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  • 1.
    Akbarpour, S.
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Hallström, S.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Metal reinforcement around fastener holes in composites2016In: ECCM 2016 - Proceeding of the 17th European Conference on Composite Materials, European Conference on Composite Materials, ECCM , 2016Conference paper (Refereed)
    Abstract [en]

    One way of improving the load capacity of bolted joints in composite components is to use metal inserts locally at the holes in order to reduce the bearing stress. In this paper an innovative local reinforcement concept is introduced where metal inserts are implemented in the form of stacked patches at the holes in order to improve the bearing strength of the composite. After doing some initial tests and a parameter study, some specimens with optimized stacked patch inserts were designed and tested. The specimens with optimized inserts show 50-60% improved bearing strength in pin-loaded tests which corresponds to a potential weight reduction of about 30%. These very promising results indicates that the efficiency of joints in composites can be improved significantly.

  • 2.
    Akbarpour, Sahar
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Reinforcement around holes in composite materials by use of patched metal inserts2019In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 225, article id 111084Article in journal (Refereed)
    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.

  • 3.
    Bull, Peter Hoaas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Curved sandwich beams with face-core debond subjected to bending moment2004In: Journal of Sandwich Structures and Materials, ISSN 1099-6362, E-ISSN 1530-7972, Vol. 6, no 2, p. 115-127Article in journal (Refereed)
    Abstract [en]

    Curved sandwich beams subjected to opening bending moment are studied. Face-core debonds of varying size are introduced at the compressively loaded face sheet and the structural integrity is investigated. Analytical and finite element models are compared in order to identify the governing failure modes of the beams. A simple expression is presented as a tool for getting a quick estimate of the severity of an interface crack in a curved sandwich beam. Five different configurations of beams are tested experimentally in a custom made bending rig.

  • 4.
    Bull, Peter Hoaas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    High-velocity and quasi-static impact of large sandwich panels2004In: Journal of Sandwich Structures and Materials, ISSN 1099-6362, E-ISSN 1530-7972, Vol. 6, no 2, p. 97-113Article in journal (Refereed)
    Abstract [en]

    An investigation of the response of sandwich structures subjected to impact velocities of virtually 0 m/s and approximately 1000 m/s is conducted. The higher velocity exceeds both the longitudinal and the transverse wave propagation velocities of the core material in the sandwich panels. The objective is to investigate the possibility to simulate the damage from ballistic impact of sandwich panels through quasi-static experiments. Panels are impacted using a 40 mm Bofors AA gun and, using a similar projectile, other panels are indented quasi-statically. Energy absorption is measured in both test series. After impact, the panels are tested in in-plane compression together with one undamaged panel for reference. Residual strength of impacted panels is analyzed by finite element analysis. It is shown that the damage from high-velocity impact is limited, and it is possible to regain most of the undamaged strength by repair.

  • 5.
    Burvall, Anna
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Larsson, Daniel H.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Lundström, Ulf
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Stig, Fredrik
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hertz, Hans M.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Phase-retrieval methods with applications in composite-material tomography2013In: 11th International Conference On X-Ray Microscopy (XRM2012), Institute of Physics Publishing (IOPP), 2013, p. 012015-Conference paper (Refereed)
    Abstract [en]

    In-line phase-contrast x-ray imaging is emerging as a method for observing small details when the contrast in absorption x-ray imaging is low. It gives images with strong edge enhancement, and phase retrieval is necessary to obtain quantitative thickness information. In particular for tomography, clarity can be enhanced by phase retrieval, as here demonstrated on a 3D-weave reinforced composite material. Seven suitable phase-retrieval methods are identified and integrated into a single method, where each version is marked by variations in particular steps. The general method and its variations are outlined and a comparison shows which methods are most suitable in different situations.

