Typical bonded joints

Figure 29.2 shows a range of bonded joints and possible modifications which can be made to reduce or eliminate the effect of cleavage and peel stresses.

The following notes should be regarded as of a general nature.

(a) Avoid butt joints if bond area is small.

(b) Scarfed joint provides increased bonding area.

(c) Simple lap joint with in-line forces.

(d) Alternative lap joint with offset loading.

(e) Tapered lap joint.

(f) Bracket bonded to a fixed surface where peel is likely.

(g) Repositioned bracket strengthens joint.

(h) and (j) Cleavage loading eliminated by the addition of a component in shear.

(k) and (l) Simple improvement for safety. (m) and (n) Increase in bond area reinforces corner joint.

Quite obviously practical considerations involve a study of the forces applicable and acceptable appearance of the finished assembly.

Figure 29.4 shows two tubular applications. In (a), a cylindrical plug is used to join two tubes in a structure. An example of a tapered tubular joint is given in (b). The taper ensures that the adhesive is not pushed out of the assembly.

The joint permits a long bond line and does not impede fluid flow.

A selection of bonded stiffeners are shown in Fig. 29.5. These can be used to reduce vibration and deflection of thin sheet materials. When the flanges on the stiffened sections do deflect with the sheet, little difficulty from peel results due to the area of the bond. Corrugated backings can provide complete flatness over the entire area. If a corrugated insert is sandwiched between two flat sheets (the second sheet is indicated by dotted lines) as indicated in example (a) then a structure of lightweight and high strength can be manufactured from adhesive bonding. There are many aircraft applications. Standard strip, angles, tee sections and formed channels are used in structural engineering.

The types of adhesive which cover the vast majority of engineering assembly applications come from the following categories.

1 Epoxies Two components are mixed in equal proportions. The adhesive and the hardener begin to cure immediately and have a usable 'pot life'. After this time the bond becomes less effective. Often used for DIY repairs. Industry uses an alternative type of epoxy which incorporates rubber of low molecular weight and is called a toughened adhesive. It has greater resistance to impact forces and peel.

This is a single component epoxy which is hardened by heat curing while the parts being bonded are clamped.

Used to bond composite materials, tubular frames and in the manufacture of components for double glazing assemblies.

2 Acrylic adhesives Four basic types:

(a) Toughened acrylics. These are two-part systems where a hardener and an adhesive are applied to the two surfaces being joined and the assembly of the joint automatically mixes them. Can be used on oily steel. Will bond glass into metal frames. Also used in railway carriage interior panels.

(b) Cyanoacrylate adhesives polymerize (solidify) by a chemical reaction which is usually initiated by atmospheric moisture, present as traces of water on the surfaces to be joined. Successful bonding depends upon ambient humidity, the type of material being bonded, the choice of adhesive, and the nature of the surface.

'Instant adhesives' and 'Superglues' are in this range of products.

Poor design

Improved design

Fig. 29.2 Typical bonded joints

Fig. 29.3 Where slotted joints are used, tapering removes the high stress concentrations caused by abrupt changes in section. Example gives a possible modification to Fig. 29.2(n)

(a)

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(c) Anaerobic adhesives automatically harden in the absence of air and are used mainly in rigid metallic joints. Many applications follow. These products are manufactured normally as single component materials.

(d) UV curing acrylics are single component adhesives where cure is effected rapidly by placing the assembly under an ultra violet lamp.

These adhesives are applied in the manufacture of printed circuit boards for surface sealing.

3 Hot melt adhesives are available in rod, sheet and powder forms. A convenient method of assembling small components which are lightly loaded. A heating gun raises the temperature of the rod and the adhesive is applied to one component. On cooling, the adhesive solidifies and the two surfaces are bonded together. These adhesives are also used in packaging equipment.

4 Solvent based contact adhesives. Here the adhesive is applied in a solvent solution to the two surfaces. The solvent evaporates leaving a tacky film and the surfaces are brought together. Applications include laminated sheet fixings in furniture manufacture.

A considerable range of options is available to the designer in the choice of suitable types of adhesive.

Precision measuring and dispensing is necessary so that the required volume, in the defined position, is applied at a given time and with consistently repeatable accuracy on a production line.

In the interests of satisfactory selection and operation, it is recommended that the manufacturer should be consulted to ensure that all technical considerations have been included in a proposed scheme.

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