Annual Ring Orientation Research Articles

On the numerical assessment of bending- and torsion-induced stresses in idealised sawn timber beams

ABSTRACT When designing timber beams for bending, shear and torsion, it is common engineering practice to use analytical formulas that neglect the cylindrical nature of the orthotropic material, the annual ring orientation, spiral grain and so on. While they result in stresses that appear reasonable, the approach is in principle theoretically flawed. In this paper a 3D finite element model is used to analyse the stress levels in sawn timber beams, taking into account the radially varying and cylindrically orthotropic material parameters, as well as a number of geometrical features. The beams are assumed to be mechanically idealised without imperfections. Parametric analyses indicate that especially the annual ring orientation has a significant impact on the maximum stresses, even though the cross-sectional stress distributions have distinct similarities with common beam theory. Furthermore, a Monte Carlo simulation is performed, randomly varying all parameters within given intervals. The simulations reveal that the maximum stresses from the finite element model, in almost all cases are noticeably larger than predicted by the analytical formulas, rendering them on the unsafe side in a theoretical sense.

Flexural behavior of wood in the transverse direction investigated using novel computer vision and machine learning approach

Abstract A deep-learning-based semantic segmentation approach (U-Net) was used to partition the anatomical features in the cross-section of hinoki (Chamaecyparis obtusa) wood during a micro three-point bending test. Using the Crocker–Grier linking algorithm, thousands of cells were successfully extracted, and several parameters (area, eccentricity, fitted ellipse aspect ratio, bounding box aspect ratio) were used to evaluate the intensity of the cells’ deformation. Thus, the 2D map of the deformation intensity distribution was constructed. By analyzing flat-sawn, quarter-sawn, and rift-sawn specimens, it was confirmed that the annual ring orientation affects the flexural behavior of wood in the transverse direction. The quarter-sawn specimens exhibited the largest modulus of elasticity (MOE) and modulus of rupture (MOR). The ray tissue aligned against the load may have contributed to the restriction of cell deformation. The rift-sawn specimens exhibited the smallest MOE and MOR, possibly owing to the loading of the specimen in the in-plane off-axial direction, which induced the shear deformation of the cell wall. For all three specimen types, the fracture had high occurrence probability in the tension part of the specimen, which exhibited large cell deformation. Therefore, the proposed method can be adapted to the prediction of wood specimen fractures. With different test wood species, this approach can be of great help in elucidating the relationship between the anatomical features and the mechanical behavior of wood to improve the effective utilization of wood resources.

Surface charring of silver fir wood cladding using an enhanced traditional Japanese Yakisugi method

The aim of this work was to better understand the ignition method of timber charring in order to improve the industrial process. Three silver fir boards were tied together to make a triangular prism, which acted as a chimney. To start the charring process, the traditional Yakisugi method uses an ignitor paper ball. This ignitor paper ball was in this research replaced with a gas burner. The gas burner supplies the required energy in an even level and provides airflow in the upward direction. The surface temperature of the samples increased from 10 to 500 °C in approximately 40 to 80 s at all recorded positions, which is considerably faster than when using a traditional method. The thickness of the charred layer and the resulting cupping effect were investigated as an indicator of the quality of the process. The charred layer produced by the gas burner method was not as thick as was achieved with the traditional method, which can be attributed to a shorter charring time. Approximately half the specimens showed cupping to the charred side, which may be related not only to a shorter charring time than previous studies, but also to the annual ring orientation of the timber. Further research should be performed on the charred layer thickness and cupping to define all relevant parameters.

