Internal Stresses in Wood Caused by Climate Variations

Abstract: This thesis is the result of a series of investigations on the hygro-mechanical behaviour of clear wood in the radial and tangential directions. Approximately half of the effort put into the investigations was concentrated on laboratory tests while the remaining effort was concentrated on modelling the observations made in the tests. The results from the investigations are published or submitted for publication in eight papers. Paper I : In this paper methods for testing the hygro-mechanical behaviour of small clear wood specimen are evaluated and discussed. The equipment used for conducting the tests is thoroughly described. It was concluded that the equipment showed good potential to perform tests on the specimen in variable climate and that the methods for testing were reliable. Paper II : Results from performed tests on small clear wood specimens in climates representative for kiln drying of sawn timber are presented in this paper. Evaluations of the tests also showed that the hygro-mechanical behaviour at high temperatures and high moisture content consists of both mechano-sorption and time induced creep. Paper III : A material model for drying wood at constant temperatures on the material point level is presented in this paper. The model was calibrated against results. It was found that the material model predicted the response from climate and load action in small specimens. Paper IV : In this paper the one dimensional material model introduced in Paper III is verified against more test results on small clear wood specimen. Also, a two dimensional model was developed and is presented in this paper. Paper V : Results from tests on small clear wood specimens for indoor climates with single cycles of humidity are presented in this paper. The mechano-sorptive strain was found to be independent of moisture level and rate of moisture change, but strongly dependent on the magnitude of moisture change. Paper VI : Results from tests on small specimens subjected to indoor climates with multiple changes of humidity are presented in this paper. A material model describing the hygro-mechanical behaviour of clear wood in radial and tangential directions for indoor conditions was derived. Paper VII : Results from simulations with the one dimensional and two dimensional material models (developed in Papers III and IV) implemented into simulation programs are shown in this paper. Paper VIII : Application of the work on material behaviour in kiln drying conditions (see Papers I, II, III, IV and VII) for practical kiln drying is presented in this paper. It is shown how simulations can be used to alter drying schedules to reduce internal stresses in drying timber.