  • 6.
    Edström, Kristina
    et al.
    KTH, School of Education and Communication in Engineering Science (ECE), Lärande.
    El Gaidi, Khalid
    KTH, School of Education and Communication in Engineering Science (ECE), Lärande.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Kuttenkeuler, Jakob
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Naval Systems.
    Integrated assessment of diciplinary and interpersonal skills - student perceptions of a novel learning experience2005In: 13th International Symposium at the Oxford Centre for Staff and Learning Development - Improved Student Learning Through Assessment, 2005Conference paper (Other academic)
  • 7.
    Edström, Kristina
    et al.
    KTH, School of Education and Communication in Engineering Science (ECE), Lärande.
    El Gaidi, Khalid
    KTH, School of Education and Communication in Engineering Science (ECE), Lärande.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Kuttenkeuler, Jakob
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Naval Systems.
    Integrated Assessment of Diciplinary, Personal and Interpersonal Skills in a Design-Build Course2005In: 1st CDIO Conference, 2005Conference paper (Other academic)
  • 8.
    Edström, Kristina
    et al.
    KTH, School of Education and Communication in Engineering Science (ECE), Lärande.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Kuttenkeuler, Jakob
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Naval Systems.
    Mini Workshop - Designing Project-Based Courses for Learning and Cost-Effective Teaching2011In: 2011 Frontiers In Education Conference (FIE), 2011Conference paper (Refereed)
    Abstract [en]

    This workshop draws on experience in the international collaboration for engineering education reform, called the CDIO Initiative, where project-based learning is a key part of the concept. The purpose of project-based courses in engineering education is to provide environments where students can develop a deeper working knowledge of technical fundamentals together with the complex skills necessary for engineering practice, or in short: where students can become engineers. In this workshop, the learning perspective is emphasized, by identifying trade-offs where there are inherent tensions between learning outcomes and other factors in project-based courses (such as project goal, product performance, technical sophistication, teacher popularity, student satisfaction). A set of principles are derived for enhancing learning and teaching in project-based courses, using concrete examples to illustrate thought-provoking implications. Each principle aims to improve both student learning outcomes and cost-effectiveness of teaching. Together the principles constitute a framework for learning-driven course design. The aim is to challenge assumptions and common practices in project-based courses, and provoke fruitful discussion among participants.

  • 9.
    Ekermann, Tomas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Mechanical characterisation of composites with 3D-woven reinforcement2015In: ICCM International Conferences on Composite Materials, International Committee on Composite Materials , 2015Conference paper (Refereed)
    Abstract [en]

    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 (GIc) through double cantilever beam tests. It is shown that GIc is greatly dependent on the amount of reinforcement in the vertical weft. 

  • 10.
    Ekermann, Tomas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Pull-off tests of CFRP T-joints with conventional and 3D reinforced fillets2019In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 223Article in journal (Refereed)
    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. Franciscangelis, C.
    et al.
    Margulis, W.
    Floridia, C.
    Rosolem, J. B.
    Salgado, F. C.
    Nyman, T.
    Petersson, M.
    Hallander, P.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Söderquist, I.
    Fruett, F.
    Vibration measurement on composite material with embedded optical fiber based on phase-OTDR2017In: Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2017 / [ed] Lynch, J P, SPIE - International Society for Optical Engineering, 2017, Vol. 10168, article id 101683QConference paper (Refereed)
    Abstract [en]

    Distributed sensors based on phase-optical time-domain reflectometry (phase-OTDR) are suitable for aircraft health monitoring due to electromagnetic interference immunity, small dimensions, low weight and flexibility. These features allow the fiber embedment into aircraft structures in a nearly non-intrusive way to measure vibrations along its length. The capability of measuring vibrations on avionics structures is of interest for what concerns the study of material fatigue or the occurrence of undesirable phenomena like flutter. In this work, we employed the phase-OTDR technique to measure vibrations ranging from some dozens of Hz to kHz in two layers of composite material board with embedded polyimide coating 0.24 numerical aperture single-mode optical fiber.

  • 12.
    Grenestedt, Joakim L
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Crack initiation from homogeneous and bimaterial corners1997In: Journal of applied mechanics, ISSN 0021-8936, E-ISSN 1528-9036, Vol. 64, no 4, p. 811-818Article in journal (Refereed)
    Abstract [en]

    Fracture initiated at a corner between two different isotropic materials is considered. A "small" crack, well within the region dominated by the asymptotic stress fields of the non-cracked corner, is modelled and the stress intensities associated with the tip of the small crack are determined. Different criteria for the direction of crack propagation are studied.

