Evaluation of transverse shrinking and swelling of bamboo using digital image correlation technique

Bamboo is structural gradient materials with hygroscopic properties. Understanding the variations in hygro-deformation behavior across the radial direction is crucial for managing the dimensional stability of bamboo. Herein, high-resolution microscopy and the digital image correlation (DIC) technique were combined to analyze the transverse shrinking and swelling of bamboo. Parenchyma cells were utilized as natural "tracking points" for DIC analysis to evaluate full-field strain at different radial positions, specifically the outer, middle and inner regions. The transverse shrinking and swelling of bamboo were found to exhibit a quasi-linear distribution in the tangential direction and a saddle-like distribution in the radial direction. The outer region, which had a higher fiber fraction, displayed greater and more isotropic hygro-deformation compared to the inner regions with fewer fibers. When relative humidity (RH) increased from 0% to 97%, the swelling ratio in the radial direction was 2.00, 1.55 and 1.22% for the outer, middle and inner regions, respectively. In the tangential direction, the swelling ratio was 1.75, 1.35 and 1.10%. The study also highlighted the differences in hygro-deformation between the fiber and parenchyma cells, with the fiber experiencing both greater shrinking and swelling compared to the parenchyma. Furthermore, the interactions between the fibers and parenchyma varied across different radial regions. Overall, this study demonstrated the use of individual bamboo cells as natural tracking points in DIC analysis and contributed to a better understanding of the relationships between bamboo and water, as well as the radial variation in shrinking and swelling of bamboo.