Hygro-mechanical response of clear softwood specimens to compression under cyclic climate Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/nc580p46s

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  • Hygro-mechanical effect is a general term applied to all phenomena that involves interaction between moisture content and mechanical characteristics of hygroscopic materials by analogy to thermo-mechanical effects. The hygromechanical effects include (but are not limited to) free shrinkage/swelling, effect of moisture on elastic modulus, strength, creep, failure mechanisms, and mechano sorption. Of those, the particular focus of this project was the mechano-sorptive effect, which is observed as an additional deformation when wooden materials are exposed to combined mechanical loading and moisture content changes. The principle objective of this research program is to create a systematic database of clearly defined hygro-mechanical characteristics for commonly used wood species. Such database is needed for accurate modeling and prediction of long-term behavior of wood and wood-based materials in structures exposed to changing environment. The specimens in this program included four softwood species: Red spruce, Douglas fir, Western hemlock and Ponderosa pine. In this project, small longitudinal specimens were exposed to compression stress along the longitudinal axis (16-22% of the ultimate compressive stress) at room temperature (about 22±1° C) and varying moisture contents (MC). Cyclic changes of MC were executed in a small climate chamber by varying the relative humidity (RH) between 90% and lower RH (40%-50%). Free shrinkage and swelling was measured on unloaded reference specimens exposed to the same conditions. In a separate reference test series, specimens were subjected to sustained compressive load at a constant humidity of 65%, in order to determine the effect of visco-elastic deformation. Some specimens for this reference tests were wrapped in Parafilm in order to protect them from even small variations of the ambient humidity. All deformations were measured by comparing successive digital images of the specimens using digital image correlation (DIC) software. The mechano-sorptive deformation characteristics were calculated by subtracting the free swelling/shrinkage and visco-elastic deformation determined on the reference specimens and specimens tested in constant humidity, respectively, from the total deformation measured on the loaded specimens tested in cyclic climate. Apart from the mechano-sorptive characteristics, the output of the test protocol included: 1) free shrinkage/swelling coefficients for longitudinal and radial directions, determined on reference specimens; and 2) elastic modulus for compression in longitudinal direction and related Poisson ratios.
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