Hygro-mechanical behaviour of earlywood and latewood in Chinese fir during moisture adsorption under tangential tensile loading

Understanding the hygro-mechanical behaviour of wood is crucial for optimising the dimensional stability of wood-based products. In this study, free swelling (ϵFS) and hygro-mechanical deformation (ϵtotal) of earlywood (EW) and latewood (LW) within the same growth ring from Chinese fir (Cunninghamia lanceolata) were investigated during stepwise adsorption from 0% to 93% RH under tangential tensile load. The load-related deformation (ϵload) was quantified by subtracting the instantaneous elastic strain and ϵFS from ϵtotal. By combining water vapour sorption behaviour, the strain increments per unit moisture content (MC) were further evaluated. Results showed that MC in LW was slightly higher than that in EW, which was associated with its higher hemicellulose content and greater OH accessibility. Compared with EW, LW showed higher ϵFS and lower ϵtotal and ϵload. With increasing MC, ϵFS, ϵtotal, and ϵload in EW and LW generally increased, and ϵload in LW remained limited below 10% MC. Further, ϵFS and ϵtotal increments per unit MC gradually increased with increasing MC, while ϵload increment per unit MC showed a fluctuation with MC, and increased sharply above 10% MC for LW.