Mixed-mode I-II crack growth in a bio-based wood adhesive bondline was investigated using a three-dimensional finite-element model with a cohesive zone formulation. Cohesive parameters - including strengths, onset displacements and fracture energies - were obtained directly from double cantilever beam experiments with uneven bending moments. These experimentally derived parameters were then implemented in the finite element model without any calibration to fit the global response, as the aim was to validate the modelling approach rather than to identify material parameters. The model reproduced stable delamination, captured the expected variation in fracture-process-zone size, provided insight into the distribution and magnitude of normal and shear stresses along the bondline from crack initiation through propagation, and showed good agreement with the global experimental response in opening-dominated (nominal Mode I) loading (phase angles psi = 0 degrees and 41 degrees). In shear-dominated mixed-mode loading (psi = 69 degrees, 85 degrees and 89 degrees), fracture resistance was overpredicted, attributed to large fracture process zones and model simplifications. Overall, the results demonstrate that a relatively simple cohesive zone model, when driven by experimentally derived cohesive laws, can capture the key trends in mixed-mode fracture response of wood-adhesive bonds.

The use of branch wood as a raw material in refined wood products has been limited. Compared with normal wood, this is due to the difference in structure from molecular level to tissue, especially for compression wood found in conifers. These differences result in distinct material behaviour. To provide greater insight into the underlying mechanisms that govern the difference in moisture-induced swelling between compression wood and opposite wood, a finite element model of a simplified predominant S2 layer was developed. To estimate the swelling of the cell wall, the model included nanoscale information from previous and more recent findings on the composition and properties of the cell wall biopolymers. Parametric investigations were carried out to determine the sensitive areas of the three-phase composite, composed of cellulose, hemicellulose, and lignin. Within reasonable bounds, it was found that the microfibril angle plays a significantly larger role in the swelling behaviour of opposite and compression wood, than the relative composition of the cell-wall biopolymers and the hygroexpansion coefficient of lignin. The latter is known to be chemically different between compression and opposite wood.

In this paper, in-situ X-ray microtomography was used to analyze liquor penetration/impregnation and delignification of wood chips during kraft pulping, allowing microstructural changes to be assessed over time. The study was conducted with sapwood of three hardwood species (alder, aspen and birch), using a reactor designed to provide liquor circulation and temperature control. Each wood sample was digested at 141 °C for four hours and, throughout this time, fifteen 3D images of the central portion of the samples were acquired. The images were segmented and used to measure lumen size, cell wall thickness and wood chip porosity. The results confirmed that vessels offered the preferred path for liquor penetration in the hardwoods. Moreover, liquor penetration from ray cells to adjacent fibers was shown to be a less efficient path for impregnation. Regarding delignification, fiber separation began first in aspen and last in alder, and the complete separation took between 1 and 1.5 h to occur in the center of the samples. The porosity of the chips increased continuously after liquor penetration, whereas cell wall thickness decreased more substantially during fiber separation, especially in aspen, but remained relatively stable afterwards. Furthermore, the position of the fibers in relation to vessels and rays did not impact the rate of delignification significantly. Overall, this work shows that in-situ tomography can be a valuable technique to move forward research on wood impregnation and on topochemistry of lignin removal during pulping.

Lignin solubility is critical for extraction, tailoring processing, and downstream applications. Understanding the factors governing its solubility helps understand the behavior of this chemically heterogeneous aromatic biopolymer that has found increasing use in recent years. This study explores the molecular weight dependence of lignin solubility across different solvents. Molecular dynamics simulations are linked to experimental solubility measurements via a ranking of solvents. This integrated approach combines the strength of atomistic insights from simulations with experimental validation. The free energy of solvation and its enthalpic and entropic contributions are determined, and specific interactions such as hydrogen bonds and π-π stacking are quantified. The results show that differences in lignin solubilization efficiency between organic solvents and water are primarily enthalpy-driven with a smaller entropic contribution. In contrast to simplified solubility parameter approaches, molecular dynamics simulations provide a holistic view of lignin solubility and are shown to be a useful tool for understanding its solubilization mechanisms.

