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

Some common adhesive systems

type

epoxy acrylic poly urethane silicone

form

paste/liquid

cure temp ( o C)

ambient

max service temp ( o C)

80-100 paste/liquid film paste/liquid liquid paste up to 200 120-170 ambient or elevated ambient or elevated ambient 120-160 120-180 120-180 80 260

advantages disadvantages

easy mixing; gap filling controlled thickness; good environmental resistance fast setting; good environmental resistance high peel strength; gap filling high peel strength; good toughness low peel strength; moderate environmental resistance improved environmental resistance refrigerated storage; brittle limited pot life moisture sensitive poor shear strength

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

30 20 10 0 80 70 60 50 40 reference joint

Joint shear strength (MPa)

small fillet large fillet tapered adherend 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

F. Matthews, in Handbook of Polymer Composites for Engineers

• 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

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, 10 6 cycles can be expected if peak stress < 25% of static strength (lap joints), or < 50% (scarf joints)

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