A numerical study of the effect of green-state moisture content on stress development in timber boards during drying

Abstract

Timber boards manufactured with a traditional sawing pattern often contain both heartwood and sapwood. In such boards, internal constraints can occur during drying because of a radial variation in green-state (GS) MC between the heartwood (30-60%) and sapwood region (120-200%). Despite such knowledge, the initial MC is seldom considered when evaluating kiln-drying schedules. The effect of GS MC on the development of tangential tensile stress during drying is studied for four types of timber boards. A numerical model was developed that can simulate transient nonlinear orthotropic moisture flow and moisture–induced stress and distortion in wood with the use of the finite element method. The stress analysis considers elastic, hygroscopic, and mechano-sorptive strain. The study shows that the GS MC does not significantly influence the maximum stress state, but that it does influence the time at which the maximum tangential tensile stress occurs at different exchange surfaces. This results in several periods in the drying schedule where unfavorable high stress situations in the tangential direction arise, which could lead to crack propagation.