Dynamic mechanical analysis of dry wood: Linear viscoelastic response region and effects of minor moisture changes

The dynamic mechanical analysis (DMA) of wood with moisture content (MC) below 1% has not yet been described. Aiming at this low MC range, the linear viscoelastic response region (LVR) of thoroughly dried wood was studied in single-cantilever bending. The LVR limit was determined as a function of grain orientation and temperature using yellow-poplar (Liriodendron tulipifera) and southern yellow pine (Pinus spp.). The LVR limit for dry wood ranged from approximately 0.03% to 0.16% strain. The LVR limit was greater for bending perpendicular to the grain than for parallel to the grain, suggesting that DMA signal quality would be better in the former case. Southern pine generally exhibited a greater LVR limit than yellow-poplar. The LVR anisotropy was greater in yellow-poplar than in southern pine. These findings suggest that detailed LVR analysis might be useful for wood analysis, or at least that regular LVR analysis is required for reliable wood DMA. The effects of wood moisture changes (between 0% and 1%) were observed in low-temperature secondary relaxations, consistent with the previous findings of others. Yellow-poplar specimens exhibited a significant storage modulus increase over a 250°C temperature range when specimen moisture increased from 0% to ∼0.7%.