| 1. |
- Ahadi, Aylin, et al.
(författare)
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Simulation of a Friction Stir Welding Process using FE technique
- 2006
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Ingår i: Report TVSM. - 0281-6679.
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Konferensbidrag (refereegranskat)abstract
- In this paper a three dimensional FE model of the friction stir welding (FSW) is presented and the FSW process is simulated. The model has been implemented in FE code Abaqus with user subroutines DLOAD, DFLUX and UMASFL. The material data used to describe the welded panels is from an aluminium alloy called AL-2024-T3 and the material is assumed to be isotropic with temperature dependent yield stress and elastic modulus. A nonlinear-coupled thermo-mechanical analysis was performed in order to study the temperature and stress distribution.
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| 2. |
- De Magistris, Federica, 1969-
(författare)
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Wood fibre deformation in combined shear and compression
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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
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| 3. |
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| 4. |
- Lindemann, Jonas, et al.
(författare)
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Using CORBA middleware in finite element software
- 2006
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Ingår i: Future Generation of Computer Systems. - : Elsevier BV. - 0167-739X. ; 22:1-2, s. 158-193
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Tidskriftsartikel (refereegranskat)abstract
- Distributed middleware technologies, such as CORBA can enable finite element software to be used in a more flexible way. Adding functionality is possible without the need for recompiling client code. Applications and libraries can expose their functionality to other applications in a language neutral way, enabling a more direct and easy transfer of data, without the need for intermediate input and output files. The CORBA software components can be easily configured and distributed transparently over the network. A sample structural mechanics code, implemented in C++ is used to illustrate these concepts. Some future directions, such as placing CORBA enabled finite element software on HPC centres are also discussed.
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| 5. |
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| 6. |
- Obaidat, Yasmeen, et al.
(författare)
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Nonlinear FE Model of the CFRP / Concrete Interface
- 2009
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Ingår i: [Host publication title missing]. - 1901-7278.
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Konferensbidrag (refereegranskat)abstract
- This paper presents a finite element analysis which is validated against laboratory tests of eight beams strengthened with externally bonded with carbon fibre reinforced plastic (CFRP) plates. The commercial numerical analysis tool Abaqus was used. Cohesive elements were used to model traction and shear behaviour of concrete/CFRP interface. The results are presented and discussed in terms of loading capacity. The results obtained from finite element models show good agreement with the experimental results.
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| 7. |
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| 8. |
- Ormarsson, Sigurdur, et al.
(författare)
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Finite element study of growth stress formation in wood and related distortion of sawn timber
- 2009
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Ingår i: Wood Science and Technology. - : Springer Science and Business Media LLC. - 0043-7719 .- 1432-5225. ; 43, s. 387-403
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Tidskriftsartikel (refereegranskat)abstract
- Lack of straightness in timber is the most frequent complaint regarding solid (and laminated) timber products worldwide. Nowadays, customers demand higher quality in the shape stability of wood products than they did earlier. The final distortion of timber boards is mostly caused by moisture-related stresses in wood (drying distortions) and growth-related stresses (distortions appearing when logs are split up to timber boards by sawing). To get more knowledge on how these distortions can be reduced in wooden products, there is a need for improved understanding of this material behaviour through good numerical tools developed from empirical data. A three-dimensional finite element board distortion model developed by Ormarsson (Doctoral thesis, Publ. 99:7, 1999) has been extended to include the influence of growth stresses by incorporating a one-dimensional finite element growth stress model developed here. The growth stress model is formulated as an axisymmetric general plane strain model where material for all new annual rings is progressively added to the tree during the analysis. The simulation results presented include how stresses are progressively generated during the tree growth, distortions related to the redistribution of growth stresses during log sawing, and distortions and stresses in drying reflecting the effects of growth stresses. The results show that growth stresses clearly vary during tree growth and also form a large stress gradient from pith to bark. This in itself can result in significant bow and crook deformations when logs are sawn into timber boards. The distortion results from the simulations match well with the results observed in reality. The parametric study also showed that the radial growth stress distribution is highly influenced by parameters such as modulus of elasticity, micro fibril angle and maturation strain.
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| 9. |
- Ormarsson, Sigurdur, 1963-, et al.
(författare)
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Modelling of growth stress generation and timber distortions related to log sawing and forced drying
- 2008
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Ingår i: 8th World Congress on Computational Mechanics and 5th European Congress on Computational Methods in Applied Sciences and Engineering.
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Konferensbidrag (refereegranskat)abstract
- Growth stresses can cause fibre collapse in living trees (often in combination with strong wind loading), internal checking resulting in end-splitting of logs, and instantaneous board distortions when the log is split into timber. How much the growth rate and growth stresses affect the final shape stability of solid timber products is not fully understood. For trees with abnormal growth conditions resulting in eccentric growth and generation of reaction wood, it is very complicated to estimate how timber products made of such material will behave during moisture variation. To study this behaviour a finite element analysis in which stress formation during normal and abnormal tree growth was simulated with the aim of better understanding of the growth stress formation.The model for progressive growth stress generation in trees with normal growth conditions is formulated as a one dimensional axisymmetric general plane strain model of the tree stem. The trunk is considered as a very long solid cone with zero shear stresses. In the model, each new (and stress free) annual ring is progressively added to the stem during the analysis. Thereafter the cell maturation is assumed to start, i.e. the crystallization of the cellulose leads to longitudinal shrinkage of the new annual ring whereas the lignification process results in transversal expansion of the fibres. Since the maturing annual ring is attached to the old and already matured rings, a strain constraint develops in the stem. The new annual ring becomes stretched longitudinally and compressed tangentially, whereas the matured rings are exposed to the opposite stress conditions. The material model used is based on the assumption of small strains and the biological maturation strains are used as a driving force for the growth stress evolutions. The aim here is also to take into account viscous effects of the wood material. ......
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| 10. |
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