Measuring Molecular Strain in Rewetted and Never-Dried Eucalypt Wood with Raman Spectroscopy

To measure growth strain in wood using Raman spectroscopy, we investigated the Raman spectra of rewetted (water-saturated) Eucalyptus regnans and green Eucalyptus quadrangulata wood during tensile tests. Partial least squares models to predict the tensile strain were built from the Raman spectra. The best model could predict the tensile strain with a root mean square error of 427.5 με. Apart from the widely reported band shift at 1095 cm^-1 upon mechanical strain, spectral changes at 1420, 1120, 895, and 456 cm^-1 were identified. The assignments of these bands were discussed in relation to the molecular deformation of cellulose. The band shift rates during tensile tests were -3.06 and -2.15 cm^-1/% for rewetted E. regnans and green E. quadrangulata wood, respectively. We successfully detected the release of the molecular growth strain in green eucalyptus wood with Raman spectroscopy by observing band shifts of the 1095 cm^-1 signal. Further, there was a moderate correlation (r = 0.48) between the growth strain measured with strain gauges and the 1095 cm^-1 band position. The precision of the prediction of growth strain using Raman spectroscopy was negatively affected by variation attributed to the inhomogeneity of wood on the millimeter scale and instrumental instability.