Simultaneous monitoring of moisture and deformation during wood drying using integrated hyperspectral imaging and digital image correlation

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Abstract

The anatomical heterogeneity of wood, particularly the structural difference between earlywood and latewood, leads to non-uniform moisture transport and differential shrinkage, often resulting in drying-induced defects. To elucidate the complex coupling mechanism between moisture distribution and mechanical deformation, this study establishes an integrated monitoring framework combining Hyperspectral Imaging (HSI) and Digital Image Correlation (DIC). A critical technical challenge, the optical interference of conventional carbon-based DIC speckle patterns with NIR moisture analysis—was resolved by utilizing a screened NIR-transparent ink. This novel patterning approach enabled high-contrast strain tracking in the visible spectrum without perturbing the underlying spectral moisture signal. Experiments conducted on Japanese cypress ( Chamaecyparis obtusa ) demonstrated that the PLS-calibrated HSI model achieved robust predictive accuracy (R 2 > 0.92, RMSE = 0.072) on independent specimens. The co-registered spatiotemporal analysis revealed that macroscopic deformation is governed by non-linear coupling mechanisms: while latewood exhibited lower initial moisture content (MC) and higher shrinkage, earlywood regions experienced transient tensile strain during the early drying stage due to asynchronous shrinkage onset. Furthermore, a "strain inversion" phenomenon was observed in later drying stages, driven by the cumulative moisture loss history relative to the fiber saturation point. This framework is intended to serve as a methodological bridge between microscale wood anatomy and macroscale drying behavior, providing a mechanistic basis for understanding drying-induced defect formation at the growth-ring scale and offering a foundation for future work aimed at optimizing drying schedules for larger-dimension timber.

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