The extension of the Johnson-Kendall-Roberts (JKR) theory of elastic contact to a linear viscoelastic material is addressed with particular emphasis on the debonding phase of a JKR test. We show that the contact stress at the moving edge of the contact zone is tensile and is characterized by a stress intensity factor, KI(t). In contrast to the elastic JKR theory, KI(t) is no longer determined by the current contact area and applied load, but depends on the entire history of loading. Furthermore, KI(t) no longer uniquely determines the energy flow (G) to the material at the moving edge of the contact zone since G is coupled to the local separation or bonding process. This means that one cannot obtain the work of adhesion (W) directly from experimental data. Since the mechanics of viscoelastic deformation is invariably coupled to the separation or bonding process, the adhesion of viscoelastic bodies can no longer be completely characterized by a single parameter (i.e. W) approach. The theory presented here suggests that a better way to characterize the adhesion of viscoelastic materials is to relate the rate of bonding or debonding of surfaces to the KI(t). Procedures for obtaining KI(t) from experimental data are given for the bonding and debonding phases of both force and displacement controlled tests.
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