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  • 1.
    Akbarpour, Sahar
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Reinforcement around holes in composite materials by use of patched metal inserts2019Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 225, artikel-id 111084Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 2.
    Barbier, Christophe
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Larsson, Per-Lennart
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Östlund, Sören
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Numerical investigation of folding of coated papers2005Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 67, nr 4, s. 383-394Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Folding of coated paper is examined numerically using the finite element method. Particular emphasis is put on the behaviour of field variables relevant for cracking of the coating layers. In the numerical analysis, the basepaper is modelled as an anisotropic elastic-plastic material (both elastic and plastic anisotropy is accounted for) while the constitutive behaviour of the coating layers are approximated by classical (Mises) elastoplasticity. The numerical results suggest, among other things, that particular forms of plastic anisotropy can substantially reduce the maximum strain levels in the coating. It is also shown that delamination buckling, in the present circumstances, will have a very small influence on the strain levels in the coating layer subjected to high tensile loading.

  • 3.
    Barbier, Christophe
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Larsson, Per-Lennart
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Östlund, Sören
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    On dynamic effects at folding of coated papers2005Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 67, nr 4, s. 395-402Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Folding of coated papers is examined numerically using the finite element method. The analysis is focused on the influence from dynamic effects on the folding process. In particular, the behaviour of field variables relevant for cracking of the coating layers are studied in some detail. The results presented indicate that dynamic effects are of little importance as regards maximum strain levels in the coating but will influence the stress and strain distributions. Accordingly, a quasi-static analysis of the problem will be sufficient in order to describe many of the important features related to cracking.

  • 4.
    Barbier, Christophe
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Larsson, Per-Lennart
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Östlund, Sören
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    On the effect of high anisotropy at folding of coated papers2006Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 72, nr 3, s. 330-338Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A finite element procedure, developed in order to account for the effect of high anisotropy at folding of coated papers, is presented. The anisotropic behaviour (with very low stiffness in the thickness direction) is modelled using stiff structural elements (trusses and beams). The numerical results indicate that high elastic anisotropy leads to lower strain levels at folding than reported in previous analyses where this effect was not accounted for. High plastic anisotropy, on the other hand, will contradict this result.

  • 5. Blanco, N.
    et al.
    Gamstedt, E. Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Costa, J.
    Trias, D.
    Analysis of the mixed-mode end load split delamination test2006Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 76, nr 1-2, s. 14-20Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Composite delaminations are commonly characterized using the double cantilever beam test for mode 1, the end-notched flexure test or the end load split test for mode 11 and the mixed-mode bending test for mixed-mode. For all these tests, the mode mix remains constant and does not vary with the crack length. However, in the mixed-mode end load split test (MMELS), the delamination propagates under a varying mode mix that depends on the crack extension, which is a more realistic scenario. The MMELS test has been previously analysed by different researchers but the resulting expressions are not equivalent. A more accurate alternative analysis of the test, based on the finite element method and the virtual crack closure technique, is used in the present work for comparison. The results are compared to the predictions of approaches presented in the literature and significant findings are found for materials characterization using the MMELS test.

  • 6.
    Burman, Magnus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Colombo, C.
    Vergani, L.
    Static and fatigue characterisation of new basalt fibre reinforced composites2012Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 94, nr 3, s. 165-1174Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Basalt reinforced composites are recently developed materials. These mineral amorphousfibres are a valid alternative to carbon fibres for their lower cost, and to glass fibres for their strength.In order to use basalt reinforced composites for structural applications, it is necessary to perform amechanical characterization. With this aim in the present work experimental results of several staticand fatigue tests are described. Two polymeric matrices are taken into account, vinylester and epoxy,to assess their influence on the evaluated parameters. In parallel to these mechanical tests, also thethermal answer of the specimens to mechanical loads is evaluated by means of thermography. Thisexperimental technique allows defining the composite local heating during the application ofmechanical loads and its behaviour in details. Final discussion on obtained results is proposedfocussing the attention on basalt fibre composite behaviour, and comparing mechanical properties ofBFRP with other composite materials in glass and carbon fibres.

  • 7.
    Burman, Magnus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Kuttenkeuler, Jakob
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Marina system.
    Comparative Life Cycle Assessment (LCA) of theHull of a High Speed CraftIngår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    A comparative Life Cycle Assessment (LCA) has been performed on the hull of a 24 meter long high speed patrol craft. The aim of the study is to compare different structural materials concepts to determine differences and sensitivities in environmental impact, especially in relation to the total impact including fuel burn. The material concepts studied are aluminium and various composite combinations consisting of glass fibre and carbon fibre with vinyl ester resin both as single skins and as sandwich with a Divinycell foam core. For each material concept a unique and weight optimized structural design was employed fulfilling the DNV high speed craft design code [1]. All identical systems and components for the five hull concepts are omitted in the LCA and hence a comparative study is performed focused on the hull structure. The commercial software SimaPro have been utilized for the LCA calculations and the impact assessment method chosen is the CML Baseline 2000.

    The structural optimization carried out in [1] show that a weight reduction by to up to 50% (on the hull) could be achieved by switching from aluminium to a carbon fibre sandwich concept. The weight reduction switching from glass fibre single skin to a carbon fibre sandwich concept is roughly 20%.

