An analysis of the swelling behavior of various woods using an optical microscope and a digital image correlation method (DIC)

Abstract

Swelling behaviors in the cross sections of various woods were measured using an optical microscope and a digital image correlation method (DIC). The interactions between 'earlywood and latewood' and 'longitudinal cells and rays' were observed to make clear the mechanism of anisotropy in swelling and shrinkage. It was suggested from the observations with akamatsu (Pinus densiflora) and western hemlock (Tsuga heterophylla) that the interaction between earlywood and latewood was the major factor of swelling anisotropy. Difference of tangential swelling between earlywood and latewood was large at the edge, but became smaller with the distance from the edge. Large shear strains were observed at annual ring boundaries where the density varies discontinuously. Distribution of swelling in radial and tangential direction of keyaki (Zelkova serrata) indicated that radial swlling was restricted by rays. Observation of swelling in red meranti (Shorea sp.) showed that diameter of pores decreased greatly in radial direction with the increase of water absorption. According to the results of 8 species (5 softwoods and 3 hardwoods), it was concluded that hardwoods had smaller swelling in radial direction than softwoods because of the restriction by rays and decrease of diameter of pores in radial direction.