Wood, being a natural hygroscopic material, the interaction between wood and moisture plays a crucial role in wood processing and utilization. Moisture affects the physical and mechanical properties of wood, and is also one of the main external factors that cause wood deformation and cracking. Drying shrinkage is a common phenomenon during the processing and utilization of wood induced by moisture loss. Drying stress is the main cause of wood deformation and cracking. The shrinkage differential between tangential and radial direction and moisture content gradient of wood are two reasons induced the generation of drying stresses. In this review, the existing states of moisture in wood and the interaction between water molecules and wood components were systematically summarized. The current research progress and deficiencies in three aspects including the factors resulted in deformation and cracking in wood caused by moisture loss, the correlation between wood mechanical properties and moisture, as well as the development of deformation and cracking in wood under moisture loss were discussed. This review aims to facilitate further research on the deformation and cracking of wood under moisture loss by providing valuable insights and assistance, ultimately reducing the occurrence of wood deformation and cracking. And thus, it will enhance the overall utilization of wood resources, making wood better serve human life.

Microwave (MW) treatment is an effective method to increase refractory wood permeability, thereby reducing drying time and defects. The extent of modification depends on the damage extent of the wood microstructure. In this study, MW intensities of 43 kWh/m³ (low intensity) and 57 kWh/m³(high intensity) were adopted to treat Chinese fir lumber. Microstructural changes in wood samples were observed using scanning electron microscopy (SEM) and pore structure was characterized using mercury intrusion porosimetry (MIP). Results were as follows: After low-intensity MW treatment, parts of the bordered pit membranes in tracheids were damaged, and micro-fibrils on the margo were ruptured, while the torus basically remained intact. Micro-cracks were observed at both ends of the cross-field pit apertures, propagating to the cell walls of tracheids. The middle lamellar between ray parenchyma cells and longitudinal tracheids cracked, and the width of cracks was in the range of 1–25 μm. After high-intensity MW treatment, damage to the wood microstructure was more severe than that in the low-intensity MW treatment, with macro-cracks having a width range of 100–130 μm being generated. In addition, on the fracture surface of macro-cracks, the bordered pit membranes in tracheids fell off, cross-field pit membranes disappeared and the ray parenchyma cells were seriously damaged, exhibiting fracture of the tracheid walls. Both low-intensity and high-intensity MW treatment can increase the pore diameter corresponding to the margo capillaries (peak value increased from 674.7 nm to 831.8 nm and 1047.6 nm, respectively). The number of pores in the tracheid lumen diameter range also significantly increased. These results provide a theoretical support forMW treatment processes’ improvement and high-value utilization of Chinese fir.

Due to wood moisture sensitivity, shrinkage cracks tend to present wooden structures. These failures are caused by moisture-related shrinkage behavior. In order to avoid it, it is necessary to have a better understanding of shrinkage behavior. In this respect, studying the dimension changes in wood at different scales is of utmost significance for a better understanding of the shrinkage properties. Herein, the shrinkage behavior of Masson pines (Pinus massoniana) wood was investigated at macroscopic and cellular levels during moisture loss via digital image correlation using VIC-3D and digital microscopic systems, respectively. According to the full-field strain maps, shrinkage strain near the external face was higher than that at the internal face, which increased susceptibility to cracking at the external face of lumber. Additionally, the anisotropic shrinkage of wood was explored. The shrinkage ratio at the end of drying was about 5.5% in the tangential (T) direction and 3.5% in the radial (R) direction. However, at a cellular level, the shrinkage ratios in the T and R directions of earlywood tracheids were 7.13% and 2.46%, whereas the corresponding values for latewood tracheid were 9.27% and 5.52%, respectively. Furthermore, the maximum T/R shrinkage ratio at the macroscopic level (1.7) was found to be similar to the value of latewood tracheid (1.72). The earlywood showed high anisotropic, its T/R shrinkage ratio was 2.75. The macroscopic shrinkage was the result of the interaction of the tracheids of earlywood and latewood and was mainly dominated by latewood tracheids.

The knowledge of moisture content (MC) and drying stress are crucial parameters to control the drying process and maintain the quality of dried wood. Herein, we investigated the pattern of moisture transfer and stress development in Chinese fir during the high-temperature drying process. The MC in each layer of lumber was separated into bound water and free water via nuclear magnetic resonance (NMR), and the drying and residual stress were measured using prong test method. There was different MC in each layer along the thickness, resulting in an MC gradient that initially increased and then decreased, which is consistent with the trend of drying stress. The T₂₁ peak indicating bound water shifted to the left especially when MC was below the fiber saturation point, signifying that the discharge of moisture became difficult with prolonged drying time. The ratio of bound water to free water was different in each layer, indicating that the moisture transfer was different along the thickness. Furthermore, the residual stress was greater than the corresponding drying stress though the disparity reduced gradually, which suggests that the MC gradient was the largest affecting factor for drying stress at high MC stages but decreased to some extent as the drying process continued.

Modification of poplar (Populus adenopoda Maxim) wood by using maltodextrin (MA) combined with 1,3-dimethylol-4,5-dihydroxyethyleneurea (DM) resin was investigated in this study. The weight percent gain, dimensional stability, mechanical strength, and microscopic chemical change were determined before and after impregnation. Results indicated that the composite modifier was impregnated into the wood cell lumen, as verified by scanning electron microscopy and a change in weight percent gain. Anti-swelling efficiency was significantly improved compared with the untreated sample; however, the sample treated with MA obtained an anti-swelling efficiency that was slightly lower than that of the sample treated with sucrose. The flexural strength, flexural modulus, and compressive strength perpendicular to the grain increased with an increase in MA concentration, but the compressive modulus and impact strength were reduced. Fourier-transform infrared spectroscopy demonstrated that MA delayed the cross-linking of the DM resin cell wall, and X-ray diffraction analysis showed that the crystallinity of the cellulose crystalline region was reduced to a certain extent. In general, the poplar wood treated with combined MA and DM resin exhibited enhanced properties relative to those of the poplar wood treated with sucrose.

Wood is widely considered to be a green and renewable building material. However, many of the challenges to using wood as an engineering material arise from the shrinkage and swelling behaviors during moisture content (MC) loss or gain. In this study, the shrinkage behavior of Queensland peppermint (Eucalyptus exserta F. V. Muell) wood was investigated at the macroscopic and cellular levels, and the interdependence between the two levels was explored. Results revealed that the tangential shrinkage of 2% and radial shrinkage of 1% was observed as MC decreased from about 60% to 32% in the macroscopic level. The most obvious anisotropic shrinkage was presented at the MC region of 32–22%, and the ratio of T/R was ranged from 1.6 to 1.75 below the fiber saturation point. At the cellular level, a considerable shrinkage was observed at MC of 42%. The cell wall was shrunk, but the lumen in radial (TR plane) was expanded with MC loss. The shrinkage of the total wood cell showed a linear relationship with MC. The shrinkage ratio of T/R was maintained around 1.5 below the fiber saturation point, indicating that the MC states have little effect on the anisotropic shrinkage. Besides, the shrinkage value at the macroscopic level was slightly smaller than the cellular level, but the anisotropic shrinkage showed an opposite trend.