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Kevin Potter 2012 1
Adhesively Bonded Joints
Between Composites
Kevin Potter
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Advantages of bonded joints
Load distributed over a large area
High joint efficiency (Strength/weight ratio)
Low part count
No holes in the basic laminate
Potential for low cost manufacture
Dissimilar materials can be bonded
. without corrosion problems
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Air and water tight to some degree, although
not necessarily perfect
Fatigue lives of bonded construction tend to
be good due to the flexibility of the adhesives
With good design the joints can retain a high
level of residual strength after initial cracking
A bonded joint is generally aerodynamically
better than a rivetted joint
Advantages of bonded joints
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Disadvantages of bonded joints
Difficult to inspect non-destructively
Very sensitive to peel loading - must be avoided
Good bonds require a good fit of parts
Usually permanent and cant be disassembledStrength can be affected by temperature / humidity
Surface preparation is critical to a good bond
Clean room conditions may be called for and the
curing process must be closely controlled
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Bonding can be expensive due to labour intensive
production
Most aerospace adhesives require high cure
temperatures (>180C) that may damage some joint
materials
With poor design the joints can fail suddenly
Adhesive layer may act as an electrical insulator
causing problems when panels must be in electrical
connection , e.g. for lightning strike.
Difficult to arrive at a realistic strength by analysis
Disadvantages of bonded joints
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Molecular energy vs. interatomic distances for
different types of bond
Van der Waals
Hydrogen Bond
Bond
Energy
Interatomic Distance
Fundamentals of adhesion 1
Covalent Bond
Ionic Bond
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Repulsion
Attraction
Resultant
Energy Interatomic
distance
Attractive and repulsive forces and the resultant
for a typical molecule.
Fundamentals of adhesion 2
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Typical
Critical surface energies and wetting:
Adherend Material Typical Adhesive
PTFE 18 (mN/m) 30 - 47
PVC 40
Polyamide 46
Iron 2030
Tungsten 6800
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Adhesive types
Epoxides Primary aerospace adhesives in film and paste form
Polyurethanes Can give very high toughness systems
Cyanoacrylate Rapid cure but very brittle and low peel strength
Anaerobic Generally retention and threadlocking types
Reactive acrylic Tough and fast for automotive uses
Phenolic (Redux) The first structural adhesive for aircraft, still used
Evaporative Solvent based glues, may be used on aircraft interiors
VHB Tapes High strength double sided tape
Hot melt Higher strength and curing variants are now used
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Adhesive toughening
Crack-stopping elastomer microspheres in adhesive
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Basic joint typesSingle lap
Double lap
Scarf
Bevel
Stepped lap
Butt strap
Double butt strap
Butt
Peel
Grout
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Deformations and shear stress distribution in a single-lap bond
Totally rigid adherends -
uniform shear stress
Elastic adherends -
shear stress concentration
at ends of joint
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Effect of yield on joint stresses
For most adhesive formulations used in structural
applications (usually epoxies) the adhesive yields prior to
failure. This yielding leads to a change in the stress
distribution shown previously and makes the shear stress
more uniform across the joint.
A. Prior to yielding B. Yielding established C. Yielding complete
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Transverse (peel) stresses
For single lap joints there is an
eccentricity in the load line that leads
to distortion of the adherend geometry
and very high transverse tensions (or
peel stresses) at the ends of the joint
For double lap joints this effect is
eliminated, but there are still
transverse stresses at the ends of the
joint due to induced bending moments
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Adhesive failure modes in composite joints
Cohesive failure, wholly
within the adhesive layer, -
preferred
Failure due to induced
through thickness stresses
in the composite - very
common, not ideal
Adhesive failure at
interface - unpredictable
must avoid
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Recommended Fibre Orientations
The fibres in the layer immediately
adjacent to the adhesive layer
must be aligned in the same
direction as the load path.
If the fibres are perpendicular to
the load then premature failure
WILL occur.
If there are multiple potential
loading directions use a plain
weave cloth as the surface ply
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Design details, dos and donts
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Assembly issues 1. TolerancingRecommended bondline thicknesses range from
about 0.1mm to 1mm, although thicker bondlines are
sometimes used.
This can create problems with tolerances
All dimensions
need to be
tightly
controlled
Dimensions of
each part can
be relaxed
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Assembly issues 2. bondline controlWithout bondline control joints can be geometrically distortedor become voidy and defective
1. Use film adhesive, gives a thin and
well controlled bondline, so long as
the surfaces being bonded are flat
and parallel. Pressure needed
2. Use internal spacers in centre ofbondline, gives a thicker bondline but
some lack of flatness is OK.