  • 13.
    Grenestedt, Joakim L
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Kuttenkeuler, Jakob
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    On cracks emanating from wedges in expanded PVC foam1996In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 54, no 4, p. 445-456Article in journal (Refereed)
    Abstract [en]

    An experimental and analytical study was made on the effect of stress singularities on the strength of expanded PVC foam materials of different densities. Experiments were performed on specimens with different wedge geometries ranging from sharp cracks, with the ordinary inverted square root stress singularity, to shallow re-entrant corners with weak singularities. A brittle fracture criterion based on a generalised stress intensity factor, called Q, at the wedge tip was fit to experimental data. The critical stress intensity factor, Qcr, for crack initiation depends on the wedge geometry. This dependence was estimated from simple point-stress criteria and a criterion due to Seweryn [Brittle fracture criterion for structures with sharp notches. Engng Fracture Mech. 47, 673-681 (1994)], and good agreement with experimental data was obtained. When the point-stress criterion was applied to Mode II sharp cracks, poor agreement with published data was found. A critical study of the Mode II crack specimen was therefore initiated, leading to the conclusion that the commonly used specimen gives erroneous values of KIIc and the reason seems to be due to crack surface friction. A new Mode II crack specimen which eliminates crack surface friction was proposed and tested, and good agreement with the point-stress criterion was obtained. A criterion for homogeneous materials proved to be adequate also for the porous PVC foams.

  • 14.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    3D-textile reinforcement in composites- mechanics, modelling, pros and cons2012In: ECCM 2012 - Composites at Venice, Proceedings of the 15th European Conference on Composite Materials, European Conference on Composite Materials, ECCM , 2012Conference paper (Refereed)
    Abstract [en]

    Fibre reinforced composite materials are quite rightfully accused of suffering from poor out-of-plane mechanical properties, and delamination concerns are known to strongly reduce their competitiveness in certain types of applications. Several methods have been invented and explored aiming at improving their 3D performance but none of them has yet received broad acceptance and utilisation in practical use. One of the reasons is likely that improved out-of-plane performance by means of through-thickness reinforcement inherently compromises the in-plane performance. The paper predominantly discusses 3D reinforcement from a general point of view but some more specific results from recent work on composites containing 3D-woven fibre reinforcement are also presented. Potential benefits and drawbacks of using 3D textiles in composites are discussed and partly quantified. The tradeoffs that become accentuated when introducing 3D reinforcement in composite materials can then be approached through use of more informed design principles.

  • 15.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Glass fibre reinforced holes in laminated timber1996In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 30, p. 323-337Article in journal (Refereed)
    Abstract [en]

    An experimental investigation of glass fibre reinforcement of glued laminated timber beams is presented. A polyester resin is used both as matrix and adhesive between the reinforcement and the wood. The main part of the work considers beams with large holes tested in three-point bending. Circular and rectangular holes, centred at quarter length of the beams make the strength of wood perpendicular to the grain become critical. Great improvements of strength are obtained with the glass fibres. A comparison between various kinds and combinations of glass fibre reinforcement is made. Further, the reinforcement applied as repair of earlier cracked beams is investigated with positive results. One series of beams without holes is reinforced and tested in four-point bending.

  • 16.
    Hallström, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Grenestedt, Joakim L
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Failure analysis of laminated timber beams reinforced with glass fibre composites1997In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 31, p. 17-34Article in journal (Refereed)
    Abstract [en]

    A qualitative analysis is presented of failure, perpendicular to the grain, in laminated timber reinforced with a glass fibre composite. The study is focused on beams with holes of different shape. The stresses by corners, infinitesimal cracks and finite cracks are investigated. An initial crack model is suggested that brings about some of the phenomena observed in earlier performed experiments. A crack appears to propagate in the wood but is retarded in the reinforced beams. Eventually, the composite will fracture and failure of the beam follows. Finite element computations suggest that the reinforcement decreases the stress intensity at cracks in the wood and acts as a crack stopper. The reinforcing effect increases with the crack length. A point-stress criterion is used to predict failure in the fibre composite.

  • 17.
    Hallström, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Grenestedt, Joakim L
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Mixed mode fracture of cracks and wedge shaped notches in expanded PVC foam1998In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 88, no 4, p. 343-358Article in journal (Refereed)
    Abstract [en]

    Fracture initiated from a sharp crack or wedge shaped notch in a homogeneous material, subjected to different loading is considered. Singularities in the stress fields at edges and vertices are discussed. A point-stress criterion is used to predict fracture for sharp cracks as well as 90° wedge notches in expanded PVC foam. The point-stress criterion is formulated in a manner allowing failure predictions in general 3D stress situations. The influence of non-singular T-stress at cracks is discussed and substantiated by experimental results.

