| 1. |
- Asp, Leif, et al.
(författare)
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An experimental investigation of the influence of delamination growth on the residual strength of impacted laminates
- 2001
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 32:9, s. 1229-1235
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Tidskriftsartikel (refereegranskat)abstract
- This study concerns an experimental investigation to establish data for validation of residual strength models for impacted composite panels. The work focuses on compression tests of panels with embedded artificial delaminations at various depths. Accompanying tests on undamaged and impact-damaged panels are reported and the relevance of the tests on artificially delaminated panels is assessed. In the experiments both the artificially delaminated and the impacted plates failed by delamination growth. Consequently, the same mechanism governed failure in the two cases. Hence, the artificially delaminated plate test is reliable for validation of methods developed for analysis of the residual strength of impact-damaged panels. However, for impacted plates, the load at global plate buckling was consistently 10% lower than that of the artificially delaminated plates and 20% lower than that of the undamaged plates. Hence, conservative prediction of the global buckling load of an impacted composite panel require s methods that consider influence of stiffness reduction of the damaged zone. © 2001 Elsevier Science Ltd. All rights reserved.
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| 2. |
- Dweib, M.A., et al.
(författare)
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Fibre structure and anisotropy of glass reinforced thermoplastics
- 2000
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Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier Science Ltd, Exeter, United Kingdom. - 1359-835X .- 1878-5840. ; 31:3, s. 235-244
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Tidskriftsartikel (refereegranskat)abstract
- The fibre structure and orientation distribution of two commercially available glass mat thermoplastics reinforced by continuous glass fibre was studied to investigate the anisotropic behaviour under compression moulding and mechanical loading, and to investigate the influence of the fibre structure and orientation on the anisotropic behaviour. Circular samples were deformed into ellipses when moulded, due to the anisotropic fibre orientation. The fibre content and orientation were examined in different locations of the elliptically deformed specimens. X-ray pictures were taken of the material in order to develop images of the fibres, before and after compression moulding. In another procedure, the matrix was burned off and the fibre network structures were studied in each case. A CCD camera was used to scan the fibres as digital images to measure the orientation distribution functions of the fibres. The fibre orientation measuring process was facilitated by subroutines implemented in the source code of the public domain NIH-image analysis software using simulated Fraunhofer diffraction.
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| 3. |
- Fernberg, S. P., et al.
(författare)
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Effects of glass fiber size composition (film-former type) on transverse cracking in cross-ply laminates
- 2000
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 31:10, s. 1083-1090
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Tidskriftsartikel (refereegranskat)abstract
- Transverse cracking is an important phenomenon in the context of fluid leakage in pipes and pressure vessels. Multiple transverse cracking in [0/90](S) glass-fiber-reinforced vinylester and epoxy laminates with six different fiber surface treatments (size) is examined. Film-former composition is the variable since this component can be easily changed also in commercial size formulations. The influence of the film-former polymer on transverse cracking is significant in epoxy laminates and very strong in vinylester laminates. Both onset of transverse cracking and slope of crack density vs, strain are influenced. Remarkably low crack densities were observed in some vinylester laminates. Micromechanisms of cracking are interpreted. Correlation is established between transverse cracking behavior and interfacial shear strength measured by single fragmentation tests. The strong film-former effect is proposed to be due to a combination of improved interfacial adhesion and the plasticizing effect from the film-former on the interphase region.
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| 4. |
- Gamstedt, E. Kristofer, et al.
(författare)
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Synthesis of unsaturated polyesters for improved interfacial strength in carbon fibre composites
- 2002
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 33:9, s. 1239-1252
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Tidskriftsartikel (refereegranskat)abstract
- Carbon fibres are gaining use as reinforcement in glass fibre/polyester composites for increased stiffness as a hybrid composite. The mechanics and chemistry of the carbon fibre-polyester interface should be addressed to achieve an improvement also in fatigue performance and off-axis strength. To make better use of the versatility of unsaturated polyesters in a carbon fibre composite, a set of unsaturated polyester resins have been synthesized with different ratios of maleic anhydride, o-phthalic anhydride and 1,2-propylene glycol as precursors. The effective interfacial strength was determined by micro-Raman spectroscopy of a single-fibre composite tested in tension. The interfacial shear strength with untreated carbon fibres increased with increasing degree of unsaturation of the polyester, which is controlled by the relative amount of maleic anhydride. This can be explained by a contribution of chemical bonding of the double bonds in the polymer to the functional groups of the carbon fibre surface.
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| 5. |
- Holmberg, J.A., et al.
(författare)
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Manufacturing and performance of RTM U-beams
- 1997
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 28:6, s. 513-521
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Tidskriftsartikel (refereegranskat)abstract
- Geometrical limitations and robustness of resin transfer moulding were investigated for single curved laminates. The design of experiments approach was used to determine how the out of plane tensile strength is affected by variations in performing method, radius, fibre content and vacuum assistance. U-beams with an inner radius of 0.8 mm were manufactured and demonstrated good mechanical performance. The strength of these beams was in fact less sensitive to defects than the strength of beams with a 5 mm radius. Different preforming methods can have a strong influence on beam strength due to effects on the void content in the finished laminates. The particular preforming problems encountered are described and possible solutions are suggested and tested. For example utilisation of vacuum assistance during mould filling reduces void content, which in turn tends to reduce the sensitivity of the beam strength to other defects such as wrinkling.
