关键词: 湿热软化, 整体压缩, 层状压缩, 传热传质, 屈服应力

Abstract: Wood compression is an effective method of enhancing the plantation fast-growing wood density, strength and hardness and improving its physical and mechanical properties, which broadens its application fields. In this paper, wood compression research status, development and problems were analyzed from the aspects of wood softening mechanism and characteristics, softening point, heat and mass transfer characteristics under hot plate heating, formation of sandwich compression and fixation of compression deformation. Composition and organizational structure of wood cellular wall is the main internal factors and hydro-thermal condition is external factors affecting wood softening and compression deformation. Wood is a natural macromolecular composite and an elastic-plastic material. Dry wood with low plasticity is plasticized by moisture and heat, especially the combined action of them was more significant. As the main components of wood cellular wall, cellulose, hemicellulose and lignin characteristics and their proportions directly impact on wood plasticity, especially the lignin. The glass transition temperature and stress yield point are the most common parameters to represent wood softening. In the elastoplastic analysis of wood, the stress yield point is used for determining the plastic potential which dominates the stress-strain relationships of material in the plastic region. However, due to the complexity of wood composition and structure, the change point of stress-strain is not obvious, so the stress yield point and the yield stress is the key and difficulty point to determine the wood plastic deformation characterization. Organizational structure of wood mainly affects its heat and mass transfer. Because of the permeability differences among wood three sections, significant moisture distribution gradient and yield stress difference can be formed by conducting the treatments of drying, soaking, setting and hot plate heating. Then sandwich compressed wood can be obtained by hot press. Density of compressed layer reached above 0.8 g·cm^-3, and the density of uncompressed layer still maintained. However, the compressed layer can be formed on the surface layer or any part of the interior. Sandwich compression can reduce volumetric loss of wood during compression. By now, the deformation mechanism study of wood compression mainly focus on the overall compression, lacking of fundamental investigation on softening point and yield stress change with moisture content variation and yield stress difference under hot plate heating. To achieve the controllability of sandwich compression, heat and mass transfer under hot plate heating status and hydro-thermal gradient distribution need to further research.

Key words: hydro-thermal softening, overall compression, sandwich compression, heat and mass transfer, yield stress