     

    The LCA study performed herein shows that, regardless of hull material concept, the environmental impact is by large dominated by the usage phase due to relatively large fuel consumption. A lower structural weight will reduce the fuel consumption and hence the environmental impact. This is illustrated in fig. 1 for the aluminum hull. All different phases of the life cycle are plotted for all environmental impact categories assessed herein. The green color is related to the operation phase and the red is the manufacturing phase. As observed the operation phase is dominating.

     

    In fig. 2 are the results from the LCA presented for all hull concepts and for all environmental impact factors. The Al-hull is the concept with highest structural weight (red bars) and hence was found to have the highest environmental impact. The carbon fibre sandwich hull (green bars) had the lowest weight of the five and consequently the lowest environmental impact.

    In the normalization analysis performed, in which different impact factors are assessed using a common impact unit, three impact factors stand out as dominating, independent of hull concept; Abiotic depletion, Global warming and Acidification. All these impact factors are herein associated with the usage phase and diesel consumption. If the operation phase is omitted and only the manufacturing phase (including the material extraction/manufacturing) is studied the observation is that the aluminium concept still will have the largest environmental impact for all categories. The most significant environmental impact is now on the marine and the fresh water aquatic ecotoxicity which is associated with the aluminium raw material excavation and manufacturing processes.

     

  • 8. Czichon, S.
    et al.
    Zimmermann, Kristian
    Middendorf, P.
    Structures Engineering, Production and Mechatronics, Germany .
    Vogler, M.
    Rolfes, R.
    Three-dimensional stress and progressive failure analysis of ultra thick laminates and experimental validation2011Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 93, nr 5, s. 1394-1403Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Test methods and analysis capabilities for fibre reinforced composites are generally limited to thin laminates. However, extending the range of application of composite materials to thick laminates is essential for a multitude of possible composite structures. This paper presents an adapted three-point bending test for a new quasi isotropic stacking sequence for non crimped fabrics for the application in ultra thick laminates (UTL). In addition, numerical simulation capabilities for thick laminates using a multiscale analysis are shown. The three-point bending test setup is developed to examine the failure behaviour of 30-60 mm thick coupons. The presented numerical analysis features a ply based mesh, stacked continuum elements as well as a multiscale approach with meso scale unit cells in order to refine initial assumptions for 3D material properties. Initial stress calculations are performed on macro level using material properties from 2D tests. Extending the analysis by a multiscale approach, material properties are generated on meso level using unit cells models. Progressive failure is subsequently modelled on macro level, using the previously obtained material properties and the Juhasz failure criterion. The failure load is compared to experimental findings.

  • 9.
    Dionisi, Filippo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner. Politecnico di Milano, Italy.
    Harnden, Ross
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Zenkert, Dan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    A model to analyse deformations and stresses in structural batteries due to electrode expansions2017Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 179, s. 580-589Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In order to aid design of future structural battery components an analytical model is developed for modelling volume expansions in laminated structural batteries. Volume expansions are caused by lithium ion intercalation in carbon fibre electrodes. An extended version of Classical Lamination Plate Theory (CLPT) is used to allow analysis of unbalanced and unsymmetric lay-ups. The fibre intercalation expansions are treated analogously to a thermal problem, based on experimental data, with intercalation coefficients relating the fibre capacity linearly to its expansions. The model is validated using FEM and allows the study of the magnitude of interlaminar stresses and hence the risk of delamination damage due to the electrochemically induced expansions. It also enables global laminate deformations to be studied. This allows information about favourable lay-ups and fibre orientations that minimise deformations and the risk of delamination to be obtained. Favourable configurations for application to a solid state mechanical actuator are also given.

  • 10.
    Ekermann, Tomas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Pull-off tests of CFRP T-joints with conventional and 3D reinforced fillets2019Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 223Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 11.
    Ekh, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Schön, Joakim
    Finite Element Modeling and optimization of Load Transfer in Multi-Fastener Joints using Structural Elements2008Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 82, nr 2, s. 245-256Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A computationally effective model of a multi-fastener, single-lap, composite-to-aluminium joint has been developed by means of structural finite elements. The model is geared towards accurate predictions of load distribution between the fasteners and accounts for bolt-bole clearances, bolt clamp-up and member plate friction. Comparisons with previously conducted experiments and detailed finite element analyses validated the accuracy of the model. A parametric study was conducted where it was found that an increased stiffness mismatch between the plates generates a more uneven load distribution, while reducing the length of the overlap region has the opposite effect. Increasing the stiffness of a fastener shifts some of the load from the nearest fasteners to that particular fastener. An idealized optimization study was conducted in order to minimize bearing stresses in the joint with restrictions on the increase of joint weight and net-section stresses. Maximum bearing stress was reduced from 220 to 120 MPa and both weight and net-section stresses were decreased.

  • 12.
    Ekh, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Schön, Joakim
    Swedish Defence Research Agency, FOI, Sweden.
    Zenkert, Dan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Simple and efficient prediction of bearing failure in single shear, composite lap joints2013Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 105, s. 35-44Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A straightforward procedure to predict bearing strength in bolted composite structures has been developed. The method is based on a finite element analysis, using structural elements, followed by a post-processing procedure. Bolt-hole clearance, friction between member plates, fastener clamp-up and fastener deformation is accounted for. Forces calculated in the FE-analysis are converted into a local stress field which is used in an existing criterion to predict fiber microbuckling in the most critically loaded lamina. Predictions were compared with experiments which validated the method. The small computational cost required by the procedure suggests that the method is applicable on large scale structures and suitable to use in conjunction with iterative schemes such as optimization and statistical investigations.

