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. In such boards, internal constraints can occur during drying because of a radial variation in greenstate (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.

Three-dimensional orthotropic nonlinear transient moisture simulation for wood: analysis on the effect of scanning curves and nonlinearity

“…The user-subroutine frameworks associated with this tool are provided by finite-element software Abaqus FEA ® (Dassault Systemes, Vélizy-Villacoublay, France). The work further develops the research previously published by Florisson et al (2019). The research objectives related to the development of the simulation tool are: (1) the implementation of the single-Fickian approach and the moisture-dependent diffusion coefficient in user-subroutine UMATHT, which is usually used in the field of polymers, freezing soil and fire engineering; (2) the implementation of the moisture flux vector normal to the boundary surface in user-subroutine FILM, which is described by the moisture dependent material property named the surface emission coefficient (Yeo and Smith 2005;Yeo et al 2002); (3) implementation of a practical method in user-subroutine FILM to describe sorption hysteresis and scanning behaviour at the boundary surface.…”

“…The diffusion coefficient is the same in both the radial and tangential directions. Figure 6a presents these coefficients together with the values obtained in Florisson et al (2019). Great similarities are found between the simulation made with the standard analysis (without routines) provided by Abaqus FAE ® in combination with tabulated flow properties, and the simulation made using the user-subroutines in combination with the expressions provided by Eq.…”

“…The profiles come from Rosenkilde (2002). To validate the routines, the moisture-dependent diffusion and surface emission coefficient obtained from this analysis are compared to the coefficients obtained in a previous publication by Florisson et al (2019). In the second "Hysteresis" section, the experimentally observed hysteretic behaviour, together with its implementation into the routine that covers the boundary condition, is discussed.…”

“…When simulating moisture transport, it is generally known to incorporate the moisture dependency of flow properties, where they can have a strong effect on the speed with which MC changes and the formation of moisture gradients (Angst-Nicollier 2012; Florisson et al 2019;Toratti 1992). To show the effect regarding the properties obtained in this paper, simulation 1 from the previous section will be compared to simulations where either constant flow properties are assumed or a mixture between constant and moisture dependent properties.…”

Crack formation, strain distribution and fracture surfaces around knots in thermally modified timber loaded in static bending

“…Therefore, it is reasonable to conclude that severe damages within or between cells in thermally modified wood (TMW) are caused by mechanisms related to the shrinkage anisotropy or differential shrinkage as reported by Boonstra et al (2006a, b). Differential shrinkage can occur in timber because of the moisture gradient that develops in the cross section upon drying (Tsoumis 1991;Florisson et al 2019). Drying stresses caused by these shrinkage differences lead to radial cracking upon drying (i.e.…”