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  • 1.
    Ahmad, Maqsood
    et al.
    Base Engine & Materials Technology, Volvo Group, Gothenburg.
    Peng, Ru
    Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
    König, Mathias
    Materials Technology for Basic Engine, Scania CV, Södertälje.
    Johansson, Sten
    Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
    Bending Fatigue Behavior of Blast Cleaned Grey Cast Iron2017In: Residual Stresses 2016: ICRS-10, Materials Research Proceedings 2 (2016), 2017, Vol. 2, p. 193-198Conference paper (Refereed)
    Abstract [en]

    This paper presents a detailed study on the effect of an industrial blast cleaning process on the fatigue behavior of a grey cast iron with regard to the residual stresses and microstructural changes induced by the process. A comparison was also made to the effect of a machining operation which removed the casting skin layer. The blast cleaning process was found to greatly improve the fatigue resistance in both the low and high cycle regimes with a 75% increase in the fatigue limit. Xray diffraction measurements and scanning electron microscopic analyses showed that the improvement was mainly attributed to compressive residual stresses in a surface layer up to 800 μm in thickness in the blast cleaned specimens. The machining also gave better fatigue performance with a 30% increase in the fatigue limit, which was ascribed to the removal of the weaker casting skin layer.

  • 2.
    Björnsson, Andreas
    Linköping University, Department of Management and Engineering, Manufacturing Engineering. Linköping University, Faculty of Science & Engineering.
    Automated layup and forming of prepreg laminates2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Composite materials like carbon fiber-reinforced polymers (CFRPs) present highly appealing material properties, as they can combine high strength with low weight. In aerospace applications, these properties help to realize lightweight designs that can reduce fuel consumption. Within the aerospace industry, the use of these types of materials has increased drastically with the introduction of a new generation of commercial aircraft. This increased use of CFRP drives a need to develop more rational manufacturing methods.

    For aerospace applications, CFRP products are commonly manufactured from a material called prepreg, which consists of carbon fibers impregnated with uncured polymer resin. There are two dominant manufacturing technologies for automated manufacturing using prepreg, automated tape layup and automated fiber placement. These two technologies are not suitable for all types of products, either due to technical limitations or a combination of high investment costs and low productivity. Automation alternatives to the two dominant technologies have been attempted, but have so far had limited impact. Due to the lack of automation alternatives, manual manufacturing methods are commonly employed for the manufacturing of complex-shaped products in low to medium manufacturing volumes.

    The research presented in this thesis aims to explore how automated manufacturing systems for the manufacturing of complex CFRP products made from prepreg can be designed so that they meet the needs and requirements of the aerospace industry, and are suitable for low to medium production volumes. In order to explore the area, a demonstrator-centered research approach has been employed. A number of demonstrators, in the form of automated manufacturing cells, have been designed and tested with industrial and research partners. The demonstrators have been used to identify key methods and technologies that enable this type of manufacturing, and to analyze some of these methods and technologies in detail. The demonstrators have also been used to map challenges that affect the development of enabling methods and technologies.

    Automated manufacturing of products with complex shapes can be simplified by dividing the process into two steps. Thin layers of prepreg are laid up on top of each other to form flat laminates that are formed to the desired shape in subsequent forming operations. The key methods and technologies required to automate such a system are methods and technologies for automated prepreg layup, the automated removal of backing paper and the forming of complex shapes. The main challenges are the low structural rigidity and tacky nature of prepreg materials, the extensive quality requirements in the aerospace industry and the need for the systems to handle a wide array of prepreg shapes.

    The demonstrators show that it is possible to automate the manufacturing of complexshaped products using automated layup and forming of prepreg laminates. Tests using the demonstrators indicate that it is possible to meet the quality requirements that apply to manual manufacturing of similar products.

    List of papers
    1. Composite Manufacturing: How Improvement Work Might Lead to Renewed Product Validation
    Open this publication in new window or tab >>Composite Manufacturing: How Improvement Work Might Lead to Renewed Product Validation
    2012 (English)In: Proceedings of the 5th International Swedish Production Symposium / [ed] Mats Björkman, 2012, p. 505-513Conference paper, Published paper (Refereed)
    Abstract [en]

    High-performance polymer composites are mainly used in applications where the benefits of high strength and low weight justify the high material and manufacturing costs. Many of these applications are found today in the aerospace, space and defense industries. Most of today’s commonly used manufacturing methods within this area are highly labor intensive. Furthermore, the quality requirements from the customers require a high level of process control. The purpose of this paper is to explore how changes that are introduced in order to improve productivity in a manufacturing system are managed, particularly with regard to who takes the decision to implement a change and how a change is validated. The study is based on qualitative interviews performed at several companies that manufacture composite components for the aerospace, space and defense sectors. The findings show that the responsibility for deciding to implement a change and the need for validating it are based on many diverse and interconnected factors. Therefore, it is difficult to construct guidelines for early assessment of the scope and cost of a proposed change. Hence each individual change request must be evaluated on its own. The study also shows that the validation process can be adapted to a level that is based on the type of change. In addition, it highlights that control over process parameters in manufacturing is essential.

    Keywords
    composite manufacturing, validation, change request management
    National Category
    Production Engineering, Human Work Science and Ergonomics
    Identifiers
    urn:nbn:se:liu:diva-85268 (URN)978-91-7519-752-4 (ISBN)
    Conference
    5th International Swedish Production Symposium (SPS 2012), 6-8 November 2012, Linköping, Sweden
    Available from: 2012-11-14 Created: 2012-11-14 Last updated: 2017-05-17
    2. Automated Removal of Prepreg Backing Paper - A Sticky Problem
    Open this publication in new window or tab >>Automated Removal of Prepreg Backing Paper - A Sticky Problem
    2013 (English)In: Proceedings of the SAE 2013, Aerotech Congress and Exhibition, 24th-26th September 2013, Montreal,Canada, 2013Conference paper, Published paper (Refereed)
    Abstract [en]

    Automated solutions for manufacturing composite products based on prepreg often imply Automatic Fiber Placement or Automatic Tape Laying. These systems are generally associated with huge investments. For certain manufacturing applications it is interesting to investigate alternatives to find simpler and less costly automation. One example of an automated system could be the use of a standard industrial robot to pick single prepreg plies from an automated cutting machine and stack them to form a plane laminate. This paper is based on a case illustrating a product from the aircraft manufacturing industry. The case will demonstrate a pick and place concept on a general level and illustrate challenges that must be solved. The challenge selected to be the main focus for this paper is an automated process for backing paper removal. A literature review of different gripping technologies reveals several interesting technologies, and the most promising are tested for backing paper removal. The tests show that an automated removal process can be designed by using standard vacuum grippers in combination with mechanical clamping grippers. In order to lift the backing paper with a vacuum gripper an initial separation between the backing paper and prepreg is needed. This separation is most easily mechanically induced by bending the material. The proposed solution for automatic backing paper removal can be integrated in a manufacturing cell for manufacturing of the studied product.

    Keywords
    composite manufacturing, automation, prepreg, gripping technology
    National Category
    Production Engineering, Human Work Science and Ergonomics
    Identifiers
    urn:nbn:se:liu:diva-99344 (URN)10.4271/2013-01-2289 (DOI)
    Conference
    SAE 2013 Aerotech Congress and Exhibition, September 24-26, 2013, Montréal, Quebec, Canada
    Note

    SAE Technical Paper 2013-01-2289

    Available from: 2013-10-16 Created: 2013-10-16 Last updated: 2017-05-17Bibliographically approved
    3. Automation of Composite Manufacturing Using Off-the-shelf Solutions, Three Cases from the Aerospace Industry
    Open this publication in new window or tab >>Automation of Composite Manufacturing Using Off-the-shelf Solutions, Three Cases from the Aerospace Industry
    2015 (English)In: Proceedings of the 20th International Conference on Composite Materials, 2015Conference paper, Published paper (Other academic)
    Abstract [en]

    With an increased use of composite materials follows a need for rational, cost-efficient manufacturing processes. This paper explores how off-the-shelf solutions, developed for other purposes than composite manufacturing, can be used to build systems for automated composite manufacturing. Three demonstrators, each of them dealing with a specific type of material and all of them representing different manufacturing technologies for automated composite manufacturing, are presented and analyzed to find aspects that affect the ability to use off-the-shelf solutions. The three demonstrators target low to medium manufacturing volumes of complex products and they have been developed in collaboration with industrial partners within the aerospace industry. The conclusions drawn from the development of the demonstrators are that it is technically feasible to use off-the-shelf solutions in the three cases while adhering to the high quality standards of the industry. Furthermore three groups of aspects, quality aspects, product aspects and system aspects, which affect the ability to use off-the-shelf solutions for automated composite manufacturing, are identified.

    Keywords
    composite manufacturing, automation, off-the-shelf, aerospace
    National Category
    Production Engineering, Human Work Science and Ergonomics
    Identifiers
    urn:nbn:se:liu:diva-120300 (URN)
    Conference
    ICCM20 - The 20th International Conference on Composite Materials, 19-24th July 2015, Copenhagen Denmark
    Available from: 2015-07-27 Created: 2015-07-27 Last updated: 2017-05-17
    4. Low-cost Automation for Prepreg Handling - Two Cases from the Aerospace Industry
    Open this publication in new window or tab >>Low-cost Automation for Prepreg Handling - Two Cases from the Aerospace Industry
    2016 (English)In: SAE International Journal of Materials & Manufacturing, ISSN 1946-3979, E-ISSN 1946-3987, Vol. 9, no 1, p. 68-74Article in journal (Refereed) Published
    Abstract [en]

    With an increased use of composite materials within the aerospace industry follows a need for rational and cost-effective methods forcomposite manufacturing. Manual operations are still common for low to medium manufacturing volumes and complex products.Manual operations can for example be found in material handling, when picking prepreg plies from a cutter table and stacking them toform a plane laminate in preparation for a subsequent forming operation. Stacking operations of this kind often involves a greatnumber of different ply geometries and removal of backing paper and other protecting materials like plastic. In this paper two differentdemonstrator cells for automated picking of prepreg plies and stacking of plane laminates are presented. One demonstrator is utilizinga standard industrial robot and an advanced end-effector to handle the ply variants. The other demonstrator is using a dual arm robotwhich allow for simpler end-effector design. In combination with a previously developed system for automated removal of backingpapers both systems have shown to be capable of automatically picking prepreg plies from a plane surface and stack them to generate aflat multistack laminate. The dual arm approach has shown advantageous since it result in simpler end-effector design and a successivelay down sequence that result in good adhesion between the plies in the laminate.

