| 1. |
- Aad, G., et al.
(författare)
-
- 2016
-
Ingår i: Physical Review D. Particles and fields. - : American Physical Society. - 0556-2821 .- 1089-4918. ; 93:1
-
Tidskriftsartikel (refereegranskat)
|
|
| 2. |
- Aad, G., et al.
(författare)
-
- 2015
-
Ingår i: Physical Review D. Particles and fields. - : American Physics Society. - 0556-2821 .- 1089-4918. ; 92:11
-
Tidskriftsartikel (refereegranskat)
|
|
| 3. |
- Aad, G., et al.
(författare)
-
- 2015
-
Ingår i: Journal of High Energy Physics. - : Springer. - 1029-8479 .- 1126-6708. ; :12
-
Tidskriftsartikel (refereegranskat)
|
|
| 4. |
|
|
| 5. |
- Berg, Jan-Erik, 1957-
(författare)
-
Wood and fibre mechanics related to the thermomechanical pulping process
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
|
|
| 6. |
|
|
| 7. |
- Hellström, Lisbeth M., et al.
(författare)
-
An analytical and numerical study of some aspects of the wood chipping process
- 2009
-
Ingår i: Nordic Pulp & Paper Research Journal. - : Walter de Gruyter GmbH. - 0283-2631 .- 2000-0669. ; 24:2, s. 225-230
-
Tidskriftsartikel (refereegranskat)abstract
- In order to model the wood chipping process, the primary process parameters have been identified and their first order interaction studied. The model is analytical and incorporates, in particular, the influence of sliding friction between the wood chipping tool and the log. To estimate the accuracy of the analytical model, a Finite Element (FE) analysis of the problem considered was also performed. The analytical model and the FE analysis are both restricted to small deformations and linear elastic orthotropic material behaviour. The most severe limitation with both the analytical and the FE model is the assumption of linearly elastic material. On the other hand it is felt that existing models of anisotropic plasticity in metals are lacking too much of physical relevance, if applied to wood. The analytical model predicts the normal and shear strain distribution in the crack-plane prior to crack initiation. The analytical distributions are in reasonable agreement with the corresponding distribution of the FE analysis. Based on experimental findings, it is suggested that the stress field over the entire crack-plane, in conjunction with the stress field close to the tip of the chipping tool, are critical for chip creation, rather than just the latter.
|
|
| 8. |
- Hellström, Lisbeth M, et al.
(författare)
-
An Analytical and Numerical Study of some aspects of the Wood Chipping Process
- 2009
-
Ingår i: Nordic Pulp & Paper Research Journal. - 0283-2631 .- 2000-0669. ; 24:2, s. 225-230
-
Tidskriftsartikel (refereegranskat)abstract
- SUMMARY: In order to model the wood chipping process, the primary process parameters have been identified and their first order interaction studied. The model is analytical and incorporates, in particular, the influence of sliding friction between the wood chipping tool and the log. To estimate the accuracy of the analytical model, a Finite Element (FE) analysis of the problem considered was also performed. The analytical model and the FE analysis are both restricted to small deformations and linear elastic orthotropic material behaviour. The most severe limitation with both the analytical and the FE model is the assumption of linearly elastic material. On the other hand, it is felt that existing models of anisotropic plasticity in metals are lacking too much of physical relevance, if applied to wood. The analytical model predicts the normal and shear strain distribution in the crack-plane prior to crack initiation. The analytical distributions are in reasonable agreement with the corresponding distribution of the FE analysis. Based on experimental findings, it is suggested that the stress field over the entire crack-plane, in conjunction with the stress field close to the tip of the chipping tool, are critical for chip creation, rather than just the latter.
|
|
| 9. |
- Holmgren, Sven-Erik, et al.
(författare)
-
An Encapsulated Split Hopkinson Pressure Bar for Testing of Wood at Elevated Strain Rate, Temperature and Pressure
- 2008
-
Ingår i: Experimental techniques (Westport, Conn.). - : Springer Science and Business Media LLC. - 0732-8818 .- 1747-1567. ; 32:5, s. 44-50
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a device, based on a split Hopkinson pressure bar (SHPB) setup, by which it is possible to obtain stress vs. strain for a wood specimen at high deformation rate, high temperature and high steam pressure. The need for determining the mechanical properties of wood not only at high deformation rate but also at high temperature and pressure is motivated by the need to model the wood chip refining process in mechanical pulp-ing. At mechanical chip refining, e.g. in thermomechanical pulping processes, preheated wood chips together with added water are fed into the centre of a refiner which in essence consists of two circular discs. Most often one disc is stationary and the other is rotating. The wood chips are transported radially between the discs due to inertia. On their way, due to impacts from radial bars on the discs, they are eventually broken down to individual fibres and fibre fragments. The device presented here is an SHPB set-up, modified so that the bars and the specimen are encapsulated in a pressure vessel within which the temperature is constant. In this way effects of temperature gradients in the bars are avoided. Pilot tests have been carried out which verify the intended per-formance of the device.
|
|
| 10. |
|
|
| 11. |
- Isaksson, Per, et al.
