| 1. |
- Crocetti, Roberto, et al.
(författare)
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Experimental and numerical investigation on the shear strength of glulam
- 2010
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Ingår i: International Council for Research and Innovation in Building and Construction: Working Commission W18 ‑ Timber Structures — 2010. - Nelson : University of Canterbury.
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Konferensbidrag (refereegranskat)abstract
- According to EC5, the shear resistance of a structural timber element should be determined on the basis of the characteristic shear strength of the material, along with classical beam theory. For glulam, the characteristic strength values are given by the European standard EN 1194 [3], which assumes a direct relationship between tensile strength and shear strength of the lamination. As an example, the characteristic shear strength of glulam class GL28c, consisting of inner laminations with characteristic tensile strength ft,0,k = 14.5 MPa, would be fv,k = 0.32·(14.5)^0.8 = 2.9 MPa. However, recent investigations both on glulam members [4] and on timber members [5] have shown that the shear strength of spruce is higher than the shear strength obtained by means of the model proposed by EN1194. Moreover, the studies show that the shear strength is nearly constant, regardless the strength class of the timber material.
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| 2. |
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| 3. |
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| 4. |
- Källander, Björn, et al.
(författare)
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Fuktförändringar, klimat och mögelpåväxt vid lagring av granvirke : Laboratoriestudie och modellering av industriellt torkat virke
- 2014
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Studien avsåg att bestämma hur fuktkvoten i virke förändras under lagring i paket och vid vilka klimat och fuktkvoter mögelpåväxt uppkommer. Fyra satser virke med medelfuktkvoter mellan 18 % och 24 % skulle studeras för att säkerställa att studien omfattade virke både över och under fuktnivåer där mögel kan uppkomma. En Finit Element- modell som kan beräkna förändringar i fuktkvot i virke under lagring utvecklas och kalibreras mot mätdata för att resultaten ska kunna tillämpas på industriell produktion.Studien avsåg komplettera den tidigare studien i industrimiljö genom att dels oförstörande mäta förändringar i fuktkvotens fördelning under lagringen, dels bestämma fuktens fördelning med större noggrannhet än torrviktsmätningar för att ge underlag till kalibrering av FE-modellen.
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| 5. |
- Larsen, Finn, et al.
(författare)
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A numerical and experimental study of stress and crack development in kiln-dried wood
- 2012
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Ingår i: Proceedings of World Conference on Timber Engineering 2012.
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Konferensbidrag (refereegranskat)abstract
- Numerical and experimental investigations were carried out on well defined log-disc samples of Norway spruce consisting of both heartwood and sapwood, with the aim of gaining more adequate knowledge of stress and fracture generation during the drying process. Use of thin discs enabled a well-controlled and simplified drying history of the samples to be obtained. Experiments supported by the numerical model showed the heartwood to dry below the fibre saturation point, much earlier than the sapwood, and thus to start shrinking at a much earlier stage.
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| 6. |
- Larsen, Finn, et al.
(författare)
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Experimental and finite element study of the effect of temperature and moisture on the tangential tensile strength and fracture behavior in timber logs
- 2014
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Ingår i: Holzforschung. - : Walter de Gruyter. - 0018-3830 .- 1437-434X. ; 68:1, s. 133-140
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Tidskriftsartikel (refereegranskat)abstract
- Timber is normally dried by kiln drying, in the course of which moisture-induced stresses and fractures can occur. Cracks occur primarily in the radial direction due to tangential tensile strength (TSt) that exceeds the strength of the material. The present article reports on experiments and numerical simulations by finite element modeling (FEM) concerning the TSt and fracture behavior of Norway spruce under various climatic conditions. Thin log disc specimens were studied to simplify the description of the moisture flow in the samples. The specimens designed for TS were acclimatized to a moisture content (MC) of 18% before TSt tests at 20°C, 60°C, and 90°C were carried out. The maximum stress results of the disc simulations by FEM were compared with the experimental strength results at the same temperature levels. There is a rather good agreement between the results of modeling and experiments. The results also illustrate the strong decrease of TSt with increasing temperature at a constant MC level.
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| 7. |
- Larsen, Finn, et al.
(författare)
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Experimental Investigation of Moisture Driven Fracture in Solid Wood
- 2010
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Ingår i: WCTE - World Conference on Timber Engineering 2010. - Riva del Garda : Trees and Timber Institute. ; , s. 449-450
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Konferensbidrag (refereegranskat)abstract
- Solid timber products, containing both heartwood and sapwood, often have a high tendency to crack during the drying process. This can cause severe loss of material for the saw-mills, especially for products with large cross sectional dimensions. The cracks (e.g. end-cracks) arise, in some cases, early in the drying process and close again later in the process. It can be difficult to see the closed cracks with visual grading. This may result in too high grading of the damaged material which may cause problems for customers such as building and furniture industries. Moisture content (MC) in green wood varies within the cross section of a timber log. The MC of heartwood, for example, is considerable lower than the MC of sapwood. Shrinkage starts at different times within different parts of the cross section, which results in a complex state of strains and stresses. The moisture related crack pattern in wood often becomes quite complex because of the annual ring structure and the different MC levels within heartwood and sapwood. The focus of this work represents the cross sectional behaviour of a timber log. The main aim is to accumulate experimental results and data for the development of a finite element model to evaluate the various couplings in the hygro-mechanical problem that govern moisture driven cracking in wood.
