木材の乾燥過程で発生するAEについての二三の考察

Abstract

乾燥中の木材におけるAEの発生機構を探るため, 生材小試片を用いて乾燥と吸湿を繰り返したときのAE発生, 木口円板乾燥時のAE発生に及ぼす半径方向の切り込みの影響, 両木口面のみを蒸発面とした試料乾燥時のAE源の位置と繊維方向の含水率分布の関係について検討した。AEは生材の乾燥過程で顕著に発生し, その後の繊維飽和点以下での乾湿繰り返しではほとんど発生しなかったが, 水中浸漬後乾燥すると再び多量に発生した。円板に切り込みを入れると試料の変形や割れは著しく軽減されたが, AE事象率やAE総数には切り込みの影響が認められなかった。木口面のみから乾燥が進行する場合, AEのおもな発生域は含水率が繊維飽和点を示す付近にあり, その領域が木口面から内部へと移行するにつれてAEの発生域も内部に移行することが確かめられた。

This paper describes three preliminary experiments to examine the generation mechanism of acoustic emission (AE) from wood during drying. First, the AE generation in a drying/damping sequence was examined for small specimens of the three hardwoods, mizunara, keyaki and mizume, and of two softwoods, sugi and hemlock (Experiment I). Secondly, AE measurements during drying of disc specimens were compared between with and without the radial slits (Fig.1) , which had been expected to prevent drying checks caused by a difference in radial and tangential shrinkage (Experiment II). Lastly, the difference in arrival time of AE waves at two AE sensors and the moisture distribution along the length were measured in drying the specimens of which longitudinal surfaces were sealed (Fig.1) , and the relationship between the locations of AEs and the moisture distribution were examined (Experiment III). The gain and the threshold for AE signals were set at 60 dB and 200 mV in all experiments, unless otherwise stated. In Experiment I, a great quantity of AEs were generated during the first drying step from the green condition and few AEs occurred in the subsequent steps of damping and re-drying (Fig.3 and 4). The AE activity, however, was restored by soaking the specimen in water (Fig.5). There was little effect of the radial slits on the AE generation of the disc specimens (Fig.6) , though warping and checking were noticeably eliminated in the slitted discs. In Experiment III, AEs occurred near the both end surfaces at the early stages of drying when only the region close to the end was dried below the fiber saturation point, and their locations moved to the interior of the specimen when the moisture content in that place reached the fiber saturation point (Fig.7 and 8) . This suggests that AEs are mainly generated in the region in which the moisture content decreases to about the fiber saturation point.