# Drying Stress and Strain of Wood: A Review
## 3 Composition of Drying Strain
### Mechano-Sorptive Creep Strain  
[PERSON] [73] proposed 'Mechano-Sorptive Creep' for the first time in 1960, and their experiments tried to determine to what extent the rate of loss or gain of moisture affected the creep rate and total creep. The mechanism of mechano-sorptive creep generation is that the change of MC causes the hydrogen bond to break and recombine under the action of external force [74]. The creep strain is related to temperature [75], load [76; 77] and MC [78].  
[PERSON] [79] explored the development of a drying strain of poplar wood during conventional drying and HTD through an improved slicing method. He selected four drying temperatures (85, 105, 115 and 125 \({}^{\circ}\)C). During the whole drying process, the dry and wet bulb temperature was kept constant, and the constant wind speed was 1.0 m/s. Table 2 and Figure 4 describe the change characteristics of mechanical adsorption creep strain. After reaching the maximum, the mechanical adsorption creep strain tends to be stable. The strain responds to temperature (especially high temperature), and the resulting permanent set (i.e., hardening) tends to hinder normal shrinkage in the later stage of drying and may cause internal checks. Therefore, the maximum value of the strain may be used as the basis for high-temperature and high-humidity intermediate treatment in the process.  
[PERSON] studied the bending creep variations of _Picea abies_ during HTD and pointed out that the bending creep deformation of high-temperature (HT) dried wood is smaller than that of conventional low-temperature (LT) dried wood [80]. In [PERSON]'s research project, the HT drying was performed at a maximum temperature of 115 \({}^{\circ}\)C for approximately 30 h while the LT drying was performed at 70 \({}^{\circ}\)C for 168 h. According to the test results above, [PERSON] et al. [81] model the mechano-sorptive creep of LT-dried and HT-dried timber beams by using a spring and a single Kelvin body. Figure 5 clearly shows us how the simulation matches the test results. Besides, significant stress relief may occur along with the relatively high temperature of the wood and the slowdown of the moisture gradient [82]. [PERSON] steam-dried _Pinups radiata_ D. Don and pointed out that mechano-sorptive creep affects the dimensional stability of wood during processing and use, which is beneficial to release part of the drying stress [83].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Drying Condition (\({}^{\circ}\)C)** & **Maximum (\%)** & **Time to Reach the Maximum (h)** \\ \hline
85 \({}^{\dagger}\) & 3.65 & 5 \\
105 & 4.96 & 5 \\
115 & 3.14 & 3 \\
125 & 4.23 & 1.5 \\ \hline \hline \end{tabular} \({}^{\dagger}\) The temperature of 85 \({}^{\circ}\)C is used to simulate conventional drying, and other temperatures are used to analyze HTD.  
\end{table}
Table 2: Maximum values of mechano-sorptive creep strain of surface wafer [79].  
[PERSON] studied the dynamic free shrinkage behavior, as well as provided a theoretical basis for determining the mechanical-optical creep mechanism of drying. He sought to separate mechano-sorptive creep from viscoelastic creep deformation, and calculated the theoretical value of mechano-sorptive creep strain during conventional drying and post-drying conditioning stages [84; 85].