ミズナラの乾燥過程におけるアコースティック・エミッション

Abstract

木材の乾燥割れをアコースティック・エミッション (AE) を利用して予知するための研究の一環として, 実際の乾燥操作におけるAEの発生特性および割れの発生と密接に関係する乾燥応力とAEの関係を明らかにするため, カップ法で乾燥応力の消長を追跡しながら一定のスケジュールに従ってミズナラ板目材を乾燥し, 種々のAE特性値の時間変化, カップ量とAE特性値の関係などについて検討した。AEの発生率は零囲気の温・湿度の設定を変更するごとに大きな過渡的変動を示すものの, 全体的にみれば, 乾燥開始後スケジュールに応じて徐々にあるいは急激に増大して最大値を示し, 材の平均含水率が12 - 15％に低下したときにほとんど0になった。この変化傾向はAE信号振幅の100事象ごとの平均値の変化と一致し, 両者が最大値をとる時点は, 比較的緩やかなスケジュールの場合はカップ量が負の極値を示す少し前であった。このことは, 乾燥応力とカップ量の一般的な関係を考慮すると, 乾燥応力が極大値をとる前後に振幅の大きなAEが最も多く発生することを意味する。

Acoustic emissions (AEs) produced during wood drying have been examined by several researchers including the present authors. Their purpose is to detect the development of early stages of surface and internal checking in lumbers during drying by measuring the frequency or the intensity of AEs. This paper deals with the characteristics of AEs during the whole process of kiln drying and the relation between some AE characteristics and the drying stresses which cause the surface and internal checks. Specimens of Japanese oak were dried in a small drying chamber using two different drying schedules (Fig. 1, Table 1). The AE event count rate, the AE cumulative event count, the AE amplitude, and the wave form and spectrum of AE signals were measured during the whole drying processes (Fig. 2). The drying stresses of specimens were estimated from the deformation of a cup sample, which was half the thickness of the original board and of which all the faces except one were covered with aluminium foil (Figs. 1 and 3). Although the AE rate showed remarkable transient variations each time the dry- and wet-bulb temperatures were changed, it generally increased slowly or rapidly, depending on the drying conditions, towards a maximum rate and then decline towards zero (Fig. 4). The AE generation almost ceased at 12-15% in moisture content of specimens for the both drying schedules. The AE amplitude, the average value of 100 events, had a similar tendency to the AE rate, and for the moderate drying schedule (schedule I) their maxima appeared a little before the cup quantity reached the negative peak (Figs. 4 and 6). This implies that stronger AEs are most frequently generated when the drying stresses are the greatest, since the deformation of a cup sample usually delays to the development of drying stresses near the surface of the lumber being dried.