Abstract
Collapse is an abnormal shrinkage which occurs in some species of wood during seasoning. It is due to physical collapse of the fibre cells, and is usually seen to occur in one or more discrete earlywood bands. Surface and internal checks can be induced by differential shrinkages arising from collapse in some parts of boards. It has already been established that the onset of collapse during seasoning is a temperature dependant process. To determine how this temperature sensitivity affects collapse induced checking in Australian eucalypts during seasoning, a stress and drying model was used to compare different drying strategies for avoiding collapse induced checking. The model demonstrated that the best way of avoiding collapse checking is to dry at temperatures below the “collapse threshold temperature” for the timber. It indicated that it may be possible to dry the timber at fully collapsing high temperatures and then recover the collapse by steam reconditioning, but this strategy requires more investigation.
Zusammenfassung
Kollaps ist eine unnormale Schwindung, die in einigen Holzarten beim Trocknen auftritt. Er wird verursacht durch Kollabieren von Faserzellen, üblicherweise in einem oder mehreren Frühholzbereichen. Oberflächliche und innere Risse können danach aufgrund differentiellen Schwindens in einigen Teilen des Schnittholzes induziert werden. Es wurde bereits früher aufgezeigt, daß dieser Vorgang temperaturabhängig ist. Um zu untersuchen, wie diese Temparaturabhängigkeit den Kollaps von australischem Eukalyptusholz auslöst, wurde ein Spannungs- und Trocknungsmodell benutzt, das es ermöglicht, verschiedene Trocknungsstrategien zu vergleichen und so die Rißbildung zu vermeiden. Die optimale Strategie ist demnach das Trocken unterhalb eines Temperatur-Schwellenwertes. Dadurch könnte es möglich sein, das Schnittholz zuerst unter Kollapsbildung bei hohen Temperaturen zu trocknen, dann aber den Kollaps durch Dampfbehandlung wieder rückgängig zu machen, so daß keine Rißbildung erfolgt. Dies erfordert jedoch noch weitere Untersuchungen.
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Abbreviations
- EC:
-
Creep strain
- Ei:
-
Instantaneous strain
- Em:
-
Mechano-sorptive strain
- Eu:
-
Unconfined shrinkage strain
- EW:
-
Earlywood
- LW:
-
Latewood
- MC:
-
Moisture content
- FSP:
-
Fiber saturation point
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Innes, T.C. Pre-drying of collapse prone wood free of surface and internal checking. Holz als Roh-und Werkstoff 54, 195–199 (1996). https://doi.org/10.1007/s001070050165
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DOI: https://doi.org/10.1007/s001070050165