Bending responses of self-shaping wood bilayers: comparison between adsorption and desorption processes

Inspired by biological materials, the self-shaping wood composites induced by relative humidity (RH) have been investigated over the last decade. In this study, a wood bilayer was fabricated by assembling two layers with perpendicular fiber orientation in a series of thickness ratios and initial moisture contents (iMC). The self-shaping response of the wood bilayer was explored as a function of its configuration, and the discrepancies between moisture adsorption and desorption processes were clarified. Higher iMC limited the evolution of curvature. During both the adsorption and desorption processes, a reversed bending response was observed, and there was a hysteretic bending behavior between them. Repeatable bending was achieved during the cyclic ad/desorption process, and a larger hysteresis loop was observed at a lower thickness ratio. Finite-element analysis showed that the maximum stress occurred at the interface between the active and passive layers, and larger thickness ratios had lower maximum stress. In addition, the bilayer composed of a 200 μm passive layer and a 400 μm active layer with 0.6% iMC was found to be the most sensitive to RH change. The results of this study elucidate the moisture-dependent bending response of wood bilayers and provide the possibility of precisely controlling the curvature of self-shaping wood composites in industrial applications.