive: AV119,
A
Flexible Adhesive: 9245
one-part epoxy, manufactured by
Ciba Polymers. cured at 120 C for
an hour according to manufacturer’s
instructions.
Structural Bonding Tape (SBT),
mixure of acrylic and epoxy, cured
at 140 C for 45 minutes according
to the manufacturer’s instructions.
Mechanical behavior
Shear Curve
Tensile Curve
80
70
Shear Curve
Tensile Curve
60
40
20
30
Stress MPa
Stress MPa
25
50
20
10
15
10
0
0
0,1
0,2
0,3
0,4
0,5
0,6
5
Strain
0
0
Structural Adhesive AV119
0,5
1
1,5
Strain
Flexible Adhesive SBT
2
Lap shear performance in quas-static
loading under tensile loading
1.6 mm
Adhesive Layer (0.4 mm)
F
63.5 mm
Overlap Length
Stress distributions in the adhesive
layers under tensile loading

SBT (Flexible
Adhesive)

AV119 (Structural
Adhesive)
80
25
70
20
60
50
SX
10
SY
S XY
5
S E QV
0
0
5
10
15
20
25
S tres s MP a
S tres s MP a
15
40
SX
30
SY
20
S XY
10
S E QV
0
-2
-10
1
4
7
10
13
16
-20
-5
Ove rla p L e ng th m m
Ove rla p L e ng th m m
19
22
25
Comparison of results from tensile
loading

Mild steel
adherends

14
60
12
50
10
8
SBT
AV119
6
4
Failure Load kN
Failure Load kN
Hard steel
adherends
40
AV119
20
2
10
0
0
0
20
40
60
Overlap Length mm
80
100
120
SBT
30
0
20
40
60
Overlap Length mm
80
100
120
A broken adherend from the tensile test
Lap shear performance in quas-static
loading under bending loading
P
P
A
D
C
B
Spew fillet
Adhesive layer
30
20
25
20
All dimensions in mm
30
Stress distributions in the adhesive
layers under bending loading

SBT

AV119
80
25
Stress in the adhesive layer, MPa
Stress in the adhesive layer, MPa
70
20
15
10
5
60
50
40
30
20
10
0
0
0
5
10
15
Overlap length, m m
20
25
30
0
5
10
15
Overlap length, m m
20
25
30
Comparison of curves (results) from
bending loading
1800
1600
1400
Load N
1200
1000
SBT
800
AV119
600
400
200
0
0
5
10
15
20
Crosshead Displacement mm
25
30
A broken adherend UTS= 2000 MPa
SBT stress-strain variation curve with strain-rate variation
at room temperature using a laser extensometer
30
1.93 /min
25
True Stress, MPa
0.25 /min
0.057 /min
20
15
10
5
0
0
20
40
60
True Strain (%)
80
100
120
Comparison of quasi-staic and impact
test
35
Shear Stress MPa
30
Impact test
25
20
15
10
QuasStatic T orsion test
5
0
0
0,5
1
Shear Strain
1,5
2
Vibration set-up dynamic properties of
materials
Oscilloscope
Signal
Generator
Digital
Multimeter
Power
Amplifier
Analogue
Voltmeter
Microphone
Specimen
Bracket with Cotton
Supports
Foam
Foam
Shaker
Damping performance of the adhesives
Adhesive Thickness = 0.45 mm
0.025
0.022
The SLJ Los factor
0.02
0.015
0.01
0.005
0.001
0
SBT
AV119
Adhesives
Benefiting from damping properties of
polymers (Adhesives)
Solid
Sandwich
Conclusions
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Mechanical behavior of materials plays
an important part in the performance
of aerspace structures
Correct design data are vital for reliable
predictions
Aerospace components require careful
design approach
Sometimes relativesly less strong
materials provide much more strong
structural systems (SBT, AV119)
Thank you
!
Dr. Ferhat KADIOĞLU
fkadioglu@thk.edu.tr
University of Turkish
Aeronautical Association
Mechanical Engineering Department