Distortion and visible crack formation in green and seasoned timber: influence of annual ring orientation in the cross section

This work describes distortion (bow, crook, twist, cup) and the formation of visible cracks in green and seasoned timber of Scots pine and Norway spruce. Distortion and crack formation have been evaluated according to the original location in the cross section of the log. The purpose is to describe differences in distortion and crack formation between sawn timber taken from star sawing and from traditional sawing patterns. Bow and crook develop in the green timber immediately after sawing as a result of growth stress release. Drying and conditioning in general lead to a marginal increase in bow, but crook may show a large percentage increase depending on the location of the wood in the cross section. Twist and cup do not occur directly after sawing, but arise during seasoning of the wood. In sawn wood, the juvenile wood has a major influence on the twist. This means that timber sawn close to the pith shows a greater twist than timber sawn away from the pith. This is a consequence of the fibre orientation in this timber. Cupping increases with decreasing radius of curvature of the annual rings in the cross section of the timber. Cracks occur in timber before seasoning. The pattern of crack formation is the same in both pine and spruce; timber sawn close to the pith or with pith enclosed exhibits a considerably larger number of visible cracks than timber sawn away from the pith. Cracks occur mainly on surfaces orientated to the pith, a fact that becomes clearer when timber is observed close to the pith. Seasoned star-sawn timber with a rectangular cross section has bow and crook of the same extent as or less than that of timber sawn according to traditional sawing patterns. Seasoned star-sawn timber with a triangular cross section has high bow, but the crook is low. Star-sawn timber does not show any cup and has a good shape stability according to the geometrical shape in the cross section. It also has a very small twist and shows very few visible cracks after seasoning.     

Distortion and visible crack formation in green and seasoned timber: influence of annual ring orientation in the cross section

This work describes distortion (bow, crook, twist, cup) and the formation of visible cracks in green and seasoned timber of Scots pine and Norway spruce. Distortion and crack formation have been evaluated according to the original location in the cross section of the log. The purpose is to describe differences in distortion and crack formation between sawn timber taken from star sawing and from traditional sawing patterns. Bow and crook develop in the green timber immediately after sawing as a result of growth stress release. Drying and conditioning in general lead to a marginal increase in bow, but crook may show a large percentage increase depending on the location of the wood in the cross section. Twist and cup do not occur directly after sawing, but arise during seasoning of the wood. In sawn wood, the juvenile wood has a major influence on the twist. This means that timber sawn close to the pith shows a greater twist than timber sawn away from the pith. This is a consequence of the fibre orientation in this timber. Cupping increases with decreasing radius of curvature of the annual rings in the cross section of the timber. Cracks occur in timber before seasoning. The pattern of crack formation is the same in both pine and spruce; timber sawn close to the pith or with pith enclosed exhibits a considerably larger number of visible cracks than timber sawn away from the pith. Cracks occur mainly on surfaces orientated to the pith, a fact that becomes clearer when timber is observed close to the pith. Seasoned star-sawn timber with a rectangular cross section has bow and crook of the same extent as or less than that of timber sawn according to traditional sawing patterns. Seasoned star-sawn timber with a triangular cross section has high bow, but the crook is low. Star-sawn timber does not show any cup and has a good shape stability according to the geometrical shape in the cross section. It also has a very small twist and shows very few visible cracks after seasoning.

