Finite-element-based moisture transport model for wood including free water above the fiber saturation point

Abstract

Timber constructions and engineered wood products exhibit a strong moisture-dependent behavior, which needs to be sufficiently describable by models to avoid critical designs. For wood, several models exist, able to describe the specific heat and moisture transfer mechanisms under normal service conditions as well as for particular situations during the production process, like the kiln drying of sawn timber. However, a unified model combining free water transport with the multi-Fickian approach below the fiber saturation point and the corresponding stable numerical implementation able to handle several possible states of water (bound water, water vapor and free water) and applicable to both drying and infiltration problems, is still missing.In this work, we link models for all those transport processes by reasonable coupling mechanisms. Numerical difficulties occurring at transition between different states of water are handled appropriately, leading to a stable and reliable simulation tool, which is implemented through a three-dimensional Abaqus user element. Thus, this contribution extends the applicability of heat and moisture simulation tools for wood to more critical service conditions, where wooden components might get into direct contact with free water. This may happen due to insufficient supervision during construction or in the production process of new types of wood composites. We anticipate that with the developed tool potentially critical designs can be identified by simulation instead of by time-consuming experiments or experience. This is of importance because the presence of free water and the resulting reduction in stiffness and strength is associated with a higher risk of local failure. This would allow more targeted designs of engineered wood products and efficient optimization strategies, especially with respect to the moisture behavior.