木材中のAE波の伝搬特性

Abstract

木材中を伝搬するAE波の特性を明らかにするため, シャープペンシルの芯の圧折を擬似AE源としたときのAE信号波形とスペクトル, それらの伝搬距離による変化, AE波の距離減衰, 伝搬速度などを, 数樹種の気乾材と2樹種の飽水材 (厚さ10 - 15mmの板) について調べた。AE波には縦波と横波が存在し, 繊維方向に伝搬する場合には明僚に区別された。伝搬距離が長くなるとAE信号に含まれる高周波数成分が顕著に低下した。距離による減衰は縦波よりは横波が, 繊維方向の伝搬よりは直交方向で, 気乾材よりは飽水材で著しく, また板幅によって減衰の様子が変化した。縦波の伝搬速度は既報の木材中の音速と同様の値が得られ, 繊維直交方向の速度は繊維方向の1/3前後であった。一方, 横波の速度は繊維方向で縦波の約1/4であった。

To clarify the propagation properties of acoustic emissions (AE) in wood, the signal of a breaking pencil lead was employed as an artificial AE source (Fig. 2) and the waveforms and frequency spectra of AEs propagated in air-dried and wet specimens were observed for several species (Table 1). The damping and the propagation velocity of AE waves in wood were also examined. AE waves contained both a longitudinal-wave component, which arrived first at an AE sensor, and a transverse-wave one, which arrived later, and they were distinguished clearly when propagating longitudinally (Figs. 3 and 4). AE waves attenuated and changed in form during the propagation in wood, and their components of the higher frequencies decreased remarkably with propagation distance (Fig. 3). Attenuation of AEs during the propagation in wood was greater for transverse waves than longitudinal ones, and it depended on the propagation direction in wood, the moisture content and the specimen width (Figs. 5, 6, and 8-10). Velocities of longitudinal-wave components were the same as the sound velocities reported previously, and the transverse velocities were approximately 1/3 of longitudinal ones (Table 2). The velocities of transverse waves were estimated at about 1/4 of longitudinal ones when propagating longitudinally.