A special molecular association is π-π-stacking, driven by weak interactions within aromatic compounds. The π-π-stacking interactions can occur in either a sandwich-like or T-shaped manner. In this study, a method to recognise π-π-stacking from classical molecular dynamics trajectories is developed. By applying three criteria, the method is tested for simple lignin dimer, tetramer and octamer systems, with all G units and β-O4' linkages. The criteria are geometric and based on distance between ring centroids, the angle between the planes of the two rings and the lateral displacement of the rings. In addition, a wide-angle X-ray scattering (WAXS) profile was calculated from a tetramer system, in agreement with previous experimental results. However, when the WAXS peak assigned to sandwich-shaped stacking was analysed in-depth, it was found to mainly be caused by other intramolecular structural motifs involving e.g. the α-carbon and ring carbons, rather than π-π-stacking. This finding is important for future analyses of WAXS profiles originating from lignin-based materials and shows the strength of combining X-ray scattering methods with molecular modelling.

The differential swelling seen between softwood opposite wood (OW) and its neighbouring compression wood (CW) developed in branches prompts several engineering issues such as dimensional instability and cracking. For a more efficient use of resources, the inevitable CW and OW should not be discarded or used as fuel, but incorporated into engineered wood products. Swelling is a hygroelastic phenomenon, where both the swelling and elastic properties of CW and OW are needed in order to make proper structural predictions. In this paper, swelling coefficients and moisture dependent elastic moduli for both CW and OW in the three principal material directions are provided along with measurements of moisture content, density, and microfibril angle. The small deformations necessitate the use of precise X-ray micro-computed tomography for measurements. The results indicate that CW and OW from Norway spruce branches differ in swelling, especially in longitudinal direction at low moisture content. It is noted that CW is a wood type with less pronounced anisotropic behaviour than both OW and normal wood from the stem, with the elastic moduli less sensitive to moisture changes in both longitudinal and transverse directions.

Wood branches subjected to bending develop reaction wood to accommodate both tensile and compressive stresses. For softwood, such as Norway spruce and Scots pine, compression wood (CW) develops in the lower parts, while opposite wood (OW) develops on the upper parts of the branch. It is likely that the ratio of CW to OW is optimised for mechanical load bearing by nature. This hypothesis was tested with an analytical beam model using experimental data of stiffness and strength of CW and OW at CW fractions from 0 to 100%. It was found that there is indeed a maximum bending moment capacity around 35% CW, like literature values of CW content in softwood branches. For all compositions, compressive or tensile strength of OW was governing the behaviour.

Softwood branches develop compression wood (CW) in the lower parts of the branch, while opposite wood (OW) develops on the upper. These wood types differ in structure at several length scales, among others in the chemical composition of their lignin matrix. While OW mostly contains guaiacyl (G) units, CW is known to contain a substantial fraction of 4-hydroxyphenyl (H) lignin. In this study, the impact this difference has on lignin hygroexpansion and interaction with water is studied by the means of atomistic models and molecular dynamics computer simulations of lignin systems at different levels of hydration. It was found that, despite the minor difference in chemical composition, there are differences in swelling, structure and water dynamics. CW lignin is found to have a higher uniaxial swelling coefficient, since the phase separation between lignin and water is more pronounced. This behavior is linked to structural differences, where intermolecular π -π stacking is more common in CW lignin and hydrogen bonding to water more pronounced in OW lignin. These findings are of interest for understanding the role of lignin in CW, and general understanding of moisture interaction with lignin inside wood cell walls.

