Durability: fundamentals

Aspects of this subject are also dealt with in other articles, notably Acrylic adhesives. Durability - fundamentals. Joint design general. Joint design cylindrical joints. Joint design strength and fracture perspectives. In the article on Toughened acrylic adhesives, some properties are compared with those of Epoxide adhesives and anaerobic adhesives. 

Relatively few anodic treatments find application in structural adhesive bonding where durability (see Durability - fundamentals) in hostile service conditions is of paramount importance. " ... 

Typical procedures for chromic and phosphoric acid anodizing are given in Pretreatment of aluminium, and the durability (see Durability - fundamentals) of bonds to the surfaces formed is discussed. An example of a profile of elemental composition in depth for a phosphoric acid anodized film is shown in Auger electron spectroscopy. 

It has been shown, however, that the Durability fundamentals of such bonds is questionable, particularly when stressed. Reduced dependence on mechanical fasteners will require the development of production-viable treatments for steel or the introduction of... 

Adhesive joints and coatings not uncommonly fail in an aqueous environment (see Durability fundamentals, Weathering of adhesive joints and Wedge test). One of the... 

Environmental performance The durability (see Durability - fundamentals) of cyanoacrylate adhesive bonds is reasonably good on rubbers and some polymer substrates. However, on glass and metals, both thermal and moisture durability are low. 

When optimized, these are suitable preparations for both adhesive bonding and painting their effect is to give a reproducible, oxidized surface on the aluminium. Initial adhesion is generally good, and durability fair, but not as good as with Anodizing pre-treatments (see Durability fundamentals. Pre-treatment of aluminium). 

Tapered double cantilever beam This is again mode I, but the value of dClda can be made constant by tapering of the arms of the specimen (see Fig. 2b). Thus, Gj is independent of crack length and this design is well suited to tests where the value of a is difficult to measure, for example, environmental tests (see Durability fundamentals) and dynamic fatigue tests. This is also used in ASTM D 3433. The value of Gic is given by... 

Water penetration of coatings is usually destructive. Diffusion of water through the binder or penetration through submicroscopic cracks in the film allows water to accumulate at the coating-substrate interface. If transport of the water is driven osmot-icaUy, the resulting build up of pressure will disrupt the coating and lead to failure (see Durability fundamentals). 

Fine powders stick together tenaciously. Anyone who has tried to dig through dry clay will bear witness to the powerful adhesion developed between the micrometre-diameter clay particles. Like the strength of many other adhesive systems, for example, epoxy-aluminium joints, the adhesion of these clay particles is much diminished by the addition of water, and also by the influence of pH, as when lime is added (see Durability fundamentals). 

Good surface preparation is important in this context (see Pre-treatment of metals prior to bonding), as in most examples of adhesive bonding, as it influences the durability of the bond, especially in wet areas (see Durability - fundamentals). 

The environment in which an article is used may influence bond durability (see also Durability fundamentals). Atmospheric ozone can cause time-dependent crack growth in vulcanized elastomers in addition, ozone can induce failure at a bond with certain bonding agents. Although water is only slightly soluble, it can permeate elastomers by an osmotic mechanism induced by salt-Uke impurities. As a result, the uptake in salt water is generally less than that in pure water. Rubber to metal bond failure has been found to occur in a time-dependent manner under salt water in the presence of electrochemical activity but much more slowly, if at all, in its absence (see also Cathodic disbondment). In the absence of imposed electrochemical activity, effects are likely to depend particularly on the metal used and its corrosion resistance. Provision of a bonded rubber cover layer over all metal surfaces subject to immersion is likely to enhance bond durability. 

Insulating glass or double glazed units consist of two panes of glass separated by a spacer and all are held together by edge Sealants (Fig. 1). These sealants seek to exploit the specific properties of low moisture vapour transmission, low inert-gas transmission and high durability of the sealant used (see Durability fundamentals). 

Adhesive joints may be subjected to a variety of adverse service conditions, including elevated temperature, organic solvents, water and stress (see Durability fundamentals. Durability creep rupture). Solvent-based, emulsion and melt adhesives are normally based on thermoplastics with fairly low softening temperatures. If a loaded joint is subjected to elevated temperature, failure may occur because of Creep unless the adhesive is cross-linked. Likewise, attack by organic solvents can be minimized by cross-linking. Solvent-based, emulsion and hot melt systems are available that cross-Unk after the initial bonding has been carried out. These systems provide improved in-service performance. 

On a laboratory scale, it has been demonstrated in recent years that thin films of silanes applied to metals can protect the metal from many forms of corrosion, including uniform corrosion, pitting corrosion, stress corrosion cracking, crevice corrosion and other forms, in addition to providing excellent and durable paint adhesion (see Durability - fundamentals). Such thin films are typically of not more than 300-mn thickness. They can be applied by immersion of the metal into a dilute silane solution, for example, 5% in water or water/alcohol mixtures, as not all silanes dissolve in water. Brushing, wiping or spraying application methods can also be used. 

Perhaps, the earlier materials found to have a useful capacity for adhesive bonding underwater depended upon the use of a stoichiometric excess of water-scavenging polyamide hardener in an epoxide-based adhesive. This approach can lead to the production of effective joints in the short term, but formulations of this type, which are hydrophilic in the uncured state, are also likely to absorb significant amounts of water in the cured condition. It is a widely accepted view that the extent of joint weakening in susceptible joints, quite apart from the consequences of plasticization, is a function of the water-uptake characteristics of the adhesive (see Glass transition temperature). The consequence is therefore likely to be that such joints will show poor durability in the presence of water, when rapid uptake of water may lead to equally rapid degradation of both cohesive and interfacial properties (see Durability fundamentals). 

Durability fundamentals G W CRITCHLOW Modes of failure water ingress pre-treatments... 

This chapter focuses on the nano-structured effects on electrode durabUity, i.e. ceU durability. Fundamental aging mechanisms of the electrode components are also summarized. Long-time performances of the nano-structured electrodes are introduced to reveal the nano-size effects on electrode durabUity. Furthermore, models ever reported for the prediction of durabUity are included for better understanding the influence from the nano-size scale. 

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