Durability of bonded assemblies - Study

One of the main requirements of a bonded joint is its ability to retain a significant proportion of its load-bearing capacity under the wide variety of environmental conditions it is likely to encounter during its service life.

For a bonded assembly, this function and its permanence over time (or durability) are the result of adhesion forces due to interactions between a solid (or adherent) substrate and an adhesive. These interactions lead to the creation of interphases whose properties evolve continuously between the interface and the adhesive core. The adhesion created by these bonding forces can be measured by the breaking (separation) energy of the joint formed. The same applies to loss of adhesion, which affects the life of the joint.

Unfortunately, one of the most aggressive environments for joints involving high surface energy substrates – metals, glass, ceramics – which, together with adhesives, make up common structural joints, is humidity. This is obviously one of the most common environments.

So, in many applications, which can involve some of the most critical uses of adhesive technology, bonded joints are subjected to a hygro-thermal environment which also happens to be one of the most damaging.

This is why the concept of the durability of bonded assemblies in an aqueous environment is undeniably one of the most important challenges facing the bonding community.

A bonded assembly must therefore be considered as a complex system which, over its lifetime, brings into play coupled chemistries and physics.

In this context, the approach to the durability of bonded assemblies is still mainly phenomenological, even if it leads to a few deterministic predictive models and a few reliable attempts, based on statistical analysis of experimental data, are made to express the criteria of limit of use and service life.

This approach is supported by a large body of experimental work. It consists in measuring the influence of factors which, during manufacture and in service, are responsible for the aging and damage of assemblies, and hence for the reduction in mechanical strength, loss of adhesion and even joint failure.

This study focuses on the durability of bonded assemblies subjected to a variety of stresses (chemical, mechanical, thermal, electrical, etc.). A second part deals with the predictive approach to durability. As these assemblies involve polymer adhesives, it may be useful to refer to the files devoted to the various aging processes of polymers and plastics. [AM 3 150]