Sub-T_g viscoelastic behaviour of chalcogenide glasses, anomalous viscous flow and stress relaxation

Stress relaxation and viscous flow of Ge_xSe_1-x and TAS chalcogenide glasses were studied using the "bending method". Inadequacy of common models or functions to describe relaxation process of inorganic glasses were proved and the origin of an unexpected relaxation behaviour is discussed. Even if most part of the stress relaxes for short period (10 to 20 days), a significant part still remains (from 5 to 33%) after over one year. A linear viscoelastic behaviour has been demonstrated in the sense of an independence of the relaxation function to the stress level. However, the apparent viscosity seems to increase of almost one order of magnitude in only one month (from 5 10¹⁵ to 3 10¹⁶ Pa·s for TAS fibers) independently of strain or stress level, that is characteristic of glass structural changes.