木材の乾燥過程で発生するAEについての二三の考察 (その3)

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第3報として, 数種の無機塩およびポリエチレングリコール (PEG) で処理したスギとミズナラの小試片を乾燥したときのAE発生と試片の収縮率, 水分蒸発の関係などについて検討した。スギでは処理の種類, 乾燥条件にかかわらず乾燥中に検出したAEの総数は試片の木口面の面積収縮率に比例したが, ミズナラでは両者に関係が認められなかった。AE計測のために設定したデッドタイムとAE発生の時間間隔の関係を考察した結果, ミズナラではAE事象の多量の数え落としのあることが分かった。さらに, AEセンサの通常の取り付け方法は試片の局部的な乾燥阻害を引き起こし, AE発生を長時間持続させることが明らかになった。

This is a continuation of two previous reports to examine the generation mechanism of acoustic emission (AE) during wood drying. Thin cross-cut specimens ( 2 x50x50mm) of sugi (Cryptomeria japonica D.Don) and mizunara (Quercus mongolica Fisch var. grosseserrata Rhed. et Wils) were treated with four saturated salt solutions, BaC12, NaC1, MnC12 and mgC12, and 9 and 25 % aqueous solutions of polyethylene glycol (PEG) with a molecular weight of 1000. The AE generation was monitored during drying of the treated and water-soaked specimens at a temperature of 60 ℃ and a relative humidity of 40, 60 or 80 %, and the shrinkage of specimens was measured. The gain, threshold and dead time for AE sigals were 70 dB, 200 mV and 1 ms, respectively. For specimens of sugi the AE total event count during drying increased with the shrinkage in area of cross-cut surfaces (Figs. 2, 3 and 4 ) and a proportional relation existed between them, regardless of the treatment (Fig. 5 a). It was difficult, however, to find a certain relation between the AE total event count and the shrinkage for mizunara (Figs. 5 b and 6 ) This could be ascribed to the miscounting of AE events, which is unavoidable in counting the events generated at the higher rate using a measurement system with the deadtime (Figs. 7 and 8 ). No employment of the deadtime resulted in a good relationship between the AE total event count and the shrinkage of specimens (Fig. 10). The retardation of specimen drying due to the mounting of an AE sensor and the improvement of sensor mounting are also discussed (Figs. 1, 9 and 10).