(författare)
-
A numerical and experimental study regarding the influence of some process parameters on the damage state in wood chips
- 2013
-
Ingår i: Holzforschung. - : Walter de Gruyter GmbH. - 0018-3830 .- 1437-434X. ; 67:6, s. 691-696
-
Tidskriftsartikel (refereegranskat)abstract
- The specific energy consumption during mechanical refining operation can be reduced by choosing the optimal process parameters in the wood chipping process such that a beneficial pretreatment is obtained. In the case of the utilization of a larger knife-edge angle, which is one such process parameter, the energy reduction is presumably due to the increased compressive loading parallel to the wood fibers. In the present article, a chip damage parameter D of spruce is in focus, which is relevant for cracking parallel to the fibers. D is defined and its dependence on the chip length and edge angle of the chipping knife is analyzed numerically by means of finite element analyses (FEA). The cutting force was measured in a pilot wood chipper for a number of knife-edge angles. There is a good correlation between the experimental results and those of FEA.
|
|
| 12. |
|
|
| 13. |
- Isaksson, Per, et al.
(författare)
-
Shear correction factors for corrugated core structures
- 2007
-
Ingår i: Composite structures. - : Elsevier BV. - 0263-8223 .- 1879-1085. ; 80:1, s. 123-130
-
Tidskriftsartikel (refereegranskat)abstract
- Because of the high computational cost involved when modeling a corrugated panel (in e.g., finite element analyses) the corrugated core sandwich is in this investigation homogenized and the panel is transformed to an equivalent continuous homogenous layer with effective equal properties. Mathematically, the corrugated board panel is divided into an arbitrary number of thin virtual layers. For each virtual layer, unique effective elastic modules are calculated. Then, the elastic properties in all layers are assembled together in order to be able to analyze a corrugated board as a continuous structure having equivalent mechanical properties to a real structure. It is shown that by using shear correction factors derived from an equilibrium stress field, improvements in the calculated stiffness and deflections can be achieved in circumstances when a corrugated board panel is subjected to bending. Following the algorithm outlined, the shear correction factors are easy to calculate and become a valuable tool when performing mechanical analyses of corrugated boards. The capability of the model to properly capture and simulate the mechanical behavior of corrugated boards subjected to plate bending as well as three-point-bending has been demonstrated by means of several numerical examples, which are compared to experiments on corrugated board panels of varying geometry.
|
|
| 14. |
- Isaksson, Per, et al.
(författare)
-
The onset and progression of damage in isotropic paper sheets
- 2006
-
Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 43:3-4, s. 713-726
-
Tidskriftsartikel (refereegranskat)abstract
- The validity of existing criteria and relations for the onset and evolution of damage in paper is investigated and it is found that a non - local theory has to be utilized.
|
|
| 15. |
- Krusper, Aleksandra, et al.
(författare)
-
Modelling of out-of-plane compression loading of corrugated paper board structures
- 2007
-
Ingår i: Journal of engineering mechanics. - 0733-9399 .- 1943-7889. ; 133:11, s. 1171-1177
-
Tidskriftsartikel (refereegranskat)abstract
- A simple analytical model for calculating the nonlinear deformations of a corrugated core (fluting) during out-of-plane compression is developed. The results from the analytical model are compared to a more comprehensive finite-element model and to experiments. A connection between the boundary conditions and the damage state of the corrugated board is discussed. Including a modified set of boundary conditions gives an almost perfect match of the initial stiffness to the experimental results, indicating that the core might be significantly damaged as a result of the manufacturing process. Possibly, the strength of corrugated boards might be increased about 20% if one could avoid damaging the fluting during the manufacturing process.
|
|
| 16. |
- Svensson, Birgitta A., et al.
