Strength and Stiffness of Adhesives

Adhesive systems can be divided into rigid, flexible structural and elastic bonding groups. A fourth group comprises sealants. These adhesive and sealant systems have a different, reversible elastic deformation and shear modulus (Fig. 6). Reversible elastic deformation is explained in more detail in the Glossary/ Definition of Terms at the end of this chapter. 

Even stress distribution in the bond line is a fundamental principle of bonding technology. An explanation is given in Fig. 8, where the load bearing capability of rigid, elastic and flexible structural bonding is compared with respect to the bond overlap. The load bearing capability is the area below the curve of the stress level. 

Elastic bonding exhibits a much lower stress level, but in contrast to rigid bonding, the stress distribution is rather even. By enlarging the overlap, the load-bearing capability increases almost proportionally. 

The flexible structural bonding is the combination of rigid and elastic bonding and provides optimised mechanical properties and maximum possible elongation, in combination with structural strength and stiffness. The load transfer of the flexible structural adhesives is lower than that of rigid adhesives and higher than 

A further discussion is given in the Section 6.2.1. Principal Advantages under Even Stress Distribution . 

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Numerical calculation methods (e.g. FEM) allow the prediction of the strength and stiffness of adhesive-bonded structures. This will lead in the future to new types of adhesive assemblies that have not yet been tried in practice either because of ignorance or because of reservations about adhesive bonding - not to mention other adhesive applications so innovative that we cannot begin to imagine them today. 

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