| 1. |
- Andersson, H.M., et al.
(författare)
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Application of digital speckle photography to measure thickness variations in the vacuum infusion process
- 2003
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 24:3, s. 448-455
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Tidskriftsartikel (refereegranskat)abstract
- A new method to measure the movement of the flexible bag used in vacuum infusion is presented. The method is based on an in-house developed stereoscopic digital speckle photography system (DSP). The advantage with this optical method, which is based on cross-correlation, is that the deflection of a large area can be continuously measured with a great accuracy (down to 10 μm. The method is at this stage most suited for research but can in the long run also be adopted in production control and optimization. By use of the method it was confirmed that a ditch is formed at the resin flow front and that there can be a considerable and seemingly perpetual compaction after complete filling. The existence of the ditch demonstrates that the stiffness of the reinforcement can be considerably reduced when it is wetted. Hence, the maximum fiber volume fraction can be larger than predicted from dry measurements of preform elasticity. It is likely that the overall thickness reduction after complete filling emanates from lubrication of the fibers combined with an outflow of the resin. Besides, the cross-linking starts and the polymer shrinks. Hence, the alteration in height will continue until complete cross-linking is reached.
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| 2. |
- Andersson, Magnus, et al.
(författare)
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Flow-enhancing layers in the vacuum infusion process
- 2002
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 23:5, s. 895-901
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Tidskriftsartikel (refereegranskat)abstract
- The current trend towards increased use of vacuum infusion molding for large surface-area parts has increased the interest in an advanced modeling of the process. Because the driving pressure is limited to 1 atmosphere, it is essential to evaluate possible ways to accelerate the impregnation. One way of doing this is to use layers of higher permeability within the reinforcing stack, i.e. flow-enhancing layers. We present an experimental investigation of the flow front shape when using such layers. The through-thickness flow front was observed by making a number of color marks on the glass-mats forming the reinforcing stack, which became visible when the resin reached their position. The in-plane flow front was derived from observations of the uppermost layer. It turned out that existing analytical models agree very well with the experiments if effective permeability data is used, that is, permeability obtained from vacuum infusions. However, the fill-time was nearly twice as long as predicted from permeability data obtained in a stiff tool. This rather large discrepancy may be due to certain features of a flexible mold half and is therefore a topic for further research. The lead-lag to final thickness ratio is dependent on the position of the flow front and ranges form 5 to 10 for the cases tested. Interestingly the lead-lag has a miximum close to the inlet.
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| 3. |
- Ericson, Mats L., et al.
(författare)
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The effect of microstructure on the elastic modulus and strength of performed and commercial GMTs
- 2003
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 14:1, s. 35-41
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Tidskriftsartikel (refereegranskat)abstract
- A new technique was used to fabricate performed glass fiber/polypropylene GMTs. The method utilizes thermoplastic powder and fiber roving in a spray-up procedure in which a porous perform in fabricated, heated and molded. The objective was to compare the properties of various preformed GMT composition with two commercial GMTs and to relate the mechanical properties to the microstructure of the materials. Preformed GMTs were fabricated with various fiber lengths and with or without a fiber/matrix adhesion promotor. Processing observation, microstructure, tensile creep modulus, and tensile strength of these preformed GMTs are reported. Fiber length and the addition of a fiber/matrix adhesion promotor had generally little effect on the modulus and strength of the preformed GMT. Comparisons with two structurally different commercial GMTs also showed negligible effects on modulus and strength. The major reason for this is suggested to be the inhomogeneities of the materials. The mechanical properties are controlled by local inhomogeneities rather than by the general microstructure of the material. These inhomogeneities arise from the fiber arrangement in the semi-finished sheet or perform. Since the microstructure of preformed GMT can be controlled, this material is well suited for future studies on the effect of better fiber dispersion.
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| 4. |
- Hagstrand, P.-O., et al.
(författare)
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Mechanical properties and morphology of flax fiber reinforced melamine-formaldehyde composites
- 2001
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 22:4, s. 568-578
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Tidskriftsartikel (refereegranskat)abstract
- The mechanical performance of natural fiber reinforced polymers is often limited owing to a weak fiber-matrix interface. In contrast, melamine-formaldehyde (MF) resins are well known to have a strong adhesion to most cellulose containing materials. In this Paper, nonwoven flax fiber mat reinforced and particulate filled MF composites processed by compression molding are studied and compared to a similar MF composite reinforced with glass fibers. Using flax instead of glass fibers has a somewhat negative effect on tensile performance. However, the difference is relatively small, and if density and material cost are taken into account, flax fibers become competitive. Tensile damage is quantified from the stiffness reduction during cyclic straining. Compared to glass fibers, flax fibers generate a material with a considerably lower damage rate. From scanning electron microscopy (SEM), it is found that microcracking takes place mainly in the fiber cell walls and not at the fiber-matrix interface. This suggests that the fiber-matrix adhesion is high. The materials are also compared using dynamic mechanical thermal analysis (DMTA) and water absorption measurements
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| 5. |
- Hammami, A., et al.
(författare)
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Analysis of the vacuum infusion molding process
- 2000
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 21:1, s. 28-40
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Tidskriftsartikel (refereegranskat)abstract
- The vacuum infusion molding process is becoming increasingly popular for the production of large composite parts. A comprehensive model of the process has not been proposed yet, making its optimization difficult. The flexible nature of the vacuum bag coupled to the varying pressure inside the mold cavity results in a variation of the cavity thickness during the impregnation. A complete simulation model must incorporate this phenomenon. In this paper, a complete analysis of the vacuum infusion molding process is presented. The analysis is not restricted to the theoretical aspects but also reviews the effect of the main processing parameters. The parameters investigated in this paper are thought to be those of most interest for the process, i.e. the compaction of the reinforcement, the permeability, the infusion strategy and the presence of flow enhancement layers. Following the characterization experiments, a 1-D model for the vacuum infusion molding process is presented. This model is derived assuming that an elastic equlibrium holds in the mold cavity during mold filling. Even though good agreement was found between simulation results and experiments, it is concluded that additional work is needed on the numerical model to integrate interesting findings from the experimental part.