  • 13.
    Hagnell K., Mathilda
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Langbeck, B.
    Åkermo, Malin
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Cost efficiency, integration and assembly of a generic composite aeronautical wing box2016Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 152, s. 1014-1023Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper presents a cost-efficiency study of part integration with respect to reduced assembly effort within aeronautical composite structures. The study is performed through the use, and continuous improvement upon, a previously developed cost model. Focus are on the assembly and basic inspection a wing box, part of a section of a full wing, where involved parts are all considered to be manufactured from carbon fibre reinforced plastic (CFRP). Treated cases range from traditional, mechanical joining, to high integration either through co-curing or co-bonding of composite structures. The outcome of presented cost study shows that increased integration decreases the overall production cost of said considered wing box. In general it is shown that co-curing or co-bonding reduces a number of cost-expensive assembly steps in comparison to mechanical joining.

  • 14.
    Hild, Francois
    et al.
    LMT Cachan, Paris, France.
    Larsson, Per-Lennart
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Leckie, Frederick
    University of California, USA.
    Uncoupled and coupled approaches to predict macrocrack initiation in fiber reinforced ceramic matrix composites1994Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 29, nr 4, s. 365-377Artikel i tidskrift (Refereegranskat)
  • 15.
    Horberg, Erik
    et al.
    Saab AB, Broderna Ugglas Gata, SE-58188 Linkoping, Sweden.;KTH Royal Inst Technol, Dept Aeronaut & Vehicle Engn, Teknikringen 8, SE-10044 Stockholm, Sweden..
    Nyman, Tonny
    Saab AB, Broderna Ugglas Gata, SE-58188 Linkoping, Sweden.;KTH Royal Inst Technol, Dept Aeronaut & Vehicle Engn, Teknikringen 8, SE-10044 Stockholm, Sweden..
    Åkermo, Malin
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Thickness effect on spring-in of prepreg composite L-profiles - An experimental study2019Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 209, s. 499-507Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper presents the results and analysis of an experimental study of laminate thickness effects on the spring-in 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 spring-in 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.

  • 16.
    Hörberg, Erik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Farkostteknik och Solidmekanik, Lättkonstruktioner.
    Nyman, Tonny
    Åkermo, Malin
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Farkostteknik och Solidmekanik, Lättkonstruktioner.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Farkostteknik och Solidmekanik, Lättkonstruktioner.
    Thickness effect on spring-in of prepreg composite L-profiles – An experimental study2019Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 209, s. 499-507Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 17.
    Jerpdal, Lars
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg. Volkswagen AG, Group Research, Wolfsburg, Germany.
    Åkermo, Malin
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Ståhlberg, D.
    Herzig, A.
    Process induced shape distortions of self-reinforced poly(ethylene terephthalate) composites2018Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 193, s. 29-34Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper investigates shape distortion and tensile properties of hot consolidated Self-reinforced poly(ethylene terephthalate) (SrPET) by evaluating the influence from stretching before consolidation and annealing after consolidation. Spring-in angle and warpage is measured from V-shaped samples that are hot consolidated from a woven fabric that is stretched to different degrees during forming. Following the same process conditions, tensile stiffness is measured from plane laminates. This study confirms that stretching the SrPET-material during forming enhances the tensile modulus but introduces shape distortions with negative spring-in and increases warpage. However also non-stretched SrPET components experience spring-back in the same level as glass- or carbon reinforced PET composite, which is unexpected. The tensile modulus is reduced and spring-in angle further influenced when the SrPET-samples are exposed to higher temperature after consolidation. This study shows how easily the characteristics of a component made from SrPET-material are influenced by stresses developed during material forming and further by release of these stresses when exposed to higher temperatures as in post processes or even in the use phase of the component.

  • 18.
    Kazemahvazi, Sohrab
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Russell, B. P.
    Zenkert, Dan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Impact of carbon fibre/epoxy corrugated cores2012Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 94, nr 11, s. 3300-3308Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The dynamic compressive response of corrugated carbon-fibre reinforced epoxy sandwich cores has been investigated using a Kolsky-bar set-up. Compression at quasi-static rates up to v 0=200ms -1 have been tested on three different slenderness ratios of strut. High speed photography was used to capture the failure mechanisms and relate these to the measured axial compressive stress. Experiments show significant strength enhancement as the loading rate increases. Although material rate sensitivity accounts for some of this, it has been shown that the majority of the strength enhancement is due to inertial stabilisation of the core members. Inertial strength enhancement rises non-linearly with impact velocity. The largest gains are associated with a shift to buckle modes composed of 2-3 half sine waves. The loading rates tested within this study are similar to those that are expected when a sandwich core is compressed due to a blast event.

