# Drying Stress and Strain of Wood: A Review
## 2 Wood Drying Stress
### The Generation and Development Mechanism of Wood Drying Stress  
During wood drying, the generation of drying stress can be put down to uneven internal shrinkages and mutual restraint of adjacent layers. The main causes of uneven shrinkage are the MC gradient and the shrinkage anisotropy.  
Ideally, assuming that the wood only produces an MC gradient along the thickness direction, the moisture on the surface of the wood will first drop below the FSP at the beginning of drying, and shrinkage will occur, while the MC of the adjacent wood layer is still above the FSP and there is no shrinkage. Therefore, the unshrinking layer exerts tensile stress on the dried shrinkage layer, while the adjacent inner layer is subjected to compressive stress. Wood has viscoelasticity and plasticity. [PERSON] et al. have proved that the wood would undergo plastic deformation and cracks (e.g., surface checks) when the tensile stress of the surface layer reaches the limit and becomes greater than the tensile strength of the surface layer [47; 48; 49].  
In the middle stage of drying, the surface layer cannot fully shrink, forming case-hardening, and accumulating a large amount of tensile residual deformation. The stress of the surface layer is reversed, and the core layer with MC over FSP is still under compressive stress, while the MC of the middle layer is lower than FSP and the tensile stress generates over there due to the restraint of the surface and the core layer.  
In the later stage of drying, the internal layers with lower MC are suffering the force of surface compressive stress. When the internal tensile stress of the wood reaches the maximum and is greater than the internal tensile strength, internal checks will occur due to severe plasticizing deformation of the surface layer at the initial stage of drying.  
The anisotropy of shrinkage refers to the difference between tangential shrinkage (T-Shr) and radial shrinkage (R-Shr) coefficients. [PERSON] et al. compared the development and distribution of the drying strain of _Betula platyphyllla Suk._ under the sole effect of the shrinkage anisotropy and the joint effect of shrinkage anisotropy and moisture gradient [50]. The results indicate that when there was an MC gradient and the MC was lower than 26%, the actual shrinkage strain became lower, the viscoelastic creep strain was more easily recovered and diverted and the mechano-sorptive creep strain was significantly reduced, while cracks and delamination became worse.  
Uneven drying stress is also affected by temperature gradients. [PERSON] designed a wood drying shrinkage force balance test system, testing the drying shrinkage development of _Pinus sylvestris_ using different drying methods [51]. The results suggest that the drying rate of the specimens at higher temperatures could reach the stress equilibrium state faster, and the shrinkage force changed more remarkably. A study of high-frequency convection combined heating and drying by [PERSON] et al. proved that the strain of each layer of wood increased with the increase in temperature gradient during the early and middle stages of drying, and the middle layer was also significantly affected by the temperature gradient in the middle of drying [52]. Additionally, growth stress and geometric growth characteristics will cause drying stress and deformation of wood as well.