  • 18.
    Hallström, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Kuttenkeuler, Jakob
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Naval Systems.
    Edström, Kristina
    KTH, School of Education and Communication in Engineering Science (ECE), Learning.
    The Route Towards a Sustainable Design-Implement Course2007In: 3rd CDIO Conference, 2007Conference paper (Other academic)
  • 19.
    Hallström, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Köll, Joonas
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Architecture and properties of stochastic foam models2015In: 20th International Conference on Composite Materials, 2015Conference paper (Refereed)
  • 20.
    Hallström, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Ribeiro-Ayeh, Steven
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Stochastic finite element models of foam materials2005In: SANDWICH STRUCTURES7: ADVANCING WITH SANDWICH STRUCTURES AND MATERIALS / [ed] Thomsen, OT; Bozhevolnaya, E; Lyckegaard, A, 2005, p. 935-943Conference paper (Refereed)
    Abstract [en]

    Driven by the desire to understand the influence of various mechanical properties and geometric features of foam materials, on the stiffness, failure mechanisms and fatigue life of sandwich materials, 3D finite element models of amorphous cellular structures have recently been developed and analyzed. Different algorithms were used to generate seed points of Voronoi tessellations, such as randomly distorted regular lattice distributions and totally random distributions. Periodic boundary conditions were used on representative volume elements containing hundreds of cells. Some comparison with experimental data from real foam materials was made and the agreement was found to be good. In the presented paper discrepancies between the used models and real foams are discussed and it is concluded that the adopted approach has substantial potential although there are some obvious routes for further improvement.

  • 21.
    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, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Thickness effect on spring-in of prepreg composite L-profiles - An experimental study2019In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 209, p. 499-507Article in journal (Refereed)
    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.

  • 22.
    Juntikka, Rickard
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Selection of energy absorbing materials for automotive head impact countermeasures2004In: Cellular polymers, ISSN 0262-4893, E-ISSN 1478-2421, Vol. 23, no 5, p. 263-297Article in journal (Refereed)
    Abstract [en]

    Material candidates for energy absorption in head impact countermeasures for automotive applications are evaluated using both quasi-static and dynamic test methods. Ranking of different materials turns out to be difficult since the mechanical response of a material could vary considerably with temperature, especially for polymers. Twenty-eight selected materials, including foams, honeycombs and balsa wood are tested and evaluated. The materials are subjected to a sequence of tests in order to thin out the array systematically. Quasi-static uni-axial compression is used for initial mapping of the selected materials, followed by quasi-static shear and dynamic uni-axial compression. The quasi-static test results show that balsa wood has by far the highest energy absorption capacity per unit weight but the yield strength is too high to make it suitable for the current application. The subsequent dynamic compression tests are performed for strain rates between 56 s(-1) and 120 s(-1) (impact velocities between 1.4 and 3 m/s) and temperatures in the range -20 - 60 degreesC. The test results emphasize the necessity of including both strain rate and temperature dependency to acquire reliable results from computer simulations of the selected materials.

  • 23.
    Juntikka, Rickard
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Shear characterization of sandwich core materials using four-point bending2007In: Journal of Sandwich Structures and Materials, ISSN 1099-6362, E-ISSN 1530-7972, Vol. 9, no 1, p. 67-94Article in journal (Refereed)
    Abstract [en]

    A new shear test method for sandwich core materials is proposed and evaluated. Sandwich beams are loaded in four-point bending, and the shear deformation is measured with two rotary sensors. Conditions of idealized sandwich theory are assumed to prevail, and the accuracy of the proposed methodology is thus dependent on a few mechanical and geometric relations between the sandwich constituents. The stress-strain responses for two polymer foam core materials, one relatively brittle and one relatively ductile, are extracted and compared with results from single-block shear tests of the same material batch. The new method provides several benefits with respect to the block shear test. It does not suffer from extreme stress concentrations and the specimens are tested under in-service conditions. Problems arise, however, for the ductile material, predominantly related to large deformations during the test eventually resulting in bending failure of the face sheet instead of shear failure of the core.

