Setting in wood plays an important role in the practice of wood drying. But its role is not well established and the true nature of setting in wood is problematical. Presently, the most important matter is to clarify the fundamental, physical, mechanical and Theological properties of set wood.
In order to clarify the rheological properties of set wood, the effect of tensile stress applied during drying on the Young's modulus of wood was determined by means of the vibrating reed method (70-85Hz) and the longitudinal resonance method (7-8kHz).
The Young's modulus of set wood increased in proportion to the degree of set and the rate of increase against normal wood is in the order of 1.12-1.20. On the other hand, tan δ of set wood does not depend clearly on the stress applied during drying.
Then, using the vibrating reed method, the change of Young's modulus of set wood during moisture-recovery of set was determined. The result confirms that the moisture-recovered wood loses its past history of the stress applied during drying.
Based on this fact, the moisture-recovery of set causes not only the recovery of apparent dimension of a specimen but also the recovery of some structure formed by the stress applied during drying.
These characteristic natures of set may be attributable to some reversibly changeable structure of wood in the microscopic level.
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