  • 19.
    Kelly, Gordon
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Load transfer in hybrid (bonded/bolted) composite single-lap joints2005Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 69, nr 1, s. 35-43Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The load transfer in hybrid (bolted/bonded) joints is complicated due to the difference in stiffness of the alternative load paths. The load distribution in hybrid composite single-lap joints has been predicted through use of a three-dimensional finite element model including the effects of bolt-hole contact and non-linear material behaviour. The effect of relevant joint design parameters on the load transferred by the bolt have been investigated through a finite element parameter study. Joint configurations where hybrid joining can provide improved structural performance in comparison to adhesive bonding have been identified. Experiments were performed to measure the distribution of load in a hybrid joint. A joint equipped with an instrumented bolt was used to measure the load transfer in the joint. The measured bolt load values were compared to predictions from the finite element model and the results were found to be in good agreement.

  • 20.
    Kelly, Gordon
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Quasi-static strength and fatigue life of hybrid (bonded/bolted) composite single-lap joints2006Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 72, nr 1, s. 119-129Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The strength and fatigue life of hybrid (bonded/bolted) joints with carbon-fibre reinforced plastic adherends have been investigated. The effect of adhesive material properties and laminate stacking sequence on the joint structural behaviour and failure modes were determined experimentally. Hybrid joints were shown to have greater strength, stiffness and fatigue life in comparison to adhesive bonded joints. However, the benefits were only observed in joints with lower modulus adhesives which allowed for load sharing between the adhesive and the bolt. Hybrid joints with high modulus adhesives showed no significant improvement in strength although increased fatigue life was observed due to the presence of the bolt. Three distinct stages in the fatigue life of hybrid joints were observed where the adhesive, the bolt and their combination were all contributing to the load transfer. Fatigue crack initiation was found to occur later in the hybrid joints where the bolt transferred a significant portion of the load. The failure mode of the joints was shown to be dependent upon the relation between the hybrid joint strength and the bearing strength of the laminate. Hybrid joints in which the strength greatly exceeded the laminate bearing strength failed catastrophically in net-section mode. In contrast, laminates with higher bearing strength failed in a non-catastrophic bearing mode. Careful comparison of the material bearing strength and adhesive bond strength is necessary to ensure increased joint strength and non-catastrophic failure.

  • 21.
    Kelly, Gordon
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Strength and failure mechanisms of composite laminates subject to localised transverse loading2005Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 69, nr 3, s. 301-314Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The behaviour of composite laminates subject to transverse load introduction has been investigated experimentally and numerically. The effect of the specimen size, stacking sequence and material system on the failure load was determined experimentally and the failure modes examined through fractographic analysis. Damage was found to initiate at low load levels, typically 20-30% of the failure load. The dominant initial failure mode was matrix intralaminar shear failure which occurred in sub-surface plies. The damage developed into a network of intralaminar and interlaminar shear cracks. Two different macromechanical failure modes were identified. fastener pull-through failure and global collapse of the laminate. The internal damage and ultimate failure mode were found to depend upon the laminate stacking sequence and resin system. A three-dimensional finite element model was developed to analyse the stress distribution within the laminate and predict first-ply failure. The results from the finite element model were found to be in general agreement with the experimental observations.

  • 22.
    Koissin, Vitali
    et al.
    KTH, Tidigare Institutioner, Farkost- och flygteknik.
    Shipsha, Andrey
    KTH, Tidigare Institutioner, Farkost- och flygteknik.
    Rizov, Victor Iliev
    KTH, Tidigare Institutioner, Farkost- och flygteknik.
    The inelastic quasi-static response of sandwich structures to local loading2004Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 64, nr 2, s. 129-138Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The paper addresses the inelastic quasi-static response of sandwich beams and panels with foam core to localized loads. The plane and axisymmetric formulations for local indentation or local low-velocity impact by a rigid body are considered; no overall bending is assumed. The governing equations for the face are derived using the Kirchhoff-Love static theory under the assumption that the core crushing follows elastic-ideally-plastic behavior. Analytical solutions are constructed on the basis of the principle of minimum work. The solutions allow predicting the face deflection, size of crushed core area and contact force. In general, the solutions are in good agreement with experimental data and finite element analysis.

  • 23.
    Koissin, Vitali
    et al.
    KTH, Tidigare Institutioner, Farkost- och flygteknik.
    Skvortsov, V.
    Krahmalev, S.
    Shipsha, Andrey
    KTH, Tidigare Institutioner, Farkost- och flygteknik.
    The elastic response of sandwich structures to local loading2004Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 63, nr 3-4, s. 375-385Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The paper addresses the elastic response of sandwich panels to local static and dynamic loading. The bottom face is assumed to be clamped, so that the overall bending is eliminated. The governing equations are derived using the static Lame equations for the core and the thin plate Kirchoff-Love dynamic theory for the faces. The plane and axisymmetric formulations are considered. The closed-form solutions are obtained using Fourier-Laplace (Hankel-Laplace) integral transformations for the cases of forced excitation and impact by a rigid body. The solutions allow to predict the stress-strain state of the structure. The analytical solutions demonstrate a good agreement with experimental data and finite element analysis.

  • 24. Koissin, Vitaly
    et al.
    Skvortsov, Vitaly
    Shipsha, Andrey
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Stability of the face layer of sandwich beams with sub-interface damage in the foam core2007Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 78, nr 4, s. 507-518Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper addresses the effect of local indentation/impact damage on the bearing capacity of foam core sandwich beams subjected to edgewise compression. The considered damage is in a form of through-width zone of crushed core accompanied by a residual dent in the face sheet. It is shown that such damage causes a significant reduction of compressive strength and stiffness of sandwich beams. Analytical solutions estimating the Euler's local buckling load are obtained for two typical modes of damage. These solutions are validated through experimental investigation of three sandwich configurations. The results of the analytical analysis are in agreement with the experimental data.

