A preliminary study on the detection methods of stress and strain in the process of wood drying. In the process of wood drying, both elastic stress and residual stress affect the drying quality; after the drying process is completed and the moisture content distribution across the thickness of the wood becomes uniform, the elastic strain has disappeared, and the residual stress continues to affect the drying quality after that.
1. Stress and deformation caused by inconsistent tangential and radial shrinkage of wood
Based on the characteristic that the tangential shrinkage coefficient of wood is approximately twice that of the radial direction, this paper analyzes the shrinkage, stress and deformation of three types of wood. a. Rift-sawn board: All board surfaces are radial cut surfaces, the thickness of the board is all in the tangential direction, the shrinkage is uniform, and no additional stress and deformation will be caused. b. Plain-sawn board: The outer board surface (the surface close to the bark) is close to the tangential direction, and its shrinkage amount is larger than that of the inner board surface (the surface close to the pith) which is close to the radial direction. Therefore, the board will warp towards the outer board surface during drying. However, in actual drying production, boards are stacked into a wood pile, and weight blocks are placed on the top of the pile to prevent wood warping and deformation. Due to the weight of the wood pile and the weight blocks, additional pressure is generated on the board to restrain its warping. As a result, additional tensile stress is generated on the outer board surface of the board, and additional compressive stress is generated on the inner board surface. This stress has nothing to do with the moisture content gradient. The additional tensile stress on the outer board surface is superimposed with the surface tensile stress caused by uneven moisture content, which can easily cause surface cracking on the outer board surface. c. Square timber with pith: Its four surfaces are close to tangential cut surfaces, and their shrinkage amount is larger than that in the diameter direction. During drying, the shrinkage of the four surfaces is restrained by the wood in the inner diameter direction. As a result, additional tensile stress is generated in the surface area, and additional compressive stress is generated in the central area. This stress is also independent of the moisture content gradient. This additional surface tensile stress is superimposed with the tensile stress caused by the moisture content gradient in the initial drying stage, which can easily cause surface cracking and radial cracking. Therefore, square timber with pith is very prone to defects during drying. Stress and deformation caused by uneven moisture content across the thickness of wood.