Good designs, adhesive joints

Many adhesive bonded assemblies are designed to take advantage of the generally good resistance adhesive joints have to shear stresses. 

When a complex joint is to be introduced in a structure, the ideal situation is to test that specific joint. However, this approach is very expensive. Before real joints or prototypes are built, the designer should first come up with a good prediction of the failure load based, among other things, on the basic mechanical properties of the adhesive. The basic properties can mean the elastic properties, such as the Young s modulus and the Poisson s ratio in case the analysis is linear elastic. However, for the more realistic theoretical methods that take into account the nonlinear behavior of the adhesive, the yield stress, the ultimate stress, and the failure strain are necessary. The stress-strain curve of adhesives is necessary for designing adhesive joints in order to compute the stress distribution and apply a suitable failure criterion based on continuum mechanics principles. 

The best approach to achieving good performance in bonded joints is to select the proper adhesive, design the joint properly for the specific application, and maintain rigid process control. One disadvantage in the use of adhesives is the absence of good non-destructive tests to determine the strength of adhesive-bonded joints in a wide variety of ord applications... 

From the preceding discussion, then, it appears that the optimum conditions for good adhesion include a diffuse interfacial zone and/or strong specific intermolecular interactions between phases. One may add to that (in the opinion of some) the existence of a direct physical interlocking between surfaces. With all the best-designed practical systems, however, adhesive joints... 

Fig. 2. Representations of good and bad joint design. (Reproduced with permission from A J Kinloch, Adhesion and Adhesives Science and Technology, Chapman and Hall, London, 1987)...

Conditions where three-sided adhesion occurs leads to premature failure by subjecting the elastomer to triaxial stresses. A few examples of good and bad joint design can be seen in Fig. 32. Frequently, it is necessary to prevent adhesion on one or more surfaces in order to eliminate triaxial stress. 

Figure 6.2 Schematic diagram of some good and poor joint designs. The good designs attempt to distribute the imposed loads within the adhesive layer as a combination of shear and compressive stresses avoiding tensile, cleavage and peel stresses as far as possible.

The key to design bonded joints is the knowledge of E-modulus or G-modulus. These values can be used to calculate and predict the strength of a bonded joint. In contrast, the value of the lap shear strength of a joint is of no use since this depends on the specific properties of the substrate and the geometry of the samples and thus not inherent to the adhesive properties. A good... 

The strength of a bonded joint will be determined by the strength of its weakest component, whieh is generally designed to be the adhesive. It follows that the requirements for satisfactory joint performance eu-e (a) good contact between the adhesive and substrate (b) absenee of weak layers in the joint (e) that the adhesive should possess appropriate mechanical properties. These basic requirements are implieit in the essential elements of bonding, and are related to many faetors which affect the performance of bonded assemblies as summarised in Table 4.2. 

Proper Choice of Adhesive Good Joint Design Cleanliness Wetting... 

The basic requirements for a good bond to plastic substrates are surface cleanliness, wetting of the surface by the adhesive, solidification of the adhesive, and proper selection of adhesive and joint design. These requirements are briefly defined here and then more thoroughly discussed in following sections. 

Elastic adhesives are resistant to most of the substances named in Eig. 20 for a limited period of time and generally, they can safely be used provided joints are designed in accordance with normal good practice (such as taking steps to prevent permanent exposure to condensed water in the case of window glass). 

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