Analysis of drying wood based on nondestructive measurements and numerical tools

Detta är en avhandling från Luleå : Luleå tekniska universitet

Sammanfattning: Improved understanding of moisture and mechanical behaviour is a general objective for wood drying research. The main objective of this doctoral thesis was to develop nondestructive experimental methods suitable for collecting valuable response data related to the moisture behaviour and mechanical behaviour of drying wood and to refine this information into modelling parameters. A method for simultaneous noncontact measurement of two-dimensional surface deformations and interior densities was developed. This was done using Digital Speckle Photography (DSP) and X-ray Computed Tomography (CT). Displacements and densities were used for calculation of strain and of moisture content. Experimental tests of the measurement method were done on cross sections of Scots pine. The following accuracy was stated for different properties: A typical calculated displacement error of approximately 10 micrometre was found. Strains derived from the displacements had a maximal error of 1.11 mstrain. Moisture content measuring accuracy was estimated to +-1.8% moisture content at a significance level of 0.05 in a measuring volume with the approximate size 2 x 2 x 1.5 mm3. A similar noncontact technique based only on X-ray CT scanning was developed. Displacements were then estimated from boundary movements of an object in CT images. The estimated standard deviation of the measured moisture content error for this method was 0.04% moisture content. The mean error was unknown. Two different approaches to determining moisture diffusion coefficients from the studied data were presented. The first was based on minimizing the difference between measured and computed values through an optimization scheme. This approach required an initial assumption of the functional form of the diffusion coefficient. The second approach calculated diffusion and mass transfer coefficients through direct finite difference calculations on measured moisture content data. Results on Norway spruce showed interesting local variations of the diffusion coefficient, especially near the evaporation surface. Comparisons between measured and FEM simulated data showed good results. An example showed that a multivariate method of analysis could be an effective and easy-to-use tool for untangling relationships between variables and for generating information from data. Finally, it could be stated that the methods presented will be of use to improve the understanding of the behaviour of drying wood, with the focus on moisture and mechanical properties.

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