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Internal stress measurement during drying of rubberwood lumber: effects of wet-bulb temperature in various drying strategies

Abstract To understand internal stress (IS) development within lumber during drying of rubberwood, restoring force measurements on half-sawn specimens has been performed at drying temperature of 90°C under various patterns of wet-bulb (WB) temperature: (1) single-step drying, (2) two-step drying, (3) oscillation drying, and (4) stress control drying. Two extreme single-step drying conditions designated as “severe” and “conservative” at WB temperatures of 40°C and 80°C, respectively, were applied as control drying. It was shown that the measured force, which strongly correlates with the released strain data obtained by the McMillen slice test, indicates the level of IS developed within the lumber. In two-step drying, when the time of abrupt WB temperature change from conservative to severe drying conditions is extended up to the time of stress reversal, the force detected after stress reversal decreases from the level of severe drying to that of conservative drying. Once the time of stress reversal is exceeded, the force profile is largely unaffected by the parameters of the severe drying condition. Gradual reduction in WB temperature beyond the time of peak stress also reduces the force detected later after stress reversal under severe drying conditions. Oscillation of the WB temperature between conservative and severe conditions periodically changes the magnitude of the measured force. After stress reversal, the force amplitude monotonically decreases after each drying cycle. In addition, longer oscillation cycles seem to produce larger initial force amplitudes, which decline quicker than those created under the shorter ones. The possible application of the measured force as a controlling parameter of rubberwood drying has also been demonstrated.