| 1. |
- Akbarpour, Sahar, et al.
(författare)
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Reinforcement around holes in composite materials by use of patched metal inserts
- 2019
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Ingår i: Composite structures. - : Elsevier. - 0263-8223 .- 1879-1085. ; 225
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Tidskriftsartikel (refereegranskat)abstract
- Metal inserts are sometimes used to improve the load carrying capacity of bolted joints in composite materials. In this paper a new concept is introduced where inserts are built during composite manufacturing by integrating stacked metal patches at locations where holes are to be made after consolidation. Initial tests and a parameter study enable more informed design, and specimens with improved stacked inserts are then produced and tested. The specimens with inserts show up to 60% strength improvement in pin-loaded tests. In addition to the experimental work, finite element analysis is performed to investigate the stress fields and the failure mechanisms. The model indicates that the singular stresses at the multi-material corner points are governing for the strength and give indications of the failure mechanisms. Some basic analytical estimates are also presented.
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| 2. |
- Ekermann, Tomas, et al.
(författare)
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Mechanical Characterisation Of Composites With 3D-Woven Reinforcement
- 2015
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Ingår i: 20Th International Conference On Composite Materials. - : AALBORG UNIV PRESS.
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Konferensbidrag (refereegranskat)abstract
- Flat specimens made of carbon/epoxy composite material are manufactured by resin transfer moulding, using 3D-woven carbon fibre preforms with a grid of warp yarns interlaced with both horizontal and vertical wefts. The aim of the study is to bring more light to the coupling between the mechanical properties of the composite material and the internal fibre architecture of its 3D-woven reinforcement. Factors that are varied in the fibre architecture are the amount of fibres in the through-thickness reinforcement (vertical weft) and the warp's wavelength. Tensile, compressive, in-plane and out-of-plane shear and peel tests are performed for the mechanical characterisation. Tensile and compressive properties are found to decrease when the crimp of the warp yarns is increased, and even more so in compression. The in-plane shear strength is evaluated through use of a new test specimen, designed for the purpose. Results show that the strength is higher when the shear load is applied across the warp than across the weft, where the difference is attributed to varying fibre content in the two in-plane directions. The out-of-plane shear properties are compared through short beam shear tests and the inter-laminar shear strength (ILSS) is determined. It is shown that the ILSS increases with increasing yarn thickness in the vertical weft, which is intuitive. The peel strength is evaluated by the opening mode I interlaminar fracture toughness (G(Ic)) through double cantilever beam tests. It is shown that G(Ic) is greatly dependent on the amount of reinforcement in the vertical weft.
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| 3. |
- Ekermann, Tomas, et al.
(författare)
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Pull-off tests of CFRP T-joints with conventional and 3D reinforced fillets
- 2019
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Ingår i: Composite structures. - : ELSEVIER SCI LTD. - 0263-8223 .- 1879-1085. ; 223
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Tidskriftsartikel (refereegranskat)abstract
- A study of T-joints made of CFRP prepreg is presented where the joints contain either conventional uni-directional (UD) fillets or fillets with three-dimensional (3D) woven reinforcement in the joint cavity. Both pristine and impacted specimens are tested experimentally in a pull-off load case. The T-joints with UD fillets are stronger but also show greater spread in strength than T-joints with 3D fillets. The higher strength is attributed to the UD fillets' ability to deform transversely to their length direction and efficiently adapt to the T-joint cavity before curing. The 3D fillets do not admit the same level of transverse shape adaptability and if their cross sections do not fit the geometry of the T-joint cavity sufficiently well, local stress concentrations could emerge that reduce the strength of the T-joint. The UD fillets on the other hand are believed to be sensitive to manufacturing flaws causing the greater spread in strength. That in turn is attributed to a lack of crack-arresting capability in the UD fillet. The 3D fillets however have excellent crack-arresting properties due to their multidirectional fibre architecture. With a few exceptions the impact damages did not significantly affect the strength of the T-joints tested in this study.
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| 4. |
- Hallström, Stefan, 1968-, et al.
(författare)
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Architecture and properties of stochastic foam models
- 2015
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Ingår i: 20th International Conference on Composite Materials.
