  In this paper, the viscoelastic properties of wood under water-saturated
conditions are investigated from 10\degree C to 135\degree C using the WAVET*
apparatus. Experiments were performed via harmonic tests at two frequencies
(0.1 Hz and 1 Hz) for several hours. Four species of wood were tested in the
radial and tangential material directions: oak (Quercus sessiliflora), beech
(Fagus sylvatica), spruce (Picea abies) and fir (Abies pectinata). When the
treatment is applied for several hours, a reduction of the wood rigidity is
significant from temperature values as low as 80-90\degree C, and increases
rapidly with the temperature level. The storage modulus of oak wood is divided
by a factor two after three hours of exposure at 135\degree C. This marked
reduction in rigidity is attributed to the hydrolysis of hemicelluloses. The
softening temperature of wood is also noticeably affected by hygrothermal
treatment. After three short successive treatments up to 135\degree C, the
softening temperature of oak shifted from 79\degree C to 103\degree C, at a
frequency of 1 Hz. This reduction in mobility of wood polymers is consistent
with the condensation of lignins observed by many authors at this temperature
level. In the same conditions, fir exhibited a softening temperature decreasing
of about 4\degree C. In any case, the internal friction clearly raises.