Abstract Microscopic lab-based X-ray computed tomography (XµCT) aided finite element (FE) modelling is a popular method with increasing nature within material science to predict local material properties of heterogeneous materials, e.g. elastic, hygroexpansion and diffusion. This method is relatively new to wood and lacks a clear methodology. Research intended to optimise the XµCT aided FE process often focuses on specific aspects within this process such as the XµCT scanning, segmentation or meshing, but not the entirety of the process. The compatibility and data transfer between aspects have not been investigated to the same extent, which creates errors that propagate and negatively impact the end results. In the current study, a methodology for the XµCT aided FE process of wood is suggested and its bottlenecks are identified based on a thorough literature review. Although the complexity of wood as a material makes it difficult to automate the XµCT aided FE process, the proposed methodology can assist in a more considered design and execution of this process. The main challenges that were identified include an automatic procedure to reconstruct the fibre orientation and to perform segmentation and meshing. A combined deep-learning segmentation method with geometry-based meshing can be suggested.

Most of the physical, mechanical, and esthetic properties of wood products are affected by the drying of sawn timber.A better understanding of moisture transport in wood during kiln drying is necessary to obtain better quality products, shorter drying schedules, lower energy consumption, and a more sustainable process.Four-dimensional X-ray computed tomography (4DCT) (three-dimensional in space and one in time) with image processing techniques can be used to study the moisture content (MC) of sawn timber during kiln drying.The development of the technique is however made difficult by computational complexity and a lack of accurate experimental validation.In this study, a method relying on 4DCT has been developed using state-of-the-art image processing techniques.The method was validated by a regression analysis of the predicted MC against gravimetric measurements for different timber cross sections and different initial MC distributions on a significantly smaller scale than has previously been investigated.It is concluded that the MC can be estimated with an average uncertainty of ±4.8 percentage points on a 10 mm scale.Sawmills could ultimately benefit from a better understanding of wood-water interactions and dry sawn timber more efficiently.

Abstract Possible new innovative materials for Counter Rotating Axis Floating Tilted Turbines are studied and discussed. The 40 MW version of the Counter Rotating Axis Floating Tilted Turbine (CRAFTT) will reach as far as 80 m below sea surface and up to 400 m above. The CRAFTT is an integrated design for floating offshore wind with two turbines on the same tilted shaft where the lower turbine is mounted directly on a rotating mast integrated with floater. The upper turbine will reach altitudes of 400 m. The system is designed to be a direct drive system, eliminating need for gearbox, taking advantage of the double air-gap speed of generator. With the generator placed at lower end as ballast the incentive to reduce weight for wings, tower and blades increase. Furthermore, wood is an attractive option as it enables both low CO2 impact production and higher degree of reusability. However, fatigue properties from both mechanical and thermal cycling needs to be addressed in order to evaluate new structural materials in the context of floating wind turbines. Starting from scratch without any preconceived notions, one could consider timber as a potential option for the tower. In such a preliminary and qualitative deliberation, one can consider that the use of wood as the main load-carrying material in large structures has been proven during the last decade by the development of new high-rise wooden buildings, with even higher buildings with timber as the main structural component expected in the future, The tall wooden buildings have been made possible since wood has the advantage of having high specific mechanical properties, i.e. high strength and stiffness with respect to density in the grain direction, in addition to being renewable. Another advantage is that wood is less sensitive to fatigue than many metallic materials, since its hierarchical microstructure prevents the propagation dominant cracks when loaded in the longitudinal direction. Design against fatigue is crucial in wind turbine structures given the inevitable cyclic loading. As all materials, wood certainly has its drawbacks, the foremost being its sensitivity to moisture, which is of obvious concern in off-shore applications. Moisture has a softening effect, resulting in creep, and moisture may trigger chemical or microbial degradation. The development of barrier coatings of aluminium has shown to be very efficient in e.g. high-voltage cables and food packaging, making them impermeable to moisture and air. Such techniques should be applicable also in wood constructions. This presentation highlights the main points specific for wood as a construction material in the design of wood towers for wind turbines in offshore locations, which need to be addressed in design.

