| 1. |
- Saseendran, Sasha, et al.
(författare)
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Characterisation of viscoelastic material properties during curing processes
- 2015
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Ingår i: ICCM International Conferences on Composite Materials. - : International Committee on Composite Materials. ; 2015-July
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Konferensbidrag (refereegranskat)abstract
- The present contribution is toward systematic characterisation of the thermo-viscoelastic properties of a curing epoxy resin system, here we use Huntsman LY5052/HY5052 as model material. The main focus is to verify the existence of equivalence relation (linearity) between time, temperature and degree (time) of cure. As a starting point, the cure kinetics behaviour of the model material has been characterised using DSC equipment and the results have been used to identify parameters in a generic Kamal cure kinetics model. In the subsequent work the DSC data and Kamal model was used to carefully monitor the degree of cure in the resin in all the subsequent experiments. The thermo-viscoelastic response of the curing epoxy was characterised using a dynamic mechanical thermal analyser (DMTA). All the DMTA experiments were focused on rubbery and glassy states only, were the material was subject to tests at various isothermal and non-isothermal loading conditions. The results were used to investigate the linearity between the three factors above (time, temperature and curing time). To summarize, the results indicate that these three parameters indeed obey a linear relationship in the linear viscoelastic regime.
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| 2. |
- Asp, Leif, et al.
(författare)
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Multiscale modelling of non-crimp fabric composites
- 2012
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Ingår i: Proceedings of the ASME International Mechanical Engineering Congress and Exposition--2012. - New York : American Society of Mechanical Engineers. - 9780791845196 ; , s. 581-590
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Konferensbidrag (refereegranskat)abstract
- Damage initiation and evolution in NCF composites leading to final failure includes a multitude of mechanisms and phenomena on several length scales. From an engineering point-of-view a computational scheme where all mechanisms would be explicitly addressed is too complex and time consuming. Hence, methods for macroscopic performance prediction of NCF composites, with limited input regarding micro- And mesoscale details, are requested. In this paper, multi-scale modelling approaches for in-plane transverse strength of NCF composites are outlined and discussed. In addition a simplistic method to predict transverse tensile and compressive strength for textile composites featuring low or no fibre waviness is presented.
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| 3. |
- Edgren, FRedrik, et al.
(författare)
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Formation of damage and its effects on non-crimp fabric reinforced composites loaded in tension
- 2004
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 64:5, s. 675-692
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Tidskriftsartikel (refereegranskat)abstract
- Non-crimp fabric (NCF) composites, manufactured by resin infusion techniques are one of the most promising next generation composite materials. They offer large potential for application in primary structures as they give excellent performance at low production costs. However, before NCF composites will be efficiently used in design, detailed understanding of governing micro mechanisms must be accumulated and described by predictive models. In the present study, NCF cross-ply laminates have been tested in tension. Intralaminar cracks caused in the 90° fibre bundle layers and their effect on laminate mechanical properties have been monitored. Occurrence of 'novel' type of cracks propagating in the load direction (longitudinal cracks) is explained by a thorough FE analysis using an Representative Volume Element (RVE) approach, revealing stress concentrations caused by 0° fibre bundle waviness. Effects of damage on mechanical properties are modelled using modified micro mechanical models developed for analysis of conventional laminated composites. The analysis reveals mechanical degradation to be ruled by the crack opening displacement (COD). However, unlike traditional composites, transverse cracks do not generally extend through the entire thickness of the 90° layer, but are rather contained in single fibre bundles, limiting the COD. © 2003 Elsevier Ltd. All rights reserved.
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| 4. |
- Giannadakis, Konstantinos, et al.
(författare)
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Mechanical Performance of a Recycled Carbon Fibre/PP Composite
- 2011
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 51:5, s. 767-777
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Tidskriftsartikel (refereegranskat)abstract
- A composite made of recycled carbon fibres in recycled polypropylene matrix is studied experimentally to describe the features of the elastic and time dependent nonlinear mechanical behaviour. The properties of the developed material have a large variability to be addressed and understood. It was found that the stress-strain curves in tension are rather nonlinear at low strain rate and the strength is sensitive to strain rate. The elastic properties' reduction for this composite after loading to high strains is rather limited. More important is that even in the "elastic region" due to viscoelastic effects the slope of loading-unloading curve is not the same and that at higher stress large viscoplastic strains develop and creep rupture is typical. The time and stress dependence of viscoplastic strains was analysed and described theoretically. The viscoelastic response of the composite was analysed using creep compliance, which was found to be slightly nonlinear. © 2010 Society for Experimental Mechanics.