Investigation of rolling shear properties of cross-laminated timber (CLT) and comparison of experimental approaches

The structural behaviour of a cross laminated timber (CLT) system is dictated in-part by the rolling shear properties (both strength fr and modulus Gr) of its laminates. To investigate the rolling shear properties of CLT, short-span four-point bending tests, modified planar shear (MPS) tests with timber sandwich plates, and MPS tests with steel sandwich plates were conducted on Australian grown Eucalyptus nitens and radiata pine, as well as European grown Norway spruce. A strong positive relationship between aspect ratio and strength was found, which agrees with previous research. However, the annual ring orientation distribution rather than the aspect ratio was shown to have the most pronounced effect on the shear modulus. The load direction, which has not been accounted for by researchers, was shown to have a substantial effect on the measured rolling shear strength. When the applied load direction was parallel to the annual rings, the Eucalyptus nitens, radiata pine and Norway spruce specimens exhibited rolling shear strengths of 4.1, 2.7 and 1.7 MPa respectively. These average recorded strengths were found to decrease by 16%, 40% and 52% respectively when the load direction was changed to run perpendicular to the rings instead. By directly measuring the shear deformations of two specimens tested under different MPS approaches using Digital Image Correlation (DIC), the impact of the plate material on the recorded shear modulus was isolated. This revealed that the MPS tests with steel plates recorded average rolling shear modulus values which were as much as 42% higher than their counterparts with timber plates, as well as identifying substantial errors in laser and LVDT-like measurement tools. It is recommended that a DIC approach is employed to complement or replace a UTM or laser sensor deformation measurement approach to mitigate these potential errors.

A Numerical Study of the Stiffness and Strength of Cross-Laminated Timber Wall-to-Floor Connections under Compression Perpendicular to the Grain

The use of cross-laminated timber (CLT) in multi-story buildings is increasing due to the potential of wood to reduce green house gas emissions and the high load-bearing capacity of CLT. Compression perpendicular to the grain (CPG) in CLT is an important design aspect, especially in multi-storied platform-type CLT buildings, where CPG stress develops in CLT floors due to loads from the roof or from upper floors. Here, CPG of CLT wall-to-floor connections are studied by means of finite element modeling with elasto-plastic material behavior based on a previously validated Quadratic multi-surface (QMS) failure criterion. Model predictions were first compared with experiments on CLT connections, before the model was used in a parameter study, to investigate the influence of wall and floor thicknesses, the annual ring pattern of the boards and the number of layers in the CLT elements. The finite element model agreed well with experimental findings. Connection stiffness was overestimated, while the strength was only slightly underestimated. The parameter study revealed that the wall thickness effect on the stiffness and strength of the connection was strongest for the practically most relevant wall thicknesses between 80 and about 160 mm. It also showed that an increasing floor thickness leads to higher stiffness and strength, due to the load dispersion effect. The increase was found to be stronger for smaller wall thicknesses. The influence of the annual ring orientation, or the pith location, was assessed as well and showed that boards cut closer to the pith yielded lower stiffness and strength. The findings of the parameter study were fitted with regression equations. Finally, a dimensionless ratio of the wall-to-floor thickness was used for deriving regression equations for stiffness and strength, as well as for load and stiffness increase factors, which could be used for the engineering design of CLT connections.

Surface Modification of Spruce and Fir Sawn-Timber by Charring in the Traditional Japanese Method-Yakisugi.

The traditional Japanese method of wood surface charring was studied. To perform the surface charring, three sawn Norway spruce and Silver fir wood boards of dimension 190 × 24 × 4000 mm3 were tied together to act as a chimney and charred in a short time (3–4 min) with open flame at a temperature above 500 °C. Temperature inside the chimney was recorded on the three different positions during the charring process. Surface temperature of spruce increased from 0 °C to 500 °C in approx. 120–300 s while fir increased in approx. 100–250 s. The thickness of the charred layer and the resulting cupping effect were investigated at the different heights of the chimney to evaluate its variability. Temperature achieved during the charring process was sufficient to get a significant charred layer of 2.5 and 4.5 mm on average for spruce and fir samples, respectively. The analyzed samples showed a significant cupping effect to the charred side with no difference between the annual ring orientation of sawn boards. Spruce exhibit a more significant cupping effect when compared to fir, i.e., 3.2–6 mm and 2.2–4.5 mm, respectively. Furthermore, the pH values of charred samples increased significantly, which could be an indication of improved resistance against wood-decay fungi. For better insight into the traditional charring method, further studies should be carried out to execute the charring process in a consistent quality and therefore fully exploit its potential.

