The Role of Moisture Gradients and Time Scales in Wood Mechanosorption

Abstract

The mechanical behavior of wood has been widely investigated, but its deformation under combined load and moisture changes remains difficult to describe and predict. The resulting deformation cannot be described by the simple superposition of the individual effects, a phenomenon known as mechanosorption. While mechanosorption is generally attributed to the structural complexity of wood and the rearrangement of hydrogen bonds, key aspects of this phenomenon, including the interplay between viscoelastic and diffusive time scales and the role of internal moisture gradients, remain debated. To address these open questions, a Diffusion-Dependent Stick-Slip Fiber Bundle Model is proposed that couples moisture transport with a mechanical description of deformation mechanisms in hygroresponsive materials. The competition between viscoelastic and diffusion time scales is analyzed through dimensionless numbers across cyclic moisture periods. The results show that longer moisture cycles lead to higher cyclic strain amplitudes, which result in greater mechanosorptive strains. Simulations with and without moisture gradients presented only minor differences in global strain. While gradients induce higher hygromechanical stresses and increased damage, their influence on the overall bundle response is mitigated by the coexistence of tension and compression zones. Finally, surrogate-model-assisted fitting to experimental data showed good agreement with consecutive mechanosorptive and constant-moisture viscoelastic responses. In addition, the results show that mechanosorptive behavior varies significantly across different combinations of mechanical-response parameters, providing insight into the origin of the distinct mechanosorption magnitudes observed in different wood anatomical directions.