  • 25.
    Larsson, Per-Lennart
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    On buckling of orthotropic compressed plates with circular holes1987Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 7, nr 2, s. 103-121Artikel i tidskrift (Refereegranskat)
  • 26.
    Larsson, Per-Lennart
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    On buckling of orthotropic stretched plates with circular holes1989Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 11, nr 2, s. 121-134Artikel i tidskrift (Refereegranskat)
  • 27.
    Larsson, Per-Lennart
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Leckie, Frederick
    University of California, USA.
    Plane strain delamination growth in composite panels1992Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 20, nr 3, s. 175-184Artikel i tidskrift (Refereegranskat)
  • 28.
    Lindström, Anders
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Energy absorption of SMC/balsa sandwich panels with geometrical triggering features2010Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 92, nr 11, s. 2676-2684Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The influence of triggering topologies on the peak load and energy absorption of sandwich panels loaded in in-plane compression is investigated. Sandwich panels with different geometrical triggering features are manufactured and tested experimentally. The damage initiation in panels with grooves is investigated using finite element models. As expected the investigated triggering features reduce the extreme load peaks. A less expected result is that the plateau load following peak load tends to be higher for panels with triggering features. Both results are favourable for the crash performance of panels in vehicle applications. For panels containing no or few grooves the peak load seems to be governed by principles of fracture mechanics while initial failure in panels with a higher number of grooves appears to be controlled by the average stress.

  • 29.
    Lindström, Anders
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    In-plane compression of sandwich panels with debonds2010Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 92, nr 2, s. 532-540Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The post failure behaviour of in-plane compression loaded sandwich panels is studied by considering the structural response of such panels with symmetrically located edge debonds. A parametric finite element model is used to determine the influence of different material and geometrical properties on the failure progression, i.e. after initiation of damage. The investigated failure modes are buckling of the debonded face sheets, debond propagation and face sheet failure. The postbuckling failure mode is mainly determined by the fracture toughness of the core and the bending stiffness and strength of the face sheets. The presented approach and results can be used to determine how sandwich panels should be constituted, or not, to promote damage progression favourable for efficient energy absorption during in-plane crushing. The prolonged damage propagation is very complex as it is strongly non-linear and depends on a combination of stiffness, strength and geometry of the constituent materials.

  • 30.
    Liu, Zibo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet.
    Rumpler, Romain
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet. KTH, Skolan för teknikvetenskap (SCI), Centra, VinnExcellence Center for ECO2 Vehicle design.
    Feng, Leping
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet.
    Broadband locally resonant metamaterial sandwich plate for improved noise insulation in the coincidence region2018Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 200, s. 165-172Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A new design for locally resonant metamaterial sandwich plates is proposed in this paper for noise insulation engineering applications. A systematic method to tune the resonance frequency of local resonators is developed in order to overcome the coincidence phenomenon. This method, based on an impedance approach, additionally explains the ability to overcome the antiresonance associated with these local resonators. The influence of the radiated sound from these local resonators is further investigated with finite element (FE) models, particularly in connection with the sound transmission loss (STL) of the resulting metamaterial sandwich plates. The new sandwich design proposed emerges from these analyses, encapsulating the resonators inside the core material. In addition to overcoming the coincidence effect and limiting the noise radiation by the resonators, the proposed design allows to improve the mass ratio of the metamaterial sandwich structure. This, in turn, enables to broaden the working frequency band independently of the material adopted for the resonator. The proposed metamaterial sandwich plate thus combines improved acoustic insulation properties, while maintaining the lightweight nature of the sandwich plate and its good static properties.

  • 31.
    Melin, Niklas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Neumeister, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Measuring constitutive shear behavior of orthotropic composites and evaluation of the modified Iosipescu test2006Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 76, nr 1-2, s. 106-115Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The modified Iosipescu test uses a variable notch opening angle 0, depending on the material anisotropy and orientation, to accomplish even stress- and strain-fields in the test region. Constitutive shear properties can thus be measured more accurately, more completely, and with fewer sources of error. Here more materials and properties were studied, and a 3D optical strain- and displacement-monitoring system was used to evaluate and improve both the modified test and the novel fixture used here. It performed substantially better than the commonly used Wyoming fixture in terms of producing constant through specimen thickness conditions and preventing undesired specimen twisting. Additionally, loading-unloading behavior of two high-performance, essentially unidirectional, fiber composites was investigated to assess stiffness reduction, damage, localization, and irreversible strains. In the present of such, well controlled testing conditions are even more important.

  • 32.
    Mårtensson, Per
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg. Volvo Car Corporation, Sweden.
    Zenkert, Dan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Åkermo, Malin
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Effects of manufacturing constraints on the cost and weight efficiency of integral and differential automotive composite structures2015Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 134, s. 572-578Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The introduction of carbon fibre composites into the high volume automotive sector challenges the design process, since these components not only need to be light but also producible in a cost-efficient manner. One way forward is to introduce manufacturing constraints into the design process, but such constraints affect the freedom of design and opportunities to tailor material properties. This work examines the trade-offs between cost-effective design for manufacturing and the weight optimization of composite structures. This will be achieved by introducing restrictions to the number of plies allowed in structural optimization in order to simplify pre-operations and reduce overall manufacturing investments. Both integral and differential design solutions are considered. It was observed that differential solutions were always more cost and weight efficient than the integral solution, however too severe manufacturing constraints result in an expensive final part due to the additional weight.

