# Drying Stress and Strain of Wood: A Review
## 4 Testing Methods of Wood Drying Stress and Strain
### Modern Testing Methods  
With the development of modern detection and scanning technology, the acoustic emission method, strain gauge method and optical method are gradually applied to determine the drying stress and strain of wood. The common point of these methods is to indirectly analyze the stress and strain in the drying process through the changes in the measured parameters. The principle of the method proposed by [PERSON] using acoustic emission is to estimate the stress change by sensing and measuring the size and frequency of the wave caused by the internal stress of wood drying [26]. [PERSON] uses accelerometers and acoustic emission sensors to detect the stress wave of wood to explore the physical and mechanical properties of thermally modified wood and the influence of wave speed and acoustic emission signals [92]. [PERSON] used a strain gauge sensor to study the relationship between stress and treatment temperature, MC and relative humidity during high-temperature steam drying of _Cryptomeria fortune_[93]. [PERSON] measured the deformation of the sample under tensile action using optical pachymetry [94].  
Modern technologies of DIC and NIRS provide a new opportunity for the study of drying stress and strain. The image analysis algorithm is a non-contact point distance measurement method based on image analysis, combined with the conventional slicing method [95]. As provided in Figure 7, the principle is to magnify the multiples of the collected image before measuring, which reduces the human error in the process of measuring the length with a micrometer or vernier caliper, and can obtain a higher-accuracy tangential strain. [PERSON] et al. [96] completed the measurement of transverse shrinkage with DIC (Figure 8a). As shown in Figure 8b, the shrinkage at the tissue level was also measured  
Figure 6: Two commonly used testing methods of drying stress and strain. Dimensions are in mm [44].  
in the specific parts including the cell walls and pores to monitor the little drying stress of a small, thin specimen. [PERSON] et al. used the image analysis method to calculate the length of the specimen and analyzed the stress state at different MC stages during the drying process and the relationship between stress and each strain [97]. We can use NIRS equipment to evaluate the surface moisture content of wood [96; 98; 99]. Recently, it has been used to investigate surface drying stress and strain during wood drying [100].  
Combining the literature review abovementioned, it is not difficult to see that whether it is a direct or indirect measurement of drying stress and strain, the accuracy and speed of the measurement will be the focus of current and future research. As the mainstream and classic method of stress and strain detection in the wood drying process, the slicing method is simple to operate and is favored by researchers. However, insufficient accuracy and human error in the measurement are indeed unavoidable problems. Although the use of modern technology and equipment requires higher costs, it provides new opportunities for stress and strain detection and research.  
Figure 8: Images collected with DIC. The purposes of image collection: (**a**) measurement of transverse shrinkage, (**b**) measurement of shrinkage at the tissue level. From [PERSON] et al. [96].  
Figure 7: Digital image acquisition device.