Moisture response characteristics of the rheological behavior of heat-treated North American alder wood during the moisture absorption process

The temperature and relative humidity (RH) have an impact on the moisture absorption rate, moisture distribution, and swelling behavior of wood, which can lead to wood absorptive rheological phenomena such as swelling stress and swelling strain. This study examined the absorption and rheological characteristics of heat-treated (HT) and conventionally kiln-dried (CKD) North American alder wood under different conditions (50, 70, and 90 °C at 90 % RH) from an absolutely dry state to target moisture content (MC) levels (5 %, 10 %, and 15 %). The results showed that heat treatment significantly reduced the moisture absorption rate, the difference in MC, and the radial swelling ratio of the wood, with maximum reductions of 0.94-points, 1.54-points, and 0.89-points, respectively. The differential swelling between wood’s surface and core generates stresses and swelling strains ( ε E , ε V , ε M ). These stresses showed a trend opposite to that of the drying stress. Heat treatment notably reduced the mechano-sorptive swelling creep strain (MSCS) in North American alder wood (47.3 % lower at 70 °C and 0.7 % lower at 90 °C). In contrast, there were no significant differences in free swelling strain, elastic swelling strain, and viscoelastic swelling creep strain between HT and CKD wood.