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Effect of impact velocity on the fracture of wood as related to the mechanical pulping process
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences. (Ved- och fibermekanik)
Responsible organisation
2001 (English)In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 35, no 4, 343-351 p.Article in journal (Refereed) Published
Abstract [en]

The fibre separation step in refining is crucial for energy consumption in subsequent refining where the pulp properties are developed. The size reduction of chips during refining is dependent on refining intensity and chip strength. Factors affecting these two parameters are discussed in a literature review. The impact strength of chips and the break down of chips to separate fibres are also discussed. Specifically the effect of impact velocity on the fracture of wood has been studied by use of a falling weight impact tester. Samples were prepared from a freshly cut log of Norway spruce, Picea abies, and the impact strength was measured using an instrumented falling weight impact tester. An increase in impact velocity from about 2.7 to 4.8 m/s resulted in an increase in impact strength of about 50%.

 

Place, publisher, year, edition, pages
2001. Vol. 35, no 4, 343-351 p.
Keyword [en]
Energy consumption, Force sensors, Fracture, Impact strength, Picea abies, Velocity
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:miun:diva-1643DOI: 10.1007/s002260100100ISI: 000170441500005Scopus ID: 2-s2.0-0035423016Local ID: 250OAI: oai:DiVA.org:miun-1643DiVA: diva2:26675
Available from: 2008-12-04 Created: 2008-10-21 Last updated: 2016-10-13Bibliographically approved
In thesis
1. Wood and fibre mechanics related to the thermomechanical pulping process
Open this publication in new window or tab >>Wood and fibre mechanics related to the thermomechanical pulping process
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main objective of this thesis was to improve the understanding of some aspects on wood and fibre mechanics related to conditions in the thermomechanical pulping process. Another objective was to measure the power distribution between the rotating plates in a refiner.

 

The thesis comprises the following parts:

–A literature review aimed at describing fracture in wood and fibres as related to the thermomechanical pulping process

–An experimental study of fracture in wood under compression, at conditions similar to those in feeding of chips into preheaters and chip refiners

–An experimental study of the effect of impact velocity on the fracture of wood, related to conditions of fibre separation in the breaker bar zone in a chip refiner

–A micromechanical model of the deterioration of wood fibres, related to the development of fibre properties during the intense treatment in the small gap in the refining zone

–Measurements of the power distribution in a refiner.

 

The fracture in wood under compression was investigated by use of acoustic emission monitoring. The wood was compressed in both lateral and longitudinal directions to predict preferred modes of deformation in order to achieve desired irreversible changes in the wood structure. It was concluded that the most efficient compression direction in this respect is longitudinal. Preferable temperature at which the compression should be carried out and specific energy input needed in order to achieve substantial changes in the wood structure were also given.

 

The fibre separation step and specifically the effect of impact velocity on the fracture energy were studied by use of a falling weight impact tester. The fracture surfaces were also examined under a microscope. An increase in impact velocity resulted in an increase in fracture energy.

In the thermomechanical pulping process the fibres are subjected to lateral compression, tension and shear which causes the creation of microcracks in the fibre wall. This damage reduces the fibre wall stiffness. A simplified analytical model is presented for the prediction of the stiffness degradation due to the damage state in a wood fibre, loaded in uni-axial tension or shear. The model was based on an assumed displacement field together with the minimum total potential energy theorem. For the damage development an energy criterion was employed. The model was applied to calculate the relevant stiffness coefficients as a function of the damage state. The energy consumption in order to achieve a certain damage state in a softwood fibre by uniaxial tension or shear load was also calculated. The energy consumption was found to be dependent on the microfibril angle in the middle secondary wall, the loading case, the thicknesses of the fibre cell wall layers, and conditions such as moisture content and temperature. At conditions, prevailing at the entrance of the gap between the plates in a refiner and at relative high damage states, more energy was needed to create cracks at higher microfibril angles. The energy consumption was lower for earlywood compared to latewood fibres. For low microfibril angles, the energy consumption was lower for loading in shear compared to tension for both earlywood and latewood fibres. Material parameters, such as initial damage state and specific fracture energy, were determined by fitting of input parameters to experimental data.

Only a part of the electrical energy demand in the thermomechanical pulping process is considered to be effective in fibre separation and developing fibre properties. Therefore it is important to improve the understanding of how this energy is distributed along the refining zone.

Investigations have been carried out in a laboratory single-disc refiner. It was found that a new developed force sensor is an effective way of measuring the power distribution within the refining zone. The collected data show that the tangential force per area and consequently also the power per unit area increased with radial position.

The results in this thesis improve the understanding of the influence of some process parameters in thermomechanical pulping related wood and fibre mechanics such as loading rate, loading direction, moisture content and temperature to separate the fibres from the wood and to achieve desired irreversible changes in the fibre structure. Further, the thesis gives an insight of the spatial energy distribution in a refiner during thermomechanical pulping.

 

 

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University, 2008. 65 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 63
Keyword
Acoustic emission, Chips, Compression tests, Defibration, Disc refiners, Energy consumption, Fibre structure, Force sensors, Fracture, Impact strength, Mathematical analysis, Moisture content, Picea abies, Refining, Stiffness degradation, Strains, Temperature, Thermomechanical pulping, Velocity
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-6725 (URN)978-91-86073-15-2 (ISBN)
Public defence
2008-11-21, O 102, SCA-salen, Holmgatan 10, Åkroken, Sundsvall, 10:00 (Swedish)
Opponent
Supervisors
Available from: 2008-10-24 Created: 2008-10-22 Last updated: 2009-06-08Bibliographically approved

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