(författare)
-
Frictional testing of wood : Initial studies with a new device
- 2009
-
Ingår i: Tribology International. - : Elsevier BV. - 0301-679X .- 1879-2464. ; 42:1, s. 190-196
-
Tidskriftsartikel (refereegranskat)abstract
- In the chip refining process used for mechanical pulp production, wood fibers are treated in a flat and narrow gap between rotating plates. The process is very energy consuming and much of the electrical energy supplied to the refiner is transferred to the fiber material through friction forces. Even though friction has been discussed frequently over the years among pulp and paper researchers worldwide and held to be of great importance, little has been proven due to the complexity of the refining equipment and the extreme conditions prevailing during operation. This paper presents a new apparatus for studying the frictional properties of wood, in lab-scale, under chip refining conditions. Friction tests can be carried out in a steam atmosphere under high temperature/pressure with maximum sliding velocity as high as 150 m/s. Initial studies at room temperature showed that the coefficient of friction between spruce wood and smooth steel increased linearly with the moisture content of the wood specimens. Impregnation by wood extractives lowered the friction coefficient for dry wood surfaces sliding at high speed. When tests were performed in a saturated steam environment, pressurized to various degrees, the frictional properties were strongly affected and varied by the temperature of the surroundings � which always has been considered as constants in analytical and numerical models of process operation.
|
|
| 17. |
- Svensson, Birgitta A., et al.
(författare)
-
High Strain Rate Compression and Sliding Friction of Wood under Refining Conditions
- 2007
-
Ingår i: International Mechanical Pulping Conference 2007, TAPPI. - 9781605602936 ; , s. 983-994
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Compression and friction characteristics are needed in order to gain a better grasp of the forces acting during refining. To this end, both stress-strain relations and frictional behavior of wood were investigated under simulated chip refining conditions (hot saturated steam, high strain rate compression, and high sliding speed). Two new, custom-designed, experimental set-ups were used. Both pieces of equipment allow a testing environment of hot saturated steam. The wood-steel friction investigations indicate that when making measurements in the lower temperature region (100C-130C), surface properties such as lubrication have a great influence on the coefficients of friction. Traces of lubricating layers, comprising fatty acids, were found on friction-tested pine surfaces using a staining technique and light microscopy; in the higher-temperature region no traces of lubrication could be detected in this way.
|
|
| 18. |
- Batchelor, W. J., et al.
(författare)
-
Effect of test conditions on measured loads and displacements in zero and short span testing
- 2003
-
Konferensbidrag (refereegranskat)abstract
- Single fibre mechanical properties play a critical role in determining sheet mechanical properties, but these mechanical properties are rarely measured due to the time-consuming nature of the tests. The zero span strength is commonly used as a measure for the strength of the fibres, but the results also can depend on the test conditions. Modelling has shown that the load displacement curves are influenced by the thickness to span ratio, as there is an in-homogenous stress-field in the thickness direction of the sample. This paper will present data on the effect of grammage and clamping pressure on the loads and displacements in zero/short span tests. Clamps were designed and made for a displacement controlled load frame made by Material Testing System (MTS). These clamps can test up to ten plies of papers with a span length from 0 to 3 mm. For the sake of comparison, tests were made using a commercial zero/short span tester, which is load controlled but is limited in span length and thickness of the tested material. Both machines were found to give comparable results. Isotropic 65 gsm handsheets, 36 gsm aluminium foil and 42 gsm greaseproof paper (MD and CD) were tested as functions of clamping pressure and grammage. An intrinsic zero-span strength was defined as the y-axis intercept of plot. A recently published procedure for determining the short span stress-strain curve from zero and short span testing was evaluated for the effect of grammage, clamping pressure and span on the calculated result. It was concluded that the method only gives results that are independent of the test conditions when the conditions are carefully selected.
|
|
| 19. |
- Berg, Jan-Erik, et al.
(författare)
-
A Micromechanical Model of the Deterioration of a Wood Fibre
- 1999
-
Ingår i: Journal of Pulp and Paper Science. - 0826-6220. ; 25:2, s. 66-71
-
Tidskriftsartikel (refereegranskat)abstract
- A simple analytical model is presented in this paper for the prediction of the stiffness degradation and the damage state in a wood fibre, loaded in uniaxial tension or shear. The model is based on an assumed displacement field together with the minimum total potential energy theorem. For the damage development, an energy criterion is employed. The model is applied to a specific example and the relevant stiffness coefficients are calculated as a function of the damage state. The damage development as a function of the applied loads is also given. The results from a specific example considered indicate that a tensile load affects the stiffness degradation to a larger extent than does a shear load.
|
|
| 20. |
- Berg, Jan-Erik, et al.
