Epoxy adhesives aluminium alloy joints

Figure 6,24 Shear stress concentrations for various lap joint designs, elastic analyses for typical epoxy adhesive/aluminium alloy joints [6].

Figure 8.26 Adhesive fracture energy, Gic, versus crack velocity, a, in water for epoxy-TEPA/aluminium alloy joints [36,136].

This mechanism has been clearly identified in the case of the durability of aluminium-alloy joints. One example [10] is that of aluminium-alloy joints bonded using an aerospace epoxy-film adhesive where the aluminium-alloy substrate was subjected to a phosphoric-acid anodising (PAA) surface treatment, but where the primer (which is normally used in such an adhesive system) was omitted. Under... 

Ford Research Laboratories [15] evaluated the fatigue behaviour of aluminium alloy joints and showed the enormous benefit of using an epoxy adhesive in combination with spot welding or mechanical... 

Fig. 4.20. Effect of outdoor weathering on joint strengths (Refs. 92, 93 Adhesive epoxy polyamide film. Adherends aluminium alloy. Pretreatment Chromic-sulphuric add etch. Cure temperature 175 C.

Considering some of the studies reported in the literature, then the adhesion of epoxy adhesives to degreased, chromic acid etched and chromic acid and sulphuric acid anodized aluminium alloys appears [89,90] to involve only secondary forces, although in all cases the initial joint strength is high and, for the etched and anodized aluminium joints, the locus of joint failure is by cohesive fracture in the adhesive. 

Alwar and Nagaraja [22] and Adams et al. [10] have used an elastic finite-element method to analyse the stress distribution in butt joints loaded in tension and a typical stress distribution is shown in Fig. 6.7 for an epoxy adhesive bonding aluminium alloy substrate. The bonded area comprises two different regions. 

Figure 6.19 Influence of thickness, /la, of adhesive layer upon experimental and theoretical fracture loads of epoxy/aluminium alloy single lap joints [61].

The theoretical analyses outlined above predict that the stress concentrations will decrease and hence that the fracture stress, Tf, of lap joints will increase as the thickness, /la, of the adhesive layer increases. Generally, if all other joint parameters are held constant the value of n is predicted to be proportional to the reciprocal of the square-root of the thickness, /la, provided that the thickness is relatively small. Figure 6.19 shows [61] the predictions for three different analyses for an epoxy/aluminium alloy single lap joint and all suggest that the breaking load will increase as the value of increases. However, the experimental results for the joints, in accord with other work [38, 69-71], shows that the actual fracture load, Ff, does not increase with increasing and may even fall slightly. 

A final example is illustrated in Fig. 7.11 and shows the experimentally determined values of compliance, C, versus crack length, a, for a tapered double cantilever beam joint (Fig. 7.8) consisting of an epoxy adhesive bonding aluminium alloy substrates. As discussed in Table 7.1, this type of joint is designed to produce a constant value of dC/da and this may be readily seen. Further, the joint obeys LEFM and for the adhesive thickness, /Za, used of 0.5 mm the adhesive layer is sufficiently thin so that the experimental value of dC/da agrees well with the theoretical value deduced from Equation 7.49 given in Table 7.1. Obviously, either from the experimental or theoretical values of dClda, the value of Gu may be calculated from Equation 7.4. For those joints which obey LEFM, in the case of cracks in the centre of the adhesive layer, values of Kc may be determined via Equation 7.35 whilst for interfacial cracks Equations 7.36 to 7.39 are more appropriate. 

Thirdly, the adhesive type does appear to influence the environmental stability of the interfacial regions. For example, especially noteworthy is the often-superior joint durability shown, in the case of bonding aluminium alloys, by some of the older phenolic-based structural adhesives, compared to the more modern epoxy-based adhesives. This does not arise from the phenolics possessing lower diffusion or solubility coefficients with respect to water than the epoxies [153]. Indeed, many such phenolic adhesives are more permeable to moisture. However, it may arise from the ability of the phenolics to establish stronger, more stable intrinsic interfacial forces, as discussed later. Alternatively, it may arise from the pH of residues from phenolic-based adhesives being acid and therefore not attacking and weakening the aluminium oxide, unlike the more alkaline extracts from some of the epoxies. It is believed... 

Armstrong KB (1997) Long-term durability in water of aluminium alloy adhesive joints bonded with epoxy adhesives. Int J Adhes Adhes 17(2) 89-105 Bauer P, Roy A, Casari P, Choqueuse D, Davies P (2004) Structural mechanical testing of a fidl-size adhesively bonded motorboat. J Eng Marine Environ 218(M4) 259-266... 

---