BONDING OF COMPOSITES STRUCTURES
Strukturlabor
by Alberto Sánchez Cebrián
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 1
Outline
o State of the art
• Types of joints
• Riveting vs. adhesive
• Types of adhesive
o Materials
• Adhesive
• Composites
o Influencing parameters
•
•
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•
•
•
•
Design
Surface treatment
Manufacturing process
Thickness of bondline
Curing process
Quality of the adherent
Thickness of adherent
o Failure modes
o Quality control
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 2
Background
Bolted
Temporary
Screw
Riveting
Pressure
Permanent
Welding
Adhesive
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CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 3
Background
Temporary
Bolted
Expensive
Screw
Expensive
Riveting
State of Art
Pressure
No plates
Welding
No CFRP
Adhesive
FUTURE?
Permanent
Riveting is commonly used
due to simplicity and robustness
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 4
Background
o Adhesive bonding
Uniform load distribution
Light weight & Cheap
Less components
Rigid, sealed & isolated unions
Difficult to qualify
Curing process required
Load and temperature limits depend on adhesive, toxic material.
Surface treatment needed
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 5
Background
o Film adhesives –SoA-
o Paste adhesives
• Pressure needed
• Do not require pressure
• Typically all assembly is heated
• Wider tolerances
• Narrow tolerances
• One or two part systems
• One part system
• More difficult to apply
• Low void content
• Sensitive to void content
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 6
Materials
o Adhesive bonding is a permanent joint type.
o Two elements can be observed:
o Adherent: Material to be bonded.
o Adhesive: Material used to bond adherents.
o Stresses in a joint
Opening
In plane shear
Out-of-plane
shear
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 7
Materials: Adhesive
o Main providers:
• Henkel
• 3m
• Huntsman
o Characteristics to observe in a paste adhesive:
• Mixing ratio: Ratio of Epoxy and hardener to mix. Only in two paste adhesives.
• Pot life: Time after mixing to get certain green strength.
• Curing process: Temperature and time to carry out a complete curing process.
* For aerospace more that 95% curing is required.
• Mechanical performance in different materials and conditions.
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 8
Materials: Composites
Typical composite processing methods.
• RTM: Resin is injected into a preform sealed inside of a mold.
+ Does not require the curing in an autoclave.
- Processing is complex and expensive.
• Prepreg: Layers of composite layers are layered up on an opened mold.
+ Quality and mechanical performance is typically higher
- Curing is typically carried out in autoclave.
• Wet lamination: Impregnation of the composite is hand made.
+ Cheap and simple
- Quality is typically low.
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 9
Materials: Composites
Contact between mold and material:
• Release film
• Peel ply
• Release agent
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 10
Influencing parameters
o Design: A good design can improve allowable load.
o Surface treatment: Cleaning of CFRP is critical.
o Thickness of bondline: Best performance between 0.1-0.5 mm.
o Manufacturing process of CFRP: The manufacturing process can mean
another contamination source.
o Curing process: Fast curing can produce degradation.
o Quality of the adherent: Bad quality means an earlier delamination.
o Thickness of adherent: Thinner adherent means bigger bending moment.
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 11
Influencing parameters: Design
Adhesive joints of CFRP have a non linear
behavior. Despite they can be approached
to a traditional model, the prediction is
very complex due to the variability of the
parameters such as:
• Porosity and local disbonds.
• Thickness variations.
• Benefits from spews at the end of the
joint.
• Plastic – elastic behavior.
• …
Hart-Smith, L. J., Analysis and Design of Advanced Composite Bonded Joints, NASA CR-2218, 1974.
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 12
Influencing parameters: Surface treatment
30
Shear strength [Mpa]
RF: Release film
25
15
C: Cleaning with
acetone, water
and de-ionized
water. Then oven
drying for 60
minutes.
10
SXXX: Size of the
sand paper
20
GB: grit blasting
5
WJ: Water-jet
Pp: Peel ply
0
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CENTRE OF STRUCTURE TECHNOLOGIES
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29.10.2013, page 13
Influencing parameters: Manufacturing process
Shear stress [MPa]
30
25
20
15
10
5
0
Release film no
cleaned
Release film with Release agent no
acetone
cleaned
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Release agent
with acetone
Peel ply
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 14
Influencing parameters: Surface treatment
In previous research, it was proven that the surface treatment achieving
higher shear strength and lower scattering with CFRP composites was:
1. Mechanical treatment:
1.1. Sanding P100
1.2. Sanding P150
2. Cleaning:
2.1: Acetone cleaning
2.2. Water cleaning
2.3. De-ionized water cleaning
2.4. Oven drying
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
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Influencing parameters: Thickness of bondline
o Effect of bondline and adherent thickness on SLS performance
26
Shear strength [MPa]
24
22
20
18
16
14
Adherents thickness 1 mm
Adherents thickness 2 mm
Co-curing
12
10
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Bondline thickness [mm]
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
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Influencing parameters: Curing process
o Effect of fast curing in SLS performance
Shear strength [MPa]
30
25
20
15
10
5
0
80
100
120
140
160
180
200
Temperature [°C]
* A very thick bondline can produce a exothermal reaction.
