Question: We use several adhesives to bond a wide range of materials, but we never feel confident unless the parts break before the adhesive bond when tested. Is this the best way to judge effectiveness?
Answer: It is a common misperception that the materials should always break before the adhesive bond. This happens frequently if you are using a very strong adhesive to bond weak materials like paper or thin plastics. However, I am sure you realize that structural adhesives are widely used to bond strong substrates—such as metals like steel and aluminum—where the adhesive invariably fails before the substrates.
It is also important to recognize that an adhesive joint has two key features: first, the adhesive strength (i.e., the actual chemical or physical bonding to the surface); and second, the cohesive strength of the bulk adhesive. The real secret of designing an adhesive joint is to determine what forces are on the bond during the actual application and how the joint might fail during use.
Adhesives can be subjected to varying forces and conditions, such as shear forces, tensile forces, peel, cyclic bending or vibration, as well as temperature variations and exposure to moisture or fluids. When choosing an adhesive, you’ll need to consider all of these influences, perform appropriate accelerated or long-term testing of the assembled joint, and build in appropriate safety factors to ensure the long-term integrity of your products. Adhesive manufacturers can often give both engineering advice and chemical advice on the choice of adhesive for a particular application.