    Place, publisher, year, edition, pages
    Warrendale, USA: SAE International, 2016
    Keywords
    Composite, Manufacturing, Automation, Low-cost, Prepreg
    National Category
    Production Engineering, Human Work Science and Ergonomics
    Identifiers
    urn:nbn:se:liu:diva-121602 (URN)10.4271/2015-01-2606 (DOI)000386445700008 ()
    Available from: 2015-09-28 Created: 2015-09-28 Last updated: 2018-02-02Bibliographically approved
    5. Robot-Forming of Prepreg Stacks ‐ Development of Equipment and Methods
    Open this publication in new window or tab >>Robot-Forming of Prepreg Stacks ‐ Development of Equipment and Methods
    Show others...
    2016 (English)In: Proceedings of the 17th European Conference on Composite Materials (ECCM17), 2016Conference paper, Published paper (Other academic)
    Abstract [en]

    Within the aerospace industry the manufacturing of composite components with complex shapes, such as spars, ribs and beams are often manufactured using manual layup and forming of prepreg material. Automated processes for prepreg layup and efficient forming techniques like vacuum forming are sometimes difficult to employ to these type of products due to technical limitations. This paper describes the development of tools and the forming sequence needed to automate sequential forming of a complex shape using an industrial robot. Plane prepreg stacks are formed to the final shape using a dual-arm industrial robot equipped with rolling tools. Tests show that the developed tools and the employed sequence can be used to form stacks to the desired shape with acceptable quality.

    National Category
    Production Engineering, Human Work Science and Ergonomics
    Identifiers
    urn:nbn:se:liu:diva-129925 (URN)
    Conference
    ECCM17 - 17th European Conference on Composite Materials 26-30th June 2016, Munich, Germany
    Available from: 2016-07-01 Created: 2016-07-01 Last updated: 2017-05-17Bibliographically approved
  • 3.
    Chen, Kai
    et al.
    Nanjing University of Science and Technology, Peoples R China; Nanjing University, Peoples R China.
    Guo, Rui
    Nanjing University of Science and Technology, Peoples R China.
    Ma, Chunguang
    Nanjing University of Science and Technology, Peoples R China.
    Dai, Tingyang
    Nanjing University, Peoples R China.
    Ye, Sunjie
    Nanjing University, Peoples R China.
    Lu, Yun
    Nanjing University, Peoples R China.
    Gao, Feng
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Zhu, Jinsong
    Nanjing University, Peoples R China.
    Jiang, Wei
    Nanjing University of Science and Technology, Peoples R China.
    Self-Assembled Core-Shell Polymer Dielectric Prepared by Solution Casting Process2009In: Integrated Ferroelectrics, ISSN 1058-4587, E-ISSN 1607-8489, Vol. 113, p. 1-8Article in journal (Refereed)
    Abstract [en]

    Giant permittivity at 1 MHz, which slightly changes with temperatures, is observed in a polymer composite. The dielectric spectroscopy demonstrates that the samples are electrically heterogeneous. The microstructure observation and the ingredient analysis evidence they self assemble the conducting cores surrounded by the insulating shells. The giant-dielectric phenomenon is therefore attributed to the percolation effect. The electrically heterogeneous microstructure with effective permittivity values about 10 000 can be fabricated by a simple solution casting process in air. The composite is an attractive option to the currently used printing dielectric and the future flexible electronics.

  • 4.
    Chen, Yu-Hsiang
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Rogström, Lina
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Roa, JJ
    Departament de Ciència dels Materials i Enginyería Metal·lúrgica, Universitat Politècnica de Catalunya, EEBE-Campus Diagonal Besòs, Barcelona, Spain.
    Zhu, Jianqiang
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Schramm, Isabella
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering. Functional Materials, Department of Materials Science, Campus D3.3, Saarland University,Saarbrücken, Germany.
    Johnson, LJS
    Sandvik Coromant, SE-126 80 Stockholm, Sweden.
    Schell, N.
    Helmholtz-Zentrum Geesthacht (HZG), Geesthacht, Germany.
    Muecklich, F.
    Functional Materials, Department of Materials Science, Campus D3.3, Saarland University, Saarbrücken, Germany.
    Anglada, M. J.
    Departament de Ciència dels Materials i Enginyería Metal·lúrgica, Universitat Politècnica de Catalunya, EEBE-Campus Diagonal Besòs, Barcelona, Spain.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Thermal and mechanical stability of wurtzite-ZrA1N/cubic-TiN and wurtzite-ZrA1N/cubic-ZrN multilayers2017In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 324, p. 328-337Article in journal (Refereed)
    Abstract [en]

    The phase stability and mechanical properties of wurtzite (w)-Zr(0.25)A1(0.75)N/cubic (c)-TiN and w-Zr(0.25)A1(0.75)N/c-ZrN multilayers grown by arc evaporation are studied. Coherent interfaces with an orientation relation of c-TiN (111)[1-10]IIw-ZrAlN (0001)[11-20] form between ZrA1N and TiN sublayers during growth of the w-ZrAIN/c-TiN multilayer. During annealing at 1100 degrees C a c-Ti(Zr)N phase forms at interfaces between ZrA1N and TiN, which reduces the lattice mismatch so that the coherency and the compressive strain are partially retained, resulting in an increased hardness (32 GPa) after annealing. For the w-ZrAIN/c-ZrN multilayer, there is no coherency between sublayers leading to strain relaxation during annealing causing the hardness to drop. The retained coherency between layers and the compressive strain in the w-ZrAIN/c-TiN multilayer results in superior fracture toughness compared to the w-ZrAIN/c-ZrN multilayer as revealed by cross-sectional investigations of damage events under scratch and indentation tests. (C) 2017 Elsevier B.V. All rights reserved.

  • 5.
    Chieh-Yi, Kuo
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Fabrication and Optical Properties of ZnO Nanocrystal/GaN Quantum Well Based Hybrid Structures2012Independent thesis Advanced level (degree of Master (Two Years)), 80 credits / 120 HE creditsStudent thesis
    Abstract [en]

    Optical properties of hybrid structures based on zinc oxide nanocrystals (NCs) and Gallium Nitride quantum well (QW) has been studied. The ZnO NCs thin films on the top of GaN QW structures were fabricated using spin coating. The surface morphology was characterized by scanning electron microscopy (SEM). We have performed temperature dependence time-resolved photoluminescence (TRPL) measurements of the bare AlGaN/GaN QW structures and hybrids, containing ZnO NCs. It was found that at some temperatures the QW PL decay has shorter decay time in the presence of ZnO NCs thin film compared to the bare QW. The effect was stronger for the samples with thinner cap layers. The results are discussed in terms of three models such as exciton nonradiative energy transfer (NRET), tunneling effect, and piezoelectric field influence on the QW exciton energy.

  • 6.
    Crispin, Xavier
    Linköping University, Department of Science and Technology, Physics and Electronics. Linköping University, Faculty of Science & Engineering.
    Carbon nanotubes get high2016In: NATURE ENERGY, ISSN 2058-7546, Vol. 1, article id 16037Article in journal (Other academic)
    Abstract [en]

    Waste heat can be converted to electricity by thermoelectric generators, but their development is hindered by the lack of cheap materials with good thermoelectric properties. Now, carbon-nanotube-based materials are shown to have improved properties when purified to contain only semiconducting species and then doped.

  • 7.
    Johansson, Martin
    et al.
    Linköping University, Department of Management and Engineering, Machine Design.
    Sundqvist, Johan
    Linköping University, Department of Management and Engineering, Machine Design.
    Utveckling och design av gripdon för komposithantering2013Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Saab AB delivers solutions, products and services in military defence and civil security. In addition to the military aircraft system Gripen, the business area of Aeronautics also produces components for civil aircraft. Customers include Airbus and Boeing. The aim of the project is to design a multifunctional robot end-effector in order to investigate whether the degree of automation in the layup of a composite component can be increased. The layup is today a predominantly manual process. In the targeted production step, the pre-impregnated carbon fiber are cut into plies at a cutting table and accurately placed on a layup template. In this step, the backing paper attached to one side of the ply also has to be removed. A systematic model has been used as a foundation of the development process. The model has consisted of problem definition, a “state of the art” study, defining a design specification, and a functionality analysis. Results from the functionality analysis have then been combined into concepts and evaluated. Finally a prototype based on the winning concept has been built and tested on an actual robot. The prototype’s structure is based on aluminum profiles. The carbon fiber plies are handled with vacuum cups whose placing has been adjusted to fit the geometries of all plies. The step where plies are cut was excluded from the multifunctional end-effector since an automated cutting machine, which is already in use, was considered a more viable option at this stage. Removal of the backing paper is done in three steps. Then the ply is placed on a lay-up template and compacted by a roller to remove any unevenness and to attach the ply better to the surface. Finally, a vacuum cup lifts the previously treated corner of the backing paper and a mechanical claw secures the grip whereupon the paper is removed. A conclusion from this project is that automation of the lay-up with a robot is possible. As a continuation of this project it is suggested that an automated robot cell is constructed to test the automated layup process.

  • 8.
    Kahlin, Magnus
    Linköping University, Department of Management and Engineering. Linköping University, Faculty of Science & Engineering.
    Fatigue Performance of Additive Manufactured Ti6Al4V in Aerospace Applications2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Additive Manufacturing (AM) for metals includes is a group of production methodst hat use a layer-by-layer approach to directly manufacture final parts. In recent years, the production rate and material quality of additive manufactured materials have improved rapidly which has gained increased interest from the industry to use AM not only for prototyping, but for serial production. AM offers a greater design freedom, compared to conventional production methods, which allows for parts with new innovative design. This is very attractive to the aerospace industry, in which parts could be designed to have reduced weight and improved performance contributing to reduced fuel consumption, increased payload and extended flight range. There are, however, challenges yet to solve before the potential of AM could be fully utilized in aerospace applications. One of the major challenges is how to deal with the poor fatigue behaviour of AM material with rough as-built surface.