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Zusammenfassung Die vorliegende Arbeit beschreibt Verformung (Biegung, Längskrümmung, Verdrehung, Schüsselung), sowie die Bildung sichtbarer Risse in frischem und getrocknetem Kiefern- und Fichtenholz. Verformung und Rißbildung wurden entsprechend ihrer ursprünglichen Lage auf dem Rundholzquerschnitt beurteilt. Dabei ging es darum, Unterschiede in der Verformungs- und Rißbildung zwischen geschnittenem Holz nach dem Sternsägeverfahren, sowie aus traditioneller Sägung zu beschreiben. Verbiegung und Längskrümmung entwickeln sich in frischem Holz sofort nach dem Sägen durch Nachlassen der Wachstumsspannung. Trocknen und Konditionieren führen im Allgemeinen zu einer marginalen Zunahme der Verbiegung, wobei Längskrümmung abhängig von ihrer Lage im Querschnitt zu einem großen Prozentsatz zunimmt.Verdrehung und Schüsselung treten nicht direkt nach dem Sägen auf, sondern während der Trocknung. In geschnittenem Holz hat Juvenilholz einen beträchtlichen Einfluss auf die Verdrehung. Das bedeutet, dass nahe am Mark geschnittenes Holz eine größere Verdrehung aufweist, als bei größerem Abstand zum Mark. Dies ergibt sich aus der Faserorientierung. Schüsselung nimmt zu mit der Radiuskrümmung der Jahresringe im Querschnitt. Risse im Holz treten vor der Trocknung auf. Die Muster der Rissbildung in Kiefern- und Fichtenholz sind gleich. In Marknähe, oder das Mark einschließend geschnittenes Holz weist eine wesentlich größere Anzahl an sichtbaren Rissen auf, als in Abstand zum Mark geschnittenes Holz. Risse treten überwiegend auf Oberflächen mit Orientierung zur Markröhre hin auf – diese Tatsache wird deutlicher wenn man das Holz nahe der Markröhre beobachtet.Getrocknetes sterngesägtes Holz mit rechteckigem Querschnitt weist Verbiegung und Längskrümmung in selbem oder geringerem Maße auf, als nach traditionellem Muster gesägtes Holz. Getrocknetes sterngesägtes Holz mit Dreiecksquerschnitt hat eine hohe Verbiegung, aber die Krümmung ist niedrig. Sterngesägtes Holz weist keine Schüsselung auf, hingegen aber gute Formstabilität analog der geometrischen Form des Querschnitts. Ebenso ist die Verdrehung nach der Trocknung nur geringfügig mit einigen wenigen Rissen.

     

Correspondences between manually estimated compression wood in Norway spruce and the warp of the sawn timber

Picea abies L.) and the warping of the sawn timber in terms of bow, spring and twist as well as further deformation after ripping of the dried products. The amount of compression wood was defined and measured on logs according to the methods of the Swedish Timber Measurement Council (Regulations for measuring of round wood) and on the sawn timber according to the Nordic Timber. The impact of two different drying schedules was also investigated. The study shows that visible compression wood in both the butt end of the log and within the sawn timber was a rather poor indicator of the warp of the dried sawn timber. In no comparison did the correlation coefficient, r, exceed 0.3. In contrast to this, the correlation between the amount of compression wood and the warp of secondary products was fair, r = 0.79. This means that it should be possible to identify sawn timber less suitable for secondary processing by the amount of compression wood. The corresponding correlation between compression wood in the butt end of the log and the warp of the secondary products was r = 0.46. No significant differences could be shown in the degree of warp, as related to compression wood, between sawn timber or secondary products, dried at a wet-bulb temperature of 55 °C/117 h, LT-schedule, and a dry-bulb temperature of 110 °C/24 h, HT-schedule, respectively.

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r  = 0,79). Es sollte also m?glich sein, weniger geeignete Schnittware anhand ihres Druckholzanteils auszusortieren. Die Korrelation zwischen Druckholzanteil am Stammende und der Verwerfung im Schnittholz betrug 0,46. Für das Ausma? der Verformungen wurde kein signifikanter Unterschied zwischen den beiden Tocknungsbedingungen (55 °C/117 h und 110 °C/24 h) gefunden.