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Konferensbidrag (refereegranskat)abstract
- Recent results from numerical analysis of the microstructure and constitutive properties of foams are presented and relations between these properties and various topological differences in the cellular architecture are illustrated and discussed. The focus is on closed cell foams but parts of the work concern effects of distribution of solid between the cell walls and edges, which then also include open cell foams.The study is based on a modelling scheme where Voronoi partitioning is conducted, based on seed point distributions coming from random packings of hard spheres. The models are then brought to surface energy equilibrium through use of the Surface Evolver software. Sphere packing, partitioning of space and relaxation of the cellular models are conducted on periodic representative volume elements (RVE), typically containing in the order of 100 cells. Finite element models of the foam architecture is then performed with various sets of periodic boundary conditions in order to study the model topology, convergence rates and extract representative constitutive properties for the studied cases.The results are compared with other simulation results from the literature, experimental data and measurements made from SEM and CT micrographs. Some differences between evaluations of 2D and 3D representations of the foam materials are also illustrated and discussed.Aspects of the generation of the models are illustrated, indicating that some model results are virtually unaffected by most of the model details while other results depend strongly on the input parameters and differences between modelling techniques.Some references are also made to classical foam property estimates coming from dimension analysis of idealised foam structures where the relatively simplistic models in many cases provide surprisingly good estimates of the mechanical properties of the foam materials. There are however also some significant differences that are highlighted and explained in the presentation.
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| 5. |
- Hörberg, Erik, 1977-, et al.
(författare)
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Thickness effect on spring-in of prepreg composite L-profiles – An experimental study
- 2019
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Ingår i: Composite structures. - : Elsevier. - 0263-8223 .- 1879-1085. ; 209, s. 499-507
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the results and analysis of an experimental study of laminate thickness effects on the springin and shape distortion of thermoset composite L profiles. The primary objective is to achieve a broader understanding of how shape distortion is affected by laminate bending stiffness and part thickness of L-shaped laminates whose thickness varies between 1 and 12 mm. The larger thicknesses in particular have not received much attention in previous research. This work further aims at distinguishing the pure (geometrical) thickness effect from that of the coupled laminate bending stiffness by comparing laminates with different lay-ups. The work is performed on test specimens subjected to both a standard cure cycle and post-cure heat treatment at elevated temperatures. In parallel, finite element (FE) analysis is performed to evaluate if variation in the bending stiffness or the laminate thickness affects the predicted spring-in angle. The results clearly show springin dependence on laminate thickness and bending stiffness, whereas this dependence is not well predicted by the FE approaches. It is concluded that both effects exist and that shape distortions are more strongly related to bending stiffness than to laminate thickness.
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| 6. |
- Khokar, N., et al.
(författare)
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Increasing energy absorption and reliability of beams by improved architecture and web-flange junctions
- 2019
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Ingår i: Advanced Materials for Defense. - Baech : Trans Tech Publications Ltd. - 9783035713664 - 9783035733662 ; , s. 114-119
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Konferensbidrag (refereegranskat)abstract
- Lightweight and strong composite material beams are increasingly sought to quickly, easily, and cost-effectively transport and setup a variety of constructions such as bridges, cabins/stores/shelters, vehicles etc. For structural beams produced as conventional laminated composite materials, their weak areas tend to occur at intersections such as web-flange junctions due to absence of fibres bridging the interconnections. This drawback can however be overcome with development of profiled 3D textile reinforcements having combination architectures and constituent web-flange parts inherently mutually interconnected through fibre interlacement. In addition to general strength improvement, beams containing such novel reinforcement architectures also show increased energy absorption capability due to the mutual web-flange integration at the junctions. An 'I' and a 'flanged-triangle' cross-section beams were produced by a novel non-conventional weaving method, using carbon fibres as reinforcement, and their energy absorption capabilities were tested. These beams respectively absorbed over 50% and 300% more energy per weight in bending, compared to metal counterparts. This paper presents some relevant aspects of these innovative beams.
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| 7. |
- Khokar, N., et al.
(författare)
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Novel 3D Preform Architecture For Performance And Reliability Of Structural Beams
- 2015
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Ingår i: 20Th International Conference On Composite Materials. - : AALBORG UNIV PRESS.
