Change of released surface strain during drying with high temperature heating by radio frequency under vacuum after a local steam explosion treatment
2002
Noji, K. (Kochi-ken. Forest Experiment Station, Tosayamada (Japan)) | Masaoka, H. | Gotou, J. | Fujiwara, S. | Kanagawa, Y.
Sugi (Cryptomeria japonica D. DON) square lumber was dried at high temperature of about 70 or 90degC by the radio-frequency heating under vacuum 18.6 kPa (140 mmHg) after a local steam explosion treatment. Drying time could be shortened to about one half of the drying time in the usual radio-frequency vacuum drying at 40 to 50degC. Slight pith checking occurred during the 120degC -steam explosion. A few surface checks were observed. Test pieces were taken out at several stages of drying, and were examined in regard to the few surface checks by measuring changes of released surface strain and moisture content distribution through the thickness during drying. Surface tension stress, which was observed from changes of surface strain, increased from the middle stage of drying, and then tended to reverse to compression stress at the final period of drying. Observing the influence of pretreatment or its absence on drying stress at the surface zone, tension stress tended to be reduced by the stream explosion pretreatment. The amount of tension strain is lower with than without pretreatment. The moisture content gradient through the thickness was very shallow, and the occurrence of drying stress was therefore reduced. The moisture content at the surface zones and the core declined before the middle stage of drying. This indicated that surface stress was changed into compression stress at the final stage of drying.
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