Hygroexpansion characterisation of Norway spruce branch wood investigated by microscopic computed tomography

Norway spruce branch wood is comprised of opposite wood (OW) and compression wood (CW) on opposite sides of its cross section. These wood types show different hygroexpansion properties to facilitate an upward bend of branches. Recently a first attempt was made to characterise these properties using a combination of manual measurements and microscopic X-ray Computed Tomography (XCT) tomograms. This paper extended the study with a detailed analysis of hygroexpansion of Norway spruce branch wood at a mesoscopic level. Medical image registration was performed to analyse the sorption-induced strain using XCT tomograms of OW and CW specimens acquired at four different relative humidities (RH). The study is supplemented with average estimations of strains manually measured, of average moisture content (MC) and micro-fibril angle. The results confirmed higher average strain values in radial and tangential direction for OW than for CW, CW experienced higher longitudinal strains than OW. In both CW and OW, the radial strain follows the density distribution with higher values in latewood than in earlywood. The average MC values at the different RHs are not significantly different between OW and CW, indicating that the differences in hygroexpansion cannot be explained by differences in sorption.