Moisture changes and surface checking development during drying of Masson pine wood

Abstract

Deformation and surface-checking defects in wood are closely related to the drying stress caused by shrinkage and moisture content (MC) gradients throughout the drying process. In this study, 50-mm-thick Masson pine (Pinus massoniana Lamb.) lumber was used as the research object. The drying stress caused by shrinkage behavior and MC changes was monitored, and the development of surface checks was observed during the drying process. Results showed that when the average MC was 60%, the free water in the surface layer of lumber evaporated completely, causing the MC to decrease below the fiber saturation point (FSP). The surface layer was subjected to tension stresses, resulting in surface checks which primarily distributed in the latewood area and propagated longitudinally. When the average MC was between 30% and 40%, the combined effects of the high MC gradient and anisotropic shrinkage of the wood resulted in further expansion of the checks. As the average MC decreased to less than 20%, the MC of the core layer lumber decreased below the FSP, resulting in the closure or even disappearance of most surface checks. The critical point (average MC of 60%) of drying stress was obtained, which can benefit the fine-tuning of kiln-drying schedules and improve the drying quality of Masson pine wood.