Creep aspects of softwood from the cell-wall level to structures

Abstract: This thesis addresses the intricate mechanical behaviour of natural materials, with a particular focus on wood. Despite millennia of use, understanding the mechanical behaviour of wood materials remains challenging due to their complex microstructures. For instance, they exhibit variations in properties among samples, nonlinear behaviour under elevated loads, and are sensitive to alterations in moisture content.Wood and related natural biobased materials hold immense potential due to their renewability, cost-effectiveness, eco-friendliness, and ease of use in sustainable construction. Wood boasts remarkable stiffness and strength along its primary axis, surpassing many man-made materials in strength-to-weight ratios. However, its anisotropic and heterogeneous nature gives rise to challenges, necessitating the consideration of multiple parameters for accurate characterization to be used in design.Wood is intrinsically heterogeneous, leading to considerable variations in local stresses and deformations during loading. To address these microstructural effects on macroscopically measurable phenomena, mathematical homogenization methods, established since the 1970s, have found applications in material mechanics, including both fibre composites and wood.In recent years, there has been a growing focus on the viscoelastic behaviour of composites and timber structures, given their increased long-term use in load-carrying applications. While numerous investigations have explored the relationship between the microstructure of wood and its elastic properties, few studies have explored the connection between microstructure and viscoelastic properties.The thesis focuses on the static and, more notably, on the time-dependent mechanical properties of wood, bridging the gap from cell-wall creep to structures. It includes experiments and numerical work, culminating in the development of a material model suitable for orthotropic materials like wood. The multiscale model establishes a link between microstructural parameters and macroscopic properties, potentially applicable to various softwood species. Given the lack of shear creep data in the literature, the thesis introduces straightforward methods to characterize shear creep properties, addressing a significant knowledge gap.Furthermore, the thesis progresses from material-level experiments to higher length scales, demonstrating how the results can be applied to larger wooden structures, such as the tower for a counter-rotating axis tilted turbine. While these results require further validation in the absence of experimental data for wooden wind turbine structures, they offer useful insights into simulating creep behaviour in such applications.In conclusion, this thesis highlights the multifaceted nature of a natural material like wood, its mechanical challenges, and the promising research avenues for comprehensive understanding and practical use. The outcome provides contributions to the efficient utilization of wood in load-carrying structures and underlines the importance of ongoing research in this field.