Timber constructions and engineered wood products, such as cross-laminated timber (CLT), demonstrate significant moisture-dependent mechanical behavior. Moisture infiltration through end-grain surfaces is particularly problematic when dry-out is prohibited, impairing CLT. However, few studies examine moisture infiltration followed by moisture dry-out and current models struggle to simulate these conditions properly. Using the model of Autengruber et al. [1], including free water transport, allows for a realistic simulation of these conditions [2]. This was validated by replicating the experiments of Kalbe et al. [3]. They examined the moisture content development of several CLT plates, where end-grain surfaces were exposed to water for one week, followed by two weeks of drying in various climate conditions. The simulations were refined by calibrating the mass transfer coefficients of water vapor and free water, where in a sensitivity analysis it was shown that the most significant impact on moisture changes emerges from the latter one. Additionally, the influence of glue lines in CLT panels on moisture transport was examined, revealing minor effects on surface layers but increasing influence towards the middle layer.

REFERENCES

[1] M. Autengruber, M. Lukacevic, J. Füssl, Finite-element-based moisture transport model for wood including free water above the fiber saturation point, Int. J. Heat Mass Transfer, Vol. 161, 120228, 2020.

[2] F. Brandstätter, K. Kalbe, M. Autengruber, M. Lukacevic, T. Kalamees, A. Ruus, A. Annuk, J. Füssl, Numerical simulation of CLT moisture uptake and dry-out following water infiltration through end-grain surfaces, J. Build. Eng., Vol. 80, 108097, 2023.

[3] K. Kalbe, T. Kalamees, V. Kukk, A. Ruus, A. Annuk, Wetting circumstances, expected moisture content, and drying performance of CLT end-grain edges based on field measurements and laboratory analysis, Build. Environ., Vol. 221, 109245, 2022.

In the past years, a new method for the production of curved cross-laminated timber (CLT) has been developed, so-called “self-shaping”. In contrast to the current state of the art using cold bending of lamellae, the inherent hygroscopic properties of wood, i.e. swelling and shrinkage due to moisture content changes, are harnessed through a bilayer setup of lamellae. Hereby, the adhesive bonding of two cross-wise arranged lamellae is used to block the deformation compatibility at the interface, leading to high curvatures of the bilayer setup. Typically, a drying process is used for the shaping, as lamellae need to be dried down to service moisture content conditions regardless. During the drying process of structural bilayer components, residual stresses are induced by shaping. These developing residual stresses and strains can be heavily impacted by moisture and temperature gradients. As a result, controlling these two factors is of great importance in order to reliably compute residual stresses. State-of-the-art computational models are fairly able to predict the deformation of timber structures and their curvature during a self-shaping process. However, up to date, no investigations considering industrial kiln-drying conditions with respect to accelerated moisture gradient conditions have been conducted. In this study, 3D moisture and temperature gradients resulting from an adapted kiln-drying process are computed using the Finite Element Method. Residual stresses and strains are computed using a rheological model for wood, considering time and moisture-dependent effects such as stress relaxation through visco-elastic and mechano-sorptive creep. Quantitative residual stresses, such as longitudinal, transverse, and rolling-shear stresses in both lamellae of self-shaped wood bilayers are assessed and shown. These values are of relevant importance for consideration in the structural engineering design of curved CLT structures assembled from wood bilayers.

An improved description of the rheological behavior of wood is key for ventures into novel applications of wood with an improved degree of material activation. These efforts are not only impaired by the strongly disordered, hierarchical microstructure. Also, non-negligible effects of moisture on pretty much every constitutive relation, further complicate the accessibility by numerical simulations. It is almost superfluous to mention that variations between species, trunks, and even between positions inside the trunks, as well as imperfections, pose further challenges. As creep strains can exceed the elastic deformations by a factor of 2 or more, depending on the climatic conditions and stress states, it becomes evident, that the rheonomous reaction of wood on mechanical and hygric load requires particular attention.

On two novel applications, weaknesses of the model and today’s underlying wood physical foundation are identified – like orthotropic mechanosorptive and viscoelastic creep. We determine creep experimentally on different scales like the macro scale, the tissue scale, as well as the tracheid scale at different moisture and loading states. Parameters of moisture-dependent creep models, based on Prony series, are identified independently for the different anatomic orientations, including all 6 shear directions. In the second step, different data reduction strategies are evaluated on the experimental data of different scales to provide a comprehensive and experimentally founded basis for rheological numerical models with increased generality.

Plant movements are often the result of a combination of elastic deformation and stiffening tissues. This makes them excellent sources of inspiration for hingeless biomimetic actuators. In the framework of a biomimetic biology push process, we present the transfer of functional motion principles of hollow tubular geometries surrounded by a reticular structure. Our plant models are the recent genera Ochroma pyramidale (balsa) and Carica papaya (papaya), as well as the fossil “seed fern” Lyginopteris oldhamia, which possess a network of macroscopic fibre structures enveloping the entire stem. Asymmetries in these fibre networks, caused specifically by asymmetric growth of the secondary wood, allow the inclined stems of O. pyramidale and C. papaya to straighten. Comparable structural rearrangements occur in the cortex of L. oldhamia. This similarity in (adaptable) anatomical structures leads to the conclusion that the cortex of L. oldhamia was also able to readjust the orientation of the stem and branches in response to mechanical stress.

In general, fibre angles play a crucial role in stress-strain relationships in a tubular reticular structure. When braided tubes are subjected to internal pressure, they become shorter and thicker if the fibre angle is greater than 54.7 degrees. However, if the fibre angle is less than 54.7 degrees, they become longer and thinner. We have used simple functional demonstrators to show how insights into functional principles from living nature can be translated into plant-inspired actuators with linear or asymmetric deformation.