Acoustic emission study of within-ring internal checking in radiata pine-Wood NDT 2000

摘要

During the last eight years within-ring internal checking has become a serious problem during the drying of radiata pine sapwood boards used for appearance grade timber. A within-ring internal check extends radially through the earlywood of a single annual ring and does not penetrate through the latewood on either side. Hence its radial length is limited to the thickness of the earlywood of a single annual ring, which no'rmally lies in the range 3 to 12 mm. The length of a check can be anything from 10 mm to the complete length of the board. These checks normally do not penetrate to the surface of the boards and hence remain invisible until re-machining occurs. Consequently boards containing within-ring checks may already incur considerable transport and manufacturing costs before the problem is detected. Before it is possible to develop drying schedules that minimise the development of within-ring internal checks, it is essential to know during what stage of drying the checking in radiata pine sapwood occurs. A series of 14 radiata pine sapwood boards believed to be very susceptible to within-ring internal checking were sawn into 600 mm lengths (2 ft) with an average moisture content of 146%. These boards were randomly assigned to 5 different kiln charges. These kiln charges were dried for periods of 6, 12 and 20 hours (3 charges) according to a schedule of 120℃ dry bulb and 70℃ wet bulb with a heat up time of two hours. The boards were then sectioned to assess the severity of internal checking immediately after drying was terminated. Four boards from the six hour charge were fitted with acoustic emission equipment prior to drying and the development of internal checking was monitored. Three pins that were an integral part of the transducer casing were pressed into the wood to achieve good contact without the need for constant force clamps. The total number of checks that were counted after drying for 6, 12 and 20 hours was 93, 73 and 39 checks respectively. This shows that internal checking occurs mainly during the first six hours of drying. This was confirmed by the acoustic emission data that showed that almost all the internal checking occurred within the first six hours of high temperature drying, at the end of which the average moisture content was 72%. The maximum rate of checking occurred 1.6 to 2 hours after the start of drying, ie. when the kiln first reached its operating temperature and lasted for about half an hour. It is concluded that within-ring internal checking in radiata pine sapwood begins as soon as the wood starts to dry. The checking process was practically complete when the sapwood moisture content reached 79%, which is well above the fibre saturation point of 30%. This indicates that within-ring internal checking in radiata pine is caused by water tension and not by differential shrinkage.