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| 5. |
- Katerelos, D.G., et al.
(författare)
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Raman spectroscopy investigation of stiffness change and residual strains due to matrix cracking
- 2006
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Ingår i: Mechanics of composite materials. - : Springer Science and Business Media LLC. - 0191-5665 .- 1573-8922. ; 42:6, s. 535-546
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Tidskriftsartikel (refereegranskat)abstract
- The matrix cracking models developed for cross-ply composite laminates have been poorly extended in the past to more complex geometries used in practice, and they are still under development. In this paper, a new detailed analysis of the effect of matrix cracking on the behaviour of cross-ply and [0/45] s laminates under uniaxial tension is attempted. The model used in this work is applicable both to cross-ply laminates and unbalanced systems. It gives exact closed-form expressions for all thermomechanical properties of a general symmetric laminate with cracks in arbitrary layers. The theoretical approach is backed by experimental data obtained by microscopic strain-state variation measurements within a specimen, with using the technique of laser Raman spectroscopy. Glass-fibre-reinforced epoxy systems were investigated. Embedded aramid fibres-sensors within the 0° ply and near the 0°/θ ° interface were necessary due to the poor Raman signal of glass. Using experimental Raman data, the residual strain and the stiffness reduction are determined as functions of increase in crack density. The stiffness reduction is predicted with a high accuracy, whereas the measured residual strains are larger than predicted. The good results for the reduction in the elastic modulus show that the basic assumption of the model is accurate. The difference is explained by the viscoelastic-viscoplastic behaviour of the off-axis layer in shear, which in creases the "apparent" residual strain. © Springer Science+Business Media, Inc. 2006.
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| 6. |
- Marklund, Erik, et al.
(författare)
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Micromechanical modelling of wood fibre composites
- 2009
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Ingår i: Plastics, rubber and composites. - 1465-8011 .- 1743-2898. ; 38:2, s. 118-123
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Tidskriftsartikel (refereegranskat)abstract
- A concentric cylinder model for an N-phase composite with orthotropic properties of constituents was previously presented by the authors. With only minor modifications the model allows for including also free hygroexpansion terms in the elastic stress-strain relationship in order to deal with orthotropic phase swelling. Thus the effect of wood fibre ultrastructure and cell wall hygroelastic properties on wood fibre composite hygroexpansion may be analysed. Multiscale modelling was performed to calculate the hygroexpansion coefficients of both the fibre cell wall and the aligned wood fibre composite. Furthermore, the fibre's helical structure leads to an extension-twist coupling and thus a free fibre will deform axially and also rotate upon loading in longitudinal fibre direction. Within the composite, however, the fibre rotation will be restricted. Therefore, the decision was to compare the composite performance in the two extreme cases (i) free rotation (ii) no rotation of the fibre in the composite.
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| 7. |
- Marklund, Erik, et al.
(författare)
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Modeling the effect of helical fiber structure on wood fiber composite elastic properties
- 2009
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Ingår i: Applied Composite Materials. - : Springer Science and Business Media LLC. - 0929-189X .- 1573-4897. ; 16:4, s. 245-262
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Tidskriftsartikel (refereegranskat)abstract
- The effect of the helical wood fiber structure on in-plane composite properties has been analyzed. The used analytical concentric cylinder model is valid for an arbitrary number of phases with monoclinic material properties in a global coordinate system. The wood fiber was modeled as a three concentric cylinder assembly with lumen in the middle followed by the S3, S2 and S1 layers. Due to its helical structure the fiber tends to rotate upon loading in axial direction. In most studies on the mechanical behavior of wood fiber composites this extension-twist coupling is overlooked since it is assumed that the fiber will be restricted from rotation within the composite. Therefore, two extreme cases, first modeling fiber then modeling composite were examined: (i) free rotation and (ii) no rotation of the cylinder assembly. It was found that longitudinal fiber modulus depending on the microfibril angle in S2 layer is very sensitive with respect to restrictions for fiber rotation. In-plane Poisson’s ratio was also shown to be greatly influenced. The results were compared to a model representing the fiber by its cell wall and using classical laminate theory to model the fiber. It was found that longitudinal fiber modulus correlates quite well with results obtained with the concentric cylinder model, whereas Poisson’s ratio gave unsatisfactory matching. Finally using typical thermoset resin properties the longitudinal modulus and Poisson’s ratio of an aligned softwood fiber composite with varying fiber content were calculated for various microfibril angles in the S2 layer.
