Microcrack propagation in transverse surface from heartwood to sapwood during drying

Microcracks, which are difficult to be observed with naked eyes, occur on the transverse surface during the first stage of wood drying. The precise investigation of their appearance is critical for understanding the mechanism of crack occurrence and propagation, which influence the properties of wood. In this study, the location and time difference of microcrack occurrence on the transverse surfaces of outer heartwood to sapwood via the intermediate wood of Cryptomeria japonica were investigated. Using the developed confocal laser scanning microscope system and a digital microscope, the microcrack propagation was captured dynamically with the wide range of high-quality images; microcracks appeared initially in the heartwood region, immediately after drying. On the other hand, in sapwood, microcrack generation occurred only after microcracks appeared in intermediate wood, and the ones in heartwood started to close or were closed. Finally, most of the microcracks almost closed and some completely disappeared towards the end of the drying process. From this result, it can be established that appropriate drying conditions should be prepared during the early stage of drying to produce high-quality wooden products because microcracks can appear in heartwood, even though the moisture content of the specimen is high.