    The aim of this thesis is to increase the knowledge of how AM can be used for high performance industrial parts by investigating the fatigue behaviour of the titanium alloy Ti6Al4V produced with different AM processes. Foremost, the intention is to improve the understanding of how rough as-built AM surfaces in combination with AM built geometrical notches affects the fatigue properties.This was done by performing constant amplitude fatigue testing to compare different combinations of AM material produced by Electron Beam Melting(EBM) and Laser Sintering (LS) with machined or rough as-built surfaces with or without geometrical notches and Hot Isostatic Pressing (HIP) treatment. Furthermore, the material response can be different between constant amplitude and variable amplitude fatigue loading due to effects of overloads and local plastic deformations. The results from constant amplitude testing were used to predict the fatigue life for variable amplitude loading by cumulative damage approach and these predictions were then verified by experimental variable amplitude testing.

    The constant amplitude fatigue strength of material with rough as-built surfaces was found to be 65-75 % lower, compared to conventional wrought bar, in which HIP treatments had neglectable influence on the fatigue strength. Furthermore, the fatigue life predictions with cumulative damage calculations showed good agreement with the experimental results which indicates that a cumulative damage approach can be used, at least for a tensile dominated load sequences, to predict the fatigue behaviour of additive manufactured Ti6Al4V.

    List of papers
    1. Fatigue Behaviour of Notched Additive Manufactured Ti6Al4V with As-built Surfaces
    Open this publication in new window or tab >>Fatigue Behaviour of Notched Additive Manufactured Ti6Al4V with As-built Surfaces
    2017 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, no 101, p. 51-60Article in journal (Refereed) Published
    Abstract [en]

    Additive manufacturing (AM) allows the manufacturer to produce parts with complex geometries that are difficult to produce with conventional production methods. Generally, AM is considered to have great potential for the aerospace industry by contributing to reduced weight and lower costs. There are a number of challenges to be solved before AM can be fully utilized in the aerospace industry, and the understanding of fatigue behaviour is one of the major challenges. Although the fatigue properties of flat additive manufactured specimens with rough as-built surfaces already have been widely studied, in practice, few aerospace components have a simple flat geometry with no corners or radii that would act as stress concentrations. Therefore, the combined effect on fatigue life of a rough as-built surface and a geometrical notch needs to be established. In this study, the fatigue properties of both laser sintered and electron beam melted Ti6Al4V have been investigated and a combined effect of a rough as-built surface and a geometrical notch has been determined. In addition, hot isostatic pressing was found to have no impact on fatigue life for rough as-built surfaces. These findings can be directly applied to predict fatigue behaviour of an AM industrial component.

    Place, publisher, year, edition, pages
    Elsevier, 2017
    Keywords
    Additive manufacturing, Fatigue, Ti6Al4V, Stress concentration, Fatigue notch factor
    National Category
    Materials Engineering
    Identifiers
    urn:nbn:se:liu:diva-137163 (URN)10.1016/j.ijfatigue.2017.04.009 (DOI)000403635300007 ()
    Note

    Funding agences: Saab AB; Swedish Foundation for Strategic Research; European commission, through the Clean Sky 2 programme

    Available from: 2017-05-08 Created: 2017-05-08 Last updated: 2017-07-07Bibliographically approved
    2. Fatigue behaviour of additive manufactured Ti6Al4V, with as-built surfaces, exposed to variable amplitude loading
    Open this publication in new window or tab >>Fatigue behaviour of additive manufactured Ti6Al4V, with as-built surfaces, exposed to variable amplitude loading
    2017 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 103, p. 353-362Article in journal (Refereed) Published
    Abstract [en]

    Additive Manufacturing (AM) allows for great design freedom compared to conventional manufacturing. This is very attractive for the aerospace industry in which AM could contribute to lightweight designs and thereby reduce fuel consumption, increase payload and extend flight range. The fatigue behaviour for rough as-built AM surfaces has previously been characterized with constant amplitude testing but in aerospace applications, most parts are exposed to variable amplitude loading. The fatigue behaviour for variable amplitude is not always consistent with the behaviour for constant amplitude due to effects of overloads and local plastic deformations. Therefore, variable amplitude loading behaviour of laser sintered and electron beam melted Ti6Al4V, with rough as-built surfaces have been investigated in this study using the Short-FALSTAFF (Fighter Aircraft Loading STAndard For Fatigue) load sequence. The predicted and the experimental fatigue life was overall consistent even though most experimental results exceeded the predicted life, especially for the laser sintered material. These findings show that conventional cumulative damage fatigue life predictions give reliable predictions for AM materials with rough as-built surfaces for the type of tension dominated load sequence used. (C) 2017 Elsevier Ltd. All rights reserved.

    Place, publisher, year, edition, pages
    ELSEVIER SCI LTD, 2017
    Keywords
    Additive manufacturing; Ti6Al4V; Fatigue; Variable amplitude loading; Stress concentration
    National Category
    Applied Mechanics
    Identifiers
    urn:nbn:se:liu:diva-140783 (URN)10.1016/j.ijfatigue.2017.06.023 (DOI)000407982800034 ()
    Note

    Funding Agencies|Saab AB; Swedish Foundation for Strategic Research; European Commission, through the Clean Sky 2 programme

    Available from: 2017-09-13 Created: 2017-09-13 Last updated: 2017-11-15
  • 9.
    Kapidzic, Zlatan
    et al.
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering. Saab AB, SE-58188 Linkoping, Sweden.
    Ansell, Hans
    Saab AB, SE-58188 Linkoping, Sweden.
    Schon, Joakim
    Swedish Def Research Agency, Sweden.
    Simonsson, Kjell
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
    Fatigue bearing failure of CFRP composite in biaxially loaded bolted joints at elevated temperature2015In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 127, p. 298-307Article in journal (Refereed)
    Abstract [en]

    Hybrid composite-aluminium structures develop internal loads when exposed to elevated temperatures, due to the different thermal expansion properties of the constituent materials. In aircraft structures with long rows of bolted joints, the mechanical and the thermally induced bolt loads are oriented in different directions, creating a biaxial bearing load state. In this study, the bearing fatigue failure process and the influence of the biaxial load state on the failure are investigated. An experimental set-up was designed, where both the mechanical and the thermally induced bolt loads were applied by means of mechanical load actuators. Two-bolt, double-lap joints with quasi-isotropic carbon-epoxy composite specimens were subjected to uniaxial and biaxial cyclic loading at 90 degrees C. A microscopy study of the bearing plane revealed that the main fatigue driving mechanisms were matrix cracking and fibre-matrix debonding. Motivated by these findings, a fatigue prediction model based on the kinetic theory of fracture for polymer matrices was run in a finite element code and the results showed a satisfactory correlation to the experimental results. The biaxial loading resulted in a longer fatigue life than the uniaxial loading, for the same peak resultant force, which was explained by the smaller effective stress range in the biaxial case.

  • 10.
    Kapidzic, Zlatan
    et al.
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering. Saab AB, SE-58188 Linkoping, Sweden.
    Ansell, Hans
    Saab AB, SE-58188 Linkoping, Sweden.
    Schön, Joakim
    Swedish Def Research Agency, Sweden.
    Simonsson, Kjell
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
    Fatigue bearing failure of CFRP composite in bolted joints exposed to biaxial variable amplitude loading at elevated temperature2016In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 142, p. 71-77Article in journal (Refereed)
    Abstract [en]

    Hybrid structures than contain composite-aluminium interfaces tend to develop internal loads at elevated temperatures. In long bolted joints, the thermally induced bolt loads are superimposed onto the mechanically applied load and can induce a biaxial bearing load state. This paper presents an experimental and numerical study of the bearing fatigue failure of carbon-epoxy laminate specimens, exposed to uniaxial and biaxial variable amplitude loading at 90C. A specifically designed experimental rig was used, where both the mechanical and the thermally induced bolt loads were applied by means of mechanical load actuators. A fatigue model based on the kinetic theory of fracture for polymers, which was previously implemented for constant amplitude loading, is expanded to account for the variable amplitude load history. The results suggest that the biaxial loading gives a longer fatigue life than the uniaxial loading for the same maximum peak resultant force. This result can be utilized as a conservative dimensioning strategy by designing biaxially loaded joints in terms of maximum peak resultant bearing load using uniaxial fatigue data.

  • 11.
    Loukil, Mohamed Sahbi
    Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå.
    Experimental and numerical studies of intralaminar cracking in high performance composites2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The macroscopic failure of composite laminates subjected to tensile increasing load is preceded by initiation and evolution of several microdamage modes. The most common damage mode and the one examined in this thesis is intralaminar cracking in layers. Due to this kind of microdamage the laminate undergoes stiffness reduction when loaded in tension. For example, the elastic modulus in the loading direction and the corresponding Poisson’s ratio will decrease.

    The degradation of the elastic properties of these materials is caused by reduced stress in the damaged layer which is mainly due to two parameters: crack opening displacement (COD) and crack sliding displacement (CSD). At fixed applied load these parameters depend on the properties of the damaged and surrounding layers, on layer orientation and on thickness. When the number of cracks per unit length is high (high crack density in the layer) the COD and CSD are reduced because of to crack interaction.

    The main objective of the first paper is to investigate the effect of crack interaction on COD using FEM and to describe the identified dependence on crack density in a simple and accurate form by introducing an interaction function dependent on crack density. This interaction function together with COD of non-interactive crack gives accurate predictions of the damaged laminate properties. The application of this function to more complex laminate lay-ups is demonstrated. All these calculations are performed assuming that cracks are equidistant.

    However, the crack distribution in the damaged layer is very non-uniform, especially in the initial stage of multiple cracking. In the second paper, the earlier developed model for general symmetric laminates is generalized to account for non-uniform crack distribution. This model is used to calculate the axial modulus of cross-ply laminates with cracks in internal and surface layers. In parametric analysis the COD and CSD are calculated using FEM, considering the smallest versus the average crack spacing ratio as non-uniformity parameter. It is shown that assuming uniform distribution we obtain lower bond to elastic modulus. A “double-periodic” approach presented to calculate the COD of a crack in a non-uniform case as the average of two solutions for periodic crack systems is very accurate for cracks in internal layers, whereas for high crack density in surface layers it underestimates the modulus reduction.

