Poisson's Ratio of Wood-Based Panels and Poisson's Effect of Hygroscopic Linear Expansion under a Uniaxially Dimensional Restraint(Special Issue on Wood and Wood Based Materials)

When wood based panels are nailed and/or bonded to lumber frame, effects of mechanical restraint on the panel hygroscopic linear expansion (LE) sometimes differ at in-plane biaxial directions. Even the adverse effect that LE in one direction becomes larger than its free LE is recognized when the other direction is strongly restrained. To give a better understanding for this phenomenon, this study was designed to investigate “Poisson's effect” of LE by comparing a free LE and biaxial LEs of a uniaxially restrained specimen of medium density fiberboard (MDF). Results showed that LE perpendicular to a restrained direction was much greater than free LE and “Poisson's effect” of LE was confirmed. Also, its apparent Poisson's ratio was found to be time dependent and associated with swelling stress. Conducting a static in-plane compressive test revealed that the apparent Poisson's ratio in LE was greater than that as an elastic constant. Furthermore, Poisson's ratios by a standard test were compared among panels with different element size, namely, plywood, oriented strand board, particleboard and MDF. Poisson's ratio of these panels ranged 0.14 to 0.28 and had a tendency to be greater as element size became smaller and elements were more randomly oriented.