Prediction of timber bending strength and in-member cross-sectional stiffness variation on the basis of local wood fibre orientation

Machine strength grading of structural timber is based upon relationships between so called indicating properties (IPs) and bending strength. However, such relationships applied on the market today are rather poor. In this paper, new IPs and a new grading method resulting in more precise strength predictions are presented. The local fibre orientation on face and edge surfaces of wooden boards was identified using high resolution laser scanning. In combination with knowledge regarding basic wood material properties for each investigated board, the grain angle information enabled a calculation of the variation of the local MOE in the longitudinal direction of the boards. By integration over cross-sections along the board, an edgewise bending stiffness profile and a longitudinal stiffness profile, respectively, were calculated. A new IP was defined as the lowest bending stiffness determined along the board. For a sample of 105 boards of Norway spruce of dimension 45 × 145 × 3,600 mm³, a coefficient of determination as high as 0.68–0.71 was achieved between this new IP and bending strength. For the same sample, the coefficient of determination between global MOE, based on the first longitudinal resonance frequency and the board density, and strength was only 0.59. Furthermore, it is shown that improved accuracy when determining the stiffness profiles of boards will lead to even better predictions of bending strength. The results thus motivate both an industrial implementation of the suggested method and further research aiming at more accurately determined board stiffness profiles.     

Verhältnis zwischen Zug- und Biege-Elastizitätsmoduln von Vollholz

Based on available test results on structural timber (European Spruce) the relationships between modulus of elasticity (E) in bending according to prEN 408 and DIN 52186, between E in bending and in tension as well as between E in edgewise and flatwise bending were determined. Average E in bending acc. to prEN 408 is 9% higher than E acc. to DIN 52186 and 9% higher than E in tension. E determined in edgewise bending is about 17% higher than E determined flatwise.     

Reliability of proof-loaded Norway spruce

Strength grading of structural timber is a prerequisite for the use of timber in load-bearing constructions. The grading methods used in production today are mainly based on estimation of the modulus of elasticity (MOE) determined from measurements of either flatwise bending stiffness or resonant vibrations. By using proof-loading techniques, the yield can be increased and the reliability of the graded material can be improved. Unfortunately, this technique can cause damage to some of the graded material. This paper deals with the quantification and severity of damage caused by proof-loading of Norway spruce (Picea abies) timber. A total of 410 specimens were loaded in cyclic bending with increasing load level in each cycle until failure. The maximum loads during the tests were compared to the failure loads of a reference sample. If the failure load was lower than the maximum load, the specimen was assumed to have been damaged in the previous load-cycle and was defined as a rogue. The technique used is similar to the technique used for example in Australia. The results obtained fit well with the results obtained from Australian species. Examination of the MOE indicates no damage in the specimens at stress levels below 75% of the strength. Damage is in this case defined as a decrease in the MOE. Regarding Norway spruce grown in Sweden it can be concluded that the problem of damage due to proof-loading is small for timber graded to strength classes C24 or C30. For both classes the number of rogues can be expected to be less than 5%. For timber graded to higher strength classes the problem of damage due to proof-loading will increase.     

Verh?ltnis zwischen Zug- und Biege-Elastizit?tsmoduln von Vollholz

Zusammenfassung An vorliegenden Ergebnissen von Festigkeitsuntersuchungen an Fichtenschnittholz in Gebrauchsabmessungen wurde der Zusammenhang zwischen dem Biege-E-Modul nach prEN 408 und nach DIN 52186, dem Biege- und dem Zug-E-Modul sowie dem Biege-E-Modul bei Hochkant- und Flachkantbiegung ermittelt. Der nach prEN 408 bestimmte Biege-E-Modul war 9% h?her als der nach DIN 52186 bestimmte Biege-E-Modul. Ferner lag der Biege-E-Modul um 9% über dem Zug-E-Modul und um 17% über dem bei Flachkantbiegung bestimmten E-Modul.

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Relationship between modulus of elasticity in tension and bending for timber

Based on available test results on structural timber (European Spruce) the relationships between modulus of elasticity (E) in bending according to prEN 408 and DIN 52186, between E in bending and in tension as well as between E in edgewise and flatwise bending were determined. Average E in bending acc. to prEN 408 is 9% higher than E acc. to DIN 52186 and 9% higher than E in tension. E determined in edgewise bending is about 17% higher than E determined flatwise.

