The characterisation of wood's fracture behaviour is a challenging task due to its inherently complex microstructure and natural variability. Consequently, to accurately model wood for engineering applications, deterministic input parameters are rarely sufficient in, for example, finite element models; the stochastic nature of the material must be considered. In the present work, we aim to quantify the variability in the fracture behaviour of two wood species: Norway spruce, which is commonly used for structural purposes in Europe, and birch, which could be an advantageous complement to Norway spruce, mainly thanks to its stiffer and stronger mechanical properties. The fracture behaviour is characterised through the three parameters that govern a material's brittleness: the stiffness, the strength and the specific fracture energy. By formulating a parameter estimation problem based in probability theory, we use Bayesian optimisation to estimate statistical distributions of the fracture parameters of interest. These distributions are multi-variate distributions and thus contain information about the mean values, variability and dependence among the parameters. It is shown that by using random samples from the acquired distributions as input parameters to finite element models, variability observed in experimental testing is recovered well.

The paper focusses on a numerical study of the structural behaviour of timber-glass composite beams made of glass panes and timber flanges bonded with adhesives of different stiffness. The first part of the paper presents a brief summary of four-point bending tests of two types of timber-glass composite beams varying in length, glass thickness, timber material used for flanges, and type of adhesive used to make a bond line connection. The second part of the paper describes the finite element models developed to simulate the flexural behaviour of timber-glass composite beams. Finite element models were developed to simulate the elastic (linear) and post-failure flexural behaviour of timber-glass composite beams. Two types of analysis, implicit and explicit, were used to simulate the degradation behaviour of glass. The objectives of the numerical analyses were to investigate the influence of various parameters such as glass strength, finite element formulation, finite element mesh size, and variation of adhesive stiffness on the response of timber-glass composite beams. Finally, experimental and numerical results are compared and discussed in terms of the initial bending stiffness, load at first cracking in glass, ultimate load and crack pattern together with global progressive failure.

The development and assessment of bio-based wood adhesives face challenges due to the limitations of conventional test procedures, particularly in predicting adhesive performance in real-world applications. This study investigated the mode I cohesive law of two birch wood-biobased adhesive systems, comparing them with conventional fossil-fuel-based systems. The experimental approach, developed during the '90s at Lund University, involves direct measurement of the traction-separation relation and allows evaluation of post-peak behaviour and fracture mechanical property characterisation. SEM analysis confirmed fracture development within the bond line for all tests. The birch-fish-adhesive bonds demonstrated peak stress of approx. 4 MPa, with serrated fracture surfaces and favourable fracture properties (specific fracture energy approx. 370 Nm/m(2), brittleness approx. 40 GPa/m). Birch wood bonded with lignin-based adhesive showed lower, yet reasonable, levels of peak stress (approx. 3 MPa) but less favourable fracture properties (specific fracture energy approx. 110 Nm/m2, brittleness approx. 90 GPa/m), suggesting room for further optimisation.

Experimental tests of Cross Laminated Timber (CLT) under in-plane beam loading conditions are presented. The influence of the element layup, the individual lamination width, and the beam overhang at the supports on the shear force capacity was investigated. All the CLT beams had the same gross cross section, and a 4-point-bending test setup was used. The experimentally determined load-bearing capacities are compared with the load-bearing capacities resulting from analytical methods proposed for structural design, focusing on shear failure in the crossing areas of flatwise bonded laminations (shear failure mode III). The test results indicate no or very small influence of the element layup and the lamination width on the shear force capacity. These results partly contradict the predictions of the proposed design methods. Of the three studied beam geometry parameters, the beam overhang at the support had the greatest influence on the load-bearing capacity.

This paper presents an experimental study where the mechanical behaviour of single-dowel timber connections made of acetylated Scots pine is compared with the behaviour of connections made from untreated Scots pine. The main aim was to evaluate the influence of the acetylation on the connection brittleness and also to compare the experimental results to the design provisions of the current European structural timber code, Eurocode 5 (EC5). The experiments included embedment tests and tests with connections loaded parallel and perpendicular to the grain, and, for the latter tests, applying different end and edge distances. The acetylated wood showed a 2% increase in density and a 31% increase in embedment strength compared to the untreated wood. For tests on connections loaded parallel to the grain, all specimens made from acetylated wood failed in a brittle manner, while the connections made from untreated wood and complying with minimum end distance of the EC5 design provisions failed due to embedment failure followed by splitting involving cracking along the grain. The connections made of acetylated wood showed a 13–15% higher capacity than the corresponding specimens made from untreated wood. Thus, to fully utilize the potential of the increased embedment strength parallel to the grain, it is concluded that reinforcement of the joint, e.g., by self-tapping screws or externally applied sheet reinforcement would be necessary if the minimum end distances of EC5 are applied. The current design provisions for loading perpendicular to the grain overestimated the capacities severely with predicted characteristic values being 20–50% higher than mean values from tests for the recommended minimum edge distances. Finally, it was found that the splitting capacity in loading perpendicular to the grain was 10–18% lower for the specimens made from acetylated wood compared to the untreated wood.

