木材の乾燥応力とアコースティック・エミッション

Abstract

木材の乾燥過程で発生するアコースティック・エミッション (AE) と乾燥応力の関係を明らかにするため, 応力法で乾燥応力の消長を追跡しながらミズナラとホオノキの板目材を乾燥し, AE事象率及びAE振幅と乾燥応力の関係について検討した。また, ミズナラ板材の厚さ方向におけるAE発生源の位置標定を詰みた。乾燥雰囲気の設定を変更したときの乾燥応力の過渡的変動に対応して, AE事象率は一時的な増大 (応力反転前) または減少 (応力反転後) を示し, 両者に密接な関係が存在することがわかった。しかし, 乾燥応力の全体的な消長とAE発生は必ずしも対応せず, またAE発生の樹種による違いも認められた。乾燥初期に材表層部にあったAEの発生域は, 応力の反転後次第に内層部に移行し, AEはおもに引張応力の作用域で発生していることがわかった。

The purpose of this paper is to examine the relationship between acoutic emission (AE) from wood during drying and the drying stresses which cause the surface and internal checks. Specimens of 30 mm thickness, of Japanese oak and cucumbertree, were dried in a small drying chamber, and the AE event count rate and the AE amplitude were measured. The drying stresses of specimens were estimated from the force required to restrain a specimen from cupping (Fig.1). The specimen for this purpose was half the thickness of the original one and all of the faces except one were covered with aluminium foil. Furthermore, the source of emission during drying was located along the thickness of an oak specimen by comparing the arrival time of the signal at two AE sensors (Fig.2). The AE rate increased or decreased transiently before or after the reversal of stress in response to the transient variations of drying stress which occurred each time the dry- and wet-bulb temperatures were changed (Figs.3, 4 and 5). This confirms that there is a close relationship between AE and drying stresses. The AE rate, however, was not necessarily affected by the magnitude of stress, especially for cucumbertree (Fig.5). The locations of AEs were in the outer zone of wood at early stages of drying, and they moved towards the inner zone after the reversal of stress (Figs.7, 8 and 9). This suggests that AEs are mainly generated in the region where the wood fibers are stressed in tension.