Viscoelastic relaxations in polyepoxide joints related to the strength of bonded structures at impact rate shear loading

The aim of this paper is to show the excellent impact behavior of a modified epoxy joint on consideration of the viscoelastic relaxation processes of an adhesive. The investigation of the epoxy joint properties, over a range of strain rates ($ \mathop \gamma \limits^ \cdot $ = 10 ^?2 to 10⁴ S^?1) and temperatures (?30, 24, 60, 80°C), shows that there is a good correlation between high impact resistance and the presence of a secondary transition. We successfully applied the Bauwens approach to explain the strain rate sensitivity of the yield stress in terms of a difference in relaxation times at low and high strain rates (α and β). Our purpose is to confirm these results by applying the Escaig model, which reviews thermoset behavior in terms of a thermally activated dislocation propagation mechanism. The Bauwens approach and the Escaig model lead to the same conclusion: They indicate that there is a critical strain rate $ \mathop \gamma \limits^ \cdot $_β (T), correlated to a critical temperature T_β ($ \mathop \gamma \limits^ \cdot $), that corresponds to the limits between the two modes of deformation required to free the different kind of molecular motions implied in the deformation process, the α mode to the α + β mode. But at low temperature (?30°C), these models are no longer valid, which means that there is a heterogeneous deformation process, characterized by local molecular motions, which involve a decrease of the polymer entropy and a permanent evolution of molecular structure.