Influence of the Surface Finish on the Shear Strength of Structural Adhesive Joints and Application Criteria in Manufacturing Processes

In order to obtain the advantages of structural adhesives, a specific design of the adhesive joint is required, enhancing its performance and restricting its limitations. One of the most relevant geometrical patterns is the surface finish of the substrates as it decisively influences the mechanical properties of the joint and has a clear economical impact on the large series manufacturing processes. The contact of the adhesive with the metal (that in some cases is an activator of polymerization) and the presence of air bubbles trapped in the roughness (if abundant and larger than the polymer molecules) can lead to the appearance of cracks, causing the fracture of the adhesive layer or an adhesive failure. The objective of the present report is to analyse the influence of the roughness (as a pre-treatment before bonding) on the mechanical performance of the joint and, by means of the application of the “Value Analysis” technique, propose the surface finish that combines best both mechanical performance and suitability to the manufacturing process. The obtained results provide excellent expectations to achieve high performance of the adhesive joint with more economical and environmentally friendly surface finishes than rough machining (less waste of material, less costs in tooling and machinery, lower manufacturing times, etc.) and, therefore, enabling a better and wider use of adhesives in the industrial manufacturing processes.