Optical Measurement of Deformations in Drying Lumber

None of the destructive or nondestructive testing techniques for monitoring wood behavior during drying allow continuous quantitative measurements of the deformation and strain distribution in a section undergoing nonuniform moisture content changes in a kiln drying regime. In this study, a noncontact optical measurement technique based on digital image correlation (DIC) principles was used to visualize and measure the progressive deformation of wood subjected to simulated kiln drying conditions. A test apparatus was designed to simulate kiln drying conditions in such a way that a section of the specimen was accessible for optical measurements, and a hardwood and a softwood were dried at several temperatures. Full-field maps of principal strains were developed to illustrate the progress of the deformation gradients. The principal observations are that (1) the deformation of wood starts in the beginning of drying when the average moisture contents are well above the fiber saturation point, (2) the difference between the average deformations in the tangential and radial directions is much larger than expected, and (3) the samples swell a little in the radial direction early in drying.