Surface Characteristics of One-Sided Charred Beech Wood.

The aim of this paper was to analyze selected properties of beech wood (Fagus sylvatica L.) treated by one-sided surface charring. Specimens were one-side charred with a hot plate using several time-temperature combinations (from 200 to 400 °C). Characteristics such as colour, discoloration, surface roughness, fire resistance, total carbohydrate content at several wood layers and decay resistance were evaluated. Surface charring was applied to the radial and tangential surfaces. Colour measurements showed that the surface of the wood turned grey due to charring. In addition to colour measurements, other experiments showed significant differences between radial and tangential specimens due to their different structures. The higher the temperature used in treating them, the lower the roughness values for radial specimens, while the trend for tangential specimens was the opposite. A smoother surface is more fire resistant, so radial specimens are generally better in this regard. Tangential specimens are more susceptible during preparation to forming cracks that impair flame resistance because a continuous protective densified layer is not formed. The determination of total carbohydrates revealed significant changes at various wood depths after surface charring. These changes were more predictable in radial specimens due to the annual ring orientation, because each layer consisted of a similar earlywood/latewood ratio. Finally, when decay resistance was assessed, weight loss was found to be lower in all specimens than in the references. The results suggest that charring at a particular combination of temperature and time improved the investigated properties of the surface-modified beech.

Predictive capacity of some wood properties by near-infrared spectroscopy

ABSTRACT Near-infrared spectroscopy (NIRS) is a non-destructive method that has been used in wood property assessment. Preliminary studies have revealed its ability to predict wood density and mechanical properties, but with little attention given to the effect of surface quality and wood anisotropy. This study simulates sawmill conditions variable timber surface quality and cross-sectional annual ring orientation where NIR spectra were taken on rough and smooth surfaces of quarter-sawn and flat-sawn specimens. Two models were developed based on a mixed set which included both cross-sections, and another that was based on wood from these two cross-sections, separately. Promising predictive models were obtained for density ( = 0.66), stiffness ( = 0.78) and strength ( = 0.82). Rough surface and quarter-sawn specimens were mostly better than their counterparts. In general, density, stiffness, and strength of wood could swiftly and with relative accuracy be assessed using NIRS, especially with rough-surfaced timber.

Distortion in laminated veneer products exposed to relative-humidity variations – Experimental studies and finite-element modelling

A shortcoming of the laminated bending process is that the product may become distorted after moulding. This study focused on the influence of fibre orientation deviation for individual veneers on the distortion of a moulded shell. The distortion of 90 cross-laminated shells of the same geometrical shape, consisting of seven peeled birch veneers, were studied under relative humidity variation. All the veneers were straight-grained in the longitudinal-tangential plane, but to simulate a deviation in fibre orientation, some of the individual veneers were oriented at an angle of 7° relative to the main orientation of the other veneers in the laminate. A finite element model (FEM) was applied to study the possibility of predicting the results of a practical experiment. The study confirms the well-known fact that deviation in fibre orientation influences shape stability. The results also show how the placement of the abnormal veneer influences the degree of distortion. From this basic knowledge, some improvements in the industrial production were suggested. However, the FE model significantly underestimated the results, according to the empirical experiment, and it did not show full coherence. The survey shows the complexity of modelling the behaviour of laminated veneer products under changing climate conditions and that there is a great need to improve the material and process data to achieve accurate simulations. Examples of such parameters that may lead to distortion are density, annual ring orientation in the cross section of the veneer, the orientation of the loose and tight sides of the veneer, and parameters related to the design of the moulding tool.

