Transverse Hygroscopic Swelling of Elm Earlywood and Latewood and its Implications for Wood Product Design

Abstract

Ring-porous hardwoods contain marked anatomical differences between earlywood and latewood that can produce heterogeneous hygroscopic deformation. This exploratory study monitored growth-ring-scale hygroscopic deformation in one elm (Ulmus pumila L.) specimen using stereomicroscope-based two-dimensional digital image correlation (2D-DIC), while time-domain nuclear magnetic resonance (TD-NMR) measurements on four companion specimens were used to estimate moisture content at corresponding adsorption times. Tangential and radial displacement fields evolved progressively and remained spatially heterogeneous during adsorption. Local profiles at an earlywood position and a latewood position showed different responses, but these differences were interpreted as local deformation heterogeneity because the two positions experienced different boundary conditions. After 120 h, mean moisture content reached 25.0%, whereas median tangential and radial Green-Lagrangian swelling strains reached 0.0316 and 0.0169, respectively. In the monitored DIC specimen, tangential strain exceeded radial strain at all investigated time points, while additional strain increments were relatively small from 22 to 120 h, during which the companion specimens continued to absorb moisture. These results demonstrate transverse swelling anisotropy together with growth-ring-scale spatial heterogeneity and provide a material basis for assessing moisture-related dimensional stability and anatomical orientation in elm wood products.