A Method to Measure Moisture Induced Swelling Properties of a Single Wood Cell

Wood cells constitute the main building block in engineered wood-based materials, whose delimiting property frequently is moisture induced swelling. The hygroexpansion properties of wood cells, technically known as fibers, are used as input in predictive micromechanical models aimed for materials design. Values presented in the literature largely depend on the microfibrillar angle, the geometry of the fiber and limiting modelling assumptions. Synchrotron X-ray micro-computed tomography has recently prompted means for detailed measurements of the geometry of unconstrained individual fibers undergoing moisture-induced swelling, which makes it possible to directly quantify the hygroexpansion properties of the cell wall. In addition to a well-defined three-dimensional geometry, the present approach also accounts for large deformations and the fact that cell-wall stiffness depends on the presence of moisture. A mixed numerical-experimental approach is adopted where a finite-element updating scheme is used to simulate the swelling of an earlywood spruce fiber going from the experimental fiber geometry at 47 % relative humidity to the predicted geometry of the fiber in the wet state at 80 % relative humidity at equilibrium conditions. The hygroexpansion coefficients are identified by comparing the predicted and the experimental three-dimensional fiber geometry in the wet state. The obtained values are 0.17 strain per change in relative humidity transverse to the microfibrils in the cell wall, and 0.014 along the microfibrils.