Engineering adhesives 265
adhesive to cure fully, and must be designed to give
uniform stress. Even in a simple face-to-face joint it
must be possible to apply adhesive to one surface and
for it to remain there until the two parts are brought
together and after that until curing takes place.
These requirements highlight the need for a choice
of thin, thick or thixotropic adhesives. Design details
which may also be significant include removal of sharp
edges and substitution of a bevel or radius.
The bond line
The gap between the parts, and therefore the thickness
of the adhesive film, has an important bearing on the
characteristics of the joint. In terms of simple strength
a thick bond line will generally be a weakening feature,
since the mechanical strength of the unsupported resin
film is likely to be less than that of the substrates.
A thick bond line can however confer advantages.
The adhesive is generally more flexible than the
adherents or substrates. This is particularly so in most
engineering applications where metals or other rigid
materials can be bonded. Because of this, a thick bond
line can offer a capacity to absorb some impact energy,
thus increasing the strength of the bond under this
type of loading.
Consideration of bond line thickness leads
immediately to the question of environmental resistance.
Adhesive bonds will always be susceptible to
environmental attack and it is essential that any such
attack should not reduce the strength of the bond to an
unacceptable level. The most important factor here is
the correct choice of adhesive, but design of the joint
can make a significant difference. Thus a thick bond
line offers a ready path for access by moisture or other
solvents which might be able to diffuse through the
cured adhesive.
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.