Hygro-Mechanical Behavior of Red Spruce in Tension Parallel to the Grain
Keywords:Solid wood, tension, creep, mechano-sorption, experimental methodology
AbstractThe 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 x 25-mm x 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 mechano-sorptive 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.
Armstrong, L.D. and S. T. Kingston. 1960. Effect of moisture content changes on creep of wood. Nature185 (4716):862-863.nBazant, Z. P. 1985. Constitutive equation of wood at variable humidity and temperature. Wood Sci. Technol.19: 150-177.nBazant, Z. P., and X. Yunping. 1994. Drying creep of concrete: Constitutive model and new experiments separating its mechanisms. Mater. Struct. 27nBengtsson, C. 2001a. Short-term mechano-sorptive creep of well-defined spruce timber. Holz Roh- Werkst.59(1/2):117-128.nBengtsson, C. 2001b. Mechano-sorptive bending creep—Influence of material parameters. Holz Roh- Werkst.59(4):229-236.nBodig, J., and B. A. Jayne. 1993. Mechanics of wood and wood composites. Kreiger Publ. Co., Malabar, Fl. 711 pp.nBruck, H. A., S. R. McNeill, M.A. Sutton, and W. H. I. Peters. 1989. Digital image correlation using Newton-Raphson method of partial differential correction. Exp. Mech.28(3):261-267.nChoi, D., J. L. Thorpe, and R. B. Hanna. 1991. Image analysis to measure strain in wood and paper. Wood Sci. Technol.25(2):251-262.nChoi, S., and S.P. Shah. 1997. Measurement of deformations on concrete subjected to compression using image correlation. Exp. Mech.37(3):307-313.nGerhards, C. 1982. Effect of moisture content and temperature on the mechanical properties of wood: An analysis of immediate effects. Wood Fiber Sci.14(1):4-36.nGrossman, P. 1976. Requirements for a model that exhibits mechano-sorptive behavior. Wood Sci. Technol.10:163-168.nHisada, T. 1986. Creep and set behavior of wood related to kiln drying. Forestry and Forest Products Research Institute. Rept. No. 335. Ibaraki, Japan. 31-130.nHunt, D. G. 1997. Dimensional change and creep of spruce and consequent model requirements. Wood Sci. Technol.31(1):3-16.nLagana, R. 2005. Development of small scale experimental protocol and multi-physics model to predict the complex hygro-mechanical behavior of wood under varying climates. Ph.D. dissertation, Forest Management Dept., University of Maine, Orono, ME.nMorlier, P. ed., 1994. Creep in timber structures. Report of Rilem Technical Committee. 112-Tsc. E & FN Spon, London, UK. 149 pp.nMott, L., S. M. Shaler, and L. H. Groom. 1996. A novel technique to measure strain distributions in single wood fibers. Wood Fiber Sci.28(4)429-437.nMuszynski, L. and P. Olejniczak. 1996. A simple experimental method to determine some basic parameters for mechano-sorptive creep model for wood. Pages 479-483 in Proc. 5th International IUFRO Wood Drying Conference, August 13-17, Quebec, PQ, Canada.nMuszynski, L., R. Lopez-Anido, and S. M. Shaler. 2000. Image correlation analysis applied to measurement of shear strains in laminated composited. Pages 163-166 in Proc. SEM IX International Congress on Experimental Mechanics, June 5-7, 2000, Orlando, FL,nMuszynski, L., F. Wang, and S. M. Shaler. 2002. Short term creep tests on phenol resorcinol formaldehyde (PRF) resin undergoing moisture content changes. Wood Fiber Sci.34(4):612-624.nMuszynski, L., R. Lagana, W. Davids, and S. M. Shaler. 2005. Comments on the experimental methodology for determination of the hygro-mechanical properties of wood. Holzforschung59(2):232-239.nPerkitny, T. 1951. Research on swelling pressure of wood (in Polish). Panstwowe Wydawnictwo Rolnicze I Lesne, Warszawa.nRanson, W. F., M. A. Sutton, and W. H. Peters. 1987. Holographic and speckle interferometry. Pages 388-429 in A.S. Kobayashi, ed. SEM Handbook of Experimental Mechanics. Prentice-Hall, Inc., New Jersey.nRanta-Maunus, A. 1975. The viscoelasticity of wood at varying moisture content. Wood Sci. Technol.9:189-205.nRanta-Maunus, A. 1989. Analysis of drying stress in timber. Paperi ja Puu—Paper and Timber10:1120-1122.nRybarczyk, W. 1973. Study on the development of mathematical model of mechanical properties of some wood materials undergoing changes in their moisture content. (in Polish) Prace Instytutu Technologii Drewna66(2): 17-138.nSalin, J.-G. 1992. Numerical prediction of checking during timber drying and new mechano- sorptive creep model. Holz Roh-Werkst.50:195-200.nSchneiwind, A. 1968. Recent progress in the study of the rheology of wood. Wood Sci. Technol.2:188-206.nSutton, M. A., and Y. J. Chao. 1988. Measurement of strains in a paper tensile specimen using computer vision and digital image correlation. TAPPI J.71(3):173-175.nVendroux, G., and W. G. Knauss. 1998. Submicron deformation field measurements: Part 2. Improved digital image correlation. Exp. Mech.38(2):86-92.nWu, Q., and M. Milota. 1996. Mechano-sorptive deformation of Douglas-fir specimens under tangential tensile stress during moisture adsorption. Wood Fiber Sci.28(1): 128-132.n
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