Full-field moisture-induced strains of the different tissues of tamarack and red oak woods assessed by 3D digital image correlation

Abstract

The full-field moisture-induced strains (in-plane) and Z-displacements (out-of-plane) of the different tissues of tamarack and red oak woods were assessed by three-dimensional digital image correlation (3D-DIC). Swelling strains in tangential (Ɛxx) and radial (Ɛyy) directions and Z-displacements in axial direction were measured on the cross section of wood samples from dried condition below P2O5 to moisture adsorption condition at 20 °C and 90% RH. Global swelling coefficients (α) were calculated and compared with 3D-DIC strains. Ɛxx, Ɛyy, and Z-displacement data were extracted from earlywood (EW) and latewood (LW) of tamarack, and from three tissue zones of red oak: large vessel zone of the earlywood (EW-v), fiber-dominated (LW-Fiber) and axial parenchyma-dominated (LW-Pa) zones of the latewood. The localized full-field strains were matched with density data obtained by X-ray densitometry. Tamarack wood had a homogeneous Ɛxx, while different Ɛyys were observed for EW and LW. Red oak wood had a more complex mapping of full-field strains, in which the effects of the broad rays on Ɛxx and differences between tissues on Ɛyy appear. A discussion about the radial density profile data and the full-field strains at the tissue level as well as the anisotropic behavior of different tissues is presented for both species. An opposite behavior was found for the EW and LW of tamarack in the longitudinal direction (Z-displacement), showing that EW swells and LW shrinks after moisture adsorption. For red oak, EW-v also swelled and LW-Fiber shrank in the longitudinal direction. LW-Pa zone also shrank after adsorption but to a lower degree.