  • 24.
    Juntikka, Rickard
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Weight-balanced drop test method for characterization of dynamic properties of cellular materials2004In: International Journal of Impact Engineering, ISSN 0734-743X, E-ISSN 1879-3509, Vol. 30, no 5, p. 541-554Article in journal (Refereed)
    Abstract [en]

    A novel weight-balanced drop rig used to evaluate the response of cellular materials subject to dynamic compression is presented. The testing method utilizes approximately constant velocity throughout the major part of the compression phase and the results compare well with results from other methods, reported in the literature. The repetitiveness is excellent, the rig is simple and the results are easily extracted. The applicability of the method for determination of elastic modulus is however limited to materials with relatively low stiffness. Accurate modulus measurements for stiff materials at high strain-rates require a very rigid and lightweight test set-up.

  • 25.
    Juntikka, Rickard
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Kleiven, Svein
    KTH, School of Technology and Health (STH), Neuronic Engineering (Closed 20130701).
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Optimization of single skin surfaces for head injury prevention - a comparison of optima calculated for global versus local injury thresholds2004In: International Journal of Crashworthiness, ISSN 1358-8265, E-ISSN 1754-2111, Vol. 9, no 4, p. 365-379Article in journal (Refereed)
    Abstract [en]

    This paper describes optimizations of material properties for a bonnet-like plate using finite element calculations and the Euro-NCAP pedestrian head impact test. Four different head models were used for the impact simulations, a Euro-NCAP dummy head, a Hybrid III dummy head and two biomechanical head models exhibiting different mechanical properties for the brain tissue. The objective function was to minimize the displacement of the bonnet plate while satisfying constraints on the head injury criterion (HIC), the resultant contact force and, for the human head models, the strain in the brain tissue. An investigation was also conducted of the kinematics of the head models during impact, evaluating the energy distribution and the apparent mass. The analysis gave at hand that optimization of the plate with respect to impact with the Euro-NCAP and Hybrid III head models reached substantially, different results compared to impact with the biomechanical head models. For the latter case, the stiffness of the brain tissue influenced which constraints were active in the final solution. The investigation of the kinematics at impact showed that a substantial portion of energy was confined within the brain during impact for the biomechanical head models. The apparent mass at impact coincided with the actual mass for the rigid dummy heads while for the human head models it was roughly the mass of the skull only.

  • 26. Kazemahvazi, S.
    et al.
    Khokar, N.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Wadley, H. N. G.
    Deshpande, V. S.
    Confluent 3D-assembly of fibrous structures2016In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 127, p. 95-105Article in journal (Refereed)
    Abstract [en]

    The ability to independently control fiber alignments and structural geometry is critical for design of optimal three-dimensional (3D) fibrous structures. We present a novel method to 3D-assemble carbon fiber structures, containing no seams or adhesive joints, using a confluence of several textile methodologies. A variety of complex structural shapes with tailored fiber topologies are demonstrated to be achievable. These optimized structures are shown to have unprecedented static and dynamic strength as well as damage tolerance and ductility. For example, the energy absorption capacity of a 3D-assembled carbon fiber prismatic sandwich structure is shown to be 300% greater than a high performance metallic counterpart.

  • 27.
    Kelly, Gordon
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Bearing strength of carbon fibre/epoxy laminates: effects of bolt-hole clearance2004In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 35, no 4, p. 331-343Article in journal (Refereed)
    Abstract [en]

    The bearing strength of carbon fibre epoxy laminates manufactured from non-crimp fabric from heavy tow yarn has been investigated. The effects of laminate stacking sequence and geometry on the bearing strength have been determined experimentally together with the effect of initial bolt-hole clearance on the bearing strength at 4% hole deformation and at ultimate load. Significant reduction in bearing strength at 4% hole deformation was found for both pin-loaded and clamped laminates as a result of bolt-hole clearance. It was concluded that the effect of bolt-hole clearance is significant with regard to the design bearing strength of mechanically fastened joints. A three-dimensional non-linear finite element model was developed to investigate the effects of bolt-hole clearance on the stress field in the laminate adjacent to the hole. The magnitude and distribution of stress at the hole was found to be significantly dependent on the level of clearance.

