Full-field moisture induced deformation in Norway spruce: intra-ring variation of transverse swelling

The transverse hygro-expansion of Norway spruce wood is studied on the growth ring level using digital image correlation. This non-destructive technique offers the possibility for a contactless study of deformation fields of relatively large areas. The measured full-field strains are segmented into individual growth rings. Whereas radial strains closely follow the density progression with the maximum in the dense latewood (LW), tangential and shear strain remain constant except for positions around the edges of the sample. A simple FE three phase growth ring model is in good agreement with the experimental values. The selective activation of individual phases like earlywood (EW), transition wood and LW demonstrates that the radial hygro-expansion is dominated by the EW deformation, whereas tangential deformation is a complex interplay of expansion and compression that needs all tissues to fully develop