Volume 61, Issue 4

Wood exhibits a highly diverse microstructure. It appears as a solid-type composite material at a length scale of some micrometers, while it resembles an assembly of plate-like elements arranged in a honeycomb fashion at the length scale of some hundreds of micrometers. These structural features result in different load-carrying mechanisms at different observation scales and under different loading conditions. In this paper, we elucidate the main load-carrying mechanisms by means of a micromechanical model for softwood materials. Representing remarkable progress with respect to earlier models reported in the literature, this model is valid across various species. The model is based on tissue-independent stiffness properties of cellulose, lignin, hemicellulose, and water obtained from direct testing and lattice-dynamics analyses. Sample-specific characteristics are considered in terms of porosity and the contents of cellulose, lignin, hemicelluloses and water, which are obtained from mass density measurements, environmental scanning micrographs, analytical chemistry, and NMR spectroscopy. The model comprises three homogenization steps, two based on continuum micromechanics and one on the unit cell method. The latter represents plate-like bending and shear of the cell walls due to transverse shear loading and axial straining in the tangential stem direction. Accurate representation of these deformation modes results in accurate (orthotropic) stiffness estimates across a variety of softwood species. These stiffness predictions deviate, on average, by less than 10% from corresponding experimental results obtained from ultrasonic or quasi-static testing. Thus, the proposed model can reliably predict microscopic and macroscopic mechanical properties from internal structure and composition, and is therefore expected to significantly support wood production technology (such as drying techniques) and mechanical analyses of timber structures.

Challenges arise in finite element (FE) analyses that predict mechanical failure in wood-based materials because their structural complexity is difficult to mimic. When considered at the macro scale, wood and engineered wood composites can reasonably be assumed to behave as homogenous continua. However, accurate meso- and micro-scale representations require a different approach. Models employing discrete FEs are robust tools for detailed failure analysis, because the elements can be made to mimic the functions of morphological structures in the material. Hybrid models that meld continuum and discrete FEs also show good promise as generalised analysis tools, but as yet their development is in its infancy. In the future, beyond mechanical damage, other energy sinks also need to be included in models, and computational efficiency should be improved. In this overview, the advantages and limitations of alternative FE representations are demonstrated in terms of failure processes in wood-based materials via case analyses.

This paper details the development of morphological lattice models to simulate fracture and failure in softwood. The lattice models rationally incorporate growth-ring geometry, differences in strength and stiffness between earlywood and latewood, and variations observed in the grain direction of clear wood (grain perturbation). Details regarding the implementation of these features are presented. Grain perturbation is shown to be a significant contributor to strength and stiffness variability. Simulations demonstrate that the inclusion of growth-ring geometry and incorporation of differences in the mechanical properties of earlywood and latewood are necessary for the lattice models to predict realistic fracture paths. Results are presented for laboratory tests on small red spruce specimens loaded in parallel-to-grain tension and shear. The lattice models give good predictions of mean specimen stiffness and strength, and reasonable predictions of strength variability. The fracture paths predicted by the lattice models are in excellent agreement with the experimental observations.

Differences in fracture patterns and properties at various moisture levels are experimentally and numerically evaluated and discussed. Experiments were performed on spruce, pine, oak and beech. The influence of moisture at 98%, 80%, 65% and 30% RH and the mechanisms involved were investigated for softwoods and hardwoods subjected to opening mode I fracture using in situ and ex situ real-time environmental scanning electron microscopy (ESEM). The wedge-splitting technique was employed. To quantify the effect of humidity, fracture toughness values were obtained from ex situ tests and finite element analysis of the contact problem in wedge-splitting. In addition, lattice fracture model simulations were performed for numerical investigation of the fracture mechanisms. Distinct changes in wood fracture behaviour were observed as a function of moisture content. Fracture toughness was highest at 30% RH for all species except for oak, and showed higher values in the radial-longitudinal than in the tangential-radial direction. In green wood, water droplets moved away from the cell lumens around the crack tip. Drying of wood promotes microcracking and crack bridging as toughening mechanisms. The findings reported may be useful for further research into the interaction between moisture transfer and stress gradients in wood accompanied by moisture-crack phenomena.