    In the third paper, the thermo-elastic constants were calculated using shear lag models and variational models in a general calculation approach (GLOB-LOC) for symmetric laminates with transverse cracks in 90° layer. The comparison of these two models with FEM was presented for cross-ply and quasi-isotropic laminates.

    Using FEM, we assume linear elastic material with ideal crack geometry. Fiber bridging over the crack surface is possible which can affect COD and CSD. The only correct way to validate these assumptions is through experiments.

    The main objective of the fourth and the fifth paper is to measure these parameters for different laminate lay-ups in this way providing models with valuable information for validation of used assumptions and for defining limits of their application. In particular, the displacement field on the edge of a [90/0]s and [903/0]s carbon fiber/epoxy laminates specimens with multiple intralaminar cracks in the surface layer is studied. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry).

    List of papers
    1. Engineering expressions for thermo-elastic constants of laminates with high density of transverse cracks
    Open this publication in new window or tab >>Engineering expressions for thermo-elastic constants of laminates with high density of transverse cracks
    2013 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 48, no 1, p. 37-46Article in journal (Refereed) Published
    Abstract [en]

    Thermo-elastic constants of symmetric and balanced laminates with intralaminar cracks in 90-layers depend on the opening displacement (COD) of the crack. The COD dependence on the interaction between cracks in the same layer is studied using FEM. The COD dependence on crack density is described by interaction function in form of tanh(). This interaction function multiplied with COD of non-interactive crack is the input parameter in analytical model for thermo-elastic properties of damaged symmetric and balanced laminates. Predictions performed for cross-ply laminates with cracks in inside and in surface layers and for quasi-isotropic laminates with different position of the 90-layer are in a very good agreement with direct FEM calculations.

    Place, publisher, year, edition, pages
    Elsevier, 2013
    National Category
    Composite Science and Engineering
    Research subject
    Polymeric Composite Materials
    Identifiers
    urn:nbn:se:liu:diva-150448 (URN)10.1016/j.compositesa.2012.12.012 (DOI)000317880200005 ()2-s2.0-84873630424 (Scopus ID)
    Note

    Validerad; 2013; 20130125 (andbra)

    Available from: 2016-09-29 Created: 2018-08-23 Last updated: 2018-08-27Bibliographically approved
    2. Applicability of solutions for periodic intralaminar crack distributions to non-uniformly damaged laminates
    Open this publication in new window or tab >>Applicability of solutions for periodic intralaminar crack distributions to non-uniformly damaged laminates
    2013 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 47, no 3, p. 287-301Article in journal (Refereed) Published
    Abstract [en]

    Stiffness reduction simulation in laminates with intralaminar cracks is usually performed assuming that cracks are equidistant and crack density is the only parameter needed. However, the crack distribution in the damaged layer is very non-uniform, especially in the initial stage of multiple cracking. In this article, the earlier developed model for general symmetric laminates is generalized to account for non-uniform crack distribution. This model, in which the normalized average crack-opening and crack-sliding displacements are the main characteristics of the crack, is used to calculate the axial modulus of cross-ply laminates with cracks in internal and surface layers. In parametric analysis, the crack-opening displacement and crack-sliding displacement are calculated using finite element method, considering the smallest versus the average crack spacing ratio as non-uniformity parameter. It is shown that assuming uniform distribution, we obtain lower bond to elastic modulus. A ‘double-periodic’ approach presented to calculate the crack-opening displacement of a crack in a non-uniform case as the average of two solutions for periodic crack systems is very accurate for cracks in internal layers of cross-ply laminates, whereas for high crack density in surface layers, it underestimates the modulus reduction.

    Place, publisher, year, edition, pages
    Sage Publications, 2013
    Keywords
    Laminates, transverse cracking, crack-opening displacement, elastic properties, damage mechanics
    National Category
    Composite Science and Engineering
    Research subject
    Polymeric Composite Materials
    Identifiers
    urn:nbn:se:liu:diva-150444 (URN)10.1177/0021998312440126 (DOI)000313902100002 ()2-s2.0-84872914468 (Scopus ID)
    Note

    Validerad; 2013; 20120323 (andbra)

    Available from: 2016-09-29 Created: 2018-08-23 Last updated: 2018-08-27Bibliographically approved
    3. Thermoelastic constants of symmetric laminates with cracks in 90-layer: application of simple models
    Open this publication in new window or tab >>Thermoelastic constants of symmetric laminates with cracks in 90-layer: application of simple models
    Show others...
    2013 (English)In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 42, no 4, p. 157-166Article in journal (Refereed) Published
    Abstract [en]

    The change of thermoelastic properties of cross-ply and quasi-isotropic laminates with intralaminar cracks in layers is analysed. Predictions are performed using previously derived general expressions for stiffness of symmetric damaged laminates as dependent on crack density and crack face opening and sliding. It is shown that the average crack opening displacement can be linked with the average value of axial stress perturbation between two cracks. Using this relationship, analytical shear lag and Hashin’s models, developed for axial modulus, can be applied for calculating thermal expansion coefficients, in-plane moduli and Poisson’s ratios of damaged laminates. The approach is evaluated using finite element method and it is shown that the accuracy is rather similar to that in axial modulus calculation.

    Place, publisher, year, edition, pages
    Taylor & Francis, 2013
    National Category
    Composite Science and Engineering
    Research subject
    Polymeric Composite Materials
    Identifiers
    urn:nbn:se:liu:diva-150434 (URN)10.1179/1743289811Y.0000000064 (DOI)000318218300005 ()2-s2.0-84876901876 (Scopus ID)
    Note

    Validerad; 2013; 20130308 (andbra)

    Available from: 2016-09-29 Created: 2018-08-23 Last updated: 2018-08-27Bibliographically approved
    4. Damage characterization in glass fiber/epoxy laminates using electronic speckle pattern interferometry
    Open this publication in new window or tab >>Damage characterization in glass fiber/epoxy laminates using electronic speckle pattern interferometry
    2015 (English)In: Experimental techniques (Westport, Conn.), ISSN 0732-8818, E-ISSN 1747-1567, Vol. 39, no 2, p. 38-45Article in journal (Refereed) Published
    Abstract [en]

    The degradation of the elastic properties of composite laminates with intralaminar cracks is caused by reduced stress in the damaged layer which is mainly due to two parameters: the crack opening displacement (COD) and the crack sliding displacement (CSD). In this paper these parameters are measured experimentally, providing laminate stiffness reduction models with valuable information for validation of used assumptions and for defining limits of their application. In particular, the displacement field on the edges of a [0/ +704/ −704]s glass fiber/epoxy laminate specimens with multiple intralaminar cracks is studied, and the COD and CSD dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (electronic speckle pattern interferometry). By studying the displacement discontinuities, the crack face displacements were measured. A comparison between the COD and the CSD (for the same crack) is performed

    Place, publisher, year, edition, pages
    John Wiley & Sons, 2015
    National Category
    Composite Science and Engineering
    Research subject
    Polymeric Composite Materials
    Identifiers
    urn:nbn:se:liu:diva-150440 (URN)10.1111/ext.12013 (DOI)000351440000006 ()2-s2.0-84924981891 (Scopus ID)
    Note

    Validerad; 2015; Nivå 2; 20130111 (andbra)

    Available from: 2016-09-29 Created: 2018-08-23 Last updated: 2018-08-27Bibliographically approved
  • 12.
    Loukil, Mohamed Sahbi
    Division of Materials Science, Department of Engineering, Sciences and Mathematics, Luleå University of Technology, Luleå, Sweden.
    Microcracking in fiber composites and degradation of thermo-elastic properties of laminates2011Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The macroscopic failure of composite laminates subjected to tensile increasing load is preceded by initiation and evolution of several microdamage modes. The most common damage mode and the one examined in this thesis is intralaminar cracking in layers. Due to this kind of microdamage the laminate undergoes stiffness reduction when loaded in tension. For example, the elastic modulus in the loading direction and the corresponding Poisson’s ratio will decrease.

    The degradation of the elastic properties of these materials is caused by reduced stress in the damaged layer which is mainly due to two parameters: crack opening displacement (COD) and crack sliding displacement (CSD). At fixed applied load these parameters depend on the properties of the damaged and surrounding layers, on layer orientation and on thickness. When the number of cracks per unit length is high (high crack density in the layer) the COD and CSD are reduced because of to crack interaction.

    The main objective of the first paper is to investigate the effect of crack interaction on COD using FEM and to describe the identified dependence on crack density in a simple and accurate form by introducing an interaction function dependent on crack density. This interaction function together with COD of non-interactive crack gives accurate predictions of the damaged laminate thermo-elastic properties. The application of this function to more complex laminate lay-ups is demonstrated. All these calculations are performed assuming that cracks are equidistant.

    However, the crack distribution in the damaged layer is very non-uniform, especially in the initial stage of multiple cracking. In the second paper, the earlier developed model for general symmetric laminates is generalized to account for non-uniform crack distribution. This model is used to calculate the axial modulus of cross-ply laminates with cracks in internal and surface layers. In parametric analysis the COD and CSD are calculated using FEM, considering the smallest versus the average crack spacing ratio as non-uniformity parameter. It is shown that assuming uniform distribution we obtain lower bond to elastic modulus. A “double-periodic” approach presented to calculate the COD of a crack in a non-uniform case as the average of two solutions for periodic crack systems is very accurate for cracks in internal layers, whereas for high crack density in surface layers it underestimates the modulus reduction.

    In the third paper, the thermo-elastic constants of damaged laminates were calculated using shear lag models and variational models in a general calculation approach (GLOB-LOC) for symmetric laminates with transverse cracks in 90° layer. The comparison of these two models with FEM was presented for cross-ply and quasi-isotropic laminates.

  • 13.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, University of Lorraine, EEIGM, Nancy, France.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    ESPI analysis of crack face displacements in damaged laminates2014In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 94, p. 80-88Article in journal (Refereed)
    Abstract [en]

    Crack opening displacement (COD) and crack sliding displacement (CSD) are the two parameters that define slope of stiffness reduction with increasing crack density. The values of these parameters strongly depend on the crack geometry which is difficult to observe and quantify. In models we usually assume straight crack, no local delaminations (questionable for surface cracks) and use uniform spacing. In this paper COD of cracks in surface layers are analyzed using the displacement field on the edge and on the surface of a [90/0]s and [903/0]s carbon fiber/epoxy laminates subjected to tension. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). It is shown that including in the FEM model crack locations exactly as in the test, the COD of an individual crack is predicted accurately. The model with periodic and symmetric crack distribution significantly underestimates the COD whereas staggered crack distribution model in many cases renders very good results. Crack face sliding displacements, CSD were measured on the edge of [0/704/-704]s GF/EP laminate, showing that the sliding can not be neglected and that the COD versus CSD ratio for this laminate is similar to model predictions.