     

Comparison of mechanical properties of oriented strand board made from trembling aspen and paper birch

This study compared the performance of oriented strand boards (OSB) made from trembling aspen, a low-density hardwood species, and OSB made from paper birch, a medium-density hardwood species. The birch strands were thinner than the aspen strands to ensure a comparable specific surface. Three levels of adhesive content were used: 3.5%, 5.0%, and 7.0%. Internal bond (IB) and modulus of elasticity (MOE) and modulus of rupture (MOR) for flatwise and edgewise bending were determined. Both species performed equally well in IB (3.5% adhesive content: 0.46 MPa, 5.0%: 0.60 MPa, and 7.0%: 0.65 MPa). The values of MOE in flatwise bending were slightly lower for birch than for aspen panels (11.8 GPa for aspen and 10.6 GPa for birch), and the MOR values were not significantly different (combined 68.3 MPa). Edgewise bending properties were not significantly different for the two species with a MOE of 10.5 GPa and a MOR of 43.2 MPa.     

Untersuchungen zum Einfluss der Holzfeuchte auf ausgewählte Eigenschaften von Spanplatten und MDF

On industrially produced particle boards and MDF, the influence of wood moisture on bending MOE, bending strength and shear modulus G_zx and G_zy was determined. The equilibrium moisture content and the differential swelling of thickness proved to be smaller for MDF than for particle boards. Tensile strength and tensile MOE, shear modulus and bending strength decrease likewise with increasing wood moisture content. Furthermore the Poisson ratio μ₁₂ decreases with increasing wood moisture content.     

Steigerung der Ausbeute von Kiefern-Brettlamellen hoher Tragfähigkeit durch maschinelle Festigkeitssortierung

The increase of timber output with high load capacity by machine strength grading is essential for maximising the value output from German roundwood. Further it is possible to document the competitiveness of home produced timber compared with imported timber from Skandinavia or America. Not least the reliable definition of product quality will improve the market position of timber against other building materials as concrete, steel or glass. In the study the applicability and reliability of a machine grading system based on the grading criterions MOE in bending, density and knot measurements for Scots pine boards will be examined.     

Relationship between non destructive and static modulus of elasticity of commercial wood plastic composites

The relationship between stress wave nondestructive (NDE) modulus of elasticity (MOE) and static MOE determined by four point bending test of commercial wood-plastic composites was evaluated in this study to assess the potential of using the NDE technique as a grading tool for wood plastics as it is currently done for solid lumber. The NDE MOE was evaluated on a Metriguard Model 340 E computer system, and the static MOE of the same boards measured on an Instron Universal Tester model 5587 according to the ASTM standard D 6109. Results showed that the NDE MOE values overestimated the static MOE from 3.5% to 17.6% depending on the boardwalk type. Regression analysis showed no significant correlation between the NDE MOE and the Static MOE. These results suggest that the stress wave NDE technique may not be appropriate to estimate the static MOE of wood plastic lumber. The trend however needs to be confirmed by further testing using a larger sample size and different combination of the raw materials.     

Festigkeitsuntersuchungen an einheimischem Douglasienholz als Bauholz

From two stands in Baden-Württemberg, four Douglas-fir stems each were taken and sawn to 143 cants with cross sections 8×8 cm to 14×20 cm, as well as to planks and side boards, 4 cm thick. After grading according to DIN 4074 and drying down to 12% m.c., 111 cants were subjected to bending and compression tests. Simultaneously, MOE, bending and compression strength, as well as density and width of annual rings were determined, with 4×4 cm clear specimens, taken from the above cants. Additionally shear resistance, compression strength und nail jointing were examined. The comparison with data gained from local spruce cants showed better MOE and strength values for Douglas-fir. In contrast, splitting in nail tests was more pronounced than in spruce cants.     

Influence of cross-section and knot assessment on the strength of visually graded Norway spruce

The strength of graded timber is determined by a multitude of parameters. Properties of interest are the shape of the cross-section and the wood quality. With regard to strength, wood quality is primarily expressed in terms of knots and knot clusters which, together with the cross-section of the timber, are used to calculate knot ratios. By applying the visual grading rules as given in the German standard DIN 4074-1, the influence of different timber sizes on grading results has been analysed. Different grading approaches for joists and boards exist and are taken into account in the assessment of 5,665 specimens originating from various parts of Europe. It was shown that both the cross-section and the grading method have a major influence on the characteristic strength values of Norway spruce. Limitations of the current standard with respect to its applicability to certain cross-sections are exposed. Alternative, simple grading approaches for boards are proposed. They ensure equal strength values and yields comparable to the rather complicated board rules used nowadays.     