A new numerical method called the material point method (MPM) is well suited for modeling problems with complex geometries and with crack propagation in arbitrary directions. In this paper, these features of MPM were used to simulate transverse fracture in solid wood. The simulations were run on the scale of growth rings. The ease with which MPM handles complex geometries was helpful for modeling realistic morphologies of earlywood and latewood. Because MPM discretizes a body into material points, it was possible to go directly from a digital image of wood to a numerical model by assigning the location and properties of material points based on the intensity or color of pixels in an image. Because the description of cracks in MPM is meshless, it can handle a variety of crack propagation and direction criteria and can simulate complex crack paths that are a consequence of the morphology of the specimen. MPM simulations were run for cracks in the radial direction, the tangential direction, and at two angles to the radial direction. The specimens were loaded by axial displacement or by wedge opening. The MPM simulations fully included contact effects during wedge loading. Finally, the potential for coupling such simulations to new experiments as a tool for characterization of wood is discussed.

A three-dimensional anisotropic material model is presented that is applicable in combination with interface elements for simulation of the behavior of timber structures loaded in shear and tension perpendicular to the wood fibers. The material model can predict the stresses derived from the three-dimensional state of deformation. Determination of the algorithmic material tangent is shown. Computation of a stress component results from deformation in all directions. Furthermore, a damage model is implemented to simulate cyclic loading that yields a realistic unloading function for a cracked structure. In this case, a continuous-differentiable material formulation guarantees a robust path-following algorithm. A basic example is used to demonstrate the capability of the model to simulate the behavior of timber structures realistically and underlines the need for further research.

This paper describes methods developed to test the mechanical properties of small-diameter (3–10 mm) 1-year-old trees. Special test fixtures and procedures were designed to accommodate the small diameter and uneven shape of these young trees. The modulus of elasticity and strength in tension parallel to the grain, compression parallel to the grain, and bending were measured.

The current work expands on a three-dimensional, non-linear, stochastic finite-element model previously developed by the author. The model predicts a materially non-linear stress-strain curve for tension, compression and bending scenarios. It is based on the non-linear constitutive properties of the individual strands, which are characterized within the framework of orthotropic elasto-plasticity. The constitutive model employs the Tsai-Wu yield criterion and an associated flow rule. Failure is marked by an upper bound surface whereupon either perfect plasticity or an abrupt loss of strength and stiffness ensues. The finite element code has also extended the capacity to perform Monte Carlo simulations. The model was further developed to predict the mechanical behavior of parallel-strand lumber (PSL) made from Douglas fir. The physical features of PSL were measured and incorporated into the finite element model and the mechanical behavior of PSL was simulated. Statistical distributions for macroporosity and grain angle variation in PSL were created and included in the model as individual random variables in a stochastic and probabilistic manner. Constitutive curves for PSL were numerically generated under tensile, compressive and three-point bending conditions. Comparison of the computed and experimental data sets demonstrates the validity of the proposed modeling technique.

Cork is a unique and complex natural cellular material with many industrial applications. The purpose of this paper is to explore a new application field for the use of micro-agglomerate cork as an energy-absorbing medium. A numerical study on the energy absorption capabilities of square and circular cork-filled aluminium tubes with a width or diameter of 80 mm, length of 300 mm and variable thickness was performed with the finite element method code LS-DYNA™. The tubes were impacted uniaxially at 10 and 15 m s -1 . The same analysis was carried out on aluminium foam-filled tubes. The results demonstrate that cork filling leads to a considerable increase in the energy absorbed for both section geometries, and that tube thickness plays an important role in the deformation modes and energy absorption. The investigation revealed better results for aluminium foam-filled structures, but demonstrated that micro-agglomerate cork has high potential as an energy-absorbing medium in crash protection applications.

