Viscoelastic properties of green wood across the grain measured by harmonic tests in the range 0-95 degrees C: Hardwood vs. softwood and normal wood vs. reaction wood

The viscoelastic properties of wood have been investigated with a dynamic mechanical analyser specifically developed for wooden materials, the WAVE(T) device. Measurements were carried out on four wood species in the temperature range 0-100 degrees C at frequencies varying between 5 mHz and 10 Hz. Wood samples were tested under water-saturated conditions in the 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 internal friction were higher in the tangential than in the radial direction. This behaviour is attributed to the fact that anatomical elements act as a function of the direction. The viscoelastic behaviour of reaction wood differs from that of normal or opposite wood. Compression wood of spruce, which has a higher lignin content, is denser and stiffer in transverse directions than normal wood, and has a lower softening temperature (T,,). In tension wood, the G-layer is weakly attached to the rest of the wall layers. This may explain why the storage modulus and softening temperature of tension wood are lower than those for opposite wood. We also demonstrate that the time-temperature equivalence fits only around the transition region, i.e., between T-g and T-g + 30 degrees C. Apart from these regions, the response of wood reflects the combined effects of all its constitutive polymers, so that the equivalence is no longer valid.