Marie Johansson

Distortion of Norway spruce timber Part 1. Variation of relevant wood properties

Picea abies) trees from one fast-grown and one slow-grown stand in southern Sweden. From the trees 240 studs (45 × 70 × 2500 mm) were taken for measurement of distortion. Wood properties were measured on small specimens (13 × 13 × 200 mm) cut from the studs. Spiral grain angle was found to vary from approximately +3° (left-handed) close to pith to zero 150 mm from pith with a strong individual variation. The material from the fast-grown stand had a larger spiral grain angle compared with the slow-grown material. Spiral grain was poorly correlated to other parameters. Presence of knots had a substantial influence on longitudinal shrinkage (αl) measurements. Specimens with large knots (KAR > 33%) had almost 100% higher longitudinal shrinkage than specimens without knots. It should be pointed out, however, that measuring shrinkage in small specimens containing even small knots can create a problem with regards to the obtained results, especially results of αl. It was found that presence of compression wood in several growth rings more than doubled the longitudinal shrinkage. For the radial and tangential direction the presence of compression wood decreased shrinkage with about 30%. The ratio between tangential and longitudinal shrinkage was 49 for normal wood whereas for compression wood the ratio was 13. These results confirm the theory that the microfibril angle governs shrinkage. Longitudinal shrinkage decreased slightly with increased distance from pith whereas radial and tangential shrinkage did not display any substantial radial variation. The fast-grown material had generally a higher longitudinal shrinkage and lower transverse shrinkage than the material from the slow-grown stand. About 50% of the variation in longitudinal shrinkage was explained by radial position, density and ring width. Density and ring width did explain 60% of the variation in radial shrinkage but only 30% of the variation in tangential shrinkage.Picea abies) von einem schnell- und einem langsamwachsenden Standort wurden verwendet. Daraus wurden 240 Kanthölzer der Abmessung 45 × 70 × 2500 mm geschnitten und daran die Verwerfung bestimmt. Holzeigenschaften wurden an kleinen Proben (13 × 13 × 200 mm) gemessen, die aus diesen Kanthölzern hergestellt wurden. Der Faserwinkel variierte zwischen +3° in der Nähe der Markröhre bis 0° bei 150 mm Abstand vom Mark mit großer individueller Streubreite. Die Proben vom schnellwachsenden Standort hatten einen größeren Faserwinkel als die vom langsamwachsenden Standort. Die Korrelation des Faserwinkels zu anderen Holzeigenschaften war nur sehr schwach. Anwesenheit von Ästen hatte einen bedeutsamen Einfluß auf das longitudinale Schwinden (αl); es lag bei hohen Astanteilen (KAR > 33%) fast 100% höher als bei astreinen Proben. Es allerdings muß betont werden, daß bei der Messung an kleinen Proben schon kleine Äste problematisch sind, speziell für αl. Weiter zeigte sich, daß Anteile von Druckholz in einigen Jahrringen das longitudinale Schwinden mindestens verdoppeln. Radiales und tangentiales Schwinden wird durch Druckholzanteile um etwa 30% verringert. Das Verhältnis zwischen tangentialem und longitudinalem Schwinden betrug 49 für normales Holz, bei Druckholz lag dieses Verhältnis bei 13. Die Ergebnisse stützen die These, daß der Mikrofibrillwinkel das Schwinden regelt. Longitudinales Schwinden nahm mit zunehmender Entfernung vom Mark langsam ab, während radiales und tangentiales Schwinden keine wesentlichen Änderungen aufwiesen. Schnellwachsendes Holz zeigte allgemein höhere Schwindwerte als langsamwachsendes Material. Rund 50% des longitudinalen Schwindens werden erklärt durch die Parameter radiale Position, Dichte und Jahrringbreite. Dichte und Jahrringbreite erklären 60% des radialen Schwindens, aber nur 30% des tangentialen Schwindens.

Distortion of Norway spruce timber Part 2. Modelling twist

Picea abies). Several material parameters were also measured, such as spiral grain angle, shrinkage in all three directions, annual ring width and density. Twist in the studs was measured at four different times at different moisture contents. The amount of twist correlated well with the moisture content and was reversible throughout several moisture changes. When the moisture content decreased, the twist increased and vice versa. About 50% of the variation in twist could be explained by a single parameter, i.e. the average growth ring curvature. All studs with severe twist were cut with its centroid within a radius of 75 mm from the pith. A statistical analysis of the data shows that growth ring curvature and spiral grain angle together explained about 70% of the variation in twist. Other parameters, such as shrinkage strains, density and ring width, did not increase predictability. When using a model developed by Stevens and Johnston (1960), about 66% of the variation in twist could be explained. The model also explained twist quantitatively well. The model included curvature of the growth ring, spiral grain angle and the tangential shrinkage strain.Picea abies). Mehrere Materialeigenschaften wurden ebenfalls gemessen, und zwar: Faserwinkel, Schwinden in drei Richtungen, Jahrringbreite und Dichte. Die Verdrehung der Kanthölzer wurde zu vier verschiedenen Zeitpunkten bei unterschiedlichen Feuchtegraden gemessen. Das Ausmaß der Verdrehung war gut korreliert mit der Feuchte. Mit abnehmender Feuchte stieg die Verdrehung und umgekehrt. Rund 50% der Verdrehungswerte sind durch einen einzigen Parameter erklärt, nämlich die Jahrringkrümmung. Bei allen Kanthölzern mit starker Verdrehung lag die Mittelachse innerhalb eines Abstandes von 75 mm von der Markröhre. Die statistische Analyse ergab, daß Jahrringkrümmung und Faserwinkel zusammen ca. 70% der Variation der Verdrehung erklären. Andere Parameter wie Schwindspannungen, Dichte und Jahrringbreite erhöhten die Vorhersagbarkeit nicht. Mit Hilfe des Modells von Stevens und Johnson (1960) konnten rund 66% der Verdrehung erklärt werden. Dieses Modell lieferte auch zufriedenstellende quantitative Ergebnisse. Berücksichtigt werden dabei Jahrringkrümmung, Faserwinkel und tangentiale Schwindspannung.

