Regardless of the joint type used, it’s important to understand the different stresses that are imparted onto a bonded assembly. Adhesives and tapes perform at their best when the stress is two-dimensional to the adhesive, dispersing a load across the entire area of a bond line. Adhesives and tapes perform at their worst when the stress is one-dimensional to the adhesive, concentrating a load onto the leading edge of a bond line. Many traditional joint designs incorporate these one dimensional stresses and may require modification for an adhesive bond to be most effective.
Tensile is pull exerted equally over the entire joint. Pull direction is straight, in-plane, and away from the adhesive bond. Force is distributed across the entire area of the bond line.
Shear is pull directed across the adhesive, forcing the substrates to slide over one another. Here again, the force is in-plane, and the force is distributed across the entire area of the bond line.
Like tension, compression is a force applied to a bond that is in-plane and straight. Unlike tension, the force is being applied toward the adhesive. Force is distributed across the entire area of the bond line.
Cleavage is pull concentrated at one edge of the joint, exerting a prying force on the bond. While one end of the adhesive joint is experiencing concentrated stress on the leading edge, the other edge of the joint is theoretically under zero stress. Cleavage occurs with two rigid substrates.
Joints that are well designed for adhesives place a majority of the stress into tensile, compression or shear modes. This allows the force to be applied over the entire adhesive area. Joints placing stress into cleavage or peel concentrate the stress onto the leading edge which may lead to premature bond failures, especially if subjected to vibration, impact or fatigue.
In addition to the stress type, optimising a joint may also require consideration of the dimensions. Adhesives are tested and reported for their approximate performance in units of force per area (e.g. shear loading, Newtons per square meter) or force per length (e.g. peel, Newtons per centimeter). By configuring the bond dimensions to accommodate loads imparted per area, bond durability can be improved.
Learn how you can design adhesive joints for less stress here.
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