     

The influence of compression wood and microfibril angle on the occurrence of distortion in window frames made from radiata pine (Pinus radiata)

D. Don) was examined for the occurrence of crook, bow and twist, and divided into two categories of samples, those with severe distortion, and those with negligible distortion. Ten samples in each category were examined. Small sections of window frame were extracted from the affected areas of the samples in the case of timber showing distortion, and at random for the control samples. Compression wood was assessed by image analysis from scanned images of the end grain, to give a percentage value. Each growth ring was examined microscopically to determine the severity of the compression wood if present and to determine the average microfibril angle in each ring. The microfibril angle gradient across the timber was then calculated. Results indicated that compression wood percent and distribution, average microfibril angle, and microfibril angle gradient, had a significant influence on the occurrence of distortion in these samples. Samples showing distortion altogether had high levels of compression wood, high microfibril angles, or high compression wood or microfibril angle gradients. On average, samples with high levels of distortion had 15% visual compression wood, an average microfibril angle of 32°, and a microfibril angle gradient of 12°, compared to 7%, 24°, and 7°, respectively for the control samples. Compression wood, where present, was typically of the mild type with intercellular spaces and a highly lignified S2L region in the cell walls of tracheids. For the majority of samples identified as containing compression wood, a highly lignified S2L region in the cell wall was the only anatomical feature present. This suggests that abnormal cell wall chemistry may be a significant factor in determining the dimensional stability of sawn timber and remanufactured products.     

Radially sawn timber—the Primwood Method for improved properties

A new manufacturing system, the PrimWood Method, has been proposed to improve the utilization of wood. A basic concept within this method is the sawing pattern called star-sawing, which produces timber with both rectangular and triangular cross sections. This method facilitates an efficient production of radially sawn timber with vertical annual rings, without juvenile wood. The sawn timber produced in the PrimWood Method is used to produce high quality, knot-free solid wood panels with vertical annual rings. In this process, part of the timber is finger-jointed to form knot-free lengths which are glued together into a block. This block can then be divided according to thickness into thinner panels with vertical annual rings. The PrimWood Method has been tested in an industrial plant. The manufacturing system was designed for a sawing capacity of about 30,000 m³ logs, which corresponds to 16,800 m³ of star-sawn timber per shift per annum. The production of solid wood panel was then designed to give a volume of 5,800 m³ per annum.     

Improvement of shape stability by high-temperature treatment of Norway spruce Effects of drying at 120 °C with and without restraint on twist

During drying, timber changes its shape mainly due to shrinkage anisotropy, radial differences in longitudinal shrinkage and spiral grain. The warp, causing severe downgrading of the timber, can be reduced by restraint and appropriate climate treatments of different types. The research presented here is part of a larger project on the improvement of shape stability of Norway Spruce (Picea abies) by high-temperature treatment. In this part, a method for determining twist as well as results for a drying temperature of 120 °C are presented. The effects of presteaming, drying and steaming with and without restraint on the size of twist in Norway Spruce were investigated in laboratory scale. Short-term twist-reducing effects as well as the permanency of the reduced distortions in subsequent moisture cycling were investigated. Results show a clear dependency of twist on the distance from the pith. Furthermore, the twist is reduced in restrained specimens sawn close to the pith. This effect is permanent even after exposure to subsequent moisture cycling.     

The influence of material properties on the amount of twist of spruce wood during kiln drying

The development of twist in Norway spruce boards (Picea abies Karst.) during normal temperature kiln drying was researched. Tangentially oriented boards were sawn from diametrical radial planks, from which the basic wood properties, i.e. linear shrinkage and grain angle, were determined. The unrestrained boards were kiln dried using a selected drying schedule. During the controlled drying process the moisture content (MC) and twist of the boards were measured. Twist was generally induced by the shrinkage of wood below the fibre saturation point, and increased in proportional to the decrease in MC. The earlier appearance of twist, with regard to MC and drying time, was confirmed in the case of boards sawn from the central part of logs. The final twist amounted to between 10 and 20°/m in the case of boards close to the pith, and decreased to less than 4°/m in the case of boards sawn at a radius of 70 mm from the centre of the logs. Ring curvature had the highest impact on the final twist of the dried boards, followed by grain angle and tangential shrinkage.     