Material properties and their interrelation in chemically modified clear wood of Scots pine

Abstract The mechanical and physical properties of modified wood were assessed experimentally. Timber studs with a cross-section of 45 mm×70 mm were modified on a semi-industrial scale by four different methods: 1) acetylation (AC) with acetic anhydride; 2) modification with methylated melamine formaldehyde resin (MMF); 3) heat treatment in vegetable oil (HT); and 4) furfurylation (FA). Sapwood of Scots pine (Pinus sylvestris) with test specimen dimensions of 10×10×200 mm3 was investigated. A total of 2449 specimens were included in the study. The following properties and their correlations were studied: density, modulus of elasticity (MOEdyn) measured dynamically, equilibrium moisture content (EMC), and swelling behaviour. The properties were measured at humidity levels of 30%, 65%, and 90% RH, while the temperature was kept constant at 23°C. Significant changes in material properties took place. AC increased the density and reduced MOEdyn, EMC and swelling strain. FA increased the density and the swelling coefficient and reduced EMC and swelling strain. HT reduced the EMC, while MMF modification increased it.

Analytical model of twist in Norway spruce (Picea abies) timber

Abstract This paper presents an analytical model for twist in timber. The model is based on calculating the twist in cylindrical shells and combining these shells to create timber studs. The shells will deform when the moisture content changes, owing to tangential shrinkage perpendicular to the fibre direction and longitudinal shrinkage parallel to the fibre direction. If the fibre direction is not parallel to the shell direction, this will cause twist in the shell. The twist in each shell contributes to the twist in the stud. The model has been used for a parametric study to determine theoretically the influence of parameters such as sawing pattern, stud size, spiral grain angle variation and tangential and longitudinal shrinkage. The model has also been used to predict the twist in 196 studs of Norway spruce timber. The model was able to predict 67% of the variation in twist among these studs.

Finite element study of growth stress formation in wood and related distortion of sawn timber

Lack of straightness in timber is the most frequent complaint regarding solid (and laminated) timber products worldwide. Nowadays, customers demand higher quality in the shape stability of wood products than they did earlier. The final distortion of timber boards is mostly caused by moisture-related stresses in wood (drying distortions) and growth-related stresses (distortions appearing when logs are split up to timber boards by sawing). To get more knowledge on how these distortions can be reduced in wooden products, there is a need for improved understanding of this material behaviour through good numerical tools developed from empirical data. A three-dimensional finite element board distortion model developed by Ormarsson (Doctoral thesis, Publ. 99:7, 1999) has been extended to include the influence of growth stresses by incorporating a one-dimensional finite element growth stress model developed here. The growth stress model is formulated as an axisymmetric general plane strain model where material for all new annual rings is progressively added to the tree during the analysis. The simulation results presented include how stresses are progressively generated during the tree growth, distortions related to the redistribution of growth stresses during log sawing, and distortions and stresses in drying reflecting the effects of growth stresses. The results show that growth stresses clearly vary during tree growth and also form a large stress gradient from pith to bark. This in itself can result in significant bow and crook deformations when logs are sawn into timber boards. The distortion results from the simulations match well with the results observed in reality. The parametric study also showed that the radial growth stress distribution is highly influenced by parameters such as modulus of elasticity, micro fibril angle and maturation strain.

Numerical study of how creep and progressive stiffening affect the growth stress formation in trees

It is not fully understood how much growth stresses affect the final quality of solid timber products in terms of, e.g. shape stability. It is, for example, difficult to predict the internal growth stress field within the tree stem. Growth stresses are progressively generated during the tree growth and they are highly influenced by climate, biologic and material-related factors. To increase the knowledge of the stress formation, a finite element model was created to study how the growth stresses develop during the tree growth. The model is an axisymmetric general plane strain model where material for all new annual rings is progressively added to the tree during the analysis. The material model used is based on the theory of small strains (where strains refer to the undeformed configuration which is good approximation for strains less than 4%) where so-called biological maturation strains (growth-related strains that form in the wood fibres during their maturation) are used as a driver for the stress generation. It is formulated as an incremental material model that takes into account elastic strain, maturation strain, viscoelastic strain and progressive stiffening of the wood material. The results clearly show how the growth stresses are progressively generated during the tree growth. The inner core becomes more and more compressed, whereas the outer sapwood is subjected to slightly increased tension. The parametric study shows that the growth stresses are highly influenced by the creep behaviour and evolution of parameters such as modulus of elasticity, micro-fibril angle and maturation strain.

