A NUMERICAL STUDY OF THE EFFECT OF GREEN-STATE MOISTURE CONTENT ON STRESS DEVELOPMENT IN TIMBER BOARDS DURING DRYING S Florisson, S Ormarsson, J Vessby

Abstract

Timber boards manufactured with a traditional sawing pattern often contain both heartwood and sapwood. The difference in moisture content between heartwood (30-60%) and sapwood regions (120-200%) result in a radial moisture variation that can cause internal constraints during drying. However, the green state moisture content is seldom considered when evaluating kiln drying schedules. The developed numerical model is able to simulate stress development in timber boards, which are dried from green state to equilibrium moisture content. The model studied the effect of initial moisture content on stress development in timber boards during drying. The model operates on continuum level and consists of a coupled transient non-linear orthotropic moisture flow analysis, while a stress analysis considers elastic, hygroscopic and mechano-sorptive strain behaviour with use of the finite element method. The simulations were performed on four different timber board configurations, each defined by a unique pith location. The study shows that the green state moisture content does not necessarily lead to significant constraints, but has a positive effect on the maximum tensile stress found in tangential direction at the exchange surfaces in the beginning of the drying process. In this stage, stress development is mainly governed by shrinkage close to the surface, which is partly prevented due to regions still above the fibre saturation point. The initial MC can also influence the time when the maximum stress occurs, but not necessarily the location.