Modeling and On-line Measurement of Drying Stress of Pinus massoniana Board

A drying stress model was established by considering that the total shrinkage of wood is the sum of free shrinkage, instantaneous strain, viscoelastic strain, and mechanosorptive strain. From the stress model, the stress can be calculated once the actual wood shrinkage and moisture content gradient are known. Based on this theory, on-line measurement of the drying stress has been realized by measuring the moisture content (MC) gradient between the surface and the core layers, and the actual shrinkage of the board for Pinus massoniana.

A sensor for measuring wood shrinkage was developed based on electric resistance and strain relationship in a selected element material within the sensor. A resistance type of MC sensor was used for the MC gradient measurement. These sensors are reliable and can meet the requirement of the measurements in practical drying. The technique reported in this article for detecting drying stress from the on-line measurements of board shrinkage and MC gradient can be applied to develop optimized drying schedule in commercial drying.