The present work has experimentally determined the specific fracture energy of the hardwood species silver birch (Betula pendula), which in recent times has caught increased attention for utilization in structural applications. The single-edge-notched beam loaded in three-point-bending was utilized for evaluating the fracture energy with the work-of-fracture method. In addition to birch, Norway spruce (Picea abies) was utilized as a reference material. The effect of two different geometries of the fracture area for each species was evaluated—one triangular and one rectangular fracture area. It should be noted that the geometry of the fracture area did influence the evaluated fracture energy, and this influence was not consistent between species. This was likely in part due to manufacturing difficulties with the triangular fracture area. In addition to the experimental testing, a numerical 2d-model including linear strain-softening behavior was used for comparative simulations. The numerical 2d-models showed reasonable agreement with the experimental results regarding the global load vs. displacement response, despite their relative simple nature. The specific fracture energy for the spruce specimens was evaluated to 221 J/ m^2 and for the birch specimens to 656 J/ m^2 . Consequently, the present work implies a marked increase in specific fracture energy for birch, compared to spruce. This increase in specific fracture energy could potentially have a large influence on the failure behavior of birch when used in structural applications which is something that needs to be considered in future work.

This paper deals with the estimation of the design force in self-tapping screws used as a reinforcement of notched cross laminated timber plates (CLT).With the reinforcement, apart from an increased load-carrying capacity, a more ductile behaviour of this detail can be achieved.An analytical model based on the Timoshenko Beam Theory was developed, enabling the estimation of the axial force in the reinforcement.A parametric numerical analysis was conducted to verify the model and to provide possible ways for the calibration.The results show a good matching of the model with the numerical results.The load-bearing capacity is analysed using a simple fracture mechanics model taking into consideration the effect of the reinforcement to correctly depict the increase in performance.Theoretical and numerical results are compared with experimental findings and show acceptable correlations encouraging a further development and a subsequent implementation in EN 1995-1-1.

The research project Innovative Solutions for Cross Laminated Timber Structures (InnoCrossLam) was recently finished.The project aimed at increasing the competitiveness of CLT as a versatile engineered product, by increasing its predictability in demanding design situations not covered by the guidelines of today, or standards and codes foreseeable in the near future (e.g., second generation of European design standards).This paper summarises the main project findings in the context of innovative CLT structures, such as: i. contemporary design approaches ii. the use of (non)linear FE modelling iii.experimental investigation of complex details for CLT structures, iv.investigation of multifunctional CLT.In this paper, the motivation behind the research topics within the project InnoCrossLam is explained and backed-up by exemplary results and discussion on future work.

The paper deals with numerical investigations of load-bearing capacity and fracture behaviour of Cross Laminated Timber at in-plane shear loading.Focus is on shear failure mechanism III, i.e. failure in the crossing areas between flatwise bonded laminations, and on evaluating test methods for that failure mode.In current design provisions, this failure mode is characterised by the rolling shear strength, fv,R, and the torsional shear strength, fv,tor, and therefore testing including both transverse (rolling) shear and torsional shear is needed.Two such test set-ups are investigated, one aiming at evaluating torsional shear strength and one aiming at evaluating rolling shear strength.Full 3D finite element analyses applying a cohesive zone model approach were used to study the strength and fracture behaviour.Failure criteria for structural design, material strength parameters, and the suitability of the two test configurations to determine strength parameters are discussed in relation to findings from the numerical investigations.One main conclusion from the investigation is that expected test results using the torsional test set-up are less size dependent as compared to test results using the transverse set-up.The numerical analyses suggest a very small influence from tensile loading perpendicular to the crossing area, for both test set-ups.

Cross-laminated timber (CLT) as a prime example of an innovative product within the field of civil engineering has over the recent years attracted attention of the construction industry around the world. Timber structural systems however are more sensitive to vibrations than conventional concrete based structures. Therefore, a thorough understanding of dynamic characteristics of CLT and accurate predictive models can enhance potential of wooden structures in the construction industry. This work presents a numerical and an experimental modal analysis of a series of CLT beams, cut out from a larger CLT plate, composed of two covering layers of Norway spruce and a mid-layer of Scots pine. The experimental results are utilised for a sensitivity analysis with respect to the mechanical properties of CLT.