  • 33.
    Oddy, Carolyn
    et al.
    Department of Industrial and Materials Science, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.
    Eckermann, Tomas
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Ekh, Magnus
    Department of Industrial and Materials Science, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.
    Fagerström, Martin
    Department of Industrial and Materials Science, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner. KTH, Tidigare Institutioner (före 2005), Flygteknik.
    Stig, Fredrik
    Research Institutes of Sweden, SE-431 53 Mölndal, Sweden.
    Predicting damage initiation in 3D fibre-reinforced composites - the case for strain-based criteria2019Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, ISSN 0263-8223Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Three dimensional (3D) fibre-reinforced composites have shown weight effi- cient strength and stiffness characteristics as well as promising energy absorp- tion capabilities. In the considered class of 3D-reinforcement, vertical and horizontal weft yarns interlace warp yarns. The through-thickness reinforce- ments 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. It is shown that the extension of the LaRC05 stress-based failure criteria for unidirectional lami- nated composites, to this class of 3D-reinforced composite presents a number of challenges and leads to erroneous predictions. Analysing a mesoscale rep- resentative volume element does however indicate, that loading the 3D fibre- reinforced composite produces relatively uniform strain fields. The average strain fields of each material constituent are well predicted by an equivalent homogeneous material response. Strain based criteria inspired by LaRC05 are therefore proposed. The criteria are evaluated numerically for tensile, compressive and shear tests. Results show that their predictions for the simulated load cases are qualitatively more reasonable.

  • 34.
    Pettersson, Kaj
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Neumeister, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Gamstedt, Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Öberg, Hans
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Stiffness reduction, creep, and irreversible strains in fiber composites tested in repeated interlaminar shear2006Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 76, nr 1-2, s. 151-161Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Interlaminar shear of a unidirectional carbon fiber/epoxy composite was studied experimentally by use of a tensile inclined double notch shear (IDNS-) test-setup. The non-linear constitutive behavior, the degradation and stiffness reduction, under monotonic and repeated loading with successively increasing peak strains were investigated. Shear strains (average values) up to 7-8% prior to failure were observed in the test region together with irreversible strains approaching 2.5% when unloaded. Hysteresis loops of notable width, increasing with higher peak strains, were observed. Several shear moduli measures were investigated. A notable decay in unloading secant moduli (similar to 30-40%) was present. Tangent (re-)loading and unloading moduli could be reliably measured only with a 2 h hold time prior to each load reversal. A quite early drop of > 10% was observed in the elastic reloading tangent modulus, but not for the unloading tangent modulus. To the knowledge of the authors, no previous experiments have achieved global interlaminar shear strain levels of such magnitudes, and moreover, stress levels were higher than obtained with previously established methods. Several mechanisms were deemed to contribute to the overall strains; such were higher elastic-but also higher irreversible shear strains in the epoxy matrix and the fibers; damage within the epoxy; conceivably also in the fibers; appearance of microcracks in epoxy-rich regions; possibly separation of the interfaces, and also notable creep deformation at higher loads. None of these phenomena alone could account for the dominating part of the obtained strains. Matrix cracks (shear hackles) were found post mortem, but could not be confirmed during the test: a different damage mechanism is responsible for the early loss of elastic stiffness. Friction-like processes (sliding, viscosity) contributed substantially to the total strains.

  • 35. Rizov, V.
    et al.
    Shipsha, A.
    Zenkert, Dan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Indentation study of foam core sandwich composite panels2005Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 69, nr 1, s. 95-102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Due to their high stiffness and strength to weight ratios, composite sandwich structures have proven their usefulness in a large number of applications in various technical fields, especially in aeronautics, automotive and civil engineering. One of the main drawbacks of sandwich structures is the loss of load caring capacity due to indentation damages. The purpose of the present work is the evaluation of the response of foam core sandwich composite panels to indentation. Experimental tests were performed by a spherical indentor using an Instron universal test machine under displacement control at cross-head speed of 2 mm/min. The test specimens were manufactured by using rigid foam Rohacell WF51 with a thickness of 50 mm and glass-fiber reinforced composite face sheets with a thickness of 2.4 mm. The load-displacement response for both loading and unloading steps was recorded during the testing. The diameter of the damaged zone on the face sheet was measured after the unloading. A numerical modeling of the indentation response in axisymmetrical conditions was performed using the ABAQUS finite element computer code. The plastic response of the core material was described by the *CRUSHABLE FOAM HARDENING option. The geometrical nonlinearity was taken into account by using the *NLGEOM option. The main goal of the analysis was to predict the residual stresses and strains, and especially the magnitude of the residual dent. A good match between the finite element modeling and the experimental data was obtained. The present work should be considered as a step towards developing a more sophisticated numerical model capable of describing indentation as well as post-indentation mechanical behavior of sandwich structures. © 2004 Elsevier Ltd. All rights reserved.