(författare)
-
Effect of Temperature on Fracture of Spruce in Compression, Investigated by Use of Acoustic Emission Monitoring
- 2000
-
Ingår i: Journal of Pulp and Paper Science (JPPS). - 0826-6220. ; 26:8, s. 294-299
-
Tidskriftsartikel (refereegranskat)abstract
- Acoustic Emission (AE) monitoring during compression of wood has been used to investigate the fracture history with specific emphasis on its dependence on temperature, moisture content, strain and loading direction. The wood was compressed in both the lateral and longitudinal directions in order to select preferred modes of deformation to achieve desired irreversible changes in the wood structure. The elastic modulus, the compressive strength and the cumulated number of AE events decreased with increasing temperature. It was concluded that the most efficient loading direction is longitudinal in order to introduce flaws in wood under compression and that a longitudinal compression of 24%, corresponding to a specific energy input of 3 kWh/ton, is needed in order to achieve substantial changes in the wood structure. The compression should be carried out at temperatures well below 120°C in order to introduce many failure sites.
|
|
| 21. |
- Berg, Jan-Erik, 1957-, et al.
(författare)
-
Fibre wall crack development
- 2009
-
Ingår i: Proceedings - 2009 International Mechanical Pulping Conference, IMPC 2009. ; , s. 340-343
-
Konferensbidrag (refereegranskat)abstract
- An analytical model has been applied to calculate the energy to achieve a certain damage state in a softwood fibre wall by uniaxial tension or shear load. At conditions such as prevailing at the entrance of the gap between the plates in a refiner less energy was needed for earlywood compared to latewood fibres. The energy was lower for loading in shear compared to tension for both earlywood and latewood fibres with a low microfibril angle. This implies that it would be preferred to refine earlywood and latewood fibres separately.
|
|
| 22. |
- Berg, Jan-Erik, et al.
(författare)
-
On the energy consumption for crack development in fibre wall in disc refining - A micromechanical approach
- 2009
-
Ingår i: Holzforschung. - 0018-3830 .- 1437-434X. ; 63:2, s. 204-210
-
Tidskriftsartikel (refereegranskat)abstract
- An analytical model has been applied to calculate the acquired strain energy density in order to achieve a certain damage state in a softwood fibre by uniaxial tension or shear load. The energy density 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 is needed to create cracks at higher microfibril angles. The energy density was lower for earlywood compared to latewood fibres. For low microfibril angles, the energy density 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.
|
|
| 23. |
|
|
| 24. |
- Gradin, Per A., et al.
(författare)
-
Measurement of the Power Distribution in a Single Disc Refiner
- 1999
-
Ingår i: Journal of Pulp and Paper Science. - 0826-6220. ; 25:11, s. 384-387
-
Tidskriftsartikel (refereegranskat)abstract
- A vital parameter in the production of mechanical pulp through refining is the energy consumption. Until recently, the pulping industry has focused on the total energy consumed, without a full understanding of how this energy is distributed in the refiners. In this paper two methods are suggested by which it is possible to estimate the energy supplied at an arbitrary point along the disc radius on refiner segments. The methods are based on instrumented bar with strain gauges. In the first method, a bar is slotted so that the sensor can be viewed as consisting of a number of cantilever beams separated by slots. In the second method, the sensor consists of one continuous bar along the disc radius. The sensors are calibrated such that influence coefficients are obtained for the slotted case and an influence function is obtained for the continuous one. The tangential forces are obtained directly in the first method, while in the second they are given by an integral equation which can be solved numerically. From the knowledge of the tangential force the power distribution can be calculated. Experimental results from the use of the slotted sensor are also given.
|
|
| 25. |
- Hellström, Lisbeth M, 1969-, et al.
(författare)
-
A device for studying wood chipping under realistic conditions
- 2009
-
Ingår i: Proceedings - 2009 International Mechanical Pulping Conference, IMPC 2009. ; , s. 252-254
-
Konferensbidrag (refereegranskat)abstract
- It seems to be a consensus in the papermaking society that chip quality is very much a matter of a narrow thickness distribution. Much effort has been spent on studying the influence of certain process parameters on the thickness distribution of chips. However, when these studies have been performed in a laboratory environment, the conditions have been far from realistic when it comes to for instance cutting rates. In this paper a laboratory chipper is presented, which has been developed to admit chipping at rates used in the industry. The knife holder is instrumented in such a way that moments and forces in three directions can be measured. Results from tests, where the cutting rate is varied, is presented and it is concluded that the laboratory chipper is a versatile tool in the process of increasing the understanding of the chipping process.
|
|
