Understanding the effect of growth ring orientation on the compressive strength perpendicular to the grain of thermally treated wood

Thermal treatment (TMT) impacts the mechanical strength of wood. Investigating the compressive strength (CS) of thermally treated wood with variable growth ring orientation is helpful to better understand the effect of TMT on mechanical performance of wood. Specimens, cut from spruce wood, were classified according to two growth ring orientations, namely 10 degree (f_c10) and 40 degree (f_c40), these two situations commonly exist in practice. Six replicates from each type were thermally treated with saturated steam at two different temperatures (180 °C and 210 °C), respectively. Dimensional changes, chemical composition, and microstructure of the specimens were monitored before and after TMT using FTIR spectroscopy and optical microscopy. Compressive stress of specimens was determined and, simultaneously, strain distribution was recorded using digital image correlation. The results show that TMT caused mass loss and shrinkage in tangential and radial directions. Changes in chemical composition were mainly due to hemicellulose degradation. For both treated and untreated samples, the CS perpendicular to the grain and deformation of f_c10 samples were higher and smaller than that of f_c40 samples, respectively. TMT results in brittle wood tissue, which hampered strain transfer along growth rings and changed failure modes of samples. Mechanical strength of f_c40 samples was sensitive to be impacted by TMT. Prior to TMT, failure modes of f_c10 and f_c40 specimens were shear deformation and growth ring buckling, respectively. After TM, failure modes of all specimens changed to shear failure parallel to growth ring boundaries. These structural changes mainly occurred in earlywood. The output of this work contributes to the effective application and scientific evaluation of mechanical performance of TMT wood by considering growth ring orientations.