| 1. |
- Alnersson, Gustaf
(författare)
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Towards 3D modelling of Compression Moulding of SMC
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The automotive industry is facing ever increasing demands for reduced emissions, and lightweight solutions are thusly required. One field that has significant potential in this regard is composite materials, which can offer a good combination of weight reduction and mechanical properties. However, the rapid development cycles in the automotive industry mean that tools for numerical modeling are necessary, both regarding manufacturing processes and prediction of mechanical properties. The material that has been of interest for this work is Sheet Moulding Compound (SMC). SMC consists of sheets of resin and chopped fibres. When used for manufacturing the sheets are cut into appropriate size and shape. The cut sheets are then placed in a pre-heated mould. When this mould is closed, the sheets melt and the fibre-filled resin flows out and fills the mould cavity; the resin then cures and solidifies. A significant advantage of SMC compared to other composite solutions is that the process has comparatively short cycle times, which is a necessity for automotive applications. However, it is a rather complicated process to model numerically for a number of reasons, including the complex rheological properties of the charge, the often rather significant temperature gradients throughout the thickness, often complicated three-dimensional effects in the flow, and the chopped fibres present in the charge. These fibres will move and change orientation as the charge is pressed, which is a significant challenge to model properly.The first part of this work is a review and discussion of the difficulties described above, and some solutions that have been suggested. The second part concerns a suggestion for a three-dimensional flow model for the compression moulding process, which takes into account factors that have been suggested to influence the flow behavior, such as temperature distribution and shear strain rate. Some simulation results are presented along with comparison to previous experimental results, and similar flow patterns are observed serving as a qualitative validation. The third part concerns the expansion of this model to include the effects of the flow on the fibre orientation.
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| 2. |
- Andre, Alann, et al.
(författare)
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An experimental and numerical study of the effect of some manufacturing defects
- 2013
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Ingår i: ICCM International Conferences on Composite Materials. - : International Committee on Composite Materials. ; , s. 4105-4112
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Konferensbidrag (refereegranskat)abstract
- During the manufacturing process of composite structural parts, layer of fabrics or unidirectional prepreg may have to be cut in order to fulfil production requirements. From a general mechanical point of view, cutting fibres in a composite part has a large negative impact on the mechanical properties. However, such interventions are necessary in particular cases, for example due to draping of complex geometries. A rather extensive test program was launched to investigate the effects of defects that typically could arise during manufacturing. The overall purpose of the test program was to determine knock-down factors on strength for typical manufacturing defects that occasionally arise and sometimes are hard to avoid in production: cuts/gaps and fibre angle deviations. Four types of specimens were tested, reference, intersection of cuts in adjacent layers combined with a bolt hole, cut in a zero degree ply combined with a bolt hole and specimens with misaligned fibres. The specimens with misaligned fibres were tested with three different fibre angles. In addition to the experimental procedure, FE-analyses utilising cohesive elements were conducted, and after mechanical tests, Non Destructive Investigation (NDI) and fractographic investigations were performed. An excellent correlation between analyses and experiments were obtained.
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| 3. |
- Björnsson, Andreas, 1981-, et al.
(författare)
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Robot-forming of prepreg stacks - Development of equipment and methods
- 2016
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Ingår i: ECCM 2016 - Proceeding of the 17th European Conference on Composite Materials. - : European Conference on Composite Materials, ECCM. - 9783000533877
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Konferensbidrag (refereegranskat)abstract
- Within the aerospace industry the manufacturing of composite components with complex shapes, such as spars, ribs and beams are often manufactured using manual layup and forming of prepreg material. Automated processes for prepreg layup and efficient forming techniques like vacuum forming are sometimes difficult to employ to these type of products due to technical limitations. This paper describes the development of tools and the forming sequence needed to automate sequential forming of a complex shape using an industrial robot. Plane prepreg stacks are formed to the final shape using a dual-arm industrial robot equipped with rolling tools. Tests show that the developed tools and the employed sequence can be used to form stacks to the desired shape with acceptable quality.
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| 4. |
- Dutta, Abhik, et al.
(författare)
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FORMING OF HYBRID (UD-ASMC) COMPOSITES
- 2022
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Ingår i: ECCM 2022. - : Composite Construction Laboratory (CCLab), Ecole Polytechnique Federale de Lausanne (EPFL). ; , s. 446-453
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Konferensbidrag (refereegranskat)abstract
- Combining short and long fibre reinforcement in the same component, enables establishing an optimum combination between cost and production efficiency while still maintaining good structural characteristics of the produced component. This can be done in a single compression molding step. However, producing defect free hybrid components require a good understanding of the deformation mechanisms during manufacturing. Traditionally, unidirectional (UD)-prepreg forming models involve the study of mechanisms such as shear and ply bending. On the other hand, manufacturing of advanced SMC (ASMC) relies on enabling a significant amount of squeeze flow. However, little is known about the interaction of these mechanism. This study aims to look at the interaction between long, aligned fibres and short fibre composite material during the first phase, forming, of a compression molding process. A two-stage modelling approach consisting of draping and squeeze flow respectively, is suggested and material properties for the first stage of modelling the UD-ASMC hybrid stack is presented. The material system studied consists of a combination of UD “snap-cure” prepreg and ASMC, both having fibre volume fraction greater than 50% and are suitable for use in serial production.
