11 research outputs found
Viscoelastic properties of green wood across the grain measured by harmonic tests in the range of 0\degree C to 95\degree C. Hardwood vs. softwood and normal wood vs. reaction wood
The viscoelastic properties of wood have been investigated with a dynamic
mechanical analyser (DMA) specifically conceived for wooden materials, the
WAVET device (environmental vibration analyser for wood). Measurements were
carried out on four wood species in the temperature range of 0\degree C to
100\degree C at frequencies varying between 5 mHz and 10 Hz. Wood samples were
tested in water-saturated conditions, in radial and tangential directions. As
expected, the radial direction always revealed a higher storage modulus than
the tangential direction. Great differences were also observed in the loss
factor. The tan\delta peak and the internal friction are higher in tangential
direction than in radial direction. This behaviour is attributed to the fact
that anatomical elements act depending on the direction. Viscoelastic behaviour
of reaction wood differs from that of normal or opposite wood. Compression wood
of spruce, which has higher lignin content, is denser and stiffer in transverse
directions than normal wood, and has lower softening temperature (Tg). In
tension wood, the G-layer is weakly attached to the rest of the wall layers.
This may explain why the storage modulus and the softening temperature of
tension wood are lower than those for the opposite wood. In this work, we also
point out that the time-temperature equivalence fits only around the transition
region, i.e. between Tg and Tg + 30\degree C. Apart from these regions, the
wood response combines the effect of all constitutive polymers, so that the
equivalence is not valid anymore