Technologische Untersuchungen an Pinus radiata-Importhölzern aus Chile

Technological investigations on small clear, samples of radiata pine timber from Chile have shown lower average densities and moduli of elasticity than conifers from Central Europe. Only bending strength has been higher as compared to spruce and fir; however, the individual values scatter considerably. Piloting investigations on structural timber indicate that the grading criteria according to DIN 4074 are in principle also applicable to radiata pine. Average bending strength and particularly Young's modulus of elasticity of the structural timber investigated were lower than the comparative values of spruce. Swelling and shrinkage caused by changes in moisture are lower as compared to spruce and European redwood.     

Study of the relationship between flatwise and edgewise modull of elasticity of sawn timber as a means to improve mechanical strength grading technology

In most of the strength grading machines for sawn timber, the flatwise bending modulus of elasticity (MOE) of timber pieces is measured. Employing regression functions, their edgewise MOE is estimated on the basis of the flatwise MOE and the edgewise bending strength of the weakest part of each piece is calculated to allocate each piece to a standardised strength class. With regard to improvements in the accuracy of timber strength grading machines, it was studied to which extent structural wood characteristics and grading parameters affect the relationship between flatwise and edgewise bending MOE. Edgewise and flatwise MOE have been determined both in knotty and in knot-free sections of boards of Norway spruce. The flatwise MOE was determined in a three-point bending test as it is typically employed in strength graders. The edgewise MOE was determined in a four-point bending test. Additionlly, the MOE and density distributions over the timber cross sections were determined to model the total MOE under consideration of these distribution patterns. Shear deformation accounts for a substantial portion of the difference between flatwise and edgewise MOE. The effect of knots on the MOE could not be defined precisely. Growth ring structure and juvenile wood in the boards lead to 5 to 10% lower flatwise MOE values as compared to the edgewise MOE.     

Unexplained site variance in machine strength grading of Norway spruce structural timber

The aim of the study was to assess site effects in terms of unexplained site variance in machine strength grading of Norway spruce structural timber. The site effects were estimated for grading based on axial resonance frequency and timber length, and for grading based on dynamic modulus of elasticity. Timber was collected from 14 sites in Norway, and linear mixed models were developed based on 1188 boards. The study showed that strength grading based on axial resonance frequency and timber length leaves out effects of site that are related to altitude, latitude and site index. The variance could be reduced when the grading was based on dynamic modulus of elasticity. For both grading methods, the site effects were smaller for bending strength than for modulus of elasticity and density. Major parts of the site effects were explained by mass density, and simulations showed that it is possible to fulfil the requirements of the strength classes with a higher yield when the sorting is based on a combination of exclusion by mass density and exclusion by the frequency-based indicating property.     

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.     

Utilization of bark flours as additive in plywood manufacturing

Increasing demand for wood based panel products and shortage of wood as raw material have triggered many efforts to utilize residues generated annually by the forest industries including a large portion of bark in panel production. In this study, the effects of using bark flours as additives obtained from different wood species (walnut, chestnut, fir and spruce), having much polyphenol content, on some physical and mechanical properties and formaldehyde emission of plywood panels were examined. Wheat flour, which has been used widely as additive in plywood manufacturing, served as control. Plane tree (Platanus orientalis) logs were obtained for veneer manufacturing. Urea formaldehyde (UF) resin with 55 % solids content was used as adhesive. The bonding shear strength, bending strength, modulus of elasticity (MOE), density, equilibrium moisture content and formaldehyde emission of plywood panels were determined according to related standards. It was found that the use of flours obtained from the barks of chestnut and fir trees in the glue mixture decreased the formaldehyde emission of panels. The bonding strength values of the test panels made using the glue mixture including the flour of walnut and spruce barks as additive were lower than those of the panels with adhesive containing the flour of fir and chestnut barks. The panels manufactured with adhesives including the flour of fir bark gave the highest bending strength and modulus of elasticity values.     

The effect of global slope of grain on the bending strength of scaffold boards

A study is described, carried out on specially prepared specimens, that specifically examines the effect of slope of grain on the bending strength of scaffold boards. Each specimen was visually graded and both the modulus of rupture and modulus of elasticity in bending determined. From the effect of slope of grain on the reduction of bending strength revised visual grading rules have been suggested that include a slope of grain limit of 1 in 12. It has also been demonstrated that machine grading is as effective at determining the adverse effect of slope of grain on board strength as it is for other defects such as high rate of growth and knots.     

