People Courses 24 Strengths of Adhesive Joints

7/30/2019 People Courses 24 Strengths of Adhesive Joints

1/18


2/18

The final test for any adhesive is that itshould give joints which are strong and

durable. Although ways do exist of assessing the

quality of joints by ultrasonic non-destructive testing, the ultimate test is tomeasure the force or energy needed tobreak a joint.


3/18

Many types of joints are available andillustrated in thenFigure are single and

double laps, cylindrical butts, and 90 peels.


4/18

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.


5/18

In general, rigid adhesives are strong in shearbut weak in peel, whereas

Rubbery adhesives are resistant to peelbut creep in shear.

Rubber toughening of modern structuraladhesives improves their peel strength.


6/18

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 areobtained with thin glue-lines; optimum practical glue-line thickness wouldbe 0.10-0.15 mm.

(iii) Conditions ofcure such as time, temperature, application ofpressure.(iv) Ageing of joints prior to testing, e.g. in ambient or hot and humid conditions.(v) Joint testingconditions are most commonly ambienttemperatures 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 setcrosshead speed is greater than the rate at which the joints are strained. Inhydraulic instruments a constant loading rate (kN min - ') can be used.


7/18

Failure can be by interfacial/ adhesive failure,

cohesive failure of the adhesive, or

failure of an adherend. 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.


8/18

A simple view might be that strength will beproportional to area but this is not the case.

Wang, Ryan and Schonhorn measured thestrengths of some joints in aluminium etchedin chromic acid and bonded with an epoxideadhesive with an aliphatic amine hardener.

Strength was proportional to joint width,but a plot of strength against overlap tendedto level out as overlap increased.

strength is independentof bonded area.


9/18

The stress in each adherend falls to zero atthe free-end of the overlap, and hence the

strain decays in a proportionate manner.


10/18

The mean shear stress is 8.96MPa, but this isconcentrated to give a

maximum of 96.5MPa very near the ends.The central region bears no loads


11/18

Peeling a flexible tape from a rigid substrate, towhich it had been bonded using a flexible

adhesive.

The peeling force P is assumed to produce asteady rate of peeling.


12/18

Kaelbles treatment assumes that the tape ispivoted about the point O, such that there is a

cleavage force to the right of O, and a compressive

force just to the left.

Strength improvement obtains by:(i) increasing adhesive flexibility, i.e. reducing Y;(ii) increasing the modulus of the tape E;(iii) increasing tape thickness;(iv) increasing the thickness of the adhesive.


13/18

where m is the sum of cleavage moments and I is the moment of inertia ofthe ta e section.


14/18

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 lengthis measured.

It is particularly useful for examining the effect ofsurface treatments on wet-durability.

Crack length can be measured by holding the sampleup to light and using a plastic ruler.


15/18

Tack is the ability to bond under conditions oflight pressure and short time, and can bemeasured by

the time needed for a ball or cylinder to rolldown an inclined plane coated with theadhesive, or by a probe method.

Here a probe is lowered at a constant speed onto

the adhesive coated surface, and, after a fixeddwell time, the force needed to remove it ismeasured.


16/18

tack force increases with the criticalsurface tension of the probe surface, and

with dwell time


17/18

It is best to report the strength of a lap joint as theforce needed to break it in newtons, at the sametime specifying the joint geometry.

Example: The adherends were of aluminium alloy, which had been

degreased and etched in chromic acid, and bonded withan epoxide adhesive into 25mm square lap joints, whichwere 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%.


18/18

Documents

People Courses 24 Strengths of Adhesive Joints