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
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29.10.2013, page 17
Influencing parameters: Quality of the adherent
o Quality of the adherent can be assessed with the measure of
the roughness that can detect fracture of fibers.
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 18
Influencing parameters: Thickness of adherent
o Adherent thickness influence on SLS performance by MTM 44-1
OoA CFRP prepreg.
26
Shear strength [MPa]
24
22
20
18
16
14
Adherents thickness 1 mm
Adherents thickness 2 mm
Co-curing
12
10
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Bondline thickness [mm]
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 19
Failure modes
Cohesive
Adhesive
Adherent
Mixed
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 20
Failure modes
I. Cohesive: Fracture in the middle of the adhesive surface.
• Acetone cleaned samples tested in DCB
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 21
Failure modes
II. Adhesive: Fracture between one adherent and adhesive.
• Non treated sample tested in DCB.
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
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29.10.2013, page 22
Failure modes
III. Adherent: Fracture is located in one of the adherents.
• Grit blasted sample tested for DCB.
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CENTRE OF STRUCTURE TECHNOLOGIES
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29.10.2013, page 23
Failure modes
IV.Mixed: Different modes can be observed.
•
Peel ply sample tested for DCB.
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 24
Quality control
o
One of the biggest drawbacks of paste adhesive is quality control.
o Control of the adhesive: The assessment of a proper curing process is critical. For structural
applications in aerospace 95% curing is required. In this context, Differential scanning calorimetry
(DSC) is carried out to ensure that the process is complete.
o Control of the joint:
• Destructive testing is based in different mechanical testing.
- Single lap shear
- Wedge and double cantilever beam
- Peel test
- 3 or 4 point bending
• Non destructive testing Some techniques can be used to detect voids, but its use for
qualification is not standardized for paste adhesives.
– Ultrasounds
– X-Rays
– …
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
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29.10.2013, page 25
Quality control: Control of the adhesive
o Modeling the curing kinetics permit to predict the relation
between degree of cure, temperature and time.
1.0
5
0.9
0.8
T2 = 211 °C
3
T1 = 28 °C
0.7
Degree of cure [-]
Endothermic heat flow [mW]
4
2
1
Overall heat = 260.97 [J/g]
0
0.6
0.5
Experimental 80 °C
Model 80 °C
Experimental 100 °C
Model 100 °C
Experimental 120 °C
Model 120 °C
Experimental 140 °C
Model 140 °C
0.4
0.3
-1
0.2
-2
0
50
100
150
200
250
300
Temperature [°C]
0.1
0.0
0
𝑑𝛼
= 𝑘 · (1 − 𝛼)𝑛
𝑑𝑡
with  = Degree of cure [-]
5
10
15
20
25
30
Time [min]
t = Time [s]
k = Reaction rate [-]
n = Reaction order [-]
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CENTRE OF STRUCTURE TECHNOLOGIES
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Quality control: Control of the joint
Single lap shear
Fracture toughness (DCB)
EN 2243-1
www.structures.ethz.ch
ISO 15024
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 27
Quality control: Control of the joint
Peel test
Bending tests: 3 or 4 point bending
EN 2243-2 or ISO 4578
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 28
Quality control: Control of the joint
Void content [%]
o Effect of fast curing in porosity levels of the paste adhesive.
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
80
100
120
140
160
180
200
Temperature [°C]
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
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29.10.2013, page 29
Quality control: Control of the joint
o Effect of fast curing in size of voids of the paste adhesive.
Void average diameter [μm]
600
500
400
300
200
100
0
80
100
120
140
160
180
200
Temperature [°C]
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 30
Quality control: Control of the joint
o Effect of fast curing in SLS performance
Shear strength [MPa]
30
25
20
15
10
5
0
80
100
120
140
160
180
200
Temperature [°C]
www.structures.ethz.ch
CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 31
Questions??
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CENTRE OF STRUCTURE TECHNOLOGIES
Alberto Sánchez Cebrián
29.10.2013, page 32