Determination of fiber orientation in Norway spruce using X-ray diffraction and laser scattering

Spiral grain reduces the quality of timber since it causes twisting during drying and reduces the mechanical strength of wood products. The orientation of wood fibers in Norway spruce as a function of the distance from the pith was studied using both x-ray diffraction and light scattering. In radial-longitudinal plane upper tips of fibers were tilted towards the pith and the tilt angle increased gradually towards the bark in most of the samples. Periodic oscillations in the spiral grain angle were observed. Increased growth rate was found to increase the amplitude of this oscillation. There was no clear correlation between the angles determining the fiber orientation and other parameters like the lumen diameter, the cell wall thickness, the density of the sample, the fiber length, the circularity index of the cell lumen, or the mean microfibril angle. However, fiber orientation in tangential-longitudinal plane varied more in broad annual rings than in narrow annual rings.     

Erkennung und Messung des Reaktionsholzes bei Fichte (Picea abies (L.) Karst.) mittels Verfahren der digitalen Bildanalyse

Both for scientific analyses and for practical applications methodical developments are desirable for identifying and measuring compression wood of Norway Spruce. The objective of the present study was to develop and to test an innovative method for identifying and measuring compression wood. Methods of digital image analysis using digital photographs of discs of Norway Spruce (Picea abies (L.) Karst.) permit, to detect and to identify independently compression wood areas within single annual rings, and to calculate their area, the position given as an angle of the deviation to the direction of North and the distance from the pith. Furthermore the total area of the discs can be determined, so that the compression wood areas can be converted to proportions of the total disc area. The quantity of the images’ inaccuracy, the so-called lens distortion, was tested within the frame of the study presented, however the inaccuracy given as a percentage amounts to less than 1% and can therefore be neglected. Using this method it is possible to identify quickly, objectively and detailed compression wood of Norway Spruce, so that the knowledge about the distribution of compression wood in stems and in sawn timber as well can be extended.     

Radially sawn timber. Gluing of star-sawn triangular profiles into form-stable products with vertical annual rings

Pinus silvestris L. ) into form-stable wood products with vertical annual rings. The triangular profiles are free from pith and from most of the juvenile wood. The method is based on dried and conditioned triangular profiles. Undesired defects are removed and the wood is finger-jointed into long lengths. In the finger-jointing, consideration is given to the appearance and annual ring orientation of the jointed materials. After the jointing, the triangular profiles are planed and glued into rectangular blocks with vertical annual rings. These blocks can then e.g. be used as construction beams or be sawn up into solid wood panels.  A pilot plant has been built for the manufacture of knot-free solid wood panels based on the proposed method. Results show a total volume yield of 53.8%. Three critical production stages can be distinguished: removal of knots and defects, planing, and division of blocks into boards. These three operations are together responsible for more than 93% of the total losses in the manufacture. The removal of knots and defects meant a volume loss of 12.8%. 13% of the manufactured units were 2.1 m long without finger-jointing and free from knots and other defects. The average length of the remaining pieces used for finger-jointed units was 0.41 m.

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Effect of grain orientation and surface wetting on vertical density profiles of thermally compressed fir and spruce

The effects of grain orientation and surface wetting on wood densification by compression in a hot press were evaluated for two commercial Canadian wood species, balsam fir (Abies balsamea) and black spruce Picea mariana. The vertical density profiles (VDP) of wood densified at 180 °C could be engineered to achieve different properties depending on press closing rate, wood permeability and annual ring orientation. The lower permeability of spruce caused it to split frequently during hot pressing. For balsam fir, at a press closing time of 2 min, the compressed wood with an original grain angle of 0° (radial compression) shows widened high density bands due to collapse of low density earlywood adjacent to the dense latewood. All grain orientations show higher density areas close to the wood surfaces similar to those of wood-based composites. However, when wood was preheated without pressure for 5 min followed by a press closing time of 2 min, water migrated to and plasticized the board centre causing it to be densified while the surface density remained low. Wood surface plasticizing with water or urea solution causes some localized surface densification, but the effect was not great.     

