Effect of Stress Level on Bending Creep Behavior of Wood During Cyclic Moisture Changes
Keywords:Creep, cyclic moisture changes, bending, stress level
AbstractThis study investigated the effect of stress level on bending creep behavior of Japanese cedar (Cryptomerica japonica D. Don) during cyclic moisture changes. Tests were made at 20° with four cyclic relative humidity changes between 65% and 95% under four stress levels: 12.6, 18.5, 24.3, and 30.2 MPa, corresponding to 17, 25, 33, and 42% of short-term breaking stress, respectively. The effect of moisture content (MC) change on elastic compliance and mechano-sorptive (MS) compliance was examined. The results indicated that the total compliance revealed different behavior under various stress levels over the history of cyclic moisture changes and appeared to be greater under higher stress. Elastic compliance increased linearly with MC and affected the behavior of total compliance during MC change. As MC change increased, the MS compliance during the first adsorption and all desorption increased linearly, while during subsequent adsorption revealed a decrease in lower stress and an increase in higher stress. At a given stress level, the first adsorption led to greater MS deformation than did desorption and subsequent adsorption. The elastic parameter KE for MC effect on elastic compliance and the MS parameter KM for relationship between MS compliance and MC increased as quadratic functions of applied stress. The rate of increase in the parameters accelerated above about 25 to 33% stress level. As a result, when MC is cycled, MS compliance increase occurs at lower stress levels relative to creep at constant MC.
Armstrong, L. D. 1972. Deformation of wood in compression during moisture movement. Wood Sci. 5:81-86.nArmstrong, L. D., and G. N. Christensen. 1961. Influence of moisture change on deformation of wood under stress. Nature 191(4791):869-870.nArmstrong, L. D., and R. T. Kingston. 1962. The effect of moisture content changes on the deformation of wood under stress. Aust. J. Appl. Sci. 13:257-276.nDinwoodie, J. M., D. J. Robson, B. H. Paxton, and J. S. Higgins. 1991. Creep in chipboard. Part 8: The effect of steady-state moisture content, temperature and level of stressing on the relative creep behavior and creep modulus of a range of boards. Wood Sci. Technol. 25: 225-238.nFushitani, M. 1968. Effect of delignifying treatment on static viscoelasticity of wood. IV. Strain dependence of stress relaxation and stress dependence of creep. Mokuzai Gakkaishi 14(3):166-171.nGerhards, C. C. 1982. Effect of moisture content and temperature on the mechanical properties of wood: An analysis of immediate effect. Wood Fiber 14(1):4-36.nHearmon, R. F. S., and J. M. Paton. 1964. Moisture changes and creep of wood. Forest Prod. J. 14(8):357-359.nHunt, D. G. 1984. Creep trajectories for beech during moisture changes under load. J. Mater. Sci. 19:1456-1467.nHunt, D. G., and C. F. Shelton. 1987. Progress in the analysis of creep in wood during concurrent moisture changes. J. Mater. Sci. 22:313-320.nKingston, R. S. T., and L. N. Clarke. 1961. Some aspects of the rheological behavior of wood. I and II. Aust. J. Appl. Sci. 12(4):211-226, and 227-240.nLeicester, R. H. 1971. A rheological model for mechanosorptive deflections of beams. Wood Sci. Technol. 5: 211-220.nLu, W., and R. W. Erickson. 1996. Mechanosorptive behavior of solid wood stressed in compression perpendicular to the grain. Forest Prod. J. 46(4):63-68.nNakano, T. 1996a. A theoretical description of creep behavior during water desorption. Holzforschung 50:49-54.nNakano, T. 1996b. Viscosity and entropy change in creep during water desorption for wood. Wood Sci. Technol. 30:117-125.nNorimoto, M., and J. Giril. 1989. Wood bending using microwave heating. J. Microwave Power Electromag. Energy 24(4):203-212nOta, M., and Y. Tsubota. 1966. Studies on the fatigue of 2-ply laminated wood. I. Several investigations on the static viscoelasticity behaviors of wood subjected to bending test. Mokuzai Gakkaishi 12(1):26-29.nOzawa, M., M. Fushitani, K. Sato, and T. Koto. 1995. Stress dependence of bending creep behavior of wood under changing moisture conditions. Mokuzai Gakkaishi 41(3):281-288.nRanta-Maunus, A. 1975. The viscoelasticity of wood at varying moisture content. Wood Sci. Technol. 9:189-205.nRice, R. W., and R. L. Youngs. 1990. The mechanism and development of creep during drying of red oak. Holz Roh-Werkst. 48:73-79.nSawada, M. 1957. Studies on the mechanics of wood beams (Report 4): Bending creep in wood under dead loading. Bull. For. Exp. Sta. Meguro Tokyo 98:85-116.nSchniewind, A. P. 1968. Recent progress in the study of rheology of wood. Wood Sci. Technol. 2:188-206.nSugiyama, H. 1957. The creep deflection of wood subjected to bending under constant loading. Trans. Architect. Instit. Japan 55:60-70.nSzabo, T., and G. Ifju. 1970. Influence of stress on creep and moisture distribution in wooden beams under sorption conditions. Wood Sci. 2(3): 159-167.nWu, Q., and M. R. Milota. 1995. Rheological behavior of Douglas-fir perpendicular to the grain at elevated temperature. Wood Fiber Sci. 2(3):285-295.nWu, Q. 1996. Mechano-sorptive deformation of Douglas-fir specimens under tangential tensile stress during moisture adsorption. Wood Fiber Sci. (1):128-132.nZhou, Y., M. Fushitani, T. Kubo, and M. Ozawa. 1999. Bending creep behavior of wood during cyclic moisture changes. J. Wood Sci. 45(2): 113-119.n
The copyright of an article published in Wood and Fiber Science is transferred to the Society of Wood Science and Technology (for U. S. Government employees: to the extent transferable), effective if and when the article is accepted for publication. This transfer grants the Society of Wood Science and Technology permission to republish all or any part of the article in any form, e.g., reprints for sale, microfiche, proceedings, etc. However, the authors reserve the following as set forth in the Copyright Law:
1. All proprietary rights other than copyright, such as patent rights.
2. The right to grant or refuse permission to third parties to republish all or part of the article or translations thereof. In the case of whole articles, such third parties must obtain Society of Wood Science and Technology written permission as well. However, the Society may grant rights with respect to Journal issues as a whole.
3. The right to use all or part of this article in future works of their own, such as lectures, press releases, reviews, text books, or reprint books.