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| 8. |
- Larsen, Finn, et al.
(författare)
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Experimental study of moisture-driven distortion and fracture in solid wood
- 2010
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Ingår i: 11th International IUFRO Wood Drying Conference : Recent Advances in the Field of Wood Drying. - Luleå : Luleå tekniska universitet. ; , s. 137-144
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Konferensbidrag (refereegranskat)abstract
- Moisture-induced fracture and lack of shape stability in solid wood products are well known problems to the saw-milling and building industries. Cracks that initiate during the drying process may cause severe material losses and the building industry may be forced to use alternative building materials. The cracking caused by kiln-drying of solid timber (and round wood) is extremely difficult to predict due to the strong orthotropic and non-homogeneous characteristics of the material in combination with considerable amounts of microscopic defects which may act as crack initiators. An experimental study has been performed to reveal the cracking behaviour of Norway spruce during drying from green moisture content down to equilibrium moisture content (EMC) at a temperature of 22-24°C and a RH of 64%. The moisture related strains and crack widths were measured with a digital image correlation system (ARAMIS) on thin discs cut from a timber log. The history of the strain field over the entire cross section of the discs was measured throughout the drying period. The results showed that the thicker discs (30 mm) cracked very early during the drying process. They also cracked significantly more than the thinner ones (15 mm) and the crack patterns developed differently. For the thicker discs the early cracks which may partly be caused by the moisture gradient in the longitudinal direction of the log, closed and became invisible later during the drying process. This indicates that sealing of timber log ends in the green moisture state could significantly reduce the development of end-cracks. It was also recognized that the initial moisture content and the shrinkage properties vary significantly from pith to bark. Based on this experimental finding it can be concluded that modelling of crack propagation in solid wood must take the material inhomogeneities into account.
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| 9. |
- Larsen, Finn, et al.
(författare)
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Moisture-driven fracture in solid wood
- 2011
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Ingår i: Wood Material Science & Engineering. - : Taylor & Francis. - 1748-0272 .- 1748-0280. ; 6:1-2, s. 49-57
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Tidskriftsartikel (refereegranskat)abstract
- Moisture-induced fractures in solid timber create considerable problems for both building industries and sawmills. Cracks caused by kiln-drying of solid timber are extremely difficult to predict. This paper reports on experiments concerned with methods of reducing cracks in wood and with the cracking behaviour of Norway spruce discs. The spruce was dried from green moisture content down to equilibrium moisture content at 23°C and 64% relative humidity. Moisture-related strains and crack development were measured using a digital image correlation system, Aramis. The moisture gradient in the longitudinal direction had a major influence on crack behaviour and was quite pronounced in discs more than 30 mm thick, but much more limited in discs only 15 mm thick. Although the thicker discs often cracked very early in the drying process, many of these cracks became invisible later on in the drying process, suggesting that sealing the ends of timber logs while in the green moisture state could considerably reduce the development of end-cracks. The initial moisture content and the shrinkage properties of the wood varied markedly from pith to bark. The importance of taking material inhomogeneities into account when modelling crack propagation in solid wood is emphasized. © 2011 Taylor & Francis.
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| 10. |
- Larsen, Finn, et al.
(författare)
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Numerical and experimental study of moisture-induced stress and strain field developments in timber logs
- 2013
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Ingår i: Wood Science and Technology. - : Springer Science and Business Media LLC. - 0043-7719 .- 1432-5225. ; 47:4, s. 837-852
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Tidskriftsartikel (refereegranskat)abstract
- When solid wood dries from a green condition to a moisture content used for further processing, moisture-induced fracture and stresses can occur. The drying stresses arise because of internal deformation constraints that are strongly affected by the cross-sectional moisture gradient differential shrinkage and the inhomogeneity of the material. To obtain a better understanding of how stresses develop during climatic variations, the field histories of stresses (and strains) in cross sections in their entirety need to be studied. The present paper reports on experiments and numerical simulations concerned with analysing the development of strains and stresses during the drying of 15-mm-thick discs of Norway spruce timber log. The samples were dried at 23 °C and relative humidity of 64 % from a green condition to equilibrium moisture content. The moisture gradient in the longitudinal direction was minimised by use of thin discs simplifying the moisture history of the samples studied. The strain field history was measured throughout the drying process by use of a digital image correlation system. Numerical simulations of the samples agreed rather well with the experimental strain results obtained. The stress results also indicated where in the cross section and when fractures could be expected to occur during drying. More optimal drying schemes showed markedly reduced stress generation.
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