Finite element analysis was used to study the effective transverse modulus of solid wood for all possible end-grain patterns. The calculations accounted for cylindrical anisotropy of wood within rectangular specimens and explicitly modeled wood as a composite of earlywood and latewood. The effective modulus was significantly reduced by growth ring curvature or off-axis loading, The large changes were attributed to the low transverse shear modulus of wood. The explicit, or heterogeneous, model was compared to prior numerical methods that homogenized properties in the transverse plane. The two models gave similar effective modulus results, but a heterogeneous model was required to capture details in modulus calculations or to realistically model stress concentrations. Various numerical methods for modeling transverse stresses in wood are discussed.

The effects of ring characteristics on the compressive strength and dynamic modulus of elasticity of seven softwood species in Taiwan were examined. The results revealed good correlation between compressive strength and dynamic modulus of elasticity obtained using an ultrasonic wave technique (correlation coefficient r =0.77–0.86). Overall, compressive strength increased with decreasing ring width parameters and increasing ring density parameters. Ring density was related to compressive strength, but was not the sole factor affecting the wood strength. According to our statistical analysis, compressive strength was affected by various ring characteristics. Relationships between ring characteristics and compressive strength are influenced by the anatomic direction. Results revealed that earlywood density and minimum density in a ring are equally important variables for evaluating the compressive strength of wood.

This paper describes an experimental device designed for the accurate determination of wood/water relations on micro-samples. The moisture content of the sample is measured with a highly sensitive electronic microbalance. Moreover, the dimensions of the sample in tangential and longitudinal direction are collected continuously without contact by means of two high-speed laser scan micrometers. The device is placed in a climatic chamber. The micro-samples investigated were prepared with a diamond wire saw. A sample thickness of less than 1 mm allows the moisture content to be almost uniform during the test. The data obtained are of excellent quality and accuracy, in spite of the very small mass and dimensions of the samples. The device provides a perfect tool for investigating the dynamic interaction between relative humidity, moisture content, and shrinkage. Results collected for beech, spruce and eucalyptus are presented. Important findings include: deviation from a linear relation between shrinkage and moisture content in beech; uniqueness of the shrinkage versus moisture content curve during desorption/adsorption cycles; evidence of cell collapse in eucalyptus, especially for tension wood; and property variations within the growth ring of normal wood and compression wood of spruce.

Clonal trees from five different plantation-grown, industrially relevant hybrid poplar genotypes of the same age, grown on a common site in British Columbia, Canada, were tested for their performance in strand production and properties of oriented strand board (OSB). The results were compared against a benchmark mill-run OSB furnish derived from native aspen ( Populus tremuloides ). Variation in solid wood density among the hybrid poplar clones was shown to influence the compaction ratio and densification of the OSB, which in turn led to variation in board strength properties. After accounting for specimen density using co-variate statistical models, it was apparent that there were significant effects of genotype on bonding strength and thickness swell. Lower density wood from the fastest growing P. deltoides × P. trichocarpa (DTAC 7) clone resulted in better mat compaction and higher bond strength, whereas higher density wood from a P. trichocarpa × P. deltoides (TD 50-184) clone resulted in lower compaction and bonding strength. Flexural strength (rupture and elastic moduli) and nail pull through were not as significantly affected by either board density or genotype when adjusted for density. The study clearly demonstrates that fast grown, large diameter wood of lower initial wood density from hybrid poplar is highly suited for OSB production.

