Peel joints energy

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... 

Figure 15,9. A long lap joint peeling under the force F. As the crack penetrates further, the shaded elements A and B change their energy state.

A second paradox is the use of the word shear to describe the fracture. It is evident from the calculation used to obtain Equation (15.5) that shear is not mentioned. The joint peels but does not slide or shear (Fig. 15.11). Only tension forces and displacements are needed to explain the failure of the joint. In fact, it would be far more logical to describe this failure as a tension failure, just as the Griffith equation describes tension failure. Of course, shear stresses exist around the crack tip, as in every crack geometry known, but the energy associated with these stresses remains constant as the crack moves and therefore cannot drive the crack. 

The prestressed joint may now be tested by two methods peeling or lap stretching, as shown in Fig. 15.14. In T peeling, the joint is weaker than before as a result of the prestress. The elastic energy stored in the material helps the crack to propagate. An energy balance shows that the peel force is now given by... 

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