Relationship between local and global modulus of elasticity in bending and its consequence on structural timber grading

The study analyses the relationship between local and global modulus of elasticity and develops and evaluates different models to predict local from global modulus measurements. The mechanical tests were performed on four species commonly used in Italy for structural purposes: fir, Douglas-fir, Corsican pine and chestnut. Two or three cross-sections and two provenances were sampled for each species. A theoretical analysis showed that the local–global modulus relationship was of polynomial form with only one coefficient. The effect of the species on the relationship was significant as well as the cross-section but only for softwoods. The effect of the cross-section was explained by the presence and the size of defects in the mid span. The different models were applied and then compared by means of the optimum grading: only slight differences among models emerged. Although optimum grading was strongly dependent on the sampling and on the grade combination, for softwoods the model for species and section showed very similar results to the grading with the true local modulus; inclusion of the knot values in the model led to only slight improvements. For chestnut all models were found to be comparable.     

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.     

Bending strength of spruce glulam

A glulam strength model where the bending strength depends both on the board tensile strength and the finger joint tensile strength is a completely transparent model being particularly suitable to determine requirements for the board and for the finger joint tensile strength of glulam made of both visually or mechanically strength graded boards. This article describes the application of these principles to an alternative and new strength model for spruce glulam. In order to determine the influence of the board and finger joint strength on glulam bending strength a numerical study was performed by means of simulated glulam beams. According to the findings current requirements for boards and finger joints are insufficient to ensure the nominal strength values of glulam strength classes established in EN 1194.     

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.     

Einige Holzeigenschaften erkrankter Fichten und Tannen aus Baden-Württemberg,als Ergebnis einer flächendeckenden Erhebung

During investigations of forest disease hitherto, 128 spruce and 127 fit trees from 20 stands in Baden-Württemberg with different degrees of disease have been examined with respect to some wood properties. In addition to tests with small clear specimens the investigation included tests of the bending strength of wooden beams. The test results for bending strength, compression strength parallel to the grain, impact bending and modulus of elasticity do not shown significant differences of mean values and variations as compared to known values of healthy trees. Negative changes in the properties investigated were not found to be dependent upon the degree of disease. No relation was found between nutrition element content of needles, degree of infection with needle fungi, and the degree of disease. In contrast to fir the green sapwood of spruce with more than 70% loss of needles showed a declining moisture content and an increase in moisture content deficit.     

Fertigung und Eigenschaften von schnell abbindenden,zementgebundenen Spanplatten,hergestellt durch Begasung mit Kohlendioxid

Cement-bonded particleboards were fabricated from spruce utilizing four kinds of portland cements of various strength classes, PKZ 35 F, PZ 35 F, PZ 45 F and PZ 55, by injection with gaseous carbon dioxide during pressing. The bending strengths were determined on not dried specimens after 7 d, 14 d, and 28 d. The other technological properties were obtained from 28 d old specimens. After 7 d the bending strength of the fabricated boards amounted to about 90 percent of the ultimate strength properties. A correlation is established between the lime standard of the cements and carbon dioxide uptake of the boards. With higher cement strength class the weight—carbon dioxide uptake—, the carbon dioxide content, and the bending strength of the boards increase. On the contrary, the modulus of elasticity, the thickness swelling, and linear movement decrease. Both kinds of cement-bonded particleboards, fabricated according to conventional and carbon dioxide injection process, have comparable bending strengths. On the contrary, the pH of boards made by the last mentioned process are lower.     

Modelling local bending stiffness based on fibre orientation in sawn timber

Strength of structural timber depends to a high degree on the occurrence of knots and on the local fibre deviation around such defects. Knowledge of local fibre orientation, obtained by laser scanning, has been utilized in a previously developed machine strength grading method, but rather crude assumptions regarding the fibre orientation in the interior of boards and a mechanical model that does not capture the full compliance of knotty sections were adopted. The purpose of the present study was to suggest and verify a model with which local bending stiffness can be predicted with high accuracy. This study included development of a model of fibre orientation in the interior of boards, and application of a three-dimensional finite element model that is able to capture the compliance of the board. Verification included bending of boards in the laboratory and application of digital image correlation to obtain strain fields comparable to those obtained by finite element simulation. Results presented comprise strain fields of boards subjected to bending and calculated bending stiffness profiles along boards. Comparisons of results indicated that the model suggested here was sufficient to capture the variation of local bending stiffness along boards with very high accuracy.     

