Adherend combinations

Adherend combinations metal/glass, metal/plastics... 

Bonding of adherend combinations In the case of adherend combinations the following rules have to be observed ... 

The failure mode should not be used as the only criterion for a useful joint. Some adhesive-adherend combinations may fail adhesively, but exhibit greater strength than a similar joint bonded with a weaker adhesive that fails cohesively. The ultimate strength of a joint is a more important criterion than the mode of joint failure. An analysis of failure mode, nevertheless, can be an extremely useful tool in determining whether the failure was due to a weak boundary layer or due to improper surface preparation. 

Consult Table 5.3 (Adhesive/adherend compatibility) and, on a copy of Table 5.5 (Assessment), enter the status - R, S, P for reject, secondary and primary - of each adhesive for each adherend surface. Reject here indicates a basic adherend/ adhesive incompatibility which renders the adhesive unsuitable. A primary adhesive/adherend combination will function well, right up to the limit of the adhesive s capability. Secondary indicates that performance may not be so predictable. It should be appreciated, however, that an inherently high strength adhesive rated as secondary on a given adherend may well produce a far stronger bond than a weak adhesive rated as primary for the same material. 

Adhesive substrates. The materials to be bonded are a prime factor in determining which adhesive to use. Some adherends, such as stainless steel or wood, can be successfully bonded with a great many adhesive types other adherends, such as nylon, can be bonded with only a few. Typical adhesive-adherend combinations are listed in Table 7.31. A number in a given column indicates the particular adhesive that will bond to a... 

To illustrate the use of the finite element method, a series of single lap joints was analysed using the following combinations of adherend titanium-titanium, titanium-composite, composite-composite, and aluminium-aluminium. For each adherend combination, both an epoxy and a bismaleimide adhesive were analysed and each joint was subjected to four different test temperatures —55°C, 20°C, 130° and 180°C. In all, therefore, 32 different cases of single lap joints were considered. The thicknesses of the adherends were 2 mm for the composite, 1.6 mm for the aluminium and 1.2 mm for the titanium. The joint width was 25 mm, the length was 12.5 mm, and the bondline thickness was 0.1 mm. 

If plasticity is included in the adhesive characteristics, then an alternative failure criterion was proposed and implemented by Crocombe and Adams [9] using critical values of effective uniaxial plastic strain. The triaxial strain was expressed as an effective uniaxial strain, and then compared with the strain to failure of the bulk adhesive. Unfortunately, this too was seen to be dependent on the density of the finite element mesh, and in reality was a critical strain at a distance criterion. The choice of critical strain was also dependent on the adherends present as different plastic zone sizes will be given by different adherend combinations. The geometry considered in this paper was the peel joint. 

Manufacturers will give advice on which materials can be bonded by particular adhesives. In Table 2 some broad indications by adhesive class are given for common adherend combinations. 

Table 4. Effect of Adherend Combination on Bonding Action and...

Interlaminar shear in centre adherend Combined transverse tensile and interlaminar 81... 

It is clear that the role of surface contamination and its effects upon the resultant joint strength is complex this point has been further highlighted in the work of Olsson-Jacques et al. (1996) who studied the influence of adsorbed aviation fuel on various differently treated surfaces. It is, however, important to note that the degree of degradation is a function of the specific adhesive-primer-treatment-adherend combination used. Therefore, to ensure good adhesion, in the general case, it is recommended that contamination is removed prior to adhesive application. Some methods to achieve this for metallic and polymeric adherends are discussed in the following sections. 

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