Local variation of modulus of elasticity in timber determined on the basis of non-contact deformation measurement and scanned fibre orientation

During the last decade, the utilization of non-contact deformation measurement systems based on digital image correlation (DIC) has increased in wood related research. By measuring deformations with DIC systems, surface strain fields can be calculated. The first aim of this study concerns the possibility to detect detailed strain fields along the entire length of a wooden board subjected to pure bending and the potential of using such strain fields to determine a bending modulus of elasticity (MOE) profile along a board. Displacements were measured over 12 subareas along a flat surface of the board. For each such area, a separate local coordinate system was defined. After the transformation of locally measured coordinates to a global system, high resolution strain fields and a corresponding bending MOE profile were calculated. A second method in establishing bending MOE profiles is to use fibre angle information obtained from laser scanning and a calculation model based on integration of bending stiffness over board cross sections. Such profiles have recently been utilized for accurate strength grading. A second aim of this study was to investigate the accuracy of the bending MOE profiles determined using the latter method involving fibre angle information. Bending MOE profiles determined using the two described methods agree rather well. However, for some patterns of knot clusters, the local bending MOE, calculated on the basis of fibre angles and integration of bending stiffness, is overestimated. Hence, this research adds knowledge that may be utilized to improve the newly suggested strength grading method.

Prediction of longitudinal shrinkage and bow in Norway spruce studs using scanning techniques

Straightness is one of the most important properties for making timber an attractive material for modern mechanized building. Several studies have shown that a lack of straightness is one of the main reasons for choosing materials other than timber in the construction industry. This paper presents a way to model moisture-induced bow from longitudinal shrinkage data predicted from an analysis of images of the surface of Norway spruce studs. For this study, eight studs (45 × 95 × 2500 mm and 45 × 120 × 3000 mm) of Norway spruce timber were selected. Bow in these studs was measured at two moisture contents below the fiber saturation point. The studs were then split into three slices 11 mm thick, and the surfaces of these slices were scanned to obtain color information and images of the tracheid effect. The slices were cut into sticks with dimensions of 10 × 10 × 200 mm. The longitudinal shrinkage coefficient of these sticks was measured. A multivariate model was created to model the longitudinal shrinkage coefficient data from the information in the images. The predicted longitudinal shrinkage data was used to model bow. The mean value of the measured longitudinal shrinkage was 0.0121 (SD 0.0123). The root mean square error of prediction (RMSEP) for the multivariate model was 0.0079, which is regarded as good. Thus, it was possible to model moisture-induced bow with good accuracy using the predicted longitudinal shrinkage data.

Comparison between HT-dried and LT-dried spruce timber in terms of shape and dimensional stability

Abstract The performance of timber studs from Norway spruce (Picea abies) in terms of shape and dimensional stability was evaluated. The shape stability studied included three modes of distortion, i.e. twist, bow and spring. The dimensional stability was assessed by measuring longitudinal shrinkage and swelling properties. The study comprised 96 studs measuring 45 mm×70 mm×2500 mm from 15 butt logs. Half the studs from each log were dried using a high-temperature method (HT) at 115°C, while the other half were dried using a conventional low-temperature method (LT) at 70°C. Distortion in the studs was measured at moisture contents of 14% and 9%. The HT-dried timber had significantly lower distortion values than the LT-dried timber. As a result, 80% of the high-temperature-dried studs were able to pass the limits for distortion, while only 60% of the low-temperature-dried studs passed the same limit. Studs cut close to the pith displayed the largest twist, independent of the drying method. A simple model for predicting twist that was used in previous studies was also valid for HT-dried timber. The most important parameters in this model for both HT- and LT-dried timber were grain angle, annual ring curvature and tangential shrinkage.

Biomass equations for selected drought-tolerant eucalypts in South Africa

In the water-scarce environment of South Africa, drought-tolerant eucalypt species have the potential to contribute to the timber and biomass resource. Biomass functions are a necessary prerequisite to predict yield and carbon sequestration. In this study preliminary biomass models for Eucalyptus cladocalyx, E. gomphocephala and E. grandis · E. camaldulensis from the dry West Coast of South Africa were developed. The study was based on 33 trees, which were destructively sampled for biomass components (branchwood, stems, bark and foliage). Simultaneous regression equations based on seemingly unrelated regression were fitted to estimate biomass while ensuring additivity. Models were of the classical allometric form, ln(Y) = a+x1ln(dbh)+x2ln(h), of which the best models explained between 70% and 98% of the variation of the predicted biomass quantities. A general model for the pooled data of all species showed a good fit as well as robust model behaviour. The average biomass proportions of the stemwood, bark, branches and foliage were 60%, 6%, 29% and 5%, respectively.