The durability of adhesive joints

Abstract

One of the most important requirements of an adhesive joint is the ability to retain a significant proportion of its load-bearing capability for the long periods under the wide variety of environmental conditions encountered during its service-life. There exists a need to improve the understanding of the mechanics and mechanisms associated with the durability of adhesive joints in hostile environments, such as one of the most potentially damaging and frequently encountered, water, to further their future extensive use in engineering applications. In the present research, four important aspects in relation to the durability of adhesive joints have been investigated. These were: (a) developing sound short-term accelerated test methodologies to assess the durability of adhesive joints; (b) understanding the mechanisms of environmental attack on different types of surface pretreated adhesively bonded aluminium alloy substrates; (c) developing adhesive/primer/pretreatment systems which possess excellent long-term durability; and (d) investigating the potential of environmentally-friendly organosilanes as primers to enhance the intrinsic adhesion of adhesive joints. The results from these studies showed that: (a) constant displacement rate and cyclicfatigue tests provide excellent quantitative durability test methodologies; (b) fracture mechanics and advanced surface analysis of adhesive joints have proven surface pretreatments using phosphoric acid anodising (PAA) are far superior to those employing a grit blasting and degreasing (GBD) pretreatment which can be attributed to the increased surface area and excellent bonding morphology of the anodised oxide surface, allowing deep penetration of the viscous adhesive and impeding water ingress at the adhesive/substrate interface; (c) the durability performance of PAA pretreated adhesive joints employing a primer (PAAP) are superior to those without a primer specifically in water, and acid-based surface pretreatments are significantly advanced compared to simple GBD; and (d) the self assembling long carbon-chain silanes enhance the durability of adhesive joints via the formation of covalent bonds between the adhesive and the activated silane monolayer deposited on the substrate.