Oftentimes when transitioning to an adhesive bond from a traditional mechanical or thermal attachment method, it is necessary to redesign the joint to accommodate better tensile, compression and shear loading, removing cleavage or peel. There are a few common joint configurations that perform very well for mechanical or thermal attachment but should be improved for use with an adhesive.
Butt joints are common welding practice when assembling two cylindrical or rectangular tubes. Most of the stress is applied as cleavage, but tension may also be present if the tubes are being pulled apart. The cleavage forces in a butt joint are not well suited for adhesive bonds. Therefore joint modifications should be made when considering adhesives for these assemblies.
Cylinder-in-cylinder joints are modifications of butt joints that insert a small diameter cylinder into a larger diameter hollow cylinder. The new design places the adhesive itself into in-plane shear force, and cleavage forces are mitigated by the mechanical lock of the tubes. It is relatively straightforward to optimize bond area by adjusting the radii of the tubes or overlap length of the joint. For applications in woodworking, tongue and groove is a common method of leveraging the mechanical strength of the wood and lap shear strength of the adhesive
T-joints are very common for welding two components together, and they are a common sight when using mechanical fasteners for perpendicular mounting. When stress is applied to these joints, much of it is in cleavage applied to a long moment arm. This is a very weak joint for adhesives.
To improve a T-Joint, many adhesive joints are redesigned as mortise and tenon. With this new design, the same applied stress is now presented as shear, tension and compression of the adhesive while also providing the benefit of a mechanical lock to improve strength. If mortise and tenon is not a viable solution (such as for many metalworking applications), incorporating a flange to the perpendicular piece not only converts some of the cleavage into shear, but adds surface area over which the adhesive makes contact.
Miter joints are common practice for many applications. When force is applied, some mechanical strength is present, but there is also a large amount of out-of-plane cleavage on the moment arm. Similar to with T-joint configurations, these joints tend to be very strong and durable when using mechanical or thermal attachment methods.
As is done with modifications for T-joints, corners can be improved by adding an interlocking mechanism such as a bridle joint to distribute the stresses in-plane on the adhesive. As with other adhesive-friendly joints, cleavage is converted into shear while still maintaining the post-and-beam mechanical advantage.
Square butt joints are common welding practice when assembling two plates side-by-side. Dual action fasteners with splines are used to hold two large panels together in woodworking applications like laminate countertops.
These joint designs often suffice for thermal or mechanical attachment, but similar to tube-in-tube joints, they place undue stress on an adhesive. Lap joints are one of the most common industrial adhesive joints whether assembling two panels, bonding panels to frames, or comparing adhesives for R&D or design purposes. Cleavage from a square butt joint is now primarily shear stress applied across the bond area, although some cleavage may still exist.
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