  • 28.
    Kelly, Gordon
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Strength and failure mechanisms of composite laminates subject to localised transverse loading2005In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 69, no 3, p. 301-314Article in journal (Refereed)
    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.

  • 29. Khokar, N.
    et al.
    Winberg, F.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Novel 3D preform architecture for performance and reliability of structural beams2015In: ICCM International Conferences on Composite Materials, International Committee on Composite Materials , 2015Conference paper (Refereed)
    Abstract [en]

    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 (+/-θ°) directions relative to the beam's length direction, the flange/s have integrated fibre assemblies oriented in the beam's length and width (0°/90°) 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. .

  • 30. Khokar, Nandan
    et al.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Novel 3D preform architecture for improved performance of structural beams2015In: 20th International Conference on Composite Materials, Copenhagen, Denmark, 2015, 2015Conference paper (Other academic)
  • 31.
    Köll, Joonas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Elastic properties from equilibrium models with statistically representative polydispersityManuscript (preprint) (Other academic)
  • 32.
    Köll, Joonas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Elastic properties of equlibrium foams2016In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 113, p. 11-18Article in journal (Refereed)
    Abstract [en]

    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.

  • 33.
    Köll, Joonas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Generation of periodic stochastic foam models for numerical analysis2014In: Journal of cellular plastics (Print), ISSN 0021-955X, E-ISSN 1530-7999, Vol. 50, no 1, p. 37-54Article in journal (Refereed)
    Abstract [en]

    Stochastic cellular models of rigid foam based on Voronoi spatial partitioning are generated and investigated for potential use in numerical analysis using finite element methods. Such partitions are deterministic once a distribution of cell nuclei has been defined. A drawback is that the models tend to exhibit a significant share of short edges and small faces. Such small geometrical features are not likely to occur in real foams since they are unfavorable from a surface energy point of view and they also generate problems in numerical analysis due to associated meshing challenges. Through minimization of the surface area, using the computer software Surface Evolver, the Voronoi models are brought to better resemblance with ideal dry foam and the occurrence of small geometrical features is strongly reduced. It is generally seen that different seed point distribution algorithms result in different model topologies. The presented methodology is systematic, parameterized and the results are very promising. Good grounds are provided for modeling of real rigid foam materials, that do not necessarily fully resemble ideal dry foam.

  • 34.
    Köll, Joonas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Influence from polydispersity on the morphology of Voronoi and equilibrium foams2017In: Journal of cellular plastics (Print), ISSN 0021-955X, E-ISSN 1530-7999, Vol. 53, no 2, p. 199-214Article in journal (Refereed)
    Abstract [en]

    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.

  • 35.
    Köll, Joonas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Influence of sphere packing fraction on polydispersity an morphology of Voronoi and equilibrium foamsManuscript (preprint) (Other academic)
  • 36.
    Köll, Joonas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Morphology effects on constitutive properties of foams2011Conference paper (Refereed)
  • 37.
    Lindström, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Design for various collapse behaviour of sandwich panels subject to in-plane compressionManuscript (preprint) (Other academic)
    Abstract [en]

    Sandwich panels were designed for different collapse behaviour when compressed in the in-plane direction, by altering the material and geometrical properties. An approach previously developed by the authors was used in the design process of the sandwich panels. Panels with CSM glass fibre face sheets of two different thicknesses and two different core materials were studied both numerically and experimentally. The collapse mode of sandwich panels is mainly dependant on the relation between the fracture toughness of the core material and the bending stiffness and strength of the face sheets. By altering properties like the fracture toughness of the core material or the face sheet thickness, the failure mode could be altered. It was shown that the adopted approach can be used in design of sandwich panels for different failure behaviour.