Effect of knots on the flatwise bending stiffness of lumber members

In machine stress-rating of lumber where flatwise bending stiffness is used as a predictor of strength, it is customary to assume that the effect of knots is accounted for by stiffness. However, only few data in the published literature can be used to substantiate this claim. The present study was undertaken to evaluate the relationship between stiffness and knot size for lumber members loaded in bending with a test geometry similar to that used by grading machines. Experiments were carried out with spruce lumber specimens containing a single centerline knot. A theory-of- elasticity based model was derived for analysis purposes. Theory and experiments agreed in showing that the effect of knots on flatwise bending stiffness is very small. This low sensitivity may explain why correlations between strength and machine measured stiffness are rather poor for commercial lumber. Implications of this finding on the practice of machine stress-rating of lumber are discussed.     

Radiometrisches Verfahren für die Holzsortierung

Grading according to DIN 4074 (quality specifications for coniferous sawn timber) is of great importance both for the security of timber structures and the economic use of the raw timber. According to this standard, timber grading is based on visual characteristics. In order to objectify timber grading as far as possible, great efforts have been made to achieve timber grading by machines. GreCon, Alfeld, has developed a procedure to determine the density of timber and thus even its content of knots by isotopic γ-radiation. Tests proved that this procedure is suitable for grading of boards and planks. The results show a good correspondence between mean bending strength of machine graded and visually graded timber according to DIN 4074. It is furthermore possible to apply this procedure to other grading rules, for instance the ECE-recommendations.     

Radiometrisches Verfahren für die Holzsortierung

Grading according to DIN 4074 (quality specifications for coniferous sawn timber) is of great importance both for the security of timber structures and the economic use of the raw timber. According to this standard, timber grading is based on visual characteristics. In order to objectify timber grading as far as possible, great efforts have been made to achieve timber grading by machines. GreCon, Alfeld, has developed a procedure to determine the density of timber and thus even its content of knots by isotopic γ-radiation. Tests proved that this procedure is suitable for grading of boards and planks. The results show a good correspondence between mean bending strength of machine graded and visually graded timber according to DIN 4074. It is furthermore possible to apply this procedure to other grading rules, for instance the ECE-recommendations.     

Die Biegefestigkeit von Keilzinkenverbindungen aus Brettern der Buche (Fagus silvatica L.)

The bending strength of glulam depends on both board strength and finger joint strength. In the case of softwood glulam this fact is well known from experience. It was recently reported by the authors that visual strength grading of beech provides a characteristic glulam bending strength of 36 N/mm² and mechanical strength grading of 48 N/mm². Therefore, adequate values for the characteristic finger joint bending strength have to be met. At the time those values were unknown. It was the aim of the following project to study the influence of visual or mechanical strength grading techniques on the finger joint bending strength. Bending tests on finger joints were carried out. The specimens were manufactured from visually strength graded boards according to the German standard DIN 4074-5 and from mechanically strength graded boards according to the dynamic MOE. The test results confirm that visual strength grading provides strength class GL36 and mechanical grading provides GL48.     

Strength grading on the basis of high resolution laser scanning and dynamic excitation: a full scale investigation of performance

Effective utilization of structural timber requires grading and indicating properties (IPs) that are able to predict strength with high accuracy and machines that are able to measure the underlying board properties at a speed that corresponds to the production speed of sawmills. The aim of this research is to assess the performance of a new machine strength grading method/procedure which was recently approved for the European market and to compare the performance of it with the performance of other available techniques. The novel method is based on laser scanning utilizing the tracheid effect, in combination with data from dynamic excitation and weighing. Applied indicating properties are defined in detail and results presented include assessment of the repeatability, coefficients of determination between IPs and grade determining properties, and examples of the yield achieved in different strength classes and combinations of strength classes. The investigated sample comprised more than 900 pieces of timber of Norway spruce (Picea abies) from Sweden, Norway and Finland. For this sample the coefficient of determination between the IP to bending strength and the measured bending strength was as high as R ² = 0.69, while the coefficient of determination between dynamic modulus of elasticity (MOE) and measured bending strength was R ² = 0.53. The yield in high strength classes, C35 and above, becomes about twice as high using the new method/procedure compared to machines using dynamic MOE as IP. A comparison of the performance with what has been presented for machines that are based on X-ray in combination with dynamic excitation indicates that the new method/procedure will surpass such machines as well.     

