Acoustic emission activity of spruce sapwood becomes weaker after each dehydration-rewetting cycle

Acoustic emission (AE) during dehydration at ambient temperature was compared between fully saturated fresh Norway spruce (Picea abies (L.) Karst.) sapwood and sapwood exposed to two dehydration-rewetting cycles in order to get information about the differences in dehydration stress. AE testing was performed within a frequency range of 35-100 kHz (R6) and 100-1000 kHz (WD) at ambient conditions. AE activity became lower and AE energies weaker after each dehydration-rewetting cycle. During the first dehydration run, highest mean AE energies were detected at the beginning of dehydration. In rewetted wood, highest mean AE energies were detected towards the end of dehydration. AE of rewetted wood was also characterized by a higher percent of AE with frequencies >70 kHz and 100-175 kHz, as detected by R6 and WD transducers, respectively. It is concluded that fresh never-dried sapwood is more prone to dehydration stresses than pre-dried sapwood. Differences in AE number and AE features appear to be due to micro-mechanical failure that decreased after each dehydration-rewetting run. Dehydration stress seems also to decrease because the membranes of bordered pits, which act as a valve to avoid the breakage of the water columns inside the conduits, become weakened after each dehydration-rewetting run. Keywords: Acoustic emissions, wood drying, dehydration stress