# Drying Stress and Strain of Wood: A Review
## 2 Wood Drying Stress  
### Wood Drying Defects and Drying Stress  
There are many types of wood drying defects, which can be roughly divided into two categories [32]: (1) defects caused by shrinkage anisotropy, associated with the warping of timber boards, including cupping, bowing, twisting, spring, etc. ; and (2) defects that arise owing to uneven drying, related to the rupture of the wood tissue and delamination. The latter type of defects includes checks (surface, internal and end), end splits due to delamination, honey-combing and case-hardening. Checks are cracks that are visible at the surface but not extending across the board [33]. The difference between these three checks is shown with labels in Figure 1: Surface checks are longitudinal cracks that occur along the wood ray, generating on the outer-tangential surface of plain-sawn timber. Internal checks split along the wood ray inside and can only be found when sawing off. End checks are cracks that occur at the end of the wood [34]. Imperfect drying processes lead to defects and bad wood quality. Aggressive drying schedules make the situation worse, causing timber to crack or delaminate. Wood drying stress has been proved to be the major factor of drying defects and has become one of the indexes of drying quality. Some techniques (e.g., pre-treatment) are normally used to eliminate stress, inhibit the occurrence of defects and improve the drying quality and drying efficiency of wood. Wood pre-treatment is to treat the wood before it is put into a drying oven or other drying equipment to prevent wood cracking. The method uses the elasticity and plasticity of wood to eliminate part of the growth stress, thereby reducing the possibility of wood bending and deformation, and improving the dimensional stability of wood. The media for pre-treating wood include water, steam, microwave, etc. Relevant scholars have studied the influence of steam pre-treatment on wood drying rate and MC gradient. The research results show that steam pre-treatment can effectively accelerate the wood drying speed and alleviate the MC gradient during wood drying [35; 36; 37]. Steam pre-treatment can shorten the drying cycle by varying degrees [38; 39]. [PERSON] et al. studied the effects of high-temperature and low-humidity pre-treatment, external load treatment and sapwood coating treatment on tangential stress and drying cracking during radio-frequency/vacuum drying of tree discs [40]. The results show that high-temperature and low-humidity pre-treatment effectively avoids the occurrence of cracks on the wood surface, and external load treatment can reduce the probability of internal cracking. [PERSON] et al. studied the effect of steam pre-treatment before drying on the drying quality of rubber wood, pointing out that steam pre-treatment for 10 h can effectively increase the drying rate and reduce drying defects [41]. Figure 1: (a) Surface checks, (b) internal checks, (c) end checks. Microwave pre-treatment may significantly reduce the formation of internal/honeycombing and surface checking in drying [42]. The experiment of [PERSON] et al. witnessed the increase in the drying rate caused by microwave pre-treatment [43]. In Figure 2, it can be clearly seen that the drying time of the microwave pre-treatment sample is shorter than that of the control sample. The drying rate at an early stage was greatly increased after microwave pre-treatment, because the pre-treatment accelerated the moisture migration during the drying process, ensuring that a long-term evaporation interface was maintained at the wood surface at early stage. At the same time, acceleration of moisture movement improved the moisture distribution in the thickness direction and decreased the drying stress (Table 1). Thermally modified timber can release part of the drying stress and reduce the recovery of deformation.