Machine strength grading of Argentinean<Emphasis Type="Italic"> Eucalyptus grandis</Emphasis>

The present paper reports the results of an investigation regarding the possibility of machine strength grading sawn timber of Argentinean Eucalyptus grandis. In order to assess the usefulness of each parameter for predicting the mechanical properties of this timber species, and to propose strength, stiffness and density profiles, an empirical research project with four samples of beams and one sample of boards subjected to bending, and another sample of boards subjected to tension parallel to the grain, was carried out. The results obtained with specimens in structural sizes reveal the more important single and combined parameters for machine strength grading this timber species, and allow to analyse strength, stiffness and density profiles in comparison with the international strength class system established in the European standard EN 338.     

Strength reduction of spruce wood through slow freezing

Some selected mechanical properties of spruce wood (Picea abies L.) were determined after freezing green timber boards under different conditions. The influence of the freezing rate and the time of exposure to negative temperatures were evaluated by applying three different freezing conditions which may occur in winter when green timber is stored in an open yard. It was found that a high freezing rate (?10 °C/h) does not affect wood strengths at all, while slow freezing (by ?1 °C/h) significantly reduces all mechanical wood properties, especially MOR, MOE and the compressive strength (by 20…30 %), the Janka hardness (by 18 %), and also the tensile strength (by 10 %). A longer time of exposure, involving repeated freezing and thawing due to natural temperature variations led to further reduction of MOR, MOE and the compressive strength (up to 37 %), but without further affecting the tensile strength, shear strength and hardness. The obtained results may be useful to industrials with respect to a more careful planning of green timber purchase and storage in wintertime.     

Festigkeit von Fichten-Bauholz mit Insekten- und Pilzbefall

340 compression and 288 tension tests were carried out on 628 pieces of spruce boards with cross-sections of on the average 20×150 mm in order to investigate the effects of insect damage and fungal decay on compression and tension strength and MOE in tension and compression. Attack by wood wasp (Sirex spec.) and long-horned-borer (Tetropium spec.) reduce compression and tension strength and tension MOE largely independent of wood quality. Xyloterus lineatus attack and red-stain only reduce compression strength of largely defect-free timber. This effect decreases significantly with increasing knot ratio. Blue-stain affects neither strength nor MOE.     

The influence of compression failure on the bending, impact bending and tensile strength of spruce wood and the evaluation of non-destructive methods for early detection

Bending strength (MOR) and bending Young’s modulus (MOE) according to DIN 52186 and MOE calculated on the basis of eigenfrequency and sound velocity were tested on small clear wood specimens of Norway spruce wood with and without compression failure. One group of specimens was climatised in a normal climate of 20°C and 65% relative humidity, while the other group was stored for one month under water before testing. The MOR of specimens with compression failure decreased about 20% on average (normal climate and wet) compared with the specimens without compression failure. The MOE of the specimens with compression failure was reduced only minimally compared with the specimens without compression failure stored in a normal climate, but very distinct differences (more then 30%) were found under wet conditions. The MOE of the specimens with compression failure calculated on the basis of eigenfrequency and sound velocity were not reduced or only minimally compared with the specimens without compression failure. It is therefore not possible to detect compression failure and to determine reduction in MOR using eigenfrequency or sound velocity. In addition, impact bending (DIN 52189), tensile strength and tensile MOE (DIN 52188) were tested on small clear wood specimens of Norway spruce wood with and without compression failure. The specimens with compression failure revealed an average reduction in impact strength of about 40% and an average reduction in tensile strength of about 20% compared with the specimens without compression failure, whereas tensile MOE of the specimens with compression failure was not reduced compared with the specimens without compression failure. The detection of compression failure by computer tomography (CT) was tested on Norway spruce wood boards 10 cm in thickness, and detection by optical scanner was tested on planed Norway spruce wood boards. CT recognised large compression failures easily, whereas the scanner was not able to detect them.     

Properties of furfurylated wood (Pinus?pinaster)

Sapwood samples of Pinus pinaster wood were treated with a?70% furfuryl alcohol mixture. Weight percent gain (WPG), equilibrium moisture content, dimensional stability, MOE, bending strength, hardness, density and durability were determined. WPG was on average 38%. Equilibrium moisture content decreased more than?40%. Dimensional stability increased reaching an ASE of?45%. MOE was little affected by the treatment but bending strength increased by about?6%. Hardness increased by about?50%, and density by?37%. Mass loss due to Postia placenta and Coniophora puteana decreased by?96% and?86%, respectively. Furfurylation of Pinus pinaster wood shows an interesting potential to improve the wood quality for solid timber products.