  • 38.
    Lindström, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Energy absorption of sandwich panels containing bio-based materials2010Manuscript (preprint) (Other academic)
  • 39.
    Lindström, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Energy Absorption of Sandwich Panels Subjected to In-plane Loads2006In: 8th Biennial ASME Conference on EngineeringSystems Design and Analysis, Torino, Italy, 4-7 July, 2006: Volume 3: Dynamic Systems and Controls, Symposium on Design and Analysis of Advanced Structures, and Tribology, 2006, p. 739-747Conference paper (Refereed)
    Abstract [en]

    Theenergy absorption mechanisms of sandwich panels subjected to in-plane compressionare studied. Quasi-static experiments are performed and analysed in orderto support the development of a modelling strategy for failureinitiation and propagation in sandwich panels. The test specimens consistof balsa wood cores and glass-fibre reinforced polyester faces. Duringcompression of a tested panel, the displacement field on oneouter face is measured using a digital speckle photography (DSP)equipment. The absorbed energy is related to debonding, delamination andcrushing of the face sheets and crushing of the core.At initial failure, the load drops dramatically and is thenrelatively constant during continued compression. The energy per unit lengthnecessary for propagation of the damage is considerably lower thanfor damage initiation. Assuming that the damage propagation is uniformthrough the thickness of the panels a simple model ofdamage growth is developed. Calibration of the model is howeverdubious due to the large scatter in the experimental results.The studied material shows damage mechanisms favourable for efficient energyabsorption but the behaviour is far from being optimal.

  • 40.
    Lindström, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Energy absorption of SMC/balsa sandwich panels with geometrical triggering features2010In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 92, no 11, p. 2676-2684Article in journal (Refereed)
    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.

  • 41.
    Lindström, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    In-plane compression of sandwich panels with debonds2010In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 92, no 2, p. 532-540Article in journal (Refereed)
    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.

  • 42.
    Lindström, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Strength of In-plane Compression Loaded Sandwich Panels with DebondsManuscript (Other academic)
  • 43.
    Malmqvist, Johan
    et al.
    Chalmers University of Technology.
    Young, Pete Y.
    Massachusetts Institute of Technology.
    Hallström, Stefan
    KTH, Superseded Departments, Aeronautical Engineering.
    Kuttenkeuler, Jakob
    KTH, Superseded Departments, Aeronautical Engineering.
    Svensson, Thomas
    Linköping University.
    Lessons learned from design-build-test-based project courses2004In: Design 2004: Proceedings of the 8th International Design Conference, Vols 1-3 / [ed] Marjanovic, D, 2004, p. 665-672Conference paper (Refereed)
  • 44.
    Rajput, Moeen S
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Burman, Magnus
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Distinguishing between Strain Measurement Procedures during Compressive Testing of Foam Materials2015In: Proceedings of the 20th International Conference on Composite Materials, INTERNATIONAL COMMITTEE ON COMPOSITE MATERIALS , 2015Conference paper (Other academic)
    Abstract [en]

    The out-of-plane compressive properties of foam materials are essential for their performance as core material in sandwich structures. Accurate data for these properties are crucial for modeling and analysis of impact response and critical design assessment of e.g. sandwich structures. Current standards differ in specifying how the strains should be measured. When different standard methods are used, significant differences in test results for flatwise compression are obtained. An experimental study of the out-of-plane compressive properties (strength and modulus) of foam materials is conducted, where two types of foams are studied; Rohacell 200 Hero and Divinycell H60. Both are closed cell structured foams used for cores in sandwich materials. A review of established test methodologies is conducted and the test results show that there is a significant difference in strain values measured from the cross head displacement (termed “gross core strain”) and when extensometer directly applied onto the foam (termed “net core strain”). The results are not related to the compliance of the test rig, which is another issue outside the scope of this investigation. The effect of surface priming on the compressive behavior is further investigated followed by digital image correlation (DIC) for detailed recordings of the strain fields.

  • 45.
    Rajput, Moeen S.
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Burman, Magnus
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Evaluation of test methods and face-sheet thickness effects in damage tolerance assessment of composite sandwich platesManuscript (preprint) (Other academic)
    Abstract [en]

