The following diagrams show some examples of good and poor joint design. The good joint designs minimise the peel loads in the joint and try to keep the adhesive in compression. Poor joint designs have significant amounts of either peel, or tension in the adhesive layer, which leads to weak joints.
Practical Example - Simple Lap Joint
The single lap joint is a relatively easy method of joining that allows a large load bearing area to be created. Because of the eccentricity of the loading, bending moments are generated in the substrates at the edges of the overlap resulting in
- normal tension (peel) stresses in the adhesive at the edge of the overlap
- higher concentrations of shear stress in the adhesive at the edge of the overlap
- higher stresses in the substrates with the potential to create a plastic hinge effect at the edge of the overlap
- rotation of the overlap as illustrated in Figure 1.
The strength of lap joints as a function of overlap length and substrate thickness is shown in Figure 2.
- For short overlaps and thick adherends or substrates, adhesive shear is the main influence.
- For longer overlaps and thinner substrates, peel stresses in the adhesive have a bigger influence
- For longer overlaps joint strength is limited by yielding of the substrate.
The following example, including re-drawn figures, is based on the case study provided by WA Lees in the Adhesives Section (B15) of the Kempe’s Engineers Year-Book, 2000. Further useful information can be found within this chapter.
The adhesive system is a structural epoxy - elastic limit of 30MPa, ultimate strength 45MPa. For a joint with 2 mm steel adherends and bondline thickness of 0.8mm, a design basis is shown in Figure 3 when considering strength (Pk), adhesive elastic limit (El) and creep under constant load (Cr). The El line represents the maximum load that ensures the stress within the adhesive remains below its elastic limit, at all location. The Cr line represents the maximum load that the joint can sustain while ensuring that the stress, at the centre of the joint, remains below 2 MPa - considered acceptable to prevent creep within the adhesive.
No strength benefits are obtained at overlap lengths greater than 35mm however an overlap length of 45mm would provide the maximum load bearing capacity while ensuring that, i) no part of the adhesive is stressed above the elastic limit, and ii) the stress within the central area of the adhesive are low enough to prevent creep occurring during service.
The shear stress distribution in the adhesive for a 25mm overlap is shown in Figure 4. Line M represents the distribution when the peak load (26.7kN) is applied. Line E shows the distribution when the joint is loaded to the elastic limit of the adhesive (7.9kN). Line Cr shows the distribution when the maximum load that ensures the stress at centre of the adhesive remains below 2MPa is applied (1.3kN). This joint overlap appears to be a good compromise as it will sustain loads up to 26.7kN (possible one-off impact loads), withstand loads of 8kN (for longer exposure periods) without adhesive yielding, and sustain a continual load of 1.3kN with potentially good creep resistance - since much of the joint is stressed below 2MPa.
