Adhesive bonding

The need for joining

• Composites often offer a reduction in parts count, since more complex geometries can be manufactured than in other materials.

• However, separate manufacture and assembly may be more economic in some cases.

• Many processes have a clear size limitation.• Joining may required for access or

replacement.

Adhesive bonding

Advantages• Sealing as well as

joining.• Better in fatigue

loading.• Good damping. • Low weight penalty.

Disadvantages• Disassembly is

difficult/impossible.• Surface preparation

required.• Accurate assembly may

be required.• NDT difficult.• Possibly sensitive to

environment.

Mechanical fastening

Advantages• No surface

preparation.• Disassembly possible• Simple inspection.• Better for thick

sections.• No ‘chemistry’.

Disadvantages• High stress

concentration at hole.• Additional weight• Loss of smooth

surface.• Possible galvanic

corrosion.

Both types of joint result in stress concentrations:

fasteners in shear; local stress concentrations around hole

peel in adherends: shear and through-thickness tension in adhesive - highest at joint edges

Peeling stresses should be minimised, as composite laminates are weak in through-thickness tension:

W. Lees, Adhesives and the Engineer.

Basic bonded joint types

type form curetemp (oC)

maxservicetemp (oC)

advantages disadvantages

epoxy paste/liquid ambient 80-100 easy mixing;gap filling

low peelstrength;moderateenvironmentalresistance

paste/liquid up to 200 120-160 improvedenvironmentalresistance

film 120-170 120-180 controlledthickness;goodenvironmentalresistance

refrigeratedstorage; brittle

acrylic paste/liquid ambientorelevated

120-180 fast setting;goodenvironmentalresistance

limited pot life

poly-urethane

liquid ambientorelevated

80 high peelstrength; gapfilling

moisturesensitive

silicone paste ambient 260 high peelstrength; goodtoughness

poor shearstrength

Some common adhesive systems

How can the stress distribution in adhesive joints be modified?

• Increase the adhesive thickness

• Select adhesive with different modulus

• Increase joint length

• Modify adherend geometry

Reduction of adhesive shear stress with thicker adherends

Effect of adhesive mechanical properties on stress distribution

Asymmetric shear stress distribution with dissimilar adherends

Effect of adherend thickness on joint strength depends on geometry

F Matthews (ed), Joining of Fibre -Reinforced Plastics

Joint shear strength (MPa)

0

10

20

30

40

50

60

70

80

reference joint small fillet large fillet taperedadherend with

fillet

Permabond, 2001

Design approaches - short term, static load:

• Maximum adhesive strain must not exceed strain to failure of the adhesive.

• Overlap length should be as small as possible (for structural efficiency).

• Strain should be constant across the joint length (to make best use of adhesive area).

• Maximum strain should not be sensitive to small changes in joint length.

Design approaches - long term, fatigue load:

• Non-uniform stress distribution is desirable.

• At edges, maximum adhesive strain must not exceed allowable strain value (in fatigue).- more than 50% of the joint length should be stressed below 10% of the elastic limit of the adhesive.

Eckold, Design and Manufacture of Composite Structures, Woodhead (1994)

Eckold, Design and Manufacture of Composite Structures, Woodhead (1994)

Eckold, Design and Manufacture of Composite Structures, Woodhead (1994)

General design considerations

• Shear and peel stresses are highest at the ends of the overlap, and a minimum in the centre of the joint.

• Shear stress is essentially constant along a scarf joint.

• Maximum stresses are reduced by:- using identical adherends- using highest possible laminate stiffness- using longest possible overlap- using low modulus adhesive

F. Matthews, in Handbook of Polymer Composites for Engineers

General design considerations

• Taper the adherends to reduce maximum peel stress.

• Calculate bond shear strength to be 50% above adherend strength (reserve for environmental and fatigue effects)

• Choose ductile adhesive (higher static and fatigue strength.

• Ensure that minimum adhesive shear stress < 10% of maximum (allows creep strains to recover)

General design considerations

• Use ‘homogeneous’ rather than ‘blocked’ laminate stacking sequence

• In fatigue, 106 cycles can be expected if peak stress < 25% of static strength (lap joints), or < 50% (scarf joints)

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