Crack formation in unfinished siding of aspen (Populus tremula L.) and Norway spruce (Picea abies (L.) Karst.) during accelerated weathering

Crack formation is a serious defect that may reduce the lifetime of unfinished wood in service. An accelerated weathering test was carried out to study crack formation of solid wood board specimens of aspen (Populus tremula L.) and Norway spruce (Picea abies (L.) Karst.). The influence of the boards' distance from the pith was also studied. The aspen specimens developed a higher number of cracks than the spruce specimens. However, these cracks were shorter and less injurious than those propagating in spruce wood. It is concluded that a solid wood siding of aspen will show at least as good a performance as spruce regarding crack injuries. For both aspen and spruce, boards taken from near the pith had a lower proportion of long cracks than boards taken further out. This supports the rule that boards with a high proportion of annual rings perpendicular to the surface (the boards near to the pith) have a higher resistance to crack formation than boards with a high proportion of horizontal annual rings.     

Restraining moisture-related twist in timber structures

Timber changes its shape at varying moisture content levels. In most pieces of timber, this causes distortion, such as twist, spring, bow and cup. Twist is one of the most severe distortion modes and is reversible at varying moisture contents. Restraining a stud from twisting in built-in conditions results in additional forces in the structure. This paper presents a method for measuring the torsional moment while restraining the twist during drying in in-service conditions. The results include the development of torsional moment and the moisture content versus time, as well as the final torsional moment and the corresponding free twist. The magnitude of the torsional moment in studs results in forces affecting the surrounding structure which can then be restrained in an appropriate manner by fasteners. The development of torsional moment versus time demonstrates the importance of controlling the moisture content and the importance of a rapid building process when there is a risk of fast drying. The measured torsional moment was correlated to several measured material properties. However, a good correlation was only found between torsional moment and free twist.     

Twisting force during drying of wood

The object of this study is to investigate when wood begins to twist and the extent of the twisting force during drying. Japanese larch (Larix leptolepis Gord.) and hiba (Thujopsis dolabrata Sieb. et Zucc. var. hondae Makino) which tend to twist significantly when drying, were used in experiments to obtain multiple examples of grain angle (GA) variation in the R-direction. A measuring system to measure the twisting force during the drying of wood in real time was developed. It is thought that the time when twisting of timber by drying starts is subject to the variation in R-direction of GA; timber begins to twist when the portion with large GA begins to dry. There is concern that timber with large GA in the inner portion may become twisted during storage after kiln drying. The twisting stress measured in this study is effective for examining the load on the timber to control the twisting.     

Radially sawn timber—the Primwood Method for improved properties

A new manufacturing system, the PrimWood Method, has been proposed to improve the utilization of wood. A basic concept within this method is the sawing pattern called star-sawing, which produces timber with both rectangular and triangular cross sections. This method facilitates an efficient production of radially sawn timber with vertical annual rings, without juvenile wood. The sawn timber produced in the PrimWood Method is used to produce high quality, knot-free solid wood panels with vertical annual rings. In this process, part of the timber is finger-jointed to form knot-free lengths which are glued together into a block. This block can then be divided according to thickness into thinner panels with vertical annual rings. The PrimWood Method has been tested in an industrial plant. The manufacturing system was designed for a sawing capacity of about 30,000 m³ logs, which corresponds to 16,800 m³ of star-sawn timber per shift per annum. The production of solid wood panel was then designed to give a volume of 5,800 m³ per annum.

