Objective The valuable timber tree species Castanopsis hystrix is prone to cracking during processing and utilization, which severely affects its yield and added value. Currently, there is a lack of systematic research on the intrinsic mechanism of its susceptibility to cracking. This study aims to explore the multi-scale structural shrinkage behavior and mechanism of moisture variation on cracking in Castanopsis hystrix plantation wood, in order to provide a theoretical basis in terms of cracking prevention, wood drying, and modification.

Method To elucidate the dynamic process of wood cracking caused by moisture content changes, a non-contact full-field strain measurement system (VIC-3D) was coupled with low-field nuclear magnetic resonance (LF-NMR) to enable real-time in-situ monitoring of full-field strain distribution and moisture states (free water and bound water) of the heartwood and sapwood during the drying process. Combined with confocal laser scanning microscopy (CLSM) and small-angle X-ray scattering (SAXS), the deformation characteristics of fiber cells and the evolution of cellulose structure at different moisture content stages were analyzed, establishing a multi-scale correlation from macroscopic strain and mesoscopic moisture to microscopic structure.

Result (1) The drying shrinkage strain of Castanopsis hystrix wood decreases in a stepwise manner from the pith to the sapwood, with the strain in the heartwood being greater than that in the sapwood, and the tangential strain being greater than the radial strain. As the MC decreased to about 40%, radial microcracks appeared in both heartwood and sapwood; however, due to the increased tangential strain, numerous large and non-closing cracks appeared in the heartwood. (2) The transverse relaxation times (T₂) of both bound and free water in heartwood and sapwood decreases as the moisture content reduces. The crack propagation was closely related to the process of bound water expulsion. After further moisture loss, some cracks show a tendency to close. (3) From the cellular to molecular level, the cell walls and lumens of fiber in heartwood and sapwood underwent shrinkage. The tangential shrinkage rate of the fiber cells was higher than that in radial, and shrinkage rate in heartwood was higher than that in sapwood. The cellulose microfibril spacing decreased, with more pronounced changes observed in heartwood.

Conclusion The cracking of Castanopsis hystrix plantation wood results from the coordinated action of multi-scale shrinkage behavior induced by moisture variation. Among them, the expulsion of bound water is the key node that triggers cracking. When the moisture content drops to around 40%, the stress concentration induced by the coordinated effect leads to the initiation and propagation of microcracks. The heartwood, due to its greater dry shrinkage deformation at multiple scales, shows a more severe cracking degree. By integrating synchronous multi-field information coupling with multi-scale correlation analysis, this study elucidates the dynamic process by which moisture migration drives multi-scale shrinkage and subsequently induces cracking in Castanopsis hystrix wood, providing a scientific basis for optimizing its drying process and anti-cracking modification.