Bonded Joints Presentation

8/2/2019 Bonded Joints Presentation

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Kevin Potter 2012 1

Adhesively Bonded Joints

Between Composites

Kevin Potter

Kevin Potter 2012 2

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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Kevin Potter 2012 3

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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Kevin Potter 2012 5

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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Kevin Potter 2012 9

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


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


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


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


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


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


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


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


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?)


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.


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


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

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Bonded Joints Presentation