This chapter deals with the application of adhesives in timber engineering, including wood-based products. This work is aiming to describe the general subjects in wood gluing technology, which are characteristic to wood substrates. Firstly, an overview of the basic needs and prerequisites is given, followed by a brief overview of the characteristics of the wood material. An introduction to the general parameters influencing complex process of bond formation and performance of adhesive bonds is presented. Factors such as surface preparation, adhesive types, gluing process, and testing methods are discussed in this section together with mechanical, climatic, environmental factors, and behaviour in fire. The strength and durability characteristics of wood-adhesive bonds are covered. Some emphasis is put on the mechanical description of wood-adhesive bonds, including experimental and numerical results from previous research. The most common reasons for failure and the most commonly used procedures for inspection, testing, and quality control are described. Repair of wooden structures is discussed, followed by some examples of the use of adhesive technology in timber engineering. Finally, some possible future trends and suggestions for further reading are given.

The moisture-dependency of the fracture energy for unmodified and acetylated Scots pine (Pinus sylvestris L.) and birch (Betula pendula Roth) has been investigated. Specimens were conditioned at relative humidity levels of 20, 75, and 97%, as well as dry and water-saturated. At moisture contents below 15%, the fracture energy increased with increasing moisture content for both unmodified and acetylated wood, while it decreased for untreated wood at higher moisture contents. A significant difference in moisture-dependency was found, indicating higher fracture energy for unmodified wood compared to acetylated wood at similar moisture contents. Additionally, to assess the impact of the increased brittleness for structural applications, the fracture energy was compared at equal relative humidity levels. The largest difference was seen at 75% relative humidity with approximately 50% lower fracture energy for acetylated wood. No significant differences were found for water-saturated samples. The moisture-dependency of the fracture energy, combined with the reduced hygroscopicity of acetylated wood, is suggested to be one, but not the only, contributing factor to the lower fracture energy of acetylated wood compared to unmodified wood at equal humidity levels. These observations have importance for structural design since design codes often assess material parameters based on ambient humidity.

Numerical modeling is an efficient tool for experimental validation and for gaining a deeper understanding of complex material phenomena, especially when causal relationships are overlaid by material variability. Wood is such a highly orthotropic and complex material, which in engineering problems however is considered as macrohomogeneous. The aim of this study is to numerically investigate stress and strain states of wood in the radialtangential plane and the influence of the orthotropic material behavior on the structural response. Model validation is based on experiments performed on clear wood of Norway spruce (Picea abies) by using a biaxial test setup. Three material models were used, namely Hill's plasticity model, the Hoffman criterion and a novel quadratic multi-surface (QMS) criterion. After validation on the local material scale, the models were applied to the engineering problem of compression perpendicular to the grain for studying the effect of the unloaded length. As a novel part, the influence of the annual ring structure on the local material behavior and the global elastoplastic force-displacement behavior of wood under compression perpendicular to the grain were numerically investigated. Hill's failure criterion was found to be the least suitable at both length scales, local material behavior and global structural response. The Hoffman and the QMS criteria showed quite good agreement with the biaxial experiments in terms of force-displacement relations and strain distributions for different loading situations, especially for combinations with radial compression, while there was less agreement with experiments for the behavior of combinations with tangential compression. Application of these material models to compression perpendicular to the grain for studying the unloaded length effect yielded similar trends as observed in structural tests. A reasonable and similar force-displacement response by Hoffman and QMS criteria was observed, while Hill's model yielded significantly overestimated force carrying capacity. Differences in force-displacement response for different loading situations were well in line with literature findings and the influence of the annual ring curvature on the overall force-displacement behavior could be quantified.

In this study, static coefficients of friction for laminated veneer lumber on steel surfaces were determined experimentally. The focus was on the frictional behaviors at different pressure levels, which were studied in combination with other influencing parameters: fiber orientation, moisture content, and surface roughness. Coefficients of friction were obtained as 0.10–0.30 for a smooth steel surface and as high as 0.80 for a rough steel surface. Pressure influenced the measured coefficients of friction, and lower normal pressures yielded higher coefficients. The influence of fiber angle was observed to be moderate, although clearly detectable, thereby resulting in a higher coefficient of friction when sliding perpendicular rather than parallel to the grain. Moist specimens contained higher coefficients of friction than oven-dry specimens. The results provide realistic values for practical applications, particularly for use as input parameters of numerical simulations where the role of friction is often wrongfully considered.

Glued-in rods are important connecting and reinforcing elements in modern timber engineering used in new and existing timber structures. The complex stress distribution along the bondline between rod and wood depends on the type and properties of the adhesive and the type of load application. In this chapter, different models for the determination of the shear stress distribution along the bondline are discussed and their effects on the stress distribution and strength of the glued-in rod are evaluated. Important points on how to enhance the load-carrying capacity and reach best structural capacity, as well as the ductile failure behaviour, are discussed.