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| 5. |
- Dutta, Abhik, et al.
(författare)
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Interply friction between unidirectional carbon/epoxy prepreg plies : Influence of fibre orientation
- 2023
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Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier BV. - 1359-835X .- 1878-5840. ; 166, s. 107375-
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Tidskriftsartikel (refereegranskat)abstract
- This work investigates the role of fibre orientation on interply friction. Interply friction tests were performed on a novel snap-cure unidirectional (UD) carbon/epoxy prepreg for five fibre orientations over a broad range of normal pressures and sliding speeds. The test method used closely mimicked the actual processing condition of the prepreg prior to forming including consolidation of the interface. The results show that fibre orientation of the prepreg has a large influence on interply friction. In fact, interply friction for various fibre orientation combinations varies substantially both in magnitude and behaviour of the curve. Further, the range of this variation is dependent on processing conditions such as consolidation, normal pressure and sliding speed. These findings make a strong case for including direction dependent friction models in forming simulations and presents opportunities to tailor the laminate stacking sequence for better forming outcomes.
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| 6. |
- Ekermann, Tomas, 1984-
(författare)
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Testing and analysis of composites with 3D woven reinforcement
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Composites with three-dimensional (3D) reinforcement have several benefits, compared to laminated composites. Their through-thickness reinforcement increase the out-of-plane properties significantly and could eliminate problems with delaminations. Also, these composites have proven to have great damage tolerance and energy absorption. However, their complex yarn architectures make it challenging to predict their mechanical response and performance.In this thesis different aspects of composites with 3D woven reinforcement are explored. The focus is on a specific yarn architecture, called fully interlaced 3D weave. The results are however not only limited to that specific 3D reinforcement but could to a certain extent also be applicable to 3D reinforcement in general.Preforms with fully interlaced 3D weaves were manufactured and impregnated with epoxy. These were then examined in great detail with computer tomography (CT) to study the internal yarn architecture after impregnation. Analysis showed that the yarns were quite significantly distorted by the through-thickness compression during impregnation. The distortion was attributed to the relatively sparse weave, not supporting the through-thickness reinforcement, which therefore distorts and brings the rest of the yarns along with it. In parallel, a simulation model of the internal geometry of the manufactured material was developed. The simulation model was however not designed to include the distortions encountered in the physical material.The manufactured material and its corresponding model was tested in a tensile test setup. Two different thicknesses of the material was manufactured as well as a corresponding composite with two-dimensional (2D) reinforcement. Results showed that the material with 2D reinforcement was stiffer and stronger than the ones with 3D reinforcement, which was attributed to the lower crimp in the 2D reinforcement. A difference in stiffness between the two 3D weaves was also found and addressed to the larger amount of surface layer in the thinner weave, where vertical weft yarns are aligned with the warp direction and contributing to the overall stiffness in that direction. Failure analysis of specimens tested in the warp direction showed that initial cracks form in the boundaries of vertical weft yarns, close to the material surface. For specimens tested in the horizontal weft direction, initial cracks were found through the vertical weft yarns at the surfaces. Both these findings were supported by results from the simulation model.An application for composites with fully interlaced 3D weave was also explored, where it was integrated as a fillet in a composite T-joint. The scope here was to make a 3D reinforced fillet, having low transverse thermal expansion which would decrease the residual stresses in the fillet after curing. T-joints with conventional fillets and fillets with 3D woven reinforcement were manufactured and tested in a pull-off test. Results showed that T-joints with conventional fillets had higher strength, but also higher spread, than the ones with 3D reinforcement. The higher strength of T-joints with conventional fillets was attributed to their better ability to adapt to the T-joint cavity, while the fit was not as good for the 3D fillets. The lower spread in strength of the T-joints with 3D fillets was attributed to their lower sensitivity to minor flaws such as voids inside the fillet.
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| 7. |
- Grankäll, Tommy, 1977-, et al.
