Hygro-thermal coupled swelling in Chinese fir wood

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Abstract

Wood used in building applications undergoes dimensional changes under varying hygro-thermal conditions, which may compromise durability and in-service performance. In this study, the full-field hygro-thermal swelling of Chinese fir [ Cunninghamia lanceolata (Lamb.) Hook.] was quantitatively characterized under controlled conditions using digital image correlation technique. Experiments were conducted at temperatures of 25, 35 and 45 °C, and moisture content (MCs) of 0.6, 4.9 and 9.3 %. The results showed that hygro-thermal swelling significantly exceeded the linear superposition of thermal expansion and hygroscopic swelling, indicating a positive synergistic coupling effect that intensified with increasing temperature and MC. For instance, in latewood at 45 °C and 9.3 % MC, the hygro-thermal coupled component accounted for 22.7 % of the total radial swelling. Notably, increasing MC markedly enhanced the thermal expansion coefficients in both radial and tangential directions, which was attributed to plasticization effect that weakened intermolecular constraints and promoted segmental mobility within the cell wall. Furthermore, latewood showed greater hygro-thermal coupled swelling than earlywood, consistent with its higher hemicelluloses and greater susceptibility to moisture-assisted mechanical relaxation. These findings improve the understanding of wood-moisture relationships, and offer guidance for enhancing dimensional stability under combined temperature and moisture variations.

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