Design of Adhesive Joints Under Humid Conditions

The diffusion of water into adhesives and adhesive joints is controlled by Fick’s laws, and it is solutions to Fick’s second law in Cartesian coordinates, which are most used in this context. Such solutions can describe diffusion in both 1 and 2 dimensions. Humidites in air are conveniently controlled using saturated solutions of salts. It is the interactions of water with epoxide adhesives which have been studied most, and diffusion is often fickian, but non-fickian diffusion is not uncommon. When salt is dissolved in water, in effect it dilutes the water, and the amount of water which is absorbed is reduced. Absorption isotherms for water in epoxides are non-linear. Water can change the properties of epoxides by lowering the glass-transition temperature, forming cracks, crazes and voids. Mechanical properties are also affected. Amongst non-structural adhesives, water diffusion has been reported for some silicones and isocyanates.

The adsorption and transport of water and ions at polymer/metal interfaces has a strong impact on the adhesion of the polymer as well as on the kinetics of corrosive degradation. Although not directly correlated both wet-de-adhesion and corrosive de-adhesion are accelerated by water enrichment at the interface which promotes the ingress of ions. The discussion of these phenomena starts with the consideration of the adsorption of water on oxide surfaces, as all engineering metals are covered with an ultra-thin atmospheric oxide film, and ends with the cathodic and anodic corrosive de-adhesion mechanisms of polymers from oxide covered metals. The bridge between the water adsorption and corrosive de-adhesion reactions is built based on the separation of water and ion transport in polymeric interphases and the discussion of diffusion and migration as the principal processes.

With metallic adherends it is recognised that some form of surface treatment is required to provide a combination of high performance and long-lasting, or durable, adhesion. This is most evident when bonded structures are exposed to environmental and/or mechanical loads for a significant part of their service life. A range of treatments exists which provide enhanced durability, to one extent or another; usually qualitative intercomparisons are made between treatments with the same alloy and adhesive combination under well-defined environmental and mechanical loading conditions. For a specific adherend, the efficacy of a particular treatment is dependent upon a large number of factors. In this section, specific examples of surface treatments are discussed for aluminium and titanium alloys and carbon steels in terms of how they modify the metal surfaces and how they perform in comparative adhesion studies. Importantly, the most high performance and optimised surface treatments when applied to an appropriate alloy, and using an appropriate adhesive-primer combination, are capable of providing the levels of structural adhesion and durability required in the highly-demanding aerospace, defence and automotive sectors.

The property change of cured bulk adhesive by water immersion is described in this chapter. Preparation of bulk adhesive specimens is firstly explained because dense bulk adhesives without porosity are not easy to make. Experimental methods to determine the water diffusion coefficient and saturated water absorption are also discussed. Based on the parameters obtained from these tests, water distribution in bulk adhesives can be calculated using the finite element method. Influence of water immersion on the stress–strain relations of bulk adhesives is also discussed.

It is generally observed that, while the locus of bonding failure of well-prepared metallic adhesive joints is invariably by cohesive fracture in the adhesive layer, after environmental attack it is by apparent interfacial failure between the adhesive and the metallic substrate. The environmental stability of the adhesive-substrate interface and of the oxide, invariably present on metallic substrates, needs therefore to be considered in detail. This chapter considers the service performance of adhesive metallic bonding upon exposure to aqueous environments and reviews the mechanisms of environmental attack and methods for increasing bonding durability.

We introduce a new adhesion test method for IC molding compounds that can experimentally separate residual stress from adhesion strength. This chapter describes the dependence of the measured true adhesion strength of molding compounds on adherent materials, temperature, and moisture absorption. We also evaluate interface delamination in a moisture-absorbed package by considering the swelling of the molding compound caused by moisture absorption. The predicted interface delamination agrees well with experimental data for moisture-soaked IC packages.

Damage mechanics is a tool that is used to model the deterioration of a material under load, ultimately leading to failure. This is achieved through an association of the properties representing the mechanical behaviour of the material with the effects of loads acting upon it, often via a defined damage variable. The result is the representation of micro-damage as a reduction in the material properties in a damage or process zone in which the effects of the loads are great enough to initiate damage but less than that to cause complete failure. This will affect the response of the structure to further loading and hence damage mechanics is often applied in a progressive fashion. In this way, the method is able to predict not just the final failure load of a structure, but can also predict the state of damage and response to loading at any point in the load-time history. Two forms of damage modelling have been applied to adhesively bonded joints, those based on cohesive zone laws and those based on continuum damage mechanics, with the former being more common. These methods are now well developed and have been validated for a number of different adhesive joint types. Although there have been few applications of damage mechanics to environmentally aged joints to date, the published work indicates that it can be practically applied to the prediction of the effects of the environment, specifically moisture and temperature, on bonded joints and that the technique can offer certain advantages in the representation of the response of adhesives in these conditions compared with alternative analysis methods. Further development and validation of these techniques are still required and it would also be useful to develop simplified analysis tools based on the damage mechanics methods for the non-specialist designer. However, the results from the studies to date have clearly shown the potential of these methods.

This chapter outlines research that has been undertaken in seeking to model the combined effect of fatigue and environment on the durability of adhesively bonded structures. The main modelling approach outlined is based on progressive damage where both fatigue cycles and the environment degrade the response of the joint. In this work the progressive damage has been incorporated using a cohesive zone model with damage models for the material parameters being defined in terms of fatigue loading on the joint and the moisture distribution in the joint. The fatigue damage model is based on the maximum fatigue load but has been generalised for varying mean loads thus enabling generalised fluctuating loads to be modelled. Two configurations of joint have been extensively tested both statically and in fatigue under a range of environmental conditions and the degraded fatigue life recorded. Progressive damage finite element (FE) modelling has been undertaken on these joints and the static and fatigue response predicted. In order to assess how the damage progresses in these joints backface strain measurements have been recorded. These have been used to validate the damage predicted by the FE modelling and the good correlation serves to further validate this approach. The chapter concludes with an outline of possible developments extending the range of application of this work.