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Radial gesägtes Holz—Die Primwood Methode zur Verbesserung seiner Eigenschaften

Zusammenfassung In der vorliegenden Arbeit wird ein neues Verarbeitungsverfahren, die sogenannte Primwood Methode, zur Verbesserung der Holznutzung vorgestellt. Ein grundlegendes Konzept dieses Verfahrens ist das Einschnittmuster, genannt Sternsägen, welches Nutzholz mit rechteckigen und dreieckigen Querschnitten liefert. Diese Methode erleichtert eine wirtschaftlich effiziente Herstellung von radial geschnittenem Nutzholz mit stehenden Jahresringen ohne Juvenilholz. Das auf diese Weise hergestellte Schnittholz wird verwendet, um astfreie, qualitativ hochwertige Vollholzplatten mit stehenden Jahresringen zu produzieren. Darüberhinaus werden diese Platten mit Keilzinkung versehen, um astfreie Abschnitte zu bilden, welche zu einem Block verleimt werden. Dieser kann dann der gewünschten Dicke entsprechend in dünnere Platten mit stehenden Jahresringen aufgeteilt werden. Die Primwoodmethode wurde im industriellen Maßstab geprüft. Der Betrieb war für eine Kapazität von ungefähr 30.000 m³ Roh- Rundholz ausgelegt, was ungefähr 16.800 m³ sterngesägtem Schnittholz pro Schicht und Jahr entspricht. Anschließend wurde die Produktion von Vollholzplatten auf ein Volumen von 5.800 m³ pro Jahr eingerichtet

     

Observation of anisotropic water movement in Pinus radiata D. Don sapwood above fiber saturation using magnetic resonance micro-imaging

Magnetic resonance micro-imaging has been used to visualise the movement of water during the drying of Pinus radiata D. Don (radiata pine) wood samples of varying annual ring orientation and dimension. The drying process has shown a strong influence of annual ring orientation for thin boards with drying deviating from the classical core-shell model. Diffusion tensor micro-imaging shows the direction of greatest restriction to diffusion being in the transverse radial direction. Similarly, anisotropic values for T ₂ relaxation are observed, with values obtained when the transverse face is normal to the static magnetic field being higher than in the case when the radial or tangential faces are normal to the field.     

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.     

Observation of anisotropic water movement in<Emphasis Type="Italic"> Pinus radiata</Emphasis> D. Don sapwood above fiber saturation using magnetic resonance micro-imaging

Magnetic resonance micro-imaging has been used to visualise the movement of water during the drying of Pinus radiata D. Don (radiata pine) wood samples of varying annual ring orientation and dimension. The drying process has shown a strong influence of annual ring orientation for thin boards with drying deviating from the classical core-shell model. Diffusion tensor micro-imaging shows the direction of greatest restriction to diffusion being in the transverse radial direction. Similarly, anisotropic values for T₂ relaxation are observed, with values obtained when the transverse face is normal to the static magnetic field being higher than in the case when the radial or tangential faces are normal to the field.     

Influence of pith location on warp of lumber in sawing medium-quality sugi (Cryptomeria japonica D. Don) logs

The forest and wood industry in Japan require the development of automation technology to produce high-quality lumber from medium-quality sugi (Cryptomeria japonica D. Don) logs. This study investigates the influence of pith location on the warp of lumber. While drying reduced the bow of lumber drastically its crook remained almost unchanged. Therefore, an effective sawing method to reduce crook was sought after. Crook measured immediately after sawing increased as the distance between the center of the lumber and the pith location along the width of lumber increased. A sawing method which takes pith location into account was concluded to be effective in reducing crook. Based on these results, a new sophisticated sawing procedure to produce high-quality lumber is proposed.     

Wood shrinkage: influence of anatomy, cell wall architecture, chemical composition and cambial age

The influence of microfibril angle (MfA), density and chemical cell wall composition on shrinkage varied between the longitudinal and tangential directions as well as between wood types, namely compression wood (CW), mature wood (MW) and juvenile wood (JW). At the same MfA, CW exhibited a lower tangential shrinkage than JW, indicating the influence of the chemical composition on wood shrinkage. The chemical composition measured via FTIR micro-spectroscopy has been shown in conjunction with density to be an alternative to MfA data for shrinkage predictions. This was particularly true for wood of young cambial age for which the MfA did not correlate to shrinkage. The results indicate a possibility to reduce distortion of sawn timber by segregation using infrared (IR) and X-ray in-line measurements.