  • 36.
    Schneider, Christof
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Zenkert, Dan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Deshpande, V. S.
    Kazemahvazi, Sohrab
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg. University of Cambridge, United Kingdom.
    Bending energy absorption of self-reinforced poly(ethylene terephthalate) composite sandwich beams2016Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 140, s. 582-589Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Fully recyclable corrugated sandwich beams made from self-reinforced poly(ethylene terephthalate) SrPET are manufactured and tested in quasi-static three-point bending. For a constant areal mass, the influence of mass distribution on peak load and energy absorption is investigated. Beams with a higher proportion of their mass distributed in the core generally show higher peak loads and energy absorption. A finite element (FE) model was developed using an anisotropic visco-plastic constitutive material law. The FE predictions are in excellent agreement with the measurements. When comparing to sandwich beams with similar weight and geometry of different materials, the SrPET sandwich beams outperform corrugated sandwich beams made from aluminium in terms of peak load and energy absorption.

  • 37.
    Song, Yubao
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Vibrationsövervakning. National University of Defense Technology, China.
    Feng, Leping
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Vibrationsövervakning.
    Wen, J.
    Yu, D.
    Wen, X.
    Reduction of the sound transmission of a periodic sandwich plate using the stop band concept2015Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 128, s. 428-436Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The sound transmission of a sandwich plate and its reduction using the stop band concept are investigated in this paper. A periodic sandwich plate consisting of a host plate and the attached structures is designed. The dispersion relation and the stop band of the periodic sandwich plate are studied first. The sound transmission of the periodic and bare sandwich plates is analysed and compared. The reduction from the stop band design (i.e., periodically adding stepped resonators) on the sound transmission of the sandwich plates is studied. The reasons for this reduction are analysed. In addition, the properties of the sandwich plate with different boundary conditions are also briefly studied. The numerical results indicate that the sound transmission is significantly reduced over the stop band of the periodic sandwich plate. The improvement can also exist in the frequency range outside the stop band.

  • 38.
    Stig, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    A Modelling Framework for Composites containing 3D Reinforcement2012Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 94, nr 9, s. 2895-2901Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Composite materials reinforced with three-dimensionally (3D) woven carbon fibre textiles are investigated and the challenge and the driver for the work is to generate numerical models to predict the mechanical behaviour of these composites. The result of the final modelling stage is near authentic finite element (FE) models of representative volume elements (RVE)s of the composites. They are created by using only a small number of input parameters, such as the size of the RVE, the number of yarns and their mutual interlacing, and the yarn crimp. The FE models may then be utilised for various purposes but are here used to derive homogenised elastic mechanical properties of 3D reinforced composite materials. The correlation between the models and experiments is good, both in terms of details in the architecture and mechanical properties. There are however some deviations that could be explained by the models being more regular than the real material.

  • 39. Stig, Fredrik
    et al.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Extended framework for geometric modelling of textile architectures2019Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Three dimensional (3D) textiles are finding their way into fibre reinforced composite applications, and for good reasons; they can eliminate the hazard of delamination and enable complex reinforcement shapes. There is, therefore, a need for engineering methods to simulate these advanced textile structures during the product development phase. This is many times challenging since the textile architecture is truly 3D and not built by layers as in conventional laminated composites. The overall approach is similar as in a method previously presented by the authors, but some steps are changed that enable modelling of textiles containing strongly curved yarns with very good geometric representation. That is essential for reliable simulations of all parts of the 3D reinforced composite materials, which could then be performed at close to authentic meso level resolution. The resulting textile geometries are very similar to the real materials they represent, both in terms of variation of yarn cross section area and shape along the length of the yarns. This is demonstrated by comparison of details between the real materials and their numerical counterparts.

  • 40.
    Stig, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Influence of Crimp on 3D-woven Fibre Reinforced Composites2013Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 95, s. 114-122Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Three analytical models are developed for prediction of the longitudinal Young's modulus of carbon fibre composite materials containing fully interlaced 3D-woven textile reinforcement. Two of the non-linear models utilise rods and springs and are designed to handle an idealised periodic repetitive volume element of the composite material. For validation, an experimental study is performed, and a more detailed model is built using existing textile software (TexGen and WiseTex). All models are employed to explore the effect of three-dimensional yarn crimp on the longitudinal Young's modulus, and they give at hand that the longitudinal stiffness decreases non-linearly with increasing crimp. One of the proposed models agrees particularly well with the experimental results but there are also a few sources of discrepancy, such as shape distortions in the textile architecture.

  • 41.
    Stig, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Hallström, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
    Spatial modelling of 3D-woven textiles2012Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 94, nr 5, s. 1495-1502Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    When analysing for instance permeability and mechanical behaviour of advanced textiles, a representative description of the textiles' architecture is desired. Realistic geometric representations of textiles are however often either difficult to obtain due to limitations in modelling tools, or judged unfeasible to generate due to unreasonable modelling efforts. In this paper a scheme is presented that enables modelling of the internal strand geometry including details of the strand path and smoothly varying cross-section size and shape, on a meso-scale. The main modelling artifice is to initially model the strand perimeters as inflatable tubes in an explicit finite element simulation. The tubes are initially made slender to avoid strand inter-penetration, and then expanded under general contact conditions until the desired volume fraction of strand is reached. For validation a model is compared with pictures from a computer tomography scan of a 3-dimensionally woven carbon fibre preform. The correlation between the simulated geometry and the real sample is striking. The results also indicate that the approach is relatively robust in terms of sensitiveness to variations of input data and starting conditions.