Dynamic compression: A novel technique to reduce energy consumption during wood fiber production

Thermo-mechanical refining is a common fiber production process known for its high energy demands. Throughout this process, wood chips are subjected to repetitive shearing and compression such that the fibers separate and subsequently fibrillate. There is a growing body of research in the development of mechanical pre-treatments that reduce energy demands during chemo- and thermo-mechanical pulping, with shear/compression combinations currently standing as the most efficient method of initiating defibration. Given the common grounds between the fiberboard and paper refining processes, it could be possible to use paper pre-treatments during fiberboard pulp refining. Furthermore, as pulping fibers for fiberboard are less worked and refined, mechanical pre-treatments are assumed to be more efficient. In this study, the effectiveness of dynamic compression was assessed as a pre-treatment step before the chips enter the refiner. Shaped wet chips with an annual ring orientation of 45° were struck by a free-falling weight with a fixed potential energy using a special prototype. After refining both the reference and pre-treated chips using a pressurized disc refiner, the energy consumption of those fibers was 48% lower than that for non pre-treated chips for a comparable fiber quality.

Classification of creosote bleeding from timber bridges by means of wood anatomical factors

ABSTRACTCreosote is commonly used as a wood preservative for highway timber bridges in Norway. However, excessive creosote bleeding at various highway timber bridge sites lead to complaints, and a potentially bad reputation for wooden timber bridges. Macro-and micro-anatomical factors such as the amount of heartwood, annual ring width, annual ring orientation, ray-height and composition and resin canal area were investigated in order to classify seven timber bridges in Norway into bleeding- and non-bleeding bridges. A classification into bleeding and non-bleeding was possible for discriminant categories based on three anatomical factors analysed on wood core samples. The amount of heartwood content dominated the influencing factors, even obscuring the significance of other factors. Classification with a low amount of variables was done preferably on sample level instead of bridge level, due to the restricted number of 17 core samples per bridge.

Annual ring orientation effect on bending strength of subfossil elm wood

The effect of annual rings’ orientation on bending strength was examined on subfossil elm wood. Elm is extremely rarely found as subfossil wood, and during the last 50–60 years it has almost disappeared from natural forest stands of south-eastern Europe, due to the Elm Dutch disease. The samples were cut from approximately 670 years old subfossil elm trunk retrieved from the bed of the river Sava in the area between the villages Grebnice and Domaljevac in north Bosnia. The wood was identified to the genus level based on optical microscopy analysis of three wood sections—transverse, tangential and radial. Bending strength was determined by the three-point bending test. The load was applied to the longitudinal–tangential surface (LT) and to the longitudinal–radial surface (LR). The bending strength values of subfossil elm do not differ from the values of recent elm, despite the 700-year resting in anoxic river conditions. Bending strength in LT direction was slightly higher than bending strength in LR direction. The coefficient of variation and the standard deviation of the arithmetical mean were higher for LT direction. It was found that the measured bending strength for both directions follows Weibull distribution. Coefficient of determination of Weibull functions was close to 1 for both directions. The Weibull shape parameter was higher for LR direction.

Rolling shear modulus and strength of beech wood laminations

Abstract Previous research indicated that the rolling shear properties of European beech wood (Fagus sylvatica) are considerably higher than those of softwood. The aim of the presented investigation was to substantiate previous data on rolling shear modulus and strength of European beech wood and to further evaluate its substitution of softwoods in applications where shear properties are influential, namely as cross layers in cross-laminated timber (CLT). Further, the effect of the annual ring orientation within the boards on shear modulus and strength was of major interest. The beech specimens comprised four different sawing patterns, classified unambiguously with reference to the pith location. The shear properties were determined by 50, two-plate shear tests with specimen cross-section dimensions of 33 mm × 135 mm. A mean rolling shear modulus of 370 N mm-2 was obtained, whereby no significant detrimental effect for pith boards with cracks was observed. In agreement with continuum mechanics, the semi-quarter-sawn boards revealed the highest shear moduli whereas the quarter-sawn boards showed roughly 30% lower values. The mean rolling shear strength was 5.6 N mm-2 for all specimens, whereby pith specimens resulted in generally lower values. The 5% quantile, disregarding pith specimens, was 4.5 N mm-2. In conclusion, the rolling shear strength and modulus exceed the respective characteristic values for softwoods by roughly factors of 5 and 7, indicating great potential for beech wood cross-layers in CLT.

