Bonding – by design
26 October 2010
Adhesives are frequently the preferred choice of designers because they offer clear advantages over conventional fastening or welding. But in choosing adhesives, what should the designer bear in mind? Colin Chapman provides some useful pointers
For a bonded joint to be completely effective, it must be designed with adhesives in mind. Simply taking a design that has previously been created with mechanical fixtures in mind, and then adding an adhesive into the equation, will not necessarily work as well as it could. But before investigating some specific design considerations, it’s worth looking at some of the benefits afforded by the use of adhesives.
As far as cosmetic appearances are concerned, there are no protruding parts such as rivets, bolts or screws – and there’s none of the heat distortion or discoloration that may be caused by welding. So much for the aesthetics, what about joint strength? Because the bond created by the adhesive is continuous, there is a uniform distribution of stresses over the areas in question in a correctly designed joint. This is in contrast to local concentrations of stresses that cannot be avoided with mechanical fasteners or welds. A continuous bond produces a stiffer structure, prevents leaks and thus reduces the chances of corrosion. Adhesives also tend to absorb vibration and sound, providing valuable dampening properties.
Clearly, it is important to consider the stresses to which a joint will be subjected during its life. These include a combination of shear, compressive, tensile, or peel stresses. Adhesives are strongest in shear, compression and tension, but they perform less efficiently where peel and cleavage loading is present.
Lap joints, with their uncomplicated substrate-upon-substrate arrangement, create one of the strongest bonded assemblies. But note that failure load does not increase proportionally with the joint length or bond area. That’s because the ends of the bond resist a greater amount of stress than does the centre. Where larger bond areas are needed to carry the load, it is better to increase the bond width rather than the length of overlap. Alternatives to simple lap joints include scarf, steeped and tapered lap, and double strap arrangements.
When two or more components are to be fitted on one end of a shaft, the construction should be designed with steps between each bond area to allow easy assembly. Then, through the utilisation of retaining adhesives, the potential for fretting is reduced, while the load carrying capacity increases. In addition, tapered bonded assemblies ensure a closely controlled concentricity and are an effective way of ensuring good adhesive coverage.
In practice, stresses in cylindrical assemblies will be concentrated at the edge of the joint area. This means that increasing the joint length will not necessarily lead to a proportionally stronger assembly.
Where the materials in a cylindrical assembly have different coefficients of thermal expansion, large tensile strains can occur in the adhesive film as the assembly reaches operating temperature. Fortunately, there are three techniques that ensure the adhesive bond can accommodate thermal expansion.
First, an adhesive may be used to supplement an interference fit. Provided a small interference is maintained throughout the total operating temperature range, the assembly will function successfully. Secondly, where there is a clearance fit, the relatively low modulus and high coefficient of expansion of adhesives means that it is possible to eliminate or reduce tensile stress in the joint. Thirdly, a shrink-bonded clearance fit should be specified when the outer part has a higher coefficient of thermal expansion than the inner section. The adhesive should be applied to the male component, the female component heated and the parts assembled.
Many designs have been enhanced through the use of adhesives, but remember, a great deal of time and heartache can be avoided by incorporating various bonding technologies into the initial designs.
Colin Chapman is with Henkel, manufacturer of Loctite brand products
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