Reducing the time for drying sawn timber to a certain moisture level without deteriorating its quality is increasingly important for an economic and energy-efficient industrial timber-drying process and to support the transition to a sustainable society. It is, however, crucial to ensure that the quality of the timber, i.e. the degree of distortion, cracking, discolouration, and moisture variation within and between the pieces in a drying batch, is not compromised. Drying-simulation software tend to be too conservative in drying-rate recommendations, which has been observed in practise particularly for large cross-section timber of Norway spruce. This study investigated the drying rate and checking occurrence of centre-yielded Norway spruce planks when dried with more aggressive schedules than normally used in practise, i.e. using higher dry-bulb temperatures and/or lower relative humidities than recommended in the conventional optimisation programmes used in the sawmill industry in Sweden. The objective was to investigate the possibility to considerably reduce the total drying time without compromising the quality of the dried timber. The quality of the planks was indirectly assessed by estimating their moisture-content distributions, calculating the moisture gradients and monitoring checking. This was achieved with 4D (3D + time) X-ray computed tomography and a recently developed image processing algorithm based on elastic image registration. The key findings in this study suggest that Norway spruce timber can be dried with significantly higher temperatures and lower relative humidities than suggested by simulations, leading to reduced total drying time without inducing checking. This methodology can help to improve the design of drying schedules to reduce drying time and energy consumption while maintaining timber quality at a level accepted by the customers.

Abstract Microscopic X-ray computed tomography (XµCT) aided finite element (FE) modelling is a popular method in material science to relate material properties to heterogeneous microstructures. Recently, a methodology was developed for the XµCT aided FE modelling of wood, which characterises the process from specimen preparation to estimation of material properties. In the current research, this methodology is tested on branches of Norway spruce (Picea abies (L.) Karst.) to estimate the hygroexpansion coefficients of opposite (OW) and compression wood (CW). These properties are largely unknown and have engineering implications. The study is complemented by measurements of density, moisture content (MC) and elastic moduli. Results showed that the methodology assisted in the design of an integrated process and the identification of bottlenecks. It was seen that the level of detail of the numerical model had a strong influence on the obtained hygroexpansion properties. CW from branches showed higher density and longitudinal shrinkage coefficients, and elastic moduli less affected by MC. These differences are unlikely caused by MC, but more likely by the characteristics of the microstructure.

Mathematical models are essential for the development of schedules for the air-circulation drying of timber in Swedish sawmills, but earlier models have been shown to be conservative leading to longer drying times than necessary. In the current study, macroscopic (macro) X-ray computed tomography (CT) has been used in both the development and validation of a finite element (FE) model, to enable the macro-CT aided FE modelling of the nonlinear transient moisture flow in wood. The model uses more advanced theory than has previously been used in Swedish sawmills, by incorporating a surface emission coefficient to simulate the surface resistance to moisture flow. A single piece of Norway spruce [Picea abies (L.) Karst.] timber was subjected to that part of a traditional kiln-drying schedule, which is associated with diffusion-driven moisture transport. The incorporation of macro-CT data into the FE model resulted in a more realistic representation of the board's geometry, the initial moisture state, and the definition of material parameters. It also led to a better simulation of flow speed and moisture gradient, especially the asymmetric MC development within the cross section throughout the drying process.

The empirical test developed as validation for a new beam element model that can account for both mechanical and environmental load action in finite element analysis is presented. The testing protocol allows for the identification and analysis of contributing deflection components in bending under varying MC conditions, including mechano-sorption. The components of deflection in the shear-free span of a four-point bending test and their responses to varying moisture are evaluated with an analytical procedure. The experiment was conducted on clear, straight-grained sapwood and heartwood specimens of Norway spruce (Picea abies) (30 x 15 x 640 mm(3)). The program consisted of three phases: 1) long-term (LT) experiments under constant temperature of 60 degrees C and RH cycles between 40% and 80%, 2) a short-term static experiment to determine the variation in the sample set and the load level of the LT experiment on end-matched specimens, and 3) creep tests at 60 degrees C and constant humidity at either 40% or 80% to determine the effect of moisture on the viscoelastic creep. Mass changes and hygro-expansion measured on matched specimens were used in the analytical method. Constitutive models used for describing the material-level response to loads and moisture changes were applied to the shear-free segment of the specimens disregarding actual moisture gradients and fiber orientation inside the test specimens. A successful identification of each deflection component and isolation of mechano-sorption component was accomplished. In the 90 da of testing, the dominant component of the total deflection was the elastic component, followed by the mechano-sorptive component. Creep was found to be nonnegligible and important in the correct description of mechano-sorption. The effect of moisture on the viscoelastic behavior showed most important during loading and first stages of decreasing deflection rate phase.