The development of moisture and strain profiles during predrying of Eucalyptus nitens

ABSTRACTThis article describes one of a series of studies carried out at the University of Bio-Bio in Chile with the objective of characterizing the response of Eucalyptus nitens to drying. The ultimate goal is to minimize collapse and internal intra-ring checking in the industrial drying of Eucalyptus nitens, which currently prevents the lumber manufacturing industry in Chile from producing a large number of commercial solid wood products from this species. This study focuses, however, on the drying conditions that have been shown in previous studies and are known in the industry to minimize collapse and internal intra-ring checking. For these conditions, this study reports the development of moisture content and internal strain profiles during drying. In addition, a simple diffusion model based on the concept of effective diffusion coefficient was proposed to compare the drying rates in the radial and tangential directions. The results showed that the diffusion coefficient in the radial direction was approximately 50% higher than that in the tangential direction. Internal strains, on the other hand, developed approximately at the same time, regardless of the annual ring orientation. This indicates that it would be feasible to apply strain relief strategies simultaneously to all boards regardless of their annual ring orientation in order avoid internal intra-ring checking during predrying. This finding could be used as a benchmarking tool in the industrial drying for solid wood products of Eucalyptus nitens.

Effect of lamellas annual-ring orientation on cracking of glulam beams investigated with computer tomography and image processing

Cracks in glulam beams can reduce their strength, depending on the crack's depth, length, and location and can also be a passage for the accumulation of water and dirt. To avoid cracks, the relationship between cracks (area of cracks, mm2) and annual-ring orientation in glulam beams of spruce (Picea abies), and pine (Pinus sylvestris) with different dimensions and surface treatments was investigated using RGB images of the surface and tomography images of the cross-sections. Image processing was used to measure characteristics visible in the photos such as crack area and lamella position in the beam. Combination of lamellas in lay up was measured from computer tomography images. Four types of combinations were defined; type 1 (pith side facing outward), types 2 and 3 (pith to the same side), and type 4 (pith sides meet pith side). It was found that the area close to the glue line in the lamellas in combination type 4 is the most subject to cracking, whereas combinations of type 1 show the highest resistance to cracking. This means that type 4 should not be used in glulam beams, because the lamellas shrink apart from each other causing stresses and resulting cracks.

Effect of fatigue and annual rings’ orientation on mechanical properties of wood under cross-grain uniaxial compression

The mechanics of fresh wood with and without a fatigue pre-treatment that mimics a mechanical pulping process was experimentally studied. The mechanical properties of Norway spruce samples under compression are considered with the macroscopic stress–strain data and from local strain properties via digital image correlation technique. The results highlight the effects of the orientation of the wood annual rings compared to the loading direction and of the pre-fatigue. The wood presents a low yield point when the annual rings are tilted compared to the load axis, but the Young’s modulus and yield stress are higher when the annual rings are either parallel or perpendicular to the load direction. In the last case, buckling of softest layers occurs. The fatigue treatment makes the wood less stiff as deduced from the decreases of Young’s modulus and yield stress, whatever the orientation of annual rings. Secondly, it creates a thin and localized softened layer.