Application of kenaf core powder as a natural plywood binder has been studied. In the first instance, binderless boards made of kenaf core powder and overlaid sugi ( Cryptomeria japonica D. Don) veneers resulted in immediate veneer delamination. However, binderless boards were successfully manufactured from sugi wood powder and had an internal bonding strength of 1.69 MPa. The board properties improved with increasing fineness of the raw material, increasing pressing temperature, and increasing board density. Essential prerequisites for binderless adhesion are: (1) particles should be situated close to each other to allow the formation of chemical bonds; and (2) a high pressing temperature is necessary to melt the solid-solid interfaces between the particles and to supply the activation energy for chemical reactions. For successful adaptation of these requirements, the kenaf core powder should have an average particle size of approximately 10 μm, which then mediates close contact between veneers. Plywood bonded with kenaf core powder had an average tensile shear strength of 0.96 MPa with a wood failure rate of 82.1%. Accordingly, vibratory ball milling activates kenaf core and the powder is suitable as a natural plywood binder.

The influence of alkaline peroxide treatment has been characterized on elementally chlorine-free (ECF) bleached softwood (SW) kraft pulp. The results indicate that fiber charge increased with an increase in peroxide charge: a maximum fiber charge increment of 16.6% was obtained with 8.0% more peroxide charge on oven-dried (o.d.) pulp at 60.0°C. Two primary bleaching temperatures of 60.0°C and 90.0°C were investigated during peroxide treatment. Copper number decreased for peroxide charges of 0.5% and 1.0% at 60.0°C and 90.0°C, respectively, then increased with increasing peroxide charge. Both fiber charge and copper number approached constant values when 4.0% or higher peroxide charge was applied. Peroxide treatment on a bleached kraft pulp at 90.0°C resulted in lower fiber charge and lower intrinsic viscosity compared to treatment at 60.0°C. Sodium borohydride (NaBH 4 ) pretreatment was able to protect the fibers from being degraded during peroxide bleaching. Fiber charge and copper number were compared after peroxide treatment of ECF bleached kraft pulp to NaBH 4 -reduced ECF bleached kraft pulp. The results indicate that the carbonyl group content of fibers is favorable for improving fiber charge after peroxide treatment.

The effect of increased fiber charge on refining, cationic starch adsorption, and hornification was examined. Two pulps were investigated: (1) a softwood (SW) kraft pulp (KP) which was bleached elementally chlorine-free (ECF) and served as control; and (2) a control pulp treated with alkaline peroxide, which had a higher fiber charge. It was shown that increased fiber charge can improve the efficiency of the refining treatment, as indicated by differences in tensile index refined from 0 to 1000 revolutions. When the control pulp was refined from 4000 to 8000 revolutions, the tensile index decreased. In contrast, the tensile index of the higher fiber charge pulp (HCP) was higher under the same refining conditions. Upon addition of 2% cationic starch to both pulps, the tensile index of the control pulp increased by 13.7% and that of HCP by 23.7%. Atomic force microscopy did not reveal differences in the surface morphology of the two pulps with and without cationic starch adsorption. Peroxide treatment enhanced the fiber charge of the never-dried pulp. This was beneficial in reducing hornification when pulp was dried at 105°C. However, if the once dried pulp at 105°C was treated with peroxide, this resulted in a drastic decrease in intrinsic viscosity of the pulp and lower tensile and burst indices of the test sheets.

The reactivity and physicochemical properties of lignins are partly governed by their molar mass distribution. The development of reliable standard methods for determination of the molar mass distribution is not only relevant for designing technical lignins for specific applications, but also for monitoring and elucidating delignification and pulping processes. Size-exclusion chromatography (SEC) offers many advantages, such as wide availability, short analysis time, low sample demand, and determination of molar mass distribution over a wide range. A collaborative study has been undertaken within the “Eurolignin” European thematic network to standardise SEC analysis of technical lignins. The high-molar-mass fraction of polydisperse lignins was shown to be the main source of intra- and interlaboratory variations, depending on the gel type, elution solvent, detection mode, and calculation strategy. The reliability of two widespread systems have been tested: one based on alkali and a hydrophilic gel (e.g., TSK Toyopearl gel) and the other based on THF as solvent and polystyrene-based gels (e.g., Styragel). A set of practical recommendations has been deduced.