This study investigates the effect of thermal modification, ThermoWood Thermo-D treatment versus no treatment, on the embedment properties of Norway spruce timber (Picea abies [L.] Karst.) with dowel-type fasteners. The test specimens were reinforced to prevent splitting of the wood. The influence of density, load direction, test specimen configuration (full hole versus half hole), moisture content, gauge points and calculation method were also evaluated. Thermal modification primarily affected the embedment strength parallel to the grain, which was similar to 25% higher after thermal modification, mainly due to the change in physical properties because of the treatment, i.e. the lower equilibrium moisture content. The influence of the investigated parameters on embedment properties of thermally modified spruce followed similar trends as for unmodified spruce. It was for example seen that the density-embedment strength relationships still hold after the treatment despite the decrease in density and the increase in embedment strength parallel to the grain. However, after thermal modification, the influence of load direction on embedment strength was similar to 30% larger and the influence of calculation method (yield versus ultimate strength) on embedment strength perpendicular to the grain was similar to 10% smaller.

This paper introduces, with the development of user-subroutines in the finite-element software Abaqus FEA®, a new practical analysis tool to simulate transient nonlinear moisture transport in wood. The tool is used to revisit the calibration of moisture simulations prior to the simulation of mechanical behaviour in bending subjected to climate change. Often, this calibration does not receive sufficient attention, since the properties and mechanical behaviour are strongly moisture dependent. The calibration of the moisture transport simulation is made with the average volumetric mass data experimentally obtained on a paired specimen of Norway spruce (Picea abies) with the dimensions $$30\times 15\times 640\, {\mathrm{mm}}^{3}$$ . The data, from a 90-day period, were measured under a constant temperature of 60 °C and systematic relative humidity cycles between 40 and 80%. A practical method based on analytical expressions was used to incorporate hysteresis and scanning behaviour at the boundary surface. The simulation tool makes the single-Fickian model and Neumann boundary condition readily available and the simulations more flexible to different uses. It also allows for a smoother description of inhomogeneity of material. The analysis from the calibration showed that scanning curves associated with hysteresis cannot be neglected in the simulation. The nonlinearity of the analysis indicated that a coherent set of moisture dependent diffusion and surface emission coefficient is necessary for the correct description of moisture gradients and mass transport.

Timber boards manufactured with a traditional sawing pattern often contain both heartwood and sapwood. The difference in moisture content between heartwood (30-60%) and sapwood regions (120-200%) result in a radial moisture variation that can cause internal constraints during drying. However, the green state moisture content is seldom considered when evaluating kiln drying schedules. The developed numerical model is able to simulate stress development in timber boards, which are dried from green state to equilibrium moisture content. The model studied the effect of initial moisture content on stress development in timber boards during drying. The model operates on continuum level and consists of a coupled transient non-linear orthotropic moisture flow analysis, while a stress analysis considers elastic, hygroscopic and mechano-sorptive strain behaviour with use of the finite element method. The simulations were performed on four different timber board configurations, each defined by a unique pith location. The study shows that the green state moisture content does not necessarily lead to significant constraints, but has a positive effect on the maximum tensile stress found in tangential direction at the exchange surfaces in the beginning of the drying process. In this stage, stress development is mainly governed by shrinkage close to the surface, which is partly prevented due to regions still above the fibre saturation point. The initial MC can also influence the time when the maximum stress occurs, but not necessarily the location.

The European design standard for timber structures provides, besides obligatory safety requirements, a set of general serviceability requirements. Despite their generality, they have been proven im ...