木材の乾燥過程で発生するAEについての二三の考察 (続)

Abstract

乾燥中の木材におけるAEの発生機構を探るための研究の続報として, 木取りの異なる2種類の生材小試片を用いて乾燥と吸水を10回繰り返したときの乾燥時のAE発生, および割れを生じさせるために板目小試片の接線方向の収縮を拘束して乾燥したときのAE発生とAE源の位置について検討した。乾燥と吸水を繰り返すとAE発生が若干変化する場合もあるが, 基本的な発生パターンは乾燥回数によって変化せず, 樹種および木取りに特有のAE発生を繰り返した。収縮を拘束するとAE事象総数がわずかに増えるが, AEの発生経過は拘束しない場合と同様であった。このことおよびAE源の位置標定結果から, 乾燥過程で、検出されるAEのうち収縮拘束による応力や割れに関係するのはわずかであり, 大部分はそれらに関係なく試片全体から発生していると結論した。

This paper describes further experiments to examine the generation mechanism of acoustic emission (AE) during wood drying. First, the AE generation during repeated cycles of drying and wetting was examined for small specimens of two types, cross-cut (50 (R) × 50 (T) × 5 (L) mm) and flat-sawn (5 (R) × 50 (T) × 50 (L) mm), of mizunara, sugi and mizume (Experiment I). Secondly, flat-sawn specimens were dried under restraint of the tangential shrinkage (Figs. 1 and 2), and some AE parameters and the locations of AE sources were compared with those for control ones (Fig. 3) (Experiment II). The gain and threshold for AE signals were set at 70 dB and 200 mV in two experiments. The basic process of AE generation during wood drying hardly changed with the repetition of drying and wetting cycles, though the generation process was more or less modified probably due to some change of the physical nature of wood (Fig. 4). A great quantity of AEs were generated repeatedly in a peculiar manner depending on species and specimen types. Although drying checks were induced by restraining the shrinkage and the final AE total event count was somewhat greater for the restrained specimens than the contorl ones, curves of event count rate for both specimens were quite similar (Fig. 5). The AEs due to the checking of the restrained specimens were discernible in the AE location only when the AE generation almost ceased close to the end of the drying (Fig. 6). From these findings it was concluded that only a few AEs were generated in connection to the checking and most of AEs detected were generated irrespective of the checking from the whole of the specimen.