    Composite sandwich materials are used in load-carrying components in commercial aircraft due to their lightweight characteristics and high strength-to-weight ratio. However, composite structures are vulnerable to impact damage which can drastically reduce their load-bearing capability. Consequently, damage tolerance assessment of composite sandwich structures is of utmost importance. At present, there is a lack of standardised test methods for such assessments of sandwich structures. This study compared test methods for damage tolerance by means of compression after impact (CAI) and how the residual strength varied with face-sheet thickness. Three different test methods, including two types of CAI, were used i.e. CAI-single skin (CAI-SS) and CAI-sandwich (CAI-SW), and one four-point bending-after-impact (BAI) test method was used for residual strength measurement. A thorough experimental campaign was performed on different face-sheet configurations utilising both symmetric and asymmetric sandwich panels. Test specimens were subjected to impact energy levels causing barely visible damage. Then they were tested using the different methods and the differences in the results were highlighted. In all three test methods, the specimens failed in compression showing prominent fibre fracture and delamination. This compressive failure was initiated at the impact zone and grew across the width of the specimen, perpendicular to the loading direction. The CAI-SW test method showed a by-pass load effect due to the parallel compression of the intact face-sheet. The CAI-SS test method was used to study the effect of face-sheet thickness on the damage tolerance assessment. The BAI test method showed similar failure stresses to the CAI-SS tests. A progressive damage finite element model was used to estimate by-pass load and post impact strengths. The simulations showed good agreement with the experiments.

  • 46.
    Rajput, Moeen S
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Burman, Magnus
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Impact resistance and damage tolerance assessment of composite sandwich materials for aircraft2018Conference paper (Refereed)
  • 47.
    Rajput, Moeen S
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Burman, Magnus
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Köll, Joonas
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Compression of structural foam materials: Experimental and numerical assessment of test procedure and specimen size effects2017In: Journal of Sandwich Structures and Materials, ISSN 1099-6362, E-ISSN 1530-7972Article in journal (Refereed)
    Abstract [en]

    This study was initiated based on the observation that standardized test for flatwise compression of foam materials, give significantly different test results for the measured moduli, and that these standards to date lack adequate instructions on how the strain should be measured and what specimen size should be used. A brief review of previous work shows that existing test methodologies provide significantly different results for the compressive moduli of foams depending on how the strains are measured. A thorough experimental study of the out-of-plane compressive properties is conducted on three different closed-cell foam materials, where strains measured with two different extensometer placements, and with digital image correlation, come out significantly differently. A parametric study is also performed showing that the results vary considerably with in-plane specimen dimensions, indicating effects of finite size and localized strain at edges. Both stochastic amorphous and homogenized finite element models of foam back the experimental observations by illustrating the effects of finite size and various boundary conditions on the measured properties.

  • 48.
    Rajput, Moeen S.
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. Odqvist Laboratory, KTH.
    Burman, Magnus
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. Odqvist Laboratory, KTH.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Odqvist Laboratory, KTH.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. Odqvist Laboratory, KTH.
    Design and evaluation of a novel instrumented drop-weight rig for controlled impact testing of polymer composites2018In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 68, p. 446-455Article in journal (Refereed)
    Abstract [en]

    A drop-weight rig (DWR) intended to test the true impact response of laminated compositematerials is presented. The test setup is designed to prevent the transfer of unwanted mechanical noise, e.g. vibrations, into the load cell that is used to measure the load during the impact event. A novel catch mechanism preventing secondary impact is also implemented in the DWR design. A detailed evaluation is performed both in terms of the experimental modal and uncertainty analysis of the measured results from the DWR. The results demonstrate that the rig is capable of capturing the true impact response, providing highly resolved and noise-free force-time measurements where even subtle details of the impact event are visible. The rig also enables impact testing with good repeatability.

  • 49.
    Ribeiro-Ayeh, Steven
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Stochastic finite element models for the characterisation of effective mechanical properties of cellular solidsManuscript (preprint) (Other academic)
  • 50.
    Ribeiro-Ayeh, Steven
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Strength prediction of beams with bi-material butt-joints2003In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 70, no 12, p. 1491-1507Article in journal (Refereed)
    Abstract [en]

    Failure of bi-material interfaces is studied with the aim to quantify the influence of the induced stress concentrations on the strength of the interfaces. The suggested approach is applied to a specimen configuration where two different materials are butt-joined to form a two-material beam. Strength predictions for different interface bias are made and verified in experiments where a polymer foam material is joined with members of either aluminium or Plexiglas.The predictions are made using a simple point-stress criterion in combination with highly accurate finite element calculations. The point-stress criterion was known from earlier work to give accurate predictions of failure at cracks and notches but had to be slightly modified to become applicable for the studied configurations. Both the strength and the cross-over from local, joint-induced failure to global failure were predicted with reasonable accuracy.

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