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| 8. |
- Marklund, Erik, et al.
(författare)
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Modeling the hygroexpansion of aligned wood fiber composites
- 2009
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Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 69:7-8, s. 1108-1114
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Tidskriftsartikel (refereegranskat)abstract
- The effect of wood fiber ultrastructure and cell wall hygroelastic properties on wood fiber composite hygroexpansion has been analyzed. An analytical concentric cylinder model extended to include also free hygroexpansion of orthotropic phase materials has been used on several length scales. Using properties of the three main wood polymers, cellulose, hemicellulose and lignin the longitudinal and transverse hygroexpansion coefficients for the microfibril unit cell were obtained and the volume fraction change of the wood polymers in the microfibril unit cell depending on relative humidity was calculated. The fiber cell wall was modeled regarding each individual S1, S2 and S3 layer and the cell wall longitudinal hygroexpansion coefficient was determined depending on microfibril angle in the S2 layer. A homogenization procedure replacing the S1, S2 and S3 layers with one single layer was found not to influence the results significantly for low microfibril angles. Finally the hygroexpansion coefficients of an aligned softwood fiber composite were calculated.
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| 9. |
- Marklund, Erik, et al.
(författare)
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Nonlinear viscoelastic viscoplastic material model including stiffness degradation for hemp/lignin composites
- 2008
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Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 68:9, s. 2156-2162
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Tidskriftsartikel (refereegranskat)abstract
- In repeating tensile tests with increasing maximum strain for every loading cycle the hemp/lignin composites clearly showed a nonlinear behavior and hysteresis loops in loading and unloading. The explanation for this behavior is the inherent viscoelastic nature for this type of material, but also noticeable stiffness degradation with increasing strain level. Creep tests performed at different stress levels revealed a nonlinear viscoelastic response and after recovery viscoplastic strain was detected for high stress levels. It is demonstrated that Schapery's model is suitable to model nonlinear viscoelasticity whereas viscoplastic strain may be described by a nonlinear functional presented by Zapas and Crissman. In a creep test this functional leads to a power law with respect to time and stress. In order to include stiffness reduction due to damage Schapery's model has been modified by incorporating a maximum strain-state dependent function reflecting the elastic modulus reduction with increasing strain measured in tensile tests. A generalized incremental model of the constitutive equation for viscoelastic case has been used to validate the developed material model in a linear stress controlled loading and unloading ramp. The model successfully describes the main features for the investigated material and shows good agreement with test data within the considered stress range. © 2008 Elsevier Ltd. All rights reserved.
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| 10. |
- Marklund, Erik, et al.
(författare)
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Stiffness and strength modelling of non-crimp fabric composites
- 2011
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Ingår i: 52nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference 2011. - Red Hook : Curran Associates, Inc.. - 9781618390424 ; , s. 679-695
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Konferensbidrag (refereegranskat)abstract
- This work comprises methodologies for micro-meso stiffness modelling and limited analysis for strength prediction. It is intended to formulate some guidelines and recommendations when modelling NCF composites mechanical performance. For both micro- and meso stiffness modelling, analytical models are compared to FE investigations considering idealised structures. Important aspects when modelling matrix failure of NCF composite bundles are presented to highlight some of the challenges in future modelling. Possible failure criteria for modelling matrix failure within fibre bundles have been investigated; their strength, problems and weaknesses are revealed.
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