A Study On Wood Drying Stress-strain Model

This paper had developed two theoretical mathematics models of wood drying stress-strain, and predicted the development using MATLAB software. Moreover, the paper analyzed the mechanism of drying stress-strain and the effect of moisture content (MC), microfibril angle (MFA) and earlywood ratio on stress-strain from ultrastructural and macroscopic levels. Mathematical model and computer design method were used in the research. For modeling, MC and MFA were taken into consideration as explicit parameters in the ultrastructural model, and so were MC and ratio of every layer in the macroscopic model. The ultrastructural model can be applied to elucidate the drying properties of the thin specimen. The macroscopic model can be applied to analyses the drying characters of a specimen with asymmetric properties through the thickness. The results of the ultrastructural model showed: The longitudinal strain is less than the tangential strain when the MFA is zero. With the increase of MFA, the longitudinal strain increases and the tangential one decreases; the changing scope of tangential and longitudinal strain increases with the MFA where the MFA is small. What's more, the former is larger than the latter; When the MFA is about 0.68 arc degree, both the tangential and longitudinal strains are about 1.85%. In the region of the MFA above 0.68 arc degree, the longitudinal strain is larger than tangential one, reaching 3.4%. When the arc degree is 0.7,the tangential strain falls to about 1.6%. With the MC dropping from 30% to 0%, the tangential strain is increased by 8% to 9%, the longitudinal one's increase is less than 1%. At the same time, the strain difference is increasing in the process. The following conclusions were obtained from the macroscopic model: When the MC goes down,the differing strain of latewood is decreasing, while that of earlywood is increasing. What's more, theformer is larger than the latter. When the wood is completely dried, the T/R value of latewood is about 1.1, that of earlywood is about 2.1; the larger the earlywood ratio is, the less the cross strain is, the larger the longitudinal strain is, the less the stress is on earlywood and the larger the stress is on latewood. Besides that, the stress on earlywood is bigger than that of latewood. The overall tangential strain is 1.8 times as the radial strain. During the wood drying, the tangential strain is larger than the radial one. The strain of latewood is much more than that of earlywood in the same direction. Some simple calculations and concrete analysis will be demonstrated in the following paper.