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Konferensbidrag (refereegranskat)abstract
- Beams are among the most frequently used structural members in constructions. Presently composite beams are not available wherein the web(s) have integrated fibre assemblies oriented in bias (+/-theta degrees) directions relative to the beam's length direction, the flange/s have integrated fibre assemblies oriented in the beam's length and width (0 degrees/90 degrees) directions, and the web(s) - flange(s) have mutual through-thickness integrity at their junctions. Consequently, the competitive potential of composite beams is not fully utilised with respect to conventional counterparts built from isotropic materials. To overcome this shortcoming and bring composite beams to a new level of competitiveness, an innovative Add-on Weaving technique has been specifically developed recently. This Paper introduces the novel woven preform architecture and presents the initial results demonstrating the performance of new composite I-beams developed for the purpose. This advancement reflects maturing of the new weaving technique and its 3D fabric reinforcements and also supports its transit from lab to industry.
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| 8. |
- Köll, Joonas, et al.
(författare)
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Elastic properties of equlibrium foams
- 2016
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 113, s. 11-18
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Tidskriftsartikel (refereegranskat)abstract
- Stochastic equilibrium finite element (FE) foam models are used to study the influence of relative density and distribution of solid material between cell walls and edges on the elastic properties of foam materials. It is first established that the models contain a sufficient number of cells to ascertain isotropy and numerically and statistically robust results. It is then found that the elastic moduli are very weakly coupled to cell size variation in the models, when the latter is varied systematically. The influence from relative density and distribution of solid on the elastic parameters is considerably stronger. Analytical estimates from the literature, based on idealized cell models and dimension analysis, are matched by fitting coefficients to the FE results, providing good qualitative but relatively poor quantitative correlation. An expansion of the analytical coupling functions is then suggested in order to reduce their level of idealization. The expanded formulation shows virtually perfect agreement with the numerical results for almost the whole range of relative densities and distributions of solid in the FE parameter study. The presented analytical expression is believed to be general and provide accurate estimates of the elastic properties of a wide range of foam materials, provided that their bulk material properties and micro structure can be established. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 9. |
- Köll, Joonas, et al.
(författare)
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Influence from polydispersity on the morphology of Voronoi and equilibrium foams
- 2017
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Ingår i: Journal of cellular plastics (Print). - : Sage Publications. - 0021-955X .- 1530-7999. ; 53:2, s. 199-214
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Tidskriftsartikel (refereegranskat)abstract
- Stochastic foam models are generated from Voronoi spatial partitioning, using the centers of equi-sized hard spheres in random periodic distributions as seed points. Models with different levels of polydispersity are generated by varying the packing of the spheres. Subsequent relaxation is then performed with the Surface Evolver software which minimizes the surface area for better resemblance with real foam structures. The polydispersity of the Voronoi precursors is conserved when the models are converted into equilibrium models. The relation between the sphere packing fraction and the resulting degree of volumetric polydispersity is examined and the relations between the polydispersity and a number of associated morphology parameters are then investigated for both the Voronoi and the equilibrium models. Comparisons with data from real foams in the literature indicate that the used method is somewhat limited in terms of spread in cell volume but it provides a very controlled way of varying the foam morphology while keeping it periodic and truly stochastic. The study shows several strikingly consistent relations between the spread in cell volume and other geometric parameters, considering the stochastic nature of the models.
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| 10. |
- Oddy, Carolyn, 1994, et al.
(författare)
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Evaluation of damage initiation models for 3D-woven fibre composites
- 2019
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Ingår i: ECCM 2018 - 18th European Conference on Composite Materials. - : Applied Mechanics Laboratory.
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Konferensbidrag (refereegranskat)abstract
- Three dimensional (3D) fibre-reinforced composites have shown weight efficient strength and stiffness characteristics as well as promising energy absorption capabilities. In the considered class of 3D-reinforcement, vertical and horizontal weft yarns interlace warp yarns. The through-thickness reinforcements suppress delamination and allow for stable and progressive damage growth in a quasi-ductile manner. With the ultimate goal of developing a homogenised computational model to predict how the material will deform and eventually fail under loading, this work proposes candidates for failure initiation criteria. The criteria are evaluated numerically for tensile, compressive and shear tests. The extension of the LaRC05 stress based failure criteria to this class of 3D-woven composites is one possibility. This however, presents a number of challenges which are discussed. These challenges are related to the relative high stiffness in all directions, which produce excessively high shear components when projected onto potential off-axis failure planes. To circumvent these challenges, strain based criteria inspired by LaRC05 are formulated. Results show that strain based failure predictions for the simulated load cases are qualitatively more reasonable.
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