  • 14.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Farge, Laurent
    Institut Jean Lamour, Nancy Universite, France.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, Nancy Universite.
    Characterization of damage evolution in thermo-echanical loading of composites materials2009In: Book of abstracts: 5th EEIGM International Conference on Advanced Materials Research, November 4-5, 2009, Nancy, France, 2009Conference paper (Refereed)
    Abstract [en]

    In the experimental part, the displacement field on the edges of a [0, 554, -554]s GF/EP laminate and [302,-602]s GF/EP laminate specimens with multiple transverse cracks is visualized and analyzed on dependence of the applied mechanical load by using ESPI(Electronic Speckle Pattern Interferometry).The different displacement profiles along the tensile-axis are drawn along the specimen edges at several distances from the mid-plane corresponding to the different plies. The measurements out-of-plane deformations are effected to determine and to compare the crack opening displacement (COD) with the crack face sliding displacement (CSD). In the numerical part, the strong dependence of the average normalized (COD) on the relative stiffness and thickness of the surrounding layers, based on FE results, is described by a simple empirical expression for non-interactive cracks. The FEM simulation results obtained by ABAQUS/CAE are compared with the general expressions for calculation of elastic constants of the damaged laminate developed by Lundmark and Varna [1]. The effect of crack interaction on the crack opening displacement (COD) is also discussed in this study. The effect on (COD) is described by the introduced “interaction function” which is determined using FE analysis.

  • 15.
    Loukil, Mohamed Sahbi
    et al.
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Hussain, W.
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Kirti, A.
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Pupurs, Andrejs
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Thermoelastic constants of symmetric laminates with cracks in 90-layer: application of simple models2013In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 42, no 4, p. 157-166Article in journal (Refereed)
    Abstract [en]

    The change of thermoelastic properties of cross-ply and quasi-isotropic laminates with intralaminar cracks in layers is analysed. Predictions are performed using previously derived general expressions for stiffness of symmetric damaged laminates as dependent on crack density and crack face opening and sliding. It is shown that the average crack opening displacement can be linked with the average value of axial stress perturbation between two cracks. Using this relationship, analytical shear lag and Hashin’s models, developed for axial modulus, can be applied for calculating thermal expansion coefficients, in-plane moduli and Poisson’s ratios of damaged laminates. The approach is evaluated using finite element method and it is shown that the accuracy is rather similar to that in axial modulus calculation.

  • 16.
    Loukil, Mohamed Sahbi
    et al.
    Swerea SICOMP, Piteå, Sweden.
    Varna, Janis
    Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå, Sweden.
    Characterization of damaged composite laminates using Electronic Speckle Pattern Interferometry (ESPI)2016In: Proceedings of the 17th European Conference on Composite Materials ECCM17 - 17th European Conference on Composite Materials, 26-30th June 2016, Munich, Germany, European Conference on Composite Materials , 2016Conference paper (Refereed)
    Abstract [en]

    The degradation of the elastic properties of composite laminates with intralaminar cracks is caused by reduced stress in the damaged layer which is mainly due to two parameters: the crack opening displacement (COD) and the crack sliding displacement (CSD). In this paper these parameters are measured experimentally providing laminate stiffness reduction models with valuable information for validation of used assumptions and for defining limits of their application. In particular, the displacement field on the edges of a [0/ +704/ -704]s glass fiber/epoxy laminate specimens with multiple intralaminar cracks is studied and the COD and CSD dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). By studying the displacement discontinuities, the crack face displacements were measured. A comparison between the COD and the CSD (for the same crack) is performed.

  • 17.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Z.
    Institut Jean Lamour, University of Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Thermo-elastic properties degradation in damaged laminates with high density of transverse cracks2013Conference paper (Other academic)
    Abstract [en]

    During the service life multidirectional laminate with an arbitrary lay-up is subjected to varying thermo-mechanical loading and in result different number on intralaminar cracks can develop in different layers of the laminate. These cracks called also transverse cracks are reducing the thermo-elastic properties of laminates. A model called GLOB-LOC model was developed in [1,2] to calculate these properties for any given damage state. This analytical calculation scheme is exact if the input information is exact. However, the expressions contain two important approximate parameters (functions) which are obtained summarizing/fitting parametric analyzes results using FEM. These parameters are proportional to the average stress state change between two cracks with respect to the undamaged stress. The parameters govern the stiffness reduction of the laminate and have the physical meaning of average opening displacement (COD) and average sliding displacement (CSD) of a crack. The parameters COD and CSD depend mostly on neighboring layer thickness and stiffness which makes them robust parameters in the model. Unfortunately, the situation changes when the crack density (number of cracks per mm) is high and the distance between cracks is comparable with the crack size (cracked layer thickness). In this crack density range the cracks are interacting and the COD and CSD depend on the distance between them. In this paper the crack interaction effects on laminate stiffness are analyzed using FEM. Based on FEM results a simple interaction function which depends on the cracked and neighboring layer thickness, elastic properties and the distance between cracks is suggested. This interaction function multiplied with COD of a non-interactive crack is the input parameter in analytical model for thermo-elastic properties of damaged symmetric and balanced laminates. It is shown that these functions when used in the GLOB-LOC model give high accuracy for all thermo-elastic constants of damaged laminates. As an example, the predicted axial modulus and Poisson’s ratio of cross-ply laminates with cracks in inside 90-layer are shown for GF/EP composite in Fig.1. FEM data are shown with symbols, predictions according to the interaction function by solid curves and dotted lines represent the ply-discount model. It has to be reminded that the normalized crack density is introduced as inverse to the crack spacing normalized with ply thickness. Therefore, the values of the normalized density larger than 1 are extremely high and are with no practical interest. Data in this region are shown to demonstrate the asymptotic approaching to the ply-discount value.

  • 18.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, France.
    Applicability of solutions for periodic intralaminar crack distributions to non-uniformly damaged laminates2013In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 47, no 3, p. 287-301Article in journal (Refereed)
    Abstract [en]

    Stiffness reduction simulation in laminates with intralaminar cracks is usually performed assuming that cracks are equidistant and crack density is the only parameter needed. However, the crack distribution in the damaged layer is very non-uniform, especially in the initial stage of multiple cracking. In this article, the earlier developed model for general symmetric laminates is generalized to account for non-uniform crack distribution. This model, in which the normalized average crack-opening and crack-sliding displacements are the main characteristics of the crack, is used to calculate the axial modulus of cross-ply laminates with cracks in internal and surface layers. In parametric analysis, the crack-opening displacement and crack-sliding displacement are calculated using finite element method, considering the smallest versus the average crack spacing ratio as non-uniformity parameter. It is shown that assuming uniform distribution, we obtain lower bond to elastic modulus. A ‘double-periodic’ approach presented to calculate the crack-opening displacement of a crack in a non-uniform case as the average of two solutions for periodic crack systems is very accurate for cracks in internal layers of cross-ply laminates, whereas for high crack density in surface layers, it underestimates the modulus reduction.

  • 19.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Application of the electronic speckle pattern interferometry (espi) technique for the characterization of damage in composite laminates2012Conference paper (Refereed)
    Abstract [en]

    Composite laminates under service loading undergo complex combinations of thermal and mechanical loading leading to microdamage accumulation in the plies. The most common damage mode is intralaminar cracking in layers. The crack opening displacement (COD) and the crack sliding displacement (CSD) during loading reduce the average stress in the damaged layer, thus reducing the laminate stiffness. In other words, the elastic modulus in the loading direction and the corresponding Poisson’s ratio decrease. These parameters depend on material properties of the damaged layer and surrounding layers, on layer orientation and thickness. Previously these parameters have been calculated using finite element method (FEM) assuming linear elastic material with idealized geometry of cracks [1]. The only correct way to validate these assumptions is through experiments. The main objective of this paper is to measure these parameters for different laminate lay-ups in this way providing models with valuable information for validation of used assumptions and for defining limits of their application. In particular, the displacement field on the edges of a [0/ 704/ -704]sand [0/554/ -554]s glass fiber/epoxy laminate specimens with multiple intralaminar cracks is studied and the (COD) dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry) [2,3]. ESPI is an optical technique that provides the displacement for every point on a surface and offers the possibility to measure both, the in-plane and out-of-plane displacement without surface preparation. The displacement jumps corresponding to cracks are clearly visible and can be used to determine the opening displacement along the cracks. The effect of crack interaction on the (COD) at high crack density is also investigated.

