Elasticity in the Adhesion Mechanism

Although in Sechon 7.7, the adhesion of the peeling rubber was not affected by the elasticity of the film, and although Bradley s rule shows that elasticity of spheres has only a small influence on their adhesion (Section 4.6), these are exceptional examples. Normally adhesion is strongly influenced by elasticity. The reason is that elasticity allows movement, and this movement usually acts to enhance the cracking of the joint, hi terms of the energy balance theory of Section 

as the material stretches, more energy (i.e. force times stretch distance) is pumped into the crack, which then converts the energy into new surface. 

Two examples of this elastic stretching mechanism are shown in Fig. 7.15. 

In the first case, of the fiber pull-out, the fiber is stretched along its length by the force and this causes a crack to run along the interface. In the second example, of the butt joint, in which an elastic adhesive layer is joining two relatively rigid plates, the adhesive is stretched all through its volume and this eventually causes a crack to run in from the edge. These examples are considered in much more detail in Chapters 15 and 16. 

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