Bonding with Elastic Adhesives

Feb. 8, 2001
Elastic adhesives have high peel strength, impact resistance, and flexibility

Philippe Mazet
Marketing Manager, Transportation OEM
Sika Industry
Madison Hts., Mich.

Edited by Victoria Reitz

The photoelastic models of joint assemblies show stress distribution. In the view of a thin-layer rigid adhesive, the expansion and deflection of bonded substrates causes stress peaks at the ends of the overlaps, which is why the adhesive layer begins to break down. The greatest stress in the bolted connection is around the bolt shaft. Stresses in the elastic adhesive bond are uniformly distributed along the bondline, and all of the bond face contributes to the strength.

Elastic bonding is a relatively new fastening technique that complements traditional fastening methods. It joins two materials with a layer of permanently elastic adhesive and offers high peel strength, impact resistance, and flexibility. Use is growing in various industries, most notably in the manufacture of domestic appliances, and road and rail vehicles.

Interest is on the rise because elastic bonding is more forgiving than other adhesive techniques. When an applied force peels or pries apart plastic adhesive-joint faces, the stresses involved quickly reach critical levels. In these cases the applied load is no longer distributed over the whole bond face, but is concentrated along a narrow line at the joint edge. The material's ultimate breaking stress is rapidly exceeded and the bond tears or fails.

Thick-layer elastic-bonded joints, on the other hand, "give" under peeling forces and distribute the load over a wider area. Consequently, stresses within the bonded materials are relatively low. The high tear prop-agation strength of elastomers — even where the adhesive layer has started to tear — prevents sudden and catastrophic joint failure. This forgiving behavior means damaged adhesive joints can be identified and repaired before total failure.

As well as transferring dynamic forces to the joint, the elastic-adhesive layers also act as a sealant, preventing entry of water, salt, or other corrosive media. Adhesive-bonded joints can also easily be made to resist other chemical action.

In contrast to rigid-adhesive joints, elastic-adhesive layers deform under applied loads. This property is extremely useful for damping vibrations or displacing an external force. Exposure to heat, for example, may result in differential thermal expansion, causing adhesive-bonded components to move relative to one another. Elastic adhesives are well suited to join materials with different coefficients of linear expansion. The adhesive is able to deform, although the layer must be thick enough to accommodate that movement.

Elastic adhesives have a lower temperature resistance compared with mechanical fastening techniques. At 100 to 50°C, the adhesives are rigid with stable mechanical properties. Long-term exposure at temperatures above 90°C is not recommended. Temperature resistance is comparable to thermoplastics and thermo-plastic paint systems, and adequate for most applications under normal stress conditions (i.e., outdoor applications with no exposure to any additional or concentrated heat source).

Unlike mechanical joints, adhesive bonds do not immediately reach their maximum or ultimate strength. In many cases, however, assemblies can be handled and passed on to the next stage of processing before they attain their ultimate strength levels. On the plus side, the actual bonding process takes significantly less time than conventional joining methods.

The most convenient way to describe the mechanical properties of adhesive bonds is in terms of the generalized elasticity factor or stiffness. To calculate the spring constant for an adhesive-bonded joint, multiply the stiffness value — a dimensionless quantity — by the known joint dimensions. The adhesive joint can then be represented as a spring element in the overall structure.

Elastic-bonded joints have an adhesive strength in excess of 2 MPa. This is typically defined as the ultimate breaking stress in the tensile lap-shear test. Elongation at break — the relative displacement of bonded components before the joint fails — exceeds 200% of the applied adhesive thickness, while the shear modulus is 1 to 3 MPa. These values rank elastic adhesives between sealants and hard-setting adhesives.

The changes in elastomer strength and stiffness due to temperature and/ or stress can be roughly quantified by applying reduction factors. The adhesive strength decreases with rise in temperature, and decreases with increased exposure to static loading. As the number of load cycles increases, the amount of alternating shear stress that the adhesive layer can withstand progressively decreases until the adhesive attains its service life resistance, at around 20 million cycles.

Calculating the strength of elastic-adhesive joints
Determine the equivalent stress for a thick-layer elastic adhesive using normal stress.

V = Equivalent stress
Z = Tensile stress
π= Shear stress

Elastic adhesives can change with service temperature and exposure to stress. Applying reduction factors roughly quantifies the changes in strength and stiffness of the elastomer temperature and/or stress increase.

wa = Width of the adhesive layer
= Length of the adhesive layer
> 2 = Safety factor
B = Adhesive tensile lap-shear strength
= Reduction factor for temperature
static = Static force
t = Reduction factor for static stress
Fdynamic = Dynamic force
Z = Reduction factor for dynamic stress

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