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| 6. |
- Nilsson, Greger, et al.
(författare)
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Flow induced fiber orientation in compression molded glass mat thermoplastics
- 2000
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 21:6, s. 1007-1013
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Tidskriftsartikel (refereegranskat)abstract
- This study examines how the mechanical properties in GMT are affected by axisymmetric flow during compression molding. Two types of GMT with different architecture are used, swirled mat and short fiber GMT. Tree different grades are tested for each fiber architecture 20, 30, and 40% fiber content by weight. These are in principle the grades of GMT commercially available today. It is found that the flow reduced the tensile strength by 30 to 50% and the tensile modulus up to 30% in the flow direction. The reduction in mechanical properties, which is mainly caused by flow-induced fiber orientation, is larger at high fiber contents. The study also showed that there is no major difference in behavior between swirled mat and short fiber GMT regarding flow induced fiber orientation.
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| 7. |
- Seldén, Ragnar, et al.
(författare)
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UV aging of poly(propylene)/wood-fiber composites
- 2004
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 25:5, s. 543-553
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Tidskriftsartikel (refereegranskat)abstract
- The effect of accelerated UV-aging on properties of PP/wood-flber composites has been studied. Plates containing 0, 25 and 50 wt% wood-fibers were subjected to accelerated UV-aging in a QUV weatherometer for up to 8 weeks. Stabilization against UV-degradation was made by adding 3% of a UV stabilizer. The results showed that both the unfilled PP and the PP/wood-fiber composites displayed good UV resistance with regard to mechanical properties. The color of the PP/wood-fiber composite plates changed from brown to chalky white at the exposed area because of the formation of a thin, strongly degraded surface layer. Physical and chemical analysis of these surface layers using differential scanning calorimetry, Fourier transform infrared spectroscopy, and scanning electron microscopy showed that degradation of the PP matrix had occurred, leading to chemicrystallization and extensive surface cracking. A decrease in PP melting temperature was also noted, due to molecular chain scission and the formation of extraneous groups, such as carbonyls and hydroperoxides. These results indicate that the wood-fibers act as chromophores when incorporated in the PP matrix. Increasing the fiber content from 25 to 50 wt% increases the rate of degradation of the PP matrix by approximately a factor of two. © 2004 Society of Plastics Engineers.
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| 8. |
- Vahlund, C. Fredrik, et al.
(författare)
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Analysis of an image processing method for fiber orientation in polymer composites
- 2001
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 22:3, s. 327-336
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Tidskriftsartikel (refereegranskat)abstract
- The errors in fiber orientation distribution measurements of compression molding materials have been investigated, for generated as well as for real distributions. Because of the size of the sample, only a finite number of fibers are visible in a picture leading to a statistical error in the measurement. A method is proposed to express this error as function of the number of visible fibers and the number of subintervals the distribution is divided into. Studies of the error In a Fraunhofer/FT based fiber orientation distribution analysis have been performed, including effects of increasing number of fibers (fiber-fiber intersections) in generated pictures and investigation of the errors in real fiber images. All steps in a fiber orientation distribution analysis of SMC/GMT kind of material are described, including suitable equipment, image enhancement methods and investigation of the errors present in analysis of a real image as well as comparisons to hand-measured data.
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| 9. |
- Åkermo, Malin, et al.
(författare)
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Modeling compression molding of all-thermoplastic honeycomb core sandwich components. Part A : Model development
- 2000
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 21:2, s. 245-256
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Tidskriftsartikel (refereegranskat)abstract
- The compression molding process is studied with the aim of modeling the instantaneous degree of face-core bonding in all-thermoplastic sandwich components during molding. The theory of bonding is briefly discussed and it is concluded that for most thermoplastic materials, processing is performed at temperatures where full bond strength is seemingly immediately established at positions in full face-core contact. A two-dimensional model is developed to predict increase in contact area due to flow of melted core material during molding. Further, heat transfer during processing is modeled in order to determine the extent of melted core material. The two models are coupled into one process model and a numerical example is presented illustrating the predicted behavior of polypropylene-based sandwich components in compression molding. The process model suggests that the face-core bond strength may be significantly increased through flow of the melted core wall.
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| 10. |
- Åkermo, Malin, et al.
(författare)
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Modeling compression molding of all-thermoplastic honeycomb core sandwich components. Part B : Model verification
- 2000
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 21:2, s. 257-267
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Tidskriftsartikel (refereegranskat)abstract
- The compression molding process for manufacturing of all-thermoplastic honeycomb core sandwich components is studied with the aim of verifying a previously proposed model describing the instantaneous face-core bonding during molding. Experiments are performed with glass/polypropylene (PP) faces and PP honeycomb core and experimental data are compared to model predictions in terms of temperature and degree of face-core interfacial contact with reasonable good agreement. Model predictions show that the face-core interface bond strength may increase with up to 200% when molding at an interface temperature slightly above the crystal melting temperature of the matrix for only a few seconds. The influence of the major process parameters, including molding temperature, molding pressure and core viscosity, on the predicted bond strength is investigated using the model.
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