When compared with traditional assembly methods, adhesive bonds are superior in the presence of dynamic stress thanks, above all, to their flexible nature.  Rivets or bolts can only transmit forces over a localised area.  The adhesive alternative distributes stress and transmits forces over the entire area of the bond.  In addition, the parts to be bonded are not weakened by the drilling of holes and are effectively sealed against corrosion.  

A wide range of adhesive products with different cure mechanisms and performance properties are available for bonding. Typical candidates include anaerobic adhesives, modified acrylics, UV curable adhesives, cyanoacrylates – commonly known as superglues or instants – urethanes and silicones. 

The strength and long-term durability of a structural adhesive bond depend on the adhesive used, the materials involved and, to a lesser extent nowadays, with certain technologies, how they are prepared. Most importantly, the maintenance engineer needs to understand the loads to which the construction is exposed.  

Which load?

The external forces or loads acting on the construction can be classified according to five types of stresses: tensile, compressive, shear, peel and cleavage. For each type of loading, there is an adhesive technology which optimises the ability to withstand the forces imposed on the component in question.  

In industrial constructions, the most common loads are the last three listed. A shear force acts on a bond line when two overlapping joined parts are pulled in parallel and opposite directions.  Peel and cleavage forces are very similar and occur when a load is applied to the end of a bond line; they easily deform at least one of the parts.

The resulting stress is presented unevenly at the joint in all three types of loads.  Bonds subjected to shear loading exhibit stress peaks at both ends, while stresses in the middle region are less intense. This is the most commonly encountered load, especially in overlapped joints which are popular subjects for an adhesive solution.

In certain instances creating a thicker bond line is a good way of making the joint more compliant to sheer stress. The extra thickness spreads the shear strain over a larger dimension, resulting in less unit strain on the adhesive and therefore, less strain concentration. This is similar to using a lower modulus adhesive; a more compliant joint results in both cases.

In all cases an effective adhesive bond is characterised by uniform stress distribution which is achieved by selecting the adhesive that will deliver the best performance for that particular construction. 

Pre-treatment

How the substrate is prepared for bonding is another consideration, although ongoing development means many of today’s formulations are more tolerant of substrate contamination than their forerunners. For example, it is now perfectly possible to bond parts that are not completely free from oil.

In general however, bonded assemblies are adversely affected by incomplete contact between the adhesive and substrates and there are several pre-treatment processes available to rectify this. Surface preparation should, as a minimum, remove oil, grease or any coating whose bond strength to the substrate is apt to be less than that of the adhesive bond. In the factory environment simple abrasion and/or solvent wiping is generally sufficient for many metallic and plastic parts.

Some applications may also benefit from a primer or an activator. A primer is typically a reactive chemical dispersed in a solvent which is brushed or sprayed onto the substrate surface. The carrier solvent is then allowed to flash off, leaving an active surface for bonding. In effect it acts as a chemical bridge between substrate and adhesive, giving greater adhesion.

Adhesives that need an activator to initiate curing are generally supplied in a dispenser that ensures the optimal quantity of both elements is delivered to the bond line.  Activators do however have a role with anaerobics too, either to enhance curability or speed up the process.  

There are many things to consider in structural bonding; Henkel has consolidated key factors in its Loctite Maintenance Expert Guide: www.loctite-maintenance.co.uk.