Considerations for adhesive bonding
When selecting adhesive bonding as a joining method various factors should be considered.
- Does the joined structure need disassembly during service, for maintenance purposes?
- Are high operating temperatures predicted during service?
- Can the assembled structure be held in place during bonding and, if required, transferred into an oven for curing?
- Are adequate facilities available for surface pre-treatment and/or preparation of the substrates?
- What quality control measures will be needed?
- Will the process be cost effective when compared to other joining methods?
Once these general considerations have been addressed, the following benefits of adhesive bonding can be realised:
Joining of dissimilar materials - with the correct surface treatments most metallic, plastic and polymer composite materials can all be joined to either themselves or each other with joint strengths superior to other joining techniques.
Higher stiffness joints - the continuous nature of adhesive bonding helps reduce high stress concentrations that arise with mechanical fastening systems and results in stiffer joints and structures.
Improved fatigue life - superior fatigue resistance can be achieved compared with welded or riveted joints.
Inherent sealing - a continuous bead of adhesive can also provide a seal and, depending on the level of joint fill, crevice corrosion can be avoided.
Aesthetic benefits - the removal of spot welds or mechanical fasteners from joined metal sheet allows for seamless joints.
Loading of Adhesive Joints
Different types of loads arise in adhesive joints depending on the joint geometry and the direction of loading. These are often classified as tensile, shear, cleavage or peel as illustrated below.
In general shear loading is more desirable than either tensile, cleavage or peel.
- Tensile loading results in high concentrations of shear stress in the adhesive at the edges of the joint and the loading is difficult to apply uniformly resulting in higher concentrations of stress on one side of the joint.
- Cleavage and peel loading results in concentrations of normal tensile stress though the adhesive layer at the edge of the joint making it susceptible to failure.
- Shear loading results primarily in shear deformations of the adhesive and the stresses are less concentrated at the edges of the joint than for the other types of loading making the adhesive less susceptible to failure.
Joint type
The choice of joint type will depend on the nature of the structure that is to be created. With joint strength being significantly higher under shear deformation, it may be desirable to choose a joint geometry that minimises the tension stresses at the edges of the overlap.
- Single lap joints provide the simplest joint geometry but detrimental peel stresses still arise in this joint. Other joint geometries may be considered to reduce peel stresses.
- Double lap joint
- Scarf joint - ideal for eliminating peel stresses, however, in practice these joints are harder to create and are not suitable for use with thin sheet adherends.
- The butt joint is also a simple joint to make, however, it is not suitable for load bearing components due to its inherent weakness to tension and cleavage forces. It can, however, be used to transmit significant torsional loading.
- The T-joint is employed where two members require joining at 90 degrees to each other (or some other angle). Again, careful design is required to minimise tensile loading of the
