Morén (1994) has suggested that during the early stages of the timber drying process, some damage, small cracks or “microcracks”, develop on the wood surface and this hypothesis has been shared by others (Hukka and Tarvainen 1997). However, the so-called microcracks have not been directly observed and the hypothesis is based on the observations of the cracking behaviour later during the drying process. Up to 50% difference in the final amount of visible cracks at the end of drying has been observed depending on the way the early warm-up stage is conducted (the actual drying stage being the same). It is suggested that something happens on the surface in the early stages that makes it more vulnerable to later developing cracks. A readily acceptable explanation is the appearance of small cracks on the surface, invisible to the naked eye, that act as initial flaws and later grow into visible cracks. The name “microcracks” has been attached to these cracks as opposed to visible “macrocracks”. The width limit for the visibility of cracks with the naked eye is approximately 0.1 mm (Wahl 1999), which may be used as a practical definition for the upper limit of “microcracks”. The goal of this work is to detect microcracks on wood surfaces and to verify the microcracking hypothesis and then to quantify it. For this purpose, a measurement technique was developed, its function verified as reported in (Wahl et al. 2001), and then applied for tests of microcracking. The results shown here concern wood surfaces examined during drying, and compare the amount of microcracking in various drying conditions.
Hanhijärvi, A., Wahl, P., Räsänen, J., & Silvennoinen, R. (2003). Observation of development of microcracks on wood surface caused by drying stresses. Holzforschung, 57(5), 561-565. https://doi.org/10.1515/HF.2003.083