  This work presents an innovative experimental device conceived to
characterize the time-dependent behavior of hygroscopic materials, as wood, at
controlled moisture content and temperature. This device, the WAVET, permits
the determination of the viscoelastic properties of samples, using harmonic
tests at frequencies varying from 5.10-3 Hz to 10 Hz. Conceived to work up to 5
bar, it allows tests in dry or water-saturated conditions to be performed over
the temperature range 0\degree C to 140\degree C. In spite of these severe
working conditions, the careful device design, together with a rigorous data
analysis, allows a rigorous determination of the storage and loss modulus and
the loss factor. The results collected for several species of wood emphasize
the ability of the WAVET to underline the influence of numerous parameters,
namely specie, material direction, anatomical and macromolecular structure, on
the rheological properties and notably on the softening temperature. The WAVET
also establishes a efficient tool to follow the modifications of the
constitutive amorphous polymers of materials submitted to hydrothermal
treatments, in relation to the evolution of their viscoelastic properties.