Artificial neural network modeling for predicting final moisture content of individual Sugi (Cryptomeria japonica) samples during air-drying

Sugi (Cryptomeria japonica D. Don) lumber is known to have a large variability in final moisture content (MCf) and is difficult to dry. This study assessed the capability of artificial neural networks (ANNs) to predict the MCf of individual wood samples. An ANN model was developed based on initial moisture content, basic density, annual ring orientation, annual ring width, heartwood ratio and lightness (L * in the CIE L * a * b * system). The performance of the ANN model was compared with a principal component regression (PCR) model. The ANN model showed good agreement with the experimentally measured MCf with a higher correlation coefficient (r) and a lower root mean square error (RMSE) than the PCR model, demonstrating the importance of nonlinearity of the variables and the higher capability of the ANN model than the PCR model. By adding redness (a * ) and yellowness (b * ) and drying time to the input variables of ANNs, r and RMSE values were improved to 0.98 and 1.2 % for the training data set, and 0.85 and 2.2 % for the testing data set, respectively. Although the developed ANNs are available under the limited conditions of this study, our results suggest that the ANNs proposed offer reliable models and powerful prediction capability for the MCf, even though wood properties vary considerably and their complex interrelations are not fully elucidated.

Investigation of wood properties that influence the final moisture content of air-dried sugi (Cryptomeria japonica) using principal component regression analysis

Sugi (Cryptomeria japonica D. Don) lumber is known to have a large variability in final moisture content (MCf) and is difficult to dry. This study investigated the variability in MCf of sugi in relation to wood properties. The wood property variables included initial moisture content (MCi), basic density (BD), annual ring orientation (ARO), annual ring width (ARW), heartwood ratio (HR) and CIE L * color (L *). Sugi samples were cut from flat-sawn lumbers and air-dried; a principal component regression (PCR) model for predicting MCf was developed with the wood property variables. The wood properties that contributed to the prediction of MCf were evaluated by PCR analysis. Significant positive regression coefficients of the PCR model were observed in the MCi, BD, ARO and HR, whereas negative ones in the L *. There were no significant regression coefficients in the ARW. These results suggest that the MCi, BD, ARO and HR had a positive influence, the L * had a negative influence, and the ARW had little influence on the MCf of air-dried sugi wood. This finding is in line with the general view on the drying characteristics in relation to these wood properties.

Investigation of softwood fracture criteria at the mesoscopic scale

In order to improve the knowledge of softwood mechanical behaviour and particularly fracture mechanisms, investigation at small scales is needed. For that purpose, new numerical tools based on the real wood morphology in the transverse plane are specifically used. A mode I fracture criterion applicable to cracks oriented in the transverse plane in softwood is then studied at the mesoscopic scale, i.e. the annual ring scale. Numerical investigation combined with digital image correlation help to obtain wood fracture parameters in the annual ring. These parameters give additional knowledge and understanding of wood fracture according to local specificities such as crack tip position in an annual ring and notch orientation in relation to natural wood orientation and loading orientation. Mesoscopic fracture criteria can help our understanding of secondary crack appearance and crack arrest phenomena. Mixed mode study and coupling between mode I and mode II of fracture will be investigated further. Mixed mode fracture criteria can then help to better understand crack bifurcation phenomenon.

Effects of Outdoor Weathering and Wood Properties on Liquid Water Absorption in Uncoated Norway Spruce Cladding

A comparison of liquid water absorption in uncoated Norway spruce (Picea abies L.) claddings before and after 1 year of outdoor weathering was performed. The study was based on 150 specimens from two sites with high-productive forest and two sites with low-productive forest in southern Norway. The specimens included inner and outer boards; density, annual ring width, and proportion of heartwood were recorded. Water absorption increased after weathering. The relative change of short-term absorption was larger for outer boards than for inner boards, whereas it did not vary between origins. The outer boards also had more cracks than the inner boards after weathering. The differences between inner and outer boards were explained by different heartwood proportion and different annual ring orientation. The variability of short-term absorption increased after weathering since the sample groups with the highest initial absorption also had the largest increase after weathering. The change of long-term absorption did not vary between inner and outer boards or between origins.