A close-up view of the intra-ring variation of transverse shrinking and swelling in Chinese fir using digital image correlation without artificial speckle pattern

Herein, an intra-ring variation of shrinking and swelling in Chinese fir was analyzed through high-resolution microscopy and digital image correlation (DIC). Instead of using artificial speckle patterns, tracheid cells were utilized as natural “tracking points” for DIC analysis to assess full-field strain as a function of the relative position in the growth ring. Wide-angle X-ray diffraction characterization showed consistent microfibril angle (~ 11°) throughout the growth ring. High-resolution changes of strain were obtained as increments of hygro-deformation from earlywood (EW) to transition wood and latewood (LW). Strain concentrations were observed at the interface of EW and LW from the previous growth ring. The hygro-deformations in both radial and tangential directions decreased sharply from the previous LW until a certain position within EW, i.e., larger hygro-deformation of EW at the interface was observed due to the restraining effect of the previous LW on EW. During moisture desorption, the restraining effect and anisotropic deformation were independent of the relative humidity (RH) level. However, the restraining effect on EW was weakened with increasing RH levels during moisture absorption, resulting in greater anisotropic deformation of the restrained EW. No obvious swelling hysteresis was found when considering hygro-deformation as a function of moisture content. This research demonstrated the feasibility of using DIC without artificial speckle patterns to characterize the intra-ring shrinking and swelling. The findings provide insight into explaining wood–water relations and the adaptive potential of hierarchical wood structure in modulating hygro-deformation.