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| 6. |
- Holmberg, J.A., et al.
(författare)
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Micromechanisms of delamination failure in RTM U-beams
- 1997
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 28:8, s. 709-717
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Tidskriftsartikel (refereegranskat)abstract
- In a previous study, resin transfer moulded U-beams were subjected to opening tensile loads applied to the flanges. The failure behaviour was sensitive to the void content of the material. In the present study, we use optical microscopy of polished edges to observe mechanisms of failure in-situ as delaminations develop in the beam radius. Descriptions of these failure mechanisms are presented. Voids were found to facilitate the growth phase of the delamination process in the beam radius. The mechanisms described will be used in a forthcoming stress analysis to explain the experimentally observed failure behaviour of beams with different radii and void contents.
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| 7. |
- Joffe, Roberts, et al.
(författare)
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Strength and adhesion characteristics of elementary flax fibres with different surface treatments
- 2003
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 34:7, s. 603-612
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Tidskriftsartikel (refereegranskat)abstract
- It is known that the best flax fibres can compete in terms of mechanical properties with glass fibres. However, during the manufacturing process flax fibres are often damaged, and hence, the properties can be lowered. Furthermore, these properties change from batch to batch (depending on the time and place of harvest), which means that they are somewhat unpredictable. The most affected fibre property is strength, which can vary in very wide interval due to defects introduced by the manufacturing process. Therefore, there is a need for a simple but reliable testing procedure that allows the estimation of the strength of flax fibres, so called quality control. Regarding the final goal, that is the development of natural fibre composites, another crucial property is the fibre/matrix adhesion. The objective of this study is to investigate the possibility to use the single fibre fragmentation test to characterize strength distribution of flax fibres and to evaluate the adhesion. Untreated flax fibres and fibres coated by a special surface treatment are used. Fragmentation tests are performed on flax fibres embedded in thermoset, vinylester and polyester, resins. Results show that there is a definite improvement in interfacial strength when a fibre surface treatment is applied. Fibre strength distribution is obtained from SFFT and compared with limited results available from single flax fibre tests.
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| 8. |
- Lundström, Staffan, et al.
(författare)
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In-plane permeability measurements : a Nordic round-robin study
- 2000
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 31:1, s. 29-43
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Tidskriftsartikel (refereegranskat)abstract
- Three methods to measure the in-plane permeability are compared. The methods are based on the wetting and saturated parallel flow techniques and the wetting radial flow technique. It is found that the stability of all three methods is very good. The best repeatability in the results is obtained with the wetting parallel flow technique. A main drawback of this method is, however, that the samples are not saturated hence, the saturated parallel flow method is suggested for continued evaluation. The standard deviation from repeatable measurements of this method is 14 and 20% for the two fabrics tested. By a round-robin comparison at three laboratories it is found that the reproducibility of the saturated parallel flow technique is in the same range as the repeatability of the method. Furthermore, it is shown that the liquid tested and the pressure applied have very small influence on the results. It is clear that most of the scatter is due to the sample preparation and variations in the material. Accordingly, a continued work should be concentrated on improvements of the sample preparation, and development of methods to reduce the influence, from small errors, in sample preparation on the results.
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| 9. |
- Lundström, Staffan
(författare)
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Measurement of void collapse during resin transfer moulding
- 1997
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 28:3, s. 201-214
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Tidskriftsartikel (refereegranskat)abstract
- This paper considers the dissolution of cylindrical voids trapped between fibres during the resin transfer moulding process. To study the behaviour of the tiny voids, a system is used which consists of a transparent mould loaded with a glass fibre reinforcement. The mould is placed under a microscope and a small area of the preform is monitored during the injection of a vinylester resin. The length of the voids decreases at a relatively fast rate and they disappear in the order of minutes due to diffusion. The experimental results are rationalized through a theory showing how certain parameters influence the rate of dissolution. The investigation shows the significance of degassing the resin before injection. This is especially important when low pressures are utilized during injection.
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| 10. |
- Lundström, Staffan
(författare)
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Permeability of non-crimp stitched fabrics
- 2000
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 31:12, s. 1345-1353
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Tidskriftsartikel (refereegranskat)abstract
- Non-crimp stitched fabrics are becoming increasingly popular as reinforcing materials in high-performance composites due to their high mechanical properties. Hence, it is of interest to investigate the processability of this type of fabrics. Focus is here set on the permeability of the fabrics. The methods of this investigation are: (i) a theoretical analysis; (ii) microscopic studies of the geometry of the fabrics and (iii) measurements of the permeability. From the experiments, it is found that only small variations of the geometry change the permeability up to three times. Furthermore, the simple model proposed works surprisingly well in certain cases, while in others it fails to predict the permeability. From the experimental results and a comparison between the model and the experiments, some guidelines are presented how to design a high-permeable fabric.
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