Relationship between structural characteristics of growth rings and strain accumulation in thermally modified Douglas fir wood during compressive test

Evaluating and predicting mechanical properties according to structural characteristics of growth rings contributes to effective production and application of wood. Specimens, from Douglas fir wood, were compressed under perpendicular-to-grain load along the transverse direction of wood, and strain distribution was simultaneously recorded using digital image correlation (DIC). Growth-ring orientation, latewood ratio, width of latewood, and shear strain were obtained at hundreds of spots in earlywood adjacent to a growth ring. The compression ratio negatively relates to both the width of the latewood and the latewood ratio of the specimens. Shear strain is prone to accumulate in regions where growth-ring orientation deviates from 90° or −90°. Shear strain homogenously distributes along growth rings when growth-ring orientation is uniform and around 65°. For both with and without thermally modified specimens, there is a linear relationship between shear-strain value and structural characteristics. For the specimens without thermal modification, the location of shear-strain accumulation is possible to be predicted based on this relationship. Shear-strain accumulation is attributed to the deformation of cells in earlywood and cracks in latewood. The direction of cracks is along with wood rays and the boundary between latewood and earlywood when the growth-ring orientations are approximately 80° and 65° respectively.