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In this paper, the viscoelastic properties of wood under water-saturatedconditions 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 theradial and tangential material directions: oak (Quercus sessiliflora), beech(Fagus sylvatica), spruce (Picea abies) and fir (Abies pectinata). When thetreatment is applied for several hours, a reduction of the wood rigidity issignificant from temperature values as low as 80-90\degree C, and increasesrapidly with the temperature level. The storage modulus of oak wood is dividedby a factor two after three hours of exposure at 135\degree C. This markedreduction in rigidity is attributed to the hydrolysis of hemicelluloses. Thesoftening temperature of wood is also noticeably affected by hygrothermaltreatment. After three short successive treatments up to 135\degree C, thesoftening temperature of oak shifted from 79\degree C to 103\degree C, at afrequency of 1 Hz. This reduction in mobility of wood polymers is consistentwith the condensation of lignins observed by many authors at this temperaturelevel. In the same conditions, fir exhibited a softening temperature decreasingof about 4\degree C. In any case, the internal friction clearly raises.

Viscoelastic properties of wood across the grain measured under water-saturated conditions up to 135\degree C: evidence of thermal degradation