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Wood fibre deformation in combined shear and compression
KTH, School of Engineering Sciences (SCI), Mechanics.
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 [en]
Arcan, compression, density, energy, finite element analysis, Iosipescu, Picea abies, shear, wood
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-415OAI: oai:DiVA.org:kth-415DiVA: diva2:10931
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
List of papers
1. Combined shear and compression analysis using the Iosipescu device: analytical and experimental studies of medium density fiberboard
Open this publication in new window or tab >>Combined shear and compression analysis using the Iosipescu device: analytical and experimental studies of medium density fiberboard
2004 (English)In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 37, no 6, 509-521 p.Article in journal (Refereed) Published
Abstract [en]

The possibility of using the Iosipescu shear test device to study the combined shear and compression behavior of anisotropic materials was examined. Measurements were made using both an original (Wyoming version) and an in-house modified Iosipescu shear and compression fixture. Numerical simulation of the combined shear and compression test was carried out to verify the reliability of the modified device. The numerical results were compared with data from experiments on a medium-density fiberboard. The numerical results show good agreement with the experimental results for the shear test in all the three material directions tested. The shapes as well as the values of the strain fields were similar in the numerical and experimental results. Different rotations of the combined shear and compression device were studied using the finite element method to find the combinations that gave reliable results in shear and compression. It was found that the 45degrees rotation gave the most uniform strain fields in the section between the notches. This rotation was tested on the fiberboard.

Keyword
Compaction, Computer simulation, Finite element method, Mechanical pulp, Strain, Tensile testing, Wood, Anisotropic materials, Compression loads, Strain fields
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6259 (URN)10.1007/s00226-003-0217-1 (DOI)000220559600006 ()
Note
QC 20101005Available from: 2005-10-14 Created: 2005-10-14 Last updated: 2010-10-05Bibliographically approved
2. Combined shear and compression analysis using a modified Iosipescu shear test device: Experimental studies on dry wood
Open this publication in new window or tab >>Combined shear and compression analysis using a modified Iosipescu shear test device: Experimental studies on dry wood
2005 (English)In: Holzforschung, ISSN 0018-3830, Vol. 59, no 5, 539-545 p.Article in journal (Refereed) Published
Abstract [en]

The mechanical treatment of wood in a refiner involves a complex combination of shear and compression forces. To obtain more knowledge on this process, the possibility of using an apparatus based on the Iosipescu shear test device to measure the behaviour of wood under a combined shear and compression load was tested. In this new apparatus, different combinations of shear and compression load were achieved by different rotations of the shear test device itself. Numerical simulations of the combined shear and compression test were carried out and compared with experimental data to verify the reliability of the modified device as applied to wood. It was concluded that the new apparatus is suitable for applying different combinations of shear and compression load in testing wood samples. However, finite element analysis showed that with the use of homogeneous material in the model, the level of strain reached would be 10-fold smaller. This fact is probably due to the honeycomb structure of the wood cells, which allows for different local deformation that could not be represented by the continuous material used in the model.

Keyword
Compression, Density, Picea abies, Shear stress, Wood
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6260 (URN)10.1515/HF.2005.089 (DOI)000231701200011 ()
Note
QC 20101005Available from: 2005-10-14 Created: 2005-10-14 Last updated: 2010-10-05Bibliographically approved
3. Deformation of wet wood under combined shear and compression
Open this publication in new window or tab >>Deformation of wet wood under combined shear and compression
2005 (English)In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 39, no 6, 460-471 p.Article in journal (Refereed) Published
Abstract [en]

During refining of mechanical pulp, a process occurring at high speed at temperatures of 140-160 degrees C, the flexibility and bonding ability of wood fibres are increased. To understand the mechanical behaviour of the fibres in this operation, deformations at low speed of wet wood specimens at 50 degrees C were studied under two different combinations of shear and compression loadings. The results were compared with the behaviour of wet wood in pure compression. Some features of the deformation that occurred in earlywood were analysed using an image analysis procedure. During pure compression the cell walls bend 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 get deformed. 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 was that permanent defects in the cells themselves were introduced already in the first load cycle of the wood samples. The load combination with lower shear gave the same permanent strain as the case of pure compression but using less energy.

Keyword
Compaction, Deformation, Image analysis, Mechanical pulp, Natural fibers, Bonding ability, Interpretation, Wet woods
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6261 (URN)10.1007/s00226-005-0025-x (DOI)000232606100005 ()
Note
QC 20101005Available from: 2005-10-14 Created: 2005-10-14 Last updated: 2010-10-05Bibliographically approved
4. Mechanical behaviour of wet wood in sequences of compression and combined compression and shear
Open this publication in new window or tab >>Mechanical behaviour of wet wood in sequences of compression and combined compression and shear
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.

Keyword
wood, shear, compression, deformation, picea abies, energy consumption
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6262 (URN)10.3183/NPPRJ-2006-21-02-p231-236 (DOI)000238726200011 ()
Note
QC 20101005. Uppdaterad från submitted till published (20101005).Available from: 2005-10-14 Created: 2005-10-14 Last updated: 2010-10-05Bibliographically approved
5. Finite Element Modelling of wood cell deformation transverse to the fibre axis
Open this publication in new window or tab >>Finite Element Modelling of wood cell deformation transverse to the fibre axis
2008 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, Vol. 23, no 2, 240-246 p.Article in journal (Refereed) Published
Abstract [en]

Modelling of wet wood under compression and combined shear and compression load was performed to simulate the mechanical pulping of wood chips in refiners. Experiments have shown that the wet fibre network exhibit two different deformation modes; an S-shape mode associated with compression and a brick-shape mode associated with combined shear and compression. To study the factors governing the mechanical behaviour of the fibre network a material model with the characteristics originating from the properties of the wood polymers was developed and was used in a three-dimensional finite element analysis. The effects of material properties were investigated by comparing models with anisotropic one-layer cell walls and orthotropic multi-layer cell walls. The deformation achieved both under compression and under combined shear and compression was found to be similar independent of the material constants used or the number of layers of the cells walls. This implies that the most important factor governing the deformation pattern of the fibre network is the cell structure itself.

Keyword
deformation, fibres, finite element analysis, mechanical properties, wood
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6263 (URN)10.3183/NPPRJ-2008-23-02-p240-246 (DOI)000258174600013 ()
Note
QC 20101005. Uppdaterad från submitted till published (20101005).Available from: 2005-10-14 Created: 2005-10-14 Last updated: 2010-10-05Bibliographically approved

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