(författare)
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Geometric compensation of convex forming tools for successful final processing in concave cure tools – An experimental study
- 2019
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Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier BV. - 1359-835X .- 1878-5840. ; 116, s. 187-196
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Tidskriftsartikel (refereegranskat)abstract
- To enable shimless composite part assembly it is desired to manufacture composite spars in concave, female tools. However, due to restricted access this process is often associated with a high degree of manual operations or expensive highly specialized production equipment. This study investigates a sequential manufacturing operation where the material is automatically laid up flat and hot formed onto a convex forming tool. Thereafter it is placed in the concave tool for curing. The study investigates four different ways to geometrically compensate the forming tool to simplify seating and enhance part quality. All compensations require material movement in-between forming and curing; this movement was tracked using Micro-CT. Micrographing and optical shape measurements show that a correct compensation provides high quality parts without adding labour intensive manufacturing steps.
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| 8. |
- Grankäll, Tommy, 1977-, et al.
(författare)
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Moisture In Composite Cure Tools
- 2022
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Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier BV. - 1359-835X .- 1878-5840. ; 158
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Tidskriftsartikel (refereegranskat)abstract
- Composite cure tools are preferable to metal tools regarding in-plane thermal expansion, quicker part processing using the tool and potentially lower cost. However, durability is often questioned and moisture in the tool is often assumed to be a negative factor. In order to increase understanding of the durability of composite cure tools sorption and swelling, two high performance prepreg tooling materials using Benzoxazine and Bismaleimide resin reinforced with carbon fibre fabric were measured over a six-month period. An Epoxy-based prepreg was also studied. The experimentally acquired material characteristics were used in simulation of water content and swelling in a composite cure tool during use. It was concluded that gradient moisture content and swelling will always be present which presents a considerably different shape distortion compared to isotropic water content. The data presented may contribute to informed selection of composite cure tool material and facilitate optimisation of tool maintenance.
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| 9. |
- Grankäll, Tommy, et al.
(författare)
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The true shape of composite cure tools
- 2020
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Ingår i: Journal of Manufacturing Processes. - : Elsevier Ltd. - 1526-6125. ; 59, s. 279-286
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Tidskriftsartikel (refereegranskat)abstract
- The cure tool is a key factor for successful composite part manufacturing since the tool shape is directly reflected in the moulded part. A composite cure tool is an alternative to metal tools. Lead time is often lower and rate tools less expensive. The recurring cost per produced part in the tool is often also lower, for instance from faster autoclave cure time. However, durability is commonly considered less favourable. Beyond all, a composite cure tool is often motivated as being the better option since the thermal expansion is close to that of the part which simplifies compensation of process-induced shape distortion. But is this really accurate? Perhaps, but only if the true mould shape at cure temperature is known. This paper includes both experiments and simulation showing actual composite cure tool shape at cure temperature. Also, simulation of stress levels in the tool laminate indicate an important characteristic of durability.
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| 10. |
- Grankäll, Tommy, 1977-
(författare)
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Tooling and processing for efficient and high tolerance prepreg composite manufacturing
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Composite use is normally motivated in aircraft design by improved weight-specific mechanical properties as compared to metals. However, fatigue and dielectric properties may also be critical for material selection. In the modern world the environmental footprint, with reduced emissions in focus, will be lighter with the successful use of composites. Flight safety is the most important requirement for the aerospace industry. In order to benefit from the important properties which ensure the necessary performance of the aircraft, part materials must be according to material specification. This is not unique to the composite materials. However, since the composite material is often created simultaneously with the composite part, the manufacturing processes used are even more critical for a composite part than for a metal part. Tooling is required for most of the process steps in composite part manufacturing. The large number of tools required to complete a composite part and the relatively small series involved in most composite applications render tooling cost a major contributor to final part cost. This is especially true in the aeronautical field considering the high-quality requirements as concerns shape accuracy and laminate quality, as well as low production rates with annual production ranging from a few dozen to the rare thousands of parts. Since tooling adds a substantial contribution to the part cost, it is imperative to be aware of the different options available for each process step and how these affect process stability and cost. Göte Strindberg, a former technical director at Saab, often said “Ninety-nine per cent right can be one hundred per cent wrong in a composite manufacturing process”. Finding the correct parameters for the last per cent is key to composite part success. This has been the overall goal of this project. The beginning of the project focused on both process and tooling related improvements for hot-forming of prepreg laminates enabling new opportunities and improved ergonomics. The research results in improvements in laminate quality, for example, reduced thickness variation, and a significant increase in process efficiency. One conclusion is that an 80% reduction of cycle time can be achieved with maintained, or even improved, laminate quality. A novel method for compensation of forming tools is also presented. Research then continued with cure tool related issues. Shape fidelity and durability for composite cure tools was studied with the combined outcome of an in-depth understanding of composite cure tool performance at the elevated temperature and moisture conditions present during the use of the tools.
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