Effect of relative humidity on some physical and mechanical properties of different types of fibreboard

Equilibrium moisture content and strength properties as a function of relative humidity (RH) were measured in three types of commercial fibreboards with different densities. The measurements were made after specimens had been conditioned to equilibrium at 35, 50, 65, 80 and 95% RH and 20 °C. It was shown that the modulus of elasticity decreased slightly between 35 and 65% RH and markedly – at above 65% RH for all types of boards. A very strong decrease of modulus of elasticity was observed at about 80% RH (capillary condensation). A similar decreasing trend was observed in bending strength. Overall, high RH had a noticeably detrimental effect both on the MOE and the bending strength for all tested boards. Adequate correlation exists between bending MOE and bending strength with moisture content. The lower the density of boards the higher their equilibrium moisture content.     

Der Einfluß des Trocknungsgrades der Holzspäne auf die Festigkeit zementgebundener Spanplatten

Cement bonded particle boards were made of undried, air dried and kiln dried particles of spruce and poplar wood and the bending strengths of the boards were measured after 3, 10 and 28 days. Air dried particles of both wood species gave boards with better strength properties than undried particles. Kiln drying of the particles was found to have a substantially improving effect on the bending strength of cement bonded particle boards made of poplar particles whereas the same procedure did not produce any effect with spruce particles.     

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.     

Einfluß der Lagerung von Industrierestholz der Klefer und Fichte auf seine Eignung für die Herstellung von Spanplatten und mitteldichten Faserplatten (MDF)

Particle- and medium density fibreboards (MDF) were prepared from chips of spruce and pine, a byproduct of sawing wood in a saw mill with a profiling unti. The wood residues were stored in piles of 40 m³ volume for different time periods. The physical-technological properties of the particle- and fibrebroads prepared therefrom were evalutated and the formaldehyde release of the boards was assessed. The results reveal: In general, there is a positive influence of storage on the thickness swelling and internal bond strength of UF-bonded particleboards; the bending strength of UF-bonded particleboards showed no pronounced response to storage. The prositive influence of storage onthe thickness swelling and internal bond strength was much higher in case of boards made from pine chips than in case of spruce boards. On using PMDI as a binder for particleboards no significant influence of storage on the thickness swelling, internal bond strength and bending strength was detected. The influence of storage on the thickness swelling of UF-bonded MDF from pine fibres was much less pregnant than on that of UF-bonded particleboards. On UF-bonded MDF from spruce the influence of storage, if any, was negligible. On using PMDI as a binder for MDF there was no detectable change in the thickness swelling due to storage. The internal bond strength of UF-bonded MDF experiences only subtle positive change due to storage of pine wood chips. However, no notable change was detected in case of MDF from spruce. Storage has almost no influence on the chemical properties of boards.     

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.     

A study of strength,hardness and deformation of acetylated Scandinavian softwoods

In this work, properties such as bending strength (expressed as modulus of rupture and modulus of elasticity), hardness, and deformation under cyclic relative humidity have been studied for acetylated Scandinavian pine and spruce. The acetylation was performed with a limited amount of liquid acetic anhydride without addition of catalyst or organic cosolvent. The weight gain due to acetylation was 19.1% for the pine samples. Two kinds of spruce samples were acetylated to 18.2% (narrow annual rings) and 23.3% (broad annual rings), respectively. The results indicated that only small changes in strength were obtained. For pine, modulus of rupture decreased with about 6%, and increased with about 7% for spruce. The modulus of elasticity also decreased for pine but increased for spruce. For practical applications, these properties can be considered unaltered. Acetylated pine wood showed increased Brinell hardness. Acetylated spruce wood did not tend to deform as much as unmodified wood when exposed to moisture cycled between 40 and 90% relative humidity