Hygro-mechanical behavior of red spruce in tension parallel to the grain
The principal objective of the project was to provide a reliable testing protocol for determination of the material-level (e.g. local and decoupled from the artifacts of the test protocol) mechano-sorptive properties of wood in the longitudinal direction that could be used for modeling of the long-term structural response of wood and wood composite elements. The method also involves determination of the hygro-mechanical characteristics of free shrinkage and swelling and short-term viscoelastic characteristics from reference tests performed on matched specimens. Tensile creep tests in the longitudinal direction at varying climate conditions were performed on small (1-mm × 25-mm × 300-mm) clear specimens of red spruce (Picea rubra). All tests were conducted in a temperature-controlled environment. Optical deformation measurement techniques were used. Strains were calculated by comparing successive digital images using Digital Image Correlation (DIC) principles. The mechano-sorptive component of total strains measured on the loaded specimens was separated by: 1) subtracting free shrinkage/swelling measured on matched reference specimens; and 2) subtraction of the magnitude of viscoelastic creep measured separately on matched specimens at constant MC (in ‘dry’ and ‘wet’ conditions). The results confirmed earlier findings reported in the literature by other researchers that the effect of cumulative moisture content change on mechanosorptive compliance is not linear. However, no fundamentally different governing mechanisms during the first and consecutive moisture cycles were observed. The effects of applied stress level and initial moisture content on the mechano-sorptive response of wood in tension were found insignificant at the 95% confidence level. The experimentally determined mechano-sorptive compliances were expressed in terms of generalized rheological model equations with cumulative moisture content change (rather than time) as the independent variable. Based on these findings, a minimal testing protocol was proposed for routine determination of hygro-mechanical characteristics for other structurally important species.