3. Use fully tooled bonding jig to directly
control bondline and fillet geometry
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Detail design considerations
Adherend Materials: strength, stiffness, thermal and moisture expansion
coefficients, surface treatment.
Adhesive properties: strength, ductility and toughness.
Static, fatigue, shock and creep loading.
Environment and aging.
Analysis methods: Hand calcs, computer based methods
Variability: bond defects, surface prep, manufacturing tolerances
Inspectability
Testing
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Steel
CRFPCl
Effect of end detail on strength of steel/CFRP double lap joints
Fail load (kN/mm)
Actual Theory
0.93 1.05
0.89 1.08
0.94 1.10
---- 2.0
3.05 3.3
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Max 290MPa
Stress in through thickness direction in the CFRP for Linear FEA model
Reason for improvement in strength from geometryMax 12.3MPa
Max peel stress
12.3MPa, fails in
tension in adhesive
Max peel stress for same end
load 290MPa, fails in through
thickness tension in laminate.
In real world adhesive
yielding reduces peak stress
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Test methods
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Peel tests
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Bondline thickness 0.1 to 0.5mm
Minimise peel loads & stresses
Bond length > 30 mm
Ultimate Load/unit width < 1 kN/mm
Use stepped-lap joints for thick adherends
Use internal end-chamfer & fillets
Use Finite Element Analysis (FEA) computer. modelling to refine the geometry if needed
Recommended design practice
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Bonded joints can be very strong, but this strength is critically
dependent on surface preparation, and even minor amounts of
contaminants such as oil can destroy the bond strength.
Surfaces must always be clean and dry prior to bonding.
Surfaces are often abraded or grit-blasted prior to bonding
The use of peel ply without secondary abrasion may prove to
be ineffective
Metallic surfaces may be acid etched or subjected to other
chemical or physical pre-treatments prior to bonding.
Surface Preparation
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Estimation of joint strength 1
So long as the adhesive can yield a reasonable first
estimate ofmaximum possible joint strength can be got
from the area of the joint and the yield stress in shear.
This assumes that the joint is fully yielded prior to failure and
does not fail from through thickness tension or by adhesion
failure, or by tension in the adhesive before complete
yielding.
This may be OK for short (
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Estimation of joint strength 3
Achieving a good prediction for the strength of a bonded
joint is really rather complicated, requiring detailed stress
analysis and the application of a validated failure criterion.
This is generally beyond the capability of most
organisations, and in any case there is no universally
accepted way of carrying out the prediction.
Making test joints representative of the real joint and
testing them under the same loadings and environmental
conditions as the real joints may be the best we can do in
many cases then apply a safety factor
(but what controls the safety factor?)
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Fatigue effects 1
0
10
20
30
40
50
60
0 2 4 6
Log cycles to failure
Max
averageshear
stress
MPa
Fatigue performance for well made composite double lap joints
Dotted lines are 95% confidence limits on performance
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Fatigue effects 2
To confidently predict fatigue life a good consistent set of
fatigue data must be available and ..
Any changes in failure mode under fatigue loading at the
endurance of interest must be known and understood.
The fatigue test environment must be an accurate reflection of
the use environment so that there is a direct
correspondence between fatigue life and operational life.
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Load/lifetime curves for different failure modes, showing that differing fatigue
degradation rates for different failure modes can lead to changes in the expected
fatigue failure mode
Mode 4
Mode 3
Mode 2
Mode 1
Mode 1
Mode 2
Mode 3Mode 4
Load
Log cycles to failure
mode 1, adherend failure,
mode 2, cohesive failure,
mode 3, peel failure
mode 4, adhesive interface failure,
Fatigue effects 3
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Environmental effects, temperature and moisture
Test temperature Deg C
Average
shear
stress MPa
Unexposed
90%RH 9 weeks
5
10
15
20
30
35
40
45
50
-60 -40 -20 0 20 40 60 80 100
DLS, exposed. BU data
UD CFRP adherends
Single lap shear 3M data
CFRP cloth reinforced adherends
The single lap shear
joints are more affected
by low temperatures
than the DLS joints due
to bending/peel effects
on the increasingly
brittle adhesive, but are
similar at high
temperature.
The adhesive was 3Ms
EC3448 paste. Other
adhesives would
behave in different
ways
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Results of testing bonded I beams3 point bending (see photo)
1st trial, steel support beam yielded2nd trial, failure at 47Tons
4 point bending: Failure at 63Tons
Failure was probably in the adhesive
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Conclusion
It is possible to make reliable high strength bonded joints withcomposite adherends, BUT.
Through thickness (peel) failure in the laminate is critical and
must be avoided
Interface failure must be avoided by good surface preparation
The effects of the use environment must be accounted for
Simple and fully validated strength prediction methods are not
available and some testing will generally be required in support of
design