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 mo dified epoxy joint on consideration of the viscoelastic relaxation pro cesses of an adhesive. The investigation of the epoxy joint properties , over a range of strain rates ((gamma) over dot = 10(-2) to 10(4) s(- 1)) and temperatures (-30, 24, 60, 80 degrees 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 t o explain the strain rate sensitivity of the yield stress in terms of a difference in relaxation times at low and high strain rates (alpha a nd beta). Our purpose is to confirm these results by applying the Esca ig model, which reviews thermoset behavior in terms of a thermally act ivated dislocation propagation mechanism. The Bauwens approach and the Escaig model lead to the same conclusion: They indicate that there is a critical strain rate (gamma) over dot(beta) (T), correlated to a cr itical temperature T-beta ((gamma) over dot), that corresponds to the limits between the two modes of deformation required to free the diffe rent kind of molecular motions implied in the deformation process, the alpha mode to the alpha + beta mode. But at low temperature (30 degre es C), these models are no longer valid, which means that there is a h eterogeneous deformation process, characterized by local molecular mot ions, which involve a decrease of the polymer entropy and a permanent evolution of molecular structure.