Aluminium adhesive joints

Finally, reports are available on the durability of adhesively bonded aluminium joints... 

A related issue is whether roughening a surface increases the strength of an adhesive joint. Harris and Beevers [82] found no difference in adhesion to mild steel and aluminium alloy blasted with alumina grits of different particle sizes. Shahid and Hashim [83] used a structural epoxide adhesive with mild steel adherends in cleavage joints. The surfaces had been grit-blasted or diamond-polished, and surface profiled. The results are shown in Table 15, where all differences in strength seem to be the same, within experimental scatter. 

An example of such a fracture envelope, in this case developed for the case of mixed-mode cracking of an adhesive joint system consisting of 7075-T6 aluminium adherends bonded with a 0.4 mm thick structural epoxy appears in... 

The durability of epoxy-aluminium joints that used a homopolymerised epoxy resin was studied by researchers based in Spain [15], and the effects of relative humidity, temperature, and salt concentration analysed. The homopolymerised epoxy resin absorbed little water (1.5 wt%) because of its non-polar network structure. Increasing relative humidity and temperature enhanced water uptake, but the joint strength remained constant because of epoxy plasticisation. A saline environment was damaging to the adhesive joints because of metal corrosion, but was not significantly harmful to the epoxy resin because of the lower diffusion coefficient of salt water. The decrease in glass transition temperature of the epoxy adhesive due to water absorption was dependent upon only the amount of absorbed water and was independent of hydrothermal ageing conditions. The durability of epoxy adhesive joints made underwater has been studied [16]. 

Arrowsmith D.J., Moth D.A. and Rose S.R (1992), The enhancement of adhesive joint strength by extending the surface of anodized aluminium , Int. J. Adhesion Adhesives, 12,67-72. 

Again, the underlying reasons for the environment resistance of adhesive joints consisting of aluminium and its alloys being so highly dependent upon the choice of surface pretreatment are more conveniently discussed in Chapter 8. 

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. 

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. 

Figure 7.24 Effect of crosshead displacement rate on the measured fracture energy, Gic, and type of crack growth [108]. (a) DGEBA-TEPA adhesive (b) DGEBA-TETA adhesive. O stable brittle crack growth (Type C) A initiation and A arrest of unstable crack growth (Type B). (Test temperature = 22 °C TDCB joints aluminium alloy substrates.)...

As discussed below, the classic example of the problem of oxide stability upon exposure of bonded joints is with aluminium and its alloys and this aspect has therefore been investigated in detail by the aerospace community. Particularly, the fundamental micro-mechanisms have been studied in order to explain observations such as those shown in Fig. 8.10, which reveals the effect of three common aerospace surface pretreatments upon the subsequent durability of the adhesive joints. The three treatments which have been studied in some detail are chromic acid etch (CAE), chromic acid anodize (CAA) and phosphoric acid anodize (PAA), and details of the processes were given in Section 4.3.4.5. 

Two other points of interest are that it appears that such hydration inhibitors are only really effective on an aluminium alloy substrate which has already been pretreated by the CAE process. It would obviously be useful for the technologist if such simple primers were effective on a degreased or abraded surface. Secondly, the above work [82] also suggested that some silane-based primers could function as hydration inhibitors, so adding another possible mechanism whereby such organometallic primers might increase the service life of adhesive joints. 

Fig. 51. Adhesive shear stress distributions in step joints aluminium...

Pires I, Quintino L, Durodola JF, Beevers A (2003) Performance of bi-adhesive bonded aluminium lap joints. Int J Adhes Adhes 23 215... 

Michaloudaki M, Lehmann E, Kosteas D (2005) Neutron imaging as a tool for the non-destructive evaluation of adhesive joints in aluminium. Int J Adhes Adhes 25 257-267... 

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... 

Details are given of a thermal analysis technique, referred to as micro thermal analysis, which combines the high resolution positioning of scanning probe microscopy with some of the quantitative analysis capabilities of conventional thermal analysis. The application of this technique in characterising the thermal properties of interfaces in aluminium/epoxy resin adhesive joints and in glass fibre-reinforced epoxy resin composites is described. 7 refs. 

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