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Mechanical behaviour of wet wood in sequences of compression and combined compression and shear
STFI-Packforsk AB.
STFI-Packforsk AB.
2006 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, Vol. 21, no 2, 231-236 p.Article in journal (Refereed) Published
Abstract [en]

During the refining of mechanical pulp, a process occurring at high speed at temperatures of 140-160°C, the flexibility and bonding ability of the wood fibres increase. To understand the mechanical behaviour of the fibres in this operation, the deformation at low speed of wet wood specimens at 50°C and 90°C were studied under different combinations of shear and compression loading using a modification of the Arcan device. The deformation in earlywood was studied using an image analysis procedure together with measurements of the work done under different loading conditions. The deformation under combined shear and compression load was different from that in pure compression. In the first cycle under compression, the fibre cell walls were bent in a characteristic "S" shape, whereas under the combined load the cells deformed according to a "brick" shape. After a first cycle under combined load, the cells deformed according to the "brick" shape even when subjected to a second load under pure compression. The first deformation cycle required the largest amount of work. Since less energy was needed for the first cycle under a combined load than under a compression load, the application of a combined load as a first cycle may be a way to permanently deform fibres using less work.

Place, publisher, year, edition, pages
2006. Vol. 21, no 2, 231-236 p.
Keyword [en]
wood, shear, compression, deformation, picea abies, energy consumption
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-6262DOI: 10.3183/NPPRJ-2006-21-02-p231-236ISI: 000238726200011OAI: oai:DiVA.org:kth-6262DiVA: diva2:10929
Note
QC 20101005. Uppdaterad från submitted till published (20101005).Available from: 2005-10-14 Created: 2005-10-14 Last updated: 2010-10-05Bibliographically approved
In thesis
1. Wood fibre deformation in combined shear and compression
Open this publication in new window or tab >>Wood fibre deformation in combined shear and compression
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Mechanical pulping for producing pulps from softwood suitable for printing grade papers, like news, is a highly energy-intensive process consuming around 2000 kWh/t in electrical energy. Due to increasing energy costs and environmental issues there is a high demand for decreasing this energy consumption. The mechanical treatment of wet wood pieces in a refiner, in the mechanical pulp plant, is a complex mechanical loading. This is a process occurring between rotating discs at high speed and temperatures of 140 °C - 160 °C, where by means of shear and compression forces the fibres are separated and then made flexible, fibrillated and collapsed for good bonding ability. In this process also fines are created giving the optical properties of the paper. In mechanical pulping only a fraction of the applied energy is used for the structural changes of the wood material. Thus fundamental studies of the loading modes of wood under refining conditions and in particular under combined shear and compression loading are desired to gain more information regarding the possibility of affecting the mechanical pulping in an energy efficient way.

The possibilities to study the behaviour of wood under a combined shear and compression load were in this thesis investigated using two methods: the Iosipescu shear test and the Arcan shear test. In both apparatus different combinations of shear and compression load were achieved by different rotations of the shear test device itself. Measurements with the Iosipescu device on a medium density fibreboard showed good agreement between experimental results and numerical simulations. Finite element analysis on wood showed, however, that with the use of a homogeneous material in the model the level of strain reached would be ten times smaller than experimentally measured. This fact is probably due to the honeycomb structure of the wood cells that allows for different local deformations that could not be represented by a continuous material model. Thus to study the deformations on the fibre level of wood an experimental equipment that uses smaller samples was needed.

With a modified Arcan shear device such deformations under combined shear and compression load and in pure compression were possible showing different deformation patterns. During pure compression the cell walls bend in a characteristic “S” shape, independently of the shape of the fibre cells and their cell wall thickness. Under combined shear and compression, however, mainly the corners of the fibre cells deform giving a “brick” shape to the cells. In a second deformation performed in compression, the fibre cells follow the same deformation pattern as given by the first deformation type whether in compression or in combined shear and compression. The interpretation is that permanent defects in the cells themselves are introduced already in the first load cycle of the wood samples.

The energy used under the different loading conditions showed that the first deformation required the largest amount of energy, for all loading conditions. The deformation in compression required larger amounts of energy than the deformation in combined loads. For subsequent deformations less energy was needed for compression if a combined load had preceded it. Due to the fact that less energy is needed to start to deform wood in combined load than under compression load, the application of a combined load as a first cycle may thus be a way to permanently deform fibres using less energy.

To investigate the critical parameters determining the permanent deformation of cells, a finite element model of a network of twelve cells was developed. Special care was given to the material properties to study how the variation of the fibril angle in the different layers affects the deformation pattern of the wood fibres under the different loading conditions. The model shows that whether modelled as homogeneous linear isotropic material or as an orthotropic material defined for every layer of the cells wall, no difference in the deformation of the network of the fibres was achieved. It is probable that the deformation type is more determined by the geometry of the fibres themselves than by their material properties

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. 49 p.
Series
Trita-MEK, ISSN 0348-467X ; 2005:13
Keyword
Arcan, compression, density, energy, finite element analysis, Iosipescu, Picea abies, shear, wood
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-415 (URN)
Public defence
2005-10-20, STFI-salen, Drottning Kristinas väg 61, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20101005Available from: 2005-10-14 Created: 2005-10-14 Last updated: 2010-10-05Bibliographically approved

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