  • 42.
    Sun, Jia
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik.
    Lopez Arteaga, Ines
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik. Eindhoven University of Technology, The Netherlands.
    Kari, Leif
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik.
    Dynamic modeling of a multilayer rotating blade via quadratic layerwise theory2013Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 99, s. 276-287Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A novel dynamic model for a multilayer rotating blade mounted at an arbitrary stagger angle using a quadratic layerwise theory is developed to study structural dynamics of the blade, particularly damping properties, using various coating layer configurations. A reduced two-dimensional (2D) model is used to describe the dynamic behavior of each layer in the weak form, while the quadratic layerwise theory is applied to interpolate the transverse shear stresses along the thickness direction. Results of numerical simulations with the reduced 2D model are compared to the full three-dimensional (3D) model showing an excellent agreement, comparable to the cubic layerwise theory, for both modal analysis and frequency response calculations. Moreover, damping analyses are performed on two types of multilayer blades: two-layer (free damping) and three-layer (constrained layer), in both non-rotating and rotating situations, and, parametric analyses with varying coating thickness and rotation speed are carried out. It is shown that damping decreases as the rotation speed increases due to inertial and Coriolis effects. Furthermore, frequency loci veering as a result of the rotation speed is observed. The proposed model gives an efficient and accurate way to study the dynamic behavior of rotating multilayer structures, such as compressor blades.

  • 43.
    Velea, Marian N.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg. Transilvania University of Braşov, Romania .
    Wennhage, Per
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Zenkert, Dan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Multi-objective optimisation of vehicle bodies made of FRP sandwich structures2014Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 111, s. 75-84Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An optimisation methodology is developed and applied on a FRP sandwich body of an electric vehicle - ZBee, where single-objective and multi-objective optimisation studies are performed stepwise using a commercially available software package. The single-objective optimisation allows the identification of the load paths within the composite body, according to the loading conditions previously defined. Within the multi-objective optimisation, the optimum thickness and distribution for each of the layers that form the composite body are searched within the design space so as to obtain the best performance with respect to weight, material cost, global and local stiffness. Strength requirements are also considered as constraints within the optimisation. A conflict situation appears when several objectives are considered within the optimisation, meaning that an increased performance in one objective may often lead to a decreased performance for the others. Therefore, a trade-off between objectives is needed. The interpretation of results is partially made by using trade-off plots, the so-called Pareto frontiers. A method for the overall selection of the most beneficial solutions is proposed and applied in order to choose between the best obtained solutions according to the importance of the objectives.

  • 44. Vigueras, G.
    et al.
    Sket, F.
    Samaniego, C.
    Wu, L.
    Noels, L.
    Tjahjanto, Denny D.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Casoni, E.
    Houzeaux, G.
    Makradi, A.
    Molina-Aldareguia, J. M.
    Vázquez, M.
    Jérusalem, A.
    An XFEM/CZM implementation for massively parallel simulations of composites fracture2015Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 125, s. 542-557Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Because of their widely generalized use in many industries, composites are the subject of many research campaigns. More particularly, the development of both accurate and flexible numerical models able to capture their intrinsically multiscale modes of failure is still a challenge. The standard finite element method typically requires intensive remeshing to adequately capture the geometry of the cracks and high accuracy is thus often sacrificed in favor of scalability, and vice versa. In an effort to preserve both properties, we present here an extended finite element method (XFEM) for large scale composite fracture simulations. In this formulation, the standard FEM formulation is partially enriched by use of shifted Heaviside functions with special attention paid to the scalability of the scheme. This enrichment technique offers several benefits since the interpolation property of the standard shape function still holds at the nodes. Those benefits include (i) no extra boundary condition for the enrichment degree of freedom, and (ii) no need for transition/blending regions; both of which contribute to maintaining the scalability of the code.Two different cohesive zone models (CZM) are then adopted to capture the physics of the crack propagation mechanisms. At the intralaminar level, an extrinsic CZM embedded in the XFEM formulation is used. At the interlaminar level, an intrinsic CZM is adopted for predicting the failure. The overall framework is implemented in ALYA, a mechanics code specifically developed for large scale, massively parallel simulations of coupled multi-physics problems. The implementation of both intrinsic and extrinsic CZM models within the code is such that it conserves the extremely efficient scalability of ALYA while providing accurate physical simulations of computationally expensive phenomena. The strong scalability provided by the proposed implementation is demonstrated. The model is ultimately validated against a full experimental campaign of loading tests and X-ray tomography analyzes.

  • 45. Zetterberg, T.
    et al.
    Astrom, B. T.
    Backlund, J.
    Burman, Magnus
    KTH, Tidigare Institutioner                               , Farkost- och flygteknik.
    On design of joints between composite profiles for bridge deck applications2001Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 51, nr 1, s. 83-91Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Techniques for joining of pultruded composite profiles for bridge-deck applications are designed and analyzed. It is shown that both adhesively bonded and bolted joints can be designed to fulfill stringent requirements, but it is clear that the former is the preferred alternative. The methodology used to analyze a large composite structure composed of modular construction elements and to determine the load transfer between composite profiles is described.

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