  • 20.
    Loukil, Mohamed Sahbi
    et al.
    Institut Jean Lamour, (SI2M), Nancy, France / Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, (SI2M), Nancy, France.
    Characterization of damaged composite laminates by an optical measurement of the displacement field2012In: 6th EEIGM International Conference Advanced Materials Research: 7th and 8th November, 2011 EEIGM, Nancy, France, Bristol: Institute of Physics (IOP), 2012, Vol. 31, p. 1-7Conference paper (Refereed)
    Abstract [en]

    The degradation of the elastic properties of composite laminates with intralaminar cracks is caused by reduced stress in the damaged layer which is mainly due to two parameters: the crack opening displacement (COD) and the crack sliding displacement (CSD). In this paper these parameters are measured experimentally providing laminate stiffness reduction models with valuable information for validation of used assumptions and for defining limits of their application. In particular, the displacement field on the edges of a [0/ +704/ −704]s glass fiber/epoxy laminate specimens with multiple intralaminar cracks is studied and the COD and CSD dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). By studying the displacement discontinuities, the crack face displacements were measured. A comparison between the COD and the CSD (for the same crack) is performed

  • 21.
    Loukil, Mohamed Sahbi
    et al.
    Department of Engineering Sciences and Mathematics, Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Department of Engineering Sciences and Mathematics, Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Science et Ingénierie des Matériaux et Métallurgie (SI2M), Institut Jean Lamour, Nancy.
    Damage characterization in glass fiber/epoxy laminates using electronic speckle pattern interferometry2015In: Experimental techniques (Westport, Conn.), ISSN 0732-8818, E-ISSN 1747-1567, Vol. 39, no 2, p. 38-45Article in journal (Refereed)
    Abstract [en]

    The degradation of the elastic properties of composite laminates with intralaminar cracks is caused by reduced stress in the damaged layer which is mainly due to two parameters: the crack opening displacement (COD) and the crack sliding displacement (CSD). In this paper these parameters are measured experimentally, providing laminate stiffness reduction models with valuable information for validation of used assumptions and for defining limits of their application. In particular, the displacement field on the edges of a [0/ +704/ −704]s glass fiber/epoxy laminate specimens with multiple intralaminar cracks is studied, and the COD and CSD dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (electronic speckle pattern interferometry). By studying the displacement discontinuities, the crack face displacements were measured. A comparison between the COD and the CSD (for the same crack) is performed

  • 22.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM, Nancy Cedex, France.
    Degradation of thermo-elastic properties of non-uniformly composite laminates2012Conference paper (Other academic)
    Abstract [en]

    During the service life composite laminates undergo complex combinations of thermal and mechanical loading leading to microdamage accumulation in the plies. The most common damage mode is intralaminar cracking in layers. These cracks are reducing the thermo-elastic properties of laminates. A model called GLOB-LOC model was developed in to calculate these properties for any given damage state. This analytical calculation scheme is exact if the input information is exact. However, the expressions contain two important approximate parameters which are obtained summarizing/fitting parametric analyzes results using FEM. These parameters are proportional to the average stress state between cracks which governsthe stiffness reduction of the laminate and have the physical meaning of average opening displacement (COD) and average sliding displacement (CSD) of a crack. Using this relationship analytical shear lag and Hashin’s models, developed for axial modulus, can be applied for calculating both thermal expansion coefficients, in-plane moduli and Poisson’s ratios of damaged laminates. The approach is evaluated using FEM and it shown that the accuracy is rather similar as in axial modulus calculation. Most of the existing models are based on assumption that the crack distribution in the layer is uniform: the same distance between all cracks. Since optical observations show that it is not true, we present FEM simulation results to evaluate the effect of non-uniform crack distribution on elastic properties of RVE showing that the differences may be rather large. A “double-periodic” approach presented to calculate the COD of a crack in a non-uniform case as the average of two solutions for periodic crack systems is very accurate for cracks in internal layers, whereas for high crack density in surface layers it underestimates the modulus reduction

  • 23.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Engineering expressions for thermo-elastic constants of laminates with high density of transverse cracks2013In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 48, no 1, p. 37-46Article in journal (Refereed)
    Abstract [en]

    Thermo-elastic constants of symmetric and balanced laminates with intralaminar cracks in 90-layers depend on the opening displacement (COD) of the crack. The COD dependence on the interaction between cracks in the same layer is studied using FEM. The COD dependence on crack density is described by interaction function in form of tanh(). This interaction function multiplied with COD of non-interactive crack is the input parameter in analytical model for thermo-elastic properties of damaged symmetric and balanced laminates. Predictions performed for cross-ply laminates with cracks in inside and in surface layers and for quasi-isotropic laminates with different position of the 90-layer are in a very good agreement with direct FEM calculations.

  • 24.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Finite element analysis and experimental comparison of the crack opening displacement profile in the damaged cross-ply laminates2013In: Book of Abstracts: 7th EEIGM International Conference on Advanced Materials Research, 21–22 March 2013 in Luleå, Sweden, 2013Conference paper (Refereed)
    Abstract [en]

    During the service life composite laminates undergo complex combinations of thermal and mechanical loading leading to microdamage accumulation in the plies. The most common damage mode and the one examined in this work is intralaminar cracking in layers. The crack opening displacement (COD) and the crack sliding displacement (CSD) during loading reduce the average stress in the damaged layer, thus reducing the laminate stiffness. These parameters depend on material properties of the damaged layer and surrounding layers, on layer orientation and thickness. Previously these parameters have been calculated using finite element method (FEM) assuming linear elastic material with idealized geometry of cracks. To validate these assumptions experimentally the displacement field on the surface of a [90/0/90], [90/0]s and [903/0]s carbon fiber/epoxy laminates specimens with multiple intralaminar cracks in the surface layer is studied. The profile of the COD along the width is investigated for each single crack. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). The effect of crack interaction as well as the effect of the thickness of 90° layers on the COD is given in this paper.

  • 25.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, SI2M, Nancy-Université, France.
    Parameter determination in damaged laminate model by optical full-field measurement of the displacement using ESPI2011Conference paper (Other academic)
  • 26.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, SI2M, Nancy-Université, France.
    Thermo-elastic properties of damaged laminates at high density of cracks in layers2011In: Aerospace - the global innovation driver: 6th International Conference "Supply on the Wings" ; November 02 - 04, 2011, Frankfurt, Germany ; conference proceedings ; in conjunction with AIRTEC, International Aerospace Supply Fair, Frankfurt: airtec GmbH , 2011Conference paper (Refereed)
  • 27.
    Loukil, Mohamed Sahbi
    et al.
    Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering. Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM, Nancy Cedex, France.
    Thermo-elastic properties of non-uniformly damaged laminates at high crack density2012Conference paper (Other academic)
  • 28.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, University of Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Validation of FEM based damaged laminate model measuring crack opening displacement in cross-ply laminate using Electronic Speckle Pattern Interferometry (ESPI)2013Conference paper (Refereed)
    Abstract [en]

    Composite laminates during service undergo complex combinations of thermal and mechanical loading leading to microdamage accumulation in the plies. The most common damage mode and the one examined in this work is intralaminar cracking in layers. The crack opening displacement (COD) and the crack sliding displacement (CSD) during loading reduce the average stress in the damaged layer, thus reducing the laminate stiffness. Finite element method (FEM) studies were performed in [1,2] to understand which material and geometry parameters affect the COD and CSD most and simple empirical relationships (power law) were suggested. All these studies and analysis assume a linear elastic material with idealized geometry of cracks. The only correct way to validate these assumptions is through experiments. The effect of material properties on COD was measured experimentally using optical microscopy of loaded damaged specimens in [3,4]. It was shown that the measured average values of COD are affected by the constraining layer orientation and stiffness. The experimental determination of the average COD and CSD needs the measurement of the displacement for all points of the crack surfaces, which justifies the use of full-field measurement technique, Electronic Speckle Pattern Interferometry (ESPI). ESPI is an optical technique that provides the displacement for every point on a surface and offers the possibility to measure both, the in-plane and out-of-plane displacement without surface preparation. This technique was used in [5,6] to measure the COD for inside cracks on the specimen’s edge. It was shown that the profile of the crack on the edge is elliptical. The main objective of this paper is to study cracks in surface layers by measuring the COD along the crack path. For this reason the cross-ply laminate with surface cracks was selected. In particular, the displacement field on the surface of a [903/0]S carbon fiber/epoxy laminate specimen with multiple intralaminar cracks is studied and the relative displacement dependence on the applied mechanical load is measured. By looking to the displacement field the cracks appear as singularities and the corresponding displacement jumps are directly related to COD and CSD. The transverse cracks are parallel to the fiber orientation in the layer, which in our case corresponds to a 90 direction with respect to the tensile axis. Consequently, there is no relative sliding of the crack faces and the only displacement of these crack faces corresponds to COD. In other words, the crack displacement discontinuity measured on the surface is directly the COD.

  • 29.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Sweden.
    Ayadi, Zoubrir
    Institut Jean Lamour, SI2M, Université de Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Caractérisation de l’endommagement des matériaux composites pour l’aéronautique à partir des mesures plein champ des déplacements et modélisation: [Measurement of Crack Opening Displacement in Damaged Composite Aerospace Laminates Using ESPI]2012Conference paper (Refereed)
    Abstract [fr]

    La rupture finale d'un composite est le résultat de l'accumulation de divers mécanismes élémentaires coexistants qui dépendent notamment de la nature des matériaux constitutifs et des sollicitations mécaniques imposées. Ces mécanismes s'accompagnent généralement du phénomène de dé-cohésion de l'interface fibre-matrice, de la rupture des fibres, de la rupture transverse ou longitudinale de la matrice. Le mécanisme d’endommagement le plus facilement observable lors d’un essai de traction est la micro-fissuration des plis. Ces fissures sont parallèles à la direction des fibres et s’étendent sur toute l’épaisseur du pli. L’apparition et la croissance du nombre de ces fissures engendrent une réduction progressive de la rigidité globale du composite. L’approche GLOB-LOC est l’une des approches quipermet d’obtenir l’ensemble des constantes thermo-élastiques du composite à partir de deux paramètres: l’ouverture et le glissement moyens des lèvres des fissures, normalisés par rapport à la contrainte appliquée. Dans cette approche, ces deux paramètres sont évalués à partir d’expressions analytiques simples obtenues enanalysant les résultats issus de nombreuses modélisations par des éléments finis[1,2]. Naturellement, ce type de modélisation n’est applicable qu’à des fissures «idéales»: fissures rectilignes, isolées, perpendiculaires au plan moyen avec une pointe de fissure située à l’interface entre les plis. En pratique, les fissures réelles peuvent différer sensiblement de ce modèle idéal, notamment du fait des modes d’endommagement supplémentaires engendrés par les fissures elles-mêmes. Lorsque le composite est sollicité mécaniquement, les concentrations de contraintes en pointe de fissures peuvent favoriser la création d’une zone où le pli fissuré et le pli adjacent sont décollés (phénomène de délamination). On pourra également observer des fissures secondaires dans la matrice du pli adjacent. Il est évident que l’apparition de ces nouvelles formes d’endommagement modifiera la dépendance de l’ouverture et du glissement des lèvres des fissures avec le chargement appliqué. Il est donc nécessaire de trouver un moyen de mesure permettant l’estimation expérimentale des valeurs de l’ouverture moyenne et du glissement moyen des lèvres des fissures. En principe, ces grandeurs ne peuvent pas être déterminées à partir d’une mesure ponctuelle, la mesure du profil du déplacement le long des fissures est en effet nécessaire. Cette exigence conduit naturellement l’expérimentateur à envisager l’emploi d’une méthode optique «plein champ» pour les mesures des déplacements. L’objectif principalde ce papierest de caractériser l’endommagement des matériaux composites (vert/époxy) utilisés dans le domaine aéronautique. En utilisant l’interférométrie de speckle (ESPI), des mesures de champs de déplacements aux bords des échantillons et dans différentes couches du stratifié [3] ainsique des études par élément finis ont été effectuées dans le but de calculer l’ouverture et le glissement des lèvres des fissures. L’effet des propriétés élastiques des matériaux sur l’endommagement aussi bien quel’effet d’interaction entre les fissures ont été déterminés. Une discussion essais/calculs est enfin réalisée afin de juger la validité des hypothèses retenues.

