# Drying Stress and Strain of Wood: A Review
## 1 Introduction  
Wood drying is an inevitable part of wood processing which increases wood's physical properties, mechanical properties and dimensional stability and coating properties [1, 2, 3, 4]. Wood material after drying is more suitable for wood-based panels [5], solid wood furniture [6], construction [7], etc. Researchers have been working to optimize wood drying to remove moisture from the wood in a relatively economical way while reducing damage. Regarding the advancement of wood drying technology, the drying of veneers is moving in the direction of more efficient and energy-saving combined drying. Compared with the wood-based panel unit, the heat and mass transfer mechanism and the stress and strain response of sawn timber are more complicated in the drying process. It therefore becomes vital to predict the changes in various physical parameters in wood drying and reduce the energy consumption of drying. In the future, the method of conventional drying and combined drying will be the development direction of wood drying techniques. Moreover, monitoring and predicting moisture, stress, strain and drying defects in wood drying processes have always been hot topics. Some new developments in recent years include the use of nuclear magnetic resonance (NMR) and low-field nuclear magnetic resonance (LFNMR) to monitor moisture status and migration, and prediction of drying quality based on ANN models, etc. [8, 9, 10, 11, 12, 13]. Besides, for the special drying of waterlogged wood, the pre-treatment of impregnation is necessary [14]. Some environmentally friendly solutions have been proposed for the collapse of waterlogged wood upon drying: exploringthe possibility of organosilicons as consolidants to improve the dimensional stability of drying; adding nanotubes to the process to solve mechanical performance problems; and so on [15; 16; 17]. The physical and mechanical properties of wood are closely related to the water in wood [18], especially when the moisture content (MC) is lower than a certain critical point of MC which is termed as the fiber saturation point (FSP). Researchers have been discussing the definition of fiber saturation in wood drying. Initially, [PERSON] [19] defined the fiber FSP from the perspective of water state in cell lumens: when there is no liquid water in cell lumens, cell walls begin to dry and strength begins to decrease. However, the above phenomena cannot occur at the same MC. [PERSON] [20] argued that it is more reasonable to express the turning point (about 30%) where physical properties of wood produce strong variations. [PERSON] et al. [21] proposed to quantify FSP with the equilibrium moisture content (EMC) of wood at 100% relative humidity (RH). Based on this concept, the FSP obtained is from 38.5 to 42.5%, which has a deviation of up to 10% from the results of [PERSON] [20]. Regarding this difference, [PERSON] [22] had the following speculation: fiber saturation may not be a specific value/point, but a transitional region from the state that new water molecules cut off different H-bonds in the wood cell polymers (\(MC=0\)-30%) to the state that new water molecules are accommodated without breaking any of the cell wall polymer H-bonds (\(MC=30\)-40%). It can be seen that, as the watershed, the saturation point of wood fiber is a turning point or a transition state, which is still an unsolved mystery. In addition, [PERSON] et al. [23] claimed that the use of such terms (i.e., FSP) is somewhat misleading, because the fibers are not saturated with water, but the cell wall is. Perhaps cell wall saturation (CWS) can better describe the phenomena in this special state. This needs to be further researched and developed. Taking into account the utilization of wood value, defects in wood drying will cause significant losses to wood processors. Most of the drying defects develop due to uneven drying stress.