Transcript Strengths of Adhesive Joints

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The final test for any adhesive is that it
should give joints which are strong and
durable.
Although ways do exist of assessing the
quality of joints by ultrasonic nondestructive testing, the ultimate test is to
measure the force or energy needed to
break a joint.
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Many types of joints are available and
illustrated in thenFigure are single and
double laps, cylindrical butts, and 90˚ peels.
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There are three principal modes of fracture:
 Mode I is due to peel or cleavage forces.
 Mode II is a shearing mode,
 Mode III is a shearing mode but here shearing is
in torsion around an axis instead of along a
plane.
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In general, rigid adhesives are strong in shear
but weak in peel, whereas
Rubbery adhesives are resistant to peel
but creep in shear.
Rubber toughening of modern structural
adhesives improves their peel strength.
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Important considerations:
(i) Size of the adherends and amount of overlap.
(ii) Control of the thickness of the adhesive layer . This can be done by the use
of jigs, or by adding small glass spheres (Ballotini) or incorporating wires
(fuse wire or fishing line). Commercial film adhesives may contain knitted
or woven fabrics known as carriers (UK) or scrims (USA). Stronger joints are
obtained with thin glue-lines; optimum practical glue-line thickness would
be 0.10-0.15 mm.
(iii) Conditions of cure such as time, temperature, application of pressure.
(iv) Ageing of joints prior to testing, e.g. in ambient or hot and humid conditions.
(v) Joint testing conditions are most commonly ambient temperatures and
humidities and in a mechanical testing instrument.
constant crosshead speed, usually of a few mm per minute with single lap
joints, slipping of the adherends in the jaws can mean that the set
crosshead speed is greater than the rate at which the joints are strained. In
hydraulic instruments a constant loading rate (kN min - ') can be used.
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Failure can be by
 interfacial/ adhesive failure,
 cohesive failure of the adhesive, or
 failure of an adherend.
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In some cases there is a mixture of failure
modes.
Interfacial failure indicates that an improved
surface treatment is needed, and
if failure is cohesive the adhesive may need
strengthening with a mineral filler.
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A simple view might be that strength will be
proportional to area but this is not the case.
Wang, Ryan and Schonhorn measured the
strengths of some joints in aluminium etched
in chromic acid and bonded with an epoxide
adhesive
with anis aliphatic
amine hardener.
strength
independent
Strength was
of proportional
bonded area.to joint width,
but a plot of strength against overlap tended
to level out as overlap increased.
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The stress in each adherend falls to zero at
the free-end of the overlap, and hence the
strain decays in a proportionate manner.
The mean shear stress is 8.96MPa, but this is
concentrated to give a
maximum of 96.5MPa very near the ends.
The central region bears no loads
Peeling a flexible tape from a rigid substrate, to
which it had been bonded using a flexible
adhesive.
 The peeling force P is assumed to produce a
steady rate of peeling.
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Strength improvement obtains by:
Kaelble’s
treatment
assumes
that
theY;tape is
(i) increasing
adhesive
flexibility, i.e.
reducing
increasing
thepoint
modulusO,
of the
tape E;
pivoted(ii)about
the
such
that there is a
(iii) increasing tape thickness;
cleavage
tothethe
rightof of
and a compressive
(iv) force
increasing
thickness
theO,
adhesive.
force just to the left.
where m is the sum of cleavage moments and I is the moment of inertia of
the tape section.
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The Boeing wedge test :
Two stiff adherends are bonded together, leaving a
non-bonded section at one end; inserting a film of
polyolefin or PTFE can be useful here.
A metal wedge is forced into this to initiate a crack.
The joint is then exposed to some hostile condition
such as warm, wet air, and the increase in crack length
is measured.
It is particularly useful for examining the effect of
surface treatments on wet-durability.
Crack length can be measured by holding the sample
up to light and using a plastic ruler.
Tack is the ability to bond under conditions of
light pressure and short time, and can be
measured by
 the time needed for a ball or cylinder to roll
down an inclined plane coated with the
adhesive, or by a probe method.
 Here a probe is lowered at a constant speed onto
the adhesive coated surface, and, after a fixed
dwell time, the force needed to remove it is
measured.
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tack force increases with the critical
surface tension of the probe surface, and
with dwell time
It is best to report the strength of a lap joint as the
force needed to break it in newtons, at the same
time specifying the joint geometry.
 Example:
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 The adherends were of aluminium alloy, which had been
degreased and etched in chromic acid, and bonded with
an epoxide adhesive into 25mm square lap joints, which
were cured for 3 h at 80 ˚C. They were tested at a
crosshead speed of 6mmmin-', and all failed cohesively.
 Joint strengths (kN):
17.3, 18.7, 15.8, 20.4, 17.8, 20.4, 14.2, 15.8.
Mean = 17.5 kN. Standard deviation = 2.2 kN or 12%.