  • 30.
    Pupurs, Andrejs
    et al.
    Composites Centre Sweden, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Luleå tekniska universitet, Materialvetenskap.
    Loukil, Mohamed Sahbi
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Kahla, Hiba Ben
    Composites Centre Sweden, Luleå University of Technology, Luleå, Sweden.
    Mattsson, D.
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Effective stiffness concept in bending modeling of laminates with damage in surface 90-layers2016In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 82, p. 244-252Article in journal (Refereed)
    Abstract [en]

    Simple approach based on Classical Laminate Theory (CLT) and effective stiffness of damaged layer is suggested for bending stiffness determination of laminate with intralaminar cracks in surface 90-layers and delaminations initiated from intralaminar cracks. The effective stiffness of a layer with damage is back-calculated comparing the in-plane stiffness of a symmetric reference cross-ply laminate with and without damage. The in-plane stiffness of the damaged reference cross-ply laminate was calculated in two ways: 1) using FEM model of representative volume element (RVE) and 2) using the analytical GLOB-LOC model. The obtained effective stiffness of a layer at varying crack density and delamination length was used to calculate the A, B and D matrices in the unsymmetrically damaged laminate. The applicability of the effective stiffness in CLT to solve bending problems was validated analyzing bending of the damaged laminate in 4-point bending test which was also simulated by 3-D FEM.

  • 31.
    Rezaei, Babak
    et al.
    Amirkabir Univ Technol, Iran.
    Shoushtari, Ahmad Mousavi
    Amirkabir Univ Technol, Iran.
    Rabiee, Mohammad
    Amirkabir Univ Technol, Iran.
    Uzun, Lokman
    Linköping University, Department of Physics, Chemistry and Biology, Biosensors and Bioelectronics. Linköping University, Faculty of Science & Engineering. Hacettepe Univ, Turkey.
    Turner, Anthony
    Linköping University, Department of Physics, Chemistry and Biology, Sensor and Actuator Systems. Linköping University, Faculty of Science & Engineering.
    Mak, Wing Cheung
    Linköping University, Department of Physics, Chemistry and Biology, Sensor and Actuator Systems. Linköping University, Faculty of Science & Engineering.
    Electrochemical performance of nanofibrous highly flexible electrodes enhanced by different structural configurations2018In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 155, p. 81-90Article in journal (Refereed)
    Abstract [en]

    Due to their unique physicomechanical properties, one dimensional (1D) nanostructured conductive materials offer remarkable potential as a flexible electroactive medium for developing miniaturized electronic devices such as supercapacitors, sensors and actuators. In this work, thin films composed of nanocomposite nanofibers with two different architectures, i.e. whiskered nanofibers (WNFs) and hierarchical-structured nanofibers (H-SNFs), were fabricated and their capability to serve as flexible and bendable electrodes were evaluated. The main difference of these two architectures is how the distributions of the nano-fillers (carboxylated multiwalled carbon nanotubes, CMWCNTs) through the nanofibers, i.e. the isotropic and anisotropic arrangements, lead to WNFs and H-SNFs, respectively. The percolation threshold of conduction for the H-SNFs (composed of 0.5 wt% CMWCNTs) and the WNFs (composed of 5 wt% CMWCNTs) were 0.13 S cm(-1) and 0.07 S cm(-1), respectively. Moreover, according to the electrochemical characterizations, although the WNFs had ten orders of magnitude higher nanotube content, the electroactivity and electron transfer rate of H-SNFs was considerably higher than those of WNFs, so that the cyclic voltammetric peak currents of H-SNFs was approximately 1.6 times higher than that of WNFs. As a proof-of-concept, our results indicate that the structural configuration is a major determinative factor, which can largely dictate the final electrical and electrochemical properties of the nanocomposite nanofibers. The bending durability results showed good electrochemical performance even upon 100 bending cycles with 120 bending angles (retained 93.4% and 83.3% of the initial peak currents for H-SNFs and WNFs, respectively). These two flexible nanocomposite nanofibrous structures could be promising materials for the development of flexible electrodes for biosensing to energy storage applications. (C) 2017 Elsevier Ltd. All rights reserved.

    The full text will be freely available from 2019-12-02 15:53
  • 32.
    Shetty, Sandeep
    et al.
    Linköping University, Department of Management and Engineering. Linköping University, Faculty of Science & Engineering. Painted Body and Closures, Volvo Car Corporation.
    Nilsson, Larsgunnar
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
    An evaluation of simple techniques to model the variation in strain hardening behavior of steel2017In: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 55, no 3, p. 945-957Article in journal (Refereed)
    Abstract [en]

    It is important to consider variations in material parameters in the design of automotive structures in order to obtain a robust and reliable design. However, expensive tests are required to gain complete knowledge of the material behavior and its associated variation. Consequently, due to time and cost constraints, simplified material scatter modeling techniques based on scatter data of typical material properties provided by the material suppliers are used at early design stages in simulation-based robustness studies. The aim of this paper is to study the accuracy of the simplified scatter modeling methods in representing the real material variation. The simplified scatter modeling methods are evaluated by comparing the material scatter obtained by them to the scatter obtained by complete tensile tests, which are obtained after detailed timeconsuming experimental investigations. Furthermore, an accuracy assessment is carried out based on selected responses from an axially-crushed, square tube made from DP600 steel.

  • 33.
    Stauffer, Flurin
    et al.
    Laboratory of Biosensors and Bioelectronics, Institute for Biomedical Engineering, ETH Zurich, Zurich, Switzerland.
    Tybrandt, Klas
    Laboratory of Biosensors and Bioelectronics, Institute for Biomedical Engineering, ETH Zurich, Zurich, Switzerland.
    Bright Stretchable Alternating Current Electroluminescent Displays Based on High Permittivity Composites2016In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 28, no 33, p. 7200-7203Article in journal (Refereed)
    Abstract [en]

    A high permittivity composite is developed to enhance the brightness of stretchable electroluminescent displays. The unique two-step assembly process yields dense layers, in which the voids around the electroluminescent particles are filled with smaller high dielectric particles. A stretchable seven-segment display based on the composite is bright enough to be used under standard indoor lighting conditions.

  • 34.
    Tiwari, Ashutosh
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biosensors and Bioelectronics. Linköping University, Faculty of Science & Engineering.
    Mishra, Yogendra KumarFunctional Nanomaterials Group, Christian-Albrechts-Universität zu Kiel, Germany.Kobayashi, HisatoshiInternational Center for Materials Nanoarchitectonics, National Institute for Materials Science, Japan.Turner, AnthonyLinköping University, Department of Physics, Chemistry and Biology, Biosensors and Bioelectronics. Linköping University, Faculty of Science & Engineering.
    Intelligent nanomaterials2016Collection (editor) (Refereed)
    Abstract [en]

    Overall, this book presents a detailed and comprehensive overview of the state-of-the-art development of different nanoscale intelligent materials for advanced applications. Apart from fundamental aspects of fabrication and characterization of nanomaterials, it also covers key advanced principles involved in utilization of functionalities of these nanomaterials in appropriate forms. It is very important to develop and understand the cutting-edge principles of how to utilize nanoscale intelligent features in the desired fashion. These unique nanoscopic properties can either be accessed when the nanomaterials are prepared in the appropriate form, e.g., composites, or in integrated nanodevice form for direct use as electronic sensing devices. In both cases, the nanostructure has to be appropriately prepared, carefully handled, and properly integrated into the desired application in order to efficiently access its intelligent features. These aspects are reviewed in detail in three themed sections with relevant chapters: Nanomaterials, Fabrication and Biomedical Applications; Nanomaterials for Energy, Electronics, and Biosensing; Smart Nanocomposites, Fabrication, and Applications.

  • 35.
    Tiwari, Ashutosh
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biosensors and Bioelectronics. Linköping University, Faculty of Science & Engineering.
    Mishra, Yogendra KumarFunctional Nanomaterials, Institute for Materials Science, University of Kiel, Germany.Kobayashi, HisatoshiWPI Research center MANA, National Institute for Material Science, Tsukuba Japan.Turner, AnthonyLinköping University, Department of Physics, Chemistry and Biology, Biosensors and Bioelectronics. Linköping University, Faculty of Science & Engineering.
    Intelligent Nanomaterials, 2nd Edition.2016Collection (editor) (Refereed)
    Abstract [en]

    Overall, this book presents a detailed and comprehensive overview of the state-of-the-art development of different nanoscale intelligent materials for advanced applications. Apart from fundamental aspects of fabrication and characterization of nanomaterials, it also covers key advanced principles involved in utilization of functionalities of these nanomaterials in appropriate forms. It is very important to develop and understand the cutting-edge principles of how to utilize nanoscale intelligent features in the desired fashion. These unique nanoscopic properties can either be accessed when the nanomaterials are prepared in the appropriate form, e.g., composites, or in integrated nanodevice form for direct use as electronic sensing devices. In both cases, the nanostructure has to be appropriately prepared, carefully handled, and properly integrated into the desired application in order to efficiently access its intelligent features. These aspects are reviewed in detail in three themed sections with relevant chapters: Nanomaterials, Fabrication and Biomedical Applications; Nanomaterials for Energy, Electronics, and Biosensing; Smart Nanocomposites, Fabrication, and Applications.

