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  • 1.
    Edlund, Ulf
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Mechanical Engineering, Engineering Mechanics.
    Volgers, Pieter
    A composite ply failure model based on continuum damage mechanics2004In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 65, no 3-4, p. 347-355Article in journal (Refereed)
    Abstract [en]

    A material model including the failure behaviour is derived for a thin unidirectional (UD) composite ply. The model is derived within a thermodynamic framework and the failure behaviour is modelled using continuum damage mechanics. The following features describe the model: (i) The ply is assumed to be in a plane state of stress. (ii) Three damage variables associated with the stress in the fibre-, transverse and shear directions, respectively, are used. (iii) The plastic behaviour of the matrix material is modelled. (iv) The difference in the material response in tensile and compressive loading is modelled. (v) Rate dependent behavior of plasticity and damage (i.e. strength) is modelled.

  • 2.
    Ireman, T
    et al.
    Royal Inst Technol, Dept Aeronaut, SE-10044 Stockholm, Sweden Saab AB, SE-58188 Linkoping, Sweden.
    Ranvik, T
    Royal Inst Technol, Dept Aeronaut, SE-10044 Stockholm, Sweden Saab AB, SE-58188 Linkoping, Sweden.
    Eriksson, I
    On damage development in mechanically fastened composite laminates2000In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 49, no 2, p. 151-171Article in journal (Refereed)
    Abstract [en]

    A comprehensive experimental program was conducted to measure and characterize the development of damage in the vicinity of fastener holes in graphite/epoxy composite laminates. This was carried out to generate data which can be used for development of appropriate failure criteria. Test specimens were loaded in quasi-static cycles with successively increasing loads, and damage development in the vicinity of the bolt holes was detected using different methods such as strain measurements, acoustic emission, X-ray and microscopic examination. Several failure modes were detected in a series of events starting at load levels far below the level at which the first visible evidence of damage appeared on the load-displacement curve. Failure modes included matrix cracking, fibre fracture, delamination and kinking. (C) 2000 Elsevier Science Ltd. All rights reserved.

  • 3.
    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.

  • 4.
    Kapidzic, Zlatan
    et al.
    Linköping University, Department of Management and Engineering. 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.
    Quasi-static bearing failure of CFRP composite in biaxially loaded bolted joints2015In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 125, p. 60-71Article in journal (Refereed)
    Abstract [en]

    Hybrid composite-aluminium bolted joints develop internal loads at elevated temperatures, due to the difference in thermal expansion properties of their constituent materials. In aircraft joints, the thermally induced bolt loads are commonly directed perpendicular to the mechanical loads, inducing a biaxial bearing load state. In this work, carbon-epoxy laminate specimens were tested in uniaxial and biaxial quasi-static bearing failure experiments in a specially designed test rig, at elevated temperature. A microscopy study of a failed specimen revealed that the failure process was mainly driven by fibre kinking, although extensive matrix cracking and delaminations were also found. The experiments were simulated by three-dimensional, explicit, finite element analyses, which included intralaminar damage and delamination. The experimental and simulated bearing failure loads differed by 1.7% in the uniaxial case and 2.1% in the biaxial case. It was suggested that the load-displacement response is influenced by the interaction of all damage mechanisms. Delamination modelling was, however, not essential for the prediction of the maximal bearing strength. The same effective bearing strengths were obtained for the biaxially loaded specimens as for the uniaxially loaded ones, but the damage accumulation process and the resulting damage distributions were different. (C) 2015 Elsevier Ltd. All rights reserved.

  • 5.
    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.

  • 6.
    Kapidzic, Zlatan
    et al.
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, The Institute of Technology.
    Nilsson, Larsgunnar
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, The Institute of Technology.
    Ansell, Hans
    Saab AB, Linköping, Sweden.
    Finite element modeling of mechanically fastened composite-aluminum joints in aircraft structures2014In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 109, p. 198-210Article in journal (Refereed)
    Abstract [en]

    A three-dimensional, solid finite element model of a composite-aluminum single-lap bolted joint with a countersunk titanium fastener is developed. The model includes progressive damage behavior of the composite and a plasticity model for the metals. The response to static loading is compared to experimental results from the literature. It is shown that the model predicts the initiation and the development of the damage well, up to failure load. The model is used to evaluate the local force-displacement responses of a number of single-lap joints installed in a hybrid composite-aluminum wing-like structure. A structural model is made where the fasteners are represented by two-node connector elements which are assigned the force-displacement characteristics determined by local models. The behavior of the wing box is simulated for bending and twisting loads applied together with an increased temperature and the distribution of fastener forces and the progressive fastener failure is studied. It is shown that the fastener forces caused by the temperature difference are of significant magnitude and should be taken into account in the design of hybrid aircraft structures. It is concluded that, the account of the non-linear response of the joints results in a less conservative load distribution at ultimate failure load.

  • 7.
    Nyman, T
    et al.
    Royal Inst Technol, Dept Aeronaut, S-10044 Stockholm, Sweden Saab AB, Future Prod & Technol, S-58188 Linkoping, Sweden Aeronaut Res Inst Sweden, S-16111 Bromma, Sweden.
    Ansell, H
    Royal Inst Technol, Dept Aeronaut, S-10044 Stockholm, Sweden Saab AB, Future Prod & Technol, S-58188 Linkoping, Sweden Aeronaut Res Inst Sweden, S-16111 Bromma, Sweden.
    Blom, A
    Effects of truncation and elimination on composite fatigue life2000In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 48, no 4, p. 275-286Article in journal (Refereed)
    Abstract [en]

    An experimental investigation has been carried out regarding load-sequence effects on the fatigue life of composite structures. Different elimination levels were considered for various load spectra and for constant amplitude block loading. The spectra considered were an early design spectrum for the Gripen fighter aircraft, spectra associated with the aft fitting of the fin and to the upper fitting of the wing, and the compression dominated spectrum "Short Inverted Falstaff". Both experimental and calculated results show, that the elimination level can be set to approximately 50% of the maximum range occurring in the load sequence. This means that considerable time and cost reductions in structural verification testing can be achieved. This will also have impact on a life prediction methodology since only a characteristic number of loading states need to be considered. The resulting life for block testing, especially for a load ratio of R = -1, is highly influenced by elimination of high load ranges in the sequence. Spectrum fatigue test results accounting for ranges up to 90% of maximum range, the remaining load states eliminated, can be mapped on constant amplitude data. At chosen elimination levels, approximately 80-90% of loading states were eliminated. Calculation results with the same technique, i.e. accounting for a characteristic number of cycles, show promising results. (C) 2000 Elsevier Science Ltd. All rights reserved.

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