  • 36.
    Tiwari, Ashutosh
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biosensors and Bioelectronics. Linköping University, Faculty of Science & Engineering.
    Mishra, Yogendra Kumar
    Functional Nanomaterials, Institute for Materials Science, University of Kiel, Germany.
    Kobayashi, Hisatoshi
    WPI Research center MANA, National Institute for Material Science, Tsukuba Japan.
    Turner, Anthony
    Linköping University, Department of Physics, Chemistry and Biology, Biosensors and Bioelectronics. Linköping University, Faculty of Science & Engineering.
    Preface2016In: Intelligent Nanomaterials, 2nd Edition. / [ed] Tiwari, A., Mishra, Y.K., Kobayashi, H., Turner, A.P.F., USA: Wiley-Scrivener , 2016, p. xvii-xxChapter in book (Refereed)
  • 37.
    Tybrandt, Klas
    et al.
    Institute for Biomedical Engineering ETH Zurich, Zurich Switzerland.
    Stauffer, Flurin
    Institute for Biomedical Engineering ETH Zurich, Zurich Switzerland.
    Vörös, Janos
    Institute for Biomedical Engineering ETH Zurich, Zurich Switzerland.
    Multilayer Patterning of High Resolution Intrinsically Stretchable Electronics2016In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, p. 1-6, article id 25641Article in journal (Refereed)
    Abstract [en]

    Stretchable electronics can bridge the gap between hard planar electronic circuits and the curved, soft and elastic objects of nature. This has led to applications like conformal displays, electronic skin and soft neuroprosthetics. A remaining challenge, however, is to match the dimensions of the interfaced systems, as all require feature sizes well below 100 μm. Intrinsically stretchable nanocomposites are attractive in this context as the mechanical deformations occur on the nanoscale, although methods for patterning high performance materials have been lacking. Here we address these issues by reporting on a multilayer additive patterning approach for high resolution fabrication of stretchable electronic devices. The method yields highly conductive 30 μm tracks with similar performance to their macroscopic counterparts. Further, we demonstrate a three layer micropatterned stretchable electroluminescent display with pixel sizes down to 70 μm. These presented findings pave the way towards future developments of high definition displays, electronic skins and dense multielectrode arrays.

  • 38.
    Tybrandt, Klas
    et al.
    Institute for Biomedical Engineering, ETH Zurich, Zurich, Switzerland.
    Vörös, Janos
    Institute for Biomedical Engineering, ETH Zurich, Zurich, Switzerland.
    Fast and Efficient Fabrication of Intrinsically Stretchable Multilayer Circuit Boards by Wax Pattern Assisted Filtration2016In: Small, ISSN 1613-6810, E-ISSN 1613-6829, Vol. 12, no 2, p. 180-184Article in journal (Refereed)
    Abstract [en]

    Intrinsically stretchable multilayer circuit boards are fabricated with a fast and material efficient method based on filtration. Silver nanowire conductor patterns of outstanding performance are defined by filtration through wax printed membranes and the circuit board is assembled by subsequent transfers of the nanowires onto the elastomer substrate. The method is used to fabricate a bright stretchable light emitting diode matrix display.

  • 39.
    Valyukh, Sergiy
    et al.
    Dalarna University, Borlänge, Sweden.
    Valyukh, Iryna
    Dalarna University, Borlänge, Sweden.
    Skarp, Kent
    Spectrophotometric determination of reflective liquid crystal cell parameters2006In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 99, no 5, p. 053102-1-053102-6Article in journal (Refereed)
    Abstract [en]

    We introduce a method for the determination of the optical retardation, its wavelength dispersion, the cell twist angle, and the orientation of the input director in a reflective liquid crystal (LC) cell. These parameters are found from the extremes of a characteristic function defined as a sum of two spectral reflectivities of the LC cell placed between a pair of linear polarizers. The reflectivities are measured for two cell orientations, one of which is arbitrary and the other one is turned through 45°. Both theoretical analysis and experimental procedures are presented. Excellent agreement between the experiment and our theory has been found. The proposed method can be applied to the measurement of reflective LC cells with small and large cell gaps, as well as cells with small and large twist angles.

  • 40.
    Varna, Janis
    et al.
    Luleå tekniska universitet, Materialvetenskap.
    Joffe, Roberts
    Luleå tekniska universitet, Materialvetenskap.
    Loukil, Mohamed Sahbi
    Luleå tekniska universitet, Materialvetenskap.
    Microdamage initiation and growth in laminated composites2009Conference paper (Refereed)
  • 41.
    Varna, Janis
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Loukil, Mohamed Sahbi
    Swerea SICOMP, Sweden.
    Effective transverse modulus of a damaged layer: Potential for predicting symmetric laminate stiffness degradation2017In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 51, no 14, p. 1945-1959Article in journal (Refereed)
    Abstract [en]

    The old concept of the effective stiffness of a 90-layer with intralaminar cracks is revisited performing 3-D FEM parametric analysis of symmetric and balanced laminates. It is shown, focusing on the effective transverse modulus, that the expected dependence of this property on composite elastic properties and laminate lay-up is very weak and follows very simple rules. Calculations show that the effective longitudinal modulus and Poisson’s ratio of the layer are not affected at all by intralaminar cracking. Simple fitting curve for effective transverse modulus change with normalized crack density is obtained from analysis of GF/EP cross-ply laminate. It is shown, comparing with FEM results and experimental data, that this expression can be used as a ‘master curve’ in laminate theory to predict macroscopic elastic property change with crack density in laminates with very different lay-ups and made of different unidirectional composites

  • 42.
    Varna, Janis
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Loukil, Mohamed Sahbi
    Swerea SICOMP, Sweden.
    Stiffness prediction of damaged laminates based on the concept of laminate effective stiffness2016In: Proceedings of the 17th European Conference on Composite Materials ECCM17: 17th European Conference on Composite Materials, 26-30th June 2016, Munich, Germany, European Conference on Composite Materials , 2016Conference paper (Refereed)
    Abstract [en]

    IThe concept of the effective stiffness of a unidirectional layer with intralaminar cracks is revisited performing 3-D FEM parametric analysis of symmetric and balanced laminates with damaged 90-layer. The effective stiffness of the damaged layer is obtained from the difference between damaged and undamaged laminate stiffness. The effective longitudinal modulus and Poisson's ratio of the layer are equal to their initial values. A very simple expression for the effective transverse modulus change with normalized crack density has sufficient accuracy and generality to be used in laminate theory to predict macroscopic elastic property change with crack density in laminates with very different lay-ups and made of different UD composites.

  • 43.
    Varna, Janis
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Loukil, Mohamed Sahbi
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Z.
    Institut Jean Lamour, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy Cedex, France.
    Degradation of elastic properties of non-uniformly damaged composite laminates2013In: 7th EEIGM International Conference on Advanced Materials Research: 21–22 March 2013, LTU, Luleå, Sweden, Institute of Physics Publishing (IOPP), 2013, Vol. 48, article id 12009Conference paper (Refereed)
    Abstract [en]

    Stiffness reduction model for laminates with non-uniformly distributed intralaminar cracks is presented and used to analyze the effect of non-uniformity and the accuracy of predictions based on uniform spacing. The values of the normalized crack opening displacement (COD) as affected by the presence of other cracks are used to calculate the axial modulus of cross-ply laminates with cracks. The COD is calculated using finite element method (FEM), considering two closest neighbors of the crack and using the smallest versus the average crack spacing ratio as non-uniformity parameter. Assuming uniform spacing the axial modulus reduction is overestimated. A "double-periodic" approach is presented to calculate the COD of a crack in a non-uniform case as the average of two solutions for periodic crack systems.

  • 44.
    Yuan, Kang
    et al.
    Beijing General Research Institute of Mining and Metallurgy, Beijing, China.
    Jonnalagadda, Krisha Praveen
    Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
    Yu, Yueguang
    Beijing General Research Institute of Mining and Metallurgy, Beijing, China.
    Peng, Ru Lin
    Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
    Li, Xin-Hai
    Department of Management and Engineering, Linköping University, S.
    Ji, Xiaojuan
    Beijing General Research Institute of Mining and Metallurgy, Beijing, China.
    Shen, Jie
    Beijing General Research Institute of Mining and Metallurgy, Beijing, China.
    Thermal fatigue failure of thermal barrier coatings with a high-Cr MCrAIY bond coat2016In: Proceedings of the International Thermal Spray Conference (ITSC), 2016, Vol. 324, p. 273-278Conference paper (Refereed)
    Abstract [en]

    Thermal barrier coatings (TBCs) were air-plasma sprayed onto Hastelloy X substrates. The TBCs consisted of a high-Cr MCrAlY (M for Ni and Co) bond coat and a yttria-stabilized zirconia (YSZ) top coat. The TBC samples were thermally cycled between 100 ºC and 1100 ºC with 1 hour dwell time at 1100 ºC. The thermal fatigue failure of the TBCs was investigated via microstructure analyses. The final fatigue failure of the TBCs was caused by the formation of interface-parallel cracks in the YSZ top coat. The formation of the cracks was found to be strongly related to the oxidation behaviour of the MCrAlY bond coat. The development of the oxide layers was therefore studied in detail. A thermokinetic model was also used to deepen the understanding on the elemental diffusion behavior in the materials.

  • 45.
    Zrida, Hana
    et al.
    Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France / Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden .
    Loukil, Mohamed Sahbi
    Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France / Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden .
    Varna, Janis
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Z.
    Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Crack opening displacement determination in damaged cross-ply laminate using electronic speckle pattern interferometry (ESPI)2012In: 6th EEIGM International Conference Advanced Materials Research: 7th and 8th November, 2011 EEIGM, Nancy, France, Bristol: Institute of Physics (IOP), 2012, Vol. 31, article id 012002Conference paper (Refereed)
    Abstract [en]

    Composite laminates during service undergo complex combinations of thermal and mechanical loading leading to microdamage accumulation in the plies. The most common damage mode and the one examined in this work is intralaminar cracking in layers. The crack opening displacement (COD) and the crack sliding displacement (CSD) during loading reduce the average stress in the damaged layer, thus reducing the laminate stiffness. These parameters depend on material properties of the damaged layer and surrounding layers, on layer orientation and thickness. Previously these parameters have been calculated using finite element method (FEM) assuming linear elastic material with idealized geometry of cracks. To validate these assumptions experimentally the displacement field on the surface of a [90/0/90] carbon fiber/epoxy laminate specimens with multiple intralaminar cracks in the surface layer is studied and the COD dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). The displacement jumps corresponding to cracks are clearly visible and can be used to determine the opening displacement along the cracks. The effect of crack interaction on the COD at high crack density is also investigated.

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