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  • 1.
    Stigh, Ulf
    et al.
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Biel, Anders
    Department of Wind Energy, Technical University of Denmark, DK-4000 Roskilde, Denmark.
    Svensson, Daniel
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Cohesive zone modelling and the fracture process of structural tape2016In: Proceedia Structural Integrity, ISSN 2452-3216, Vol. 2, p. 235-244Article in journal (Refereed)
    Abstract [en]

    Structural tapes provide comparable toughness as structural adhesives at orders of magnitude lower stresses. This is potentially useful to minimize the effects of differences in thermal expansion in the joining of mixed materials. The strength properties are modelled using the cohesive zone model. Thus, a cohesive zone represents the tape, i.e. stresses in the tape are transmitted to the substrates through tractions determined by the separations of the surfaces of substrates. This simplification allows for structural analysis of large complex structures. The relation between the traction and the separation is measured experimentally using methods based on the path independence of the J-integral. Repeated experiments are performed at quasi-static loading. A mixed mode cohesive law is adapted to the experimental data. The law is implemented as a UMAT in Abaqus. Simulations show minor thermal distortions due to thermal loading and substantial structural strength in mechanical loading of a mixed material structure.

  • 2.
    Stigh, Ulf
    et al.
    University of Skövde, School of Technology and Society. University of Skövde, The Virtual Systems Research Centre.
    Svensson, Daniel
    University of Skövde, School of Technology and Society. University of Skövde, The Virtual Systems Research Centre.
    On cohesive laws for delamination of composites2010In: 14th European conference on Composite materials, Budapest: University of Technology and Economics , 2010, p. 1-10, article id 159-ECCM14Conference paper (Refereed)
    Abstract [en]

    Analysis of delamination of carbon fibre reinforced composite using cohesive models is studied. A method to measure the cohesive law associated with delamination is presented. The method allows for identification of a cohesive law fit to model the fracture process at the crack tip, i.e. not considering fibre bridging. Due to the small size of the cohesive zone, an elaborated method involving simulations of the fracture process is developed. The results show larger scatter in the parameters of the cohesive law than in the fracture energy.

  • 3.
    Svensson, Daniel
    University of Skövde, School of Technology and Society. University of Skövde, The Virtual Systems Research Centre.
    Experimental methods to determine model parameters for failure modes of CFRP2013Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The focus of this thesis is to develop methods to predict the damage response of Carbon Fibre Reinforced Polymers (CFRP). In the pursuit of reducing the manufacturing cost and weight of CFRP components, it is crucial to enable modelling of the non-linear response associated with various failure modes. Two failure modes are considered in this thesis: fibre compressive failure and interlaminar delamination. Multidirectional laminated composites are commonly used when a low weight is desired due to their high specific strength and stiffness. In a carbon/epoxy composite, almost exclusively the fibres carry the load. However, along the fibre direction, the compressive strength is considerably lower than the tensile strength. With the same reasoning, the transverse strength is considerably lower than the in-plane strength. This makes delamination and fibre compressive failure two of the major concerns in structural design. Moreover, the presence of delaminations severely reduces the compressive strength of a laminate. This can cause catastrophic failure of the structure. In Paper A, we suggest a test method for determining fracture properties associated with fibre compressive failure. A modified compact compression specimen is designed for this purpose and compressive failure takes place in a region consisting exclusively of fibres oriented parallel to the loading direction. The evaluation method is based on a generalized J-integral and full field measurements of the strain field on the surface of the specimen. Thus, the method is not restricted to small damage zones. Paper B focuses on measuring cohesive laws for delamination in pure mode loading. The cohesive laws in mode I and mode II are measured with the DCB- and ENF-specimen, respectively. With a method based on the J-integral, the energy release rate associated with the crack tip separation is measured directly. From this, the cohesive laws are derived. It is concluded that the nonlinear response at the crack tip is crucial in the evaluation of the mode II fracture energy.

  • 4.
    Svensson, Daniel
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre. Chalmers Tekniska Högskola.
    On Cohesive Modelling ofCarbon/Epoxy Composites: Delamination and Fibre Compressive Failure2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Carbon Fibre Reinforced Polymers are widely used in engineering applications where weight saving and high mechanical performance are a key factors. However, an inheren tweakness of laminated CFRP:s is there relatively low resistance to delamination. Therst part of this thesis is devoted to extract cohesive laws associated with delamination.The method is based on fracture mechanical tests and measurement of the displacementeld close to the crack tip. The second part of the thesis is concerned with mixed modecohesive modelling under small scale yielding conditions. Under such loading conditions,a robust cohesive model should conform to Linear Elastic Fracture Mechanics (LEFM). Itis shown that two design rules for mixed mode cohesive laws are sucient to achieve thisproperty. The third part focuses a failure mode entirely dierent from delamination. Theobjective is to determine cohesive laws associated with longitudinal compressive failure.Equilibrium of congurational forces is used for this purpose. Identied congurationalforces are continuously measured by monitoring the displacements eld on the specimen'slateral surface. This allows for identication of the cohesive law.

  • 5.
    Svensson, Daniel
    et al.
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Alfredsson, K. Svante
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Biel, Anders
    University of Skövde, School of Engineering Science.
    Stigh, Ulf
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Measurement of cohesive laws for interlaminar failure of CFRP2014In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 100, p. 53-62Article in journal (Refereed)
  • 6.
    Svensson, Daniel
    et al.
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Alfredsson, K. Svante
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Stigh, Ulf
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    On the ability of coupled mixed mode cohesive laws to conform to LEFM for cracks in homogeneous orthotropic solids2016In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 163, p. 426-448Article in journal (Refereed)
    Abstract [en]

    The ability of coupled cohesive laws to conform to the predictions of linear elastic fracturemechanics (LEFM) in the case of small-scale-yielding (SSY) is explored. The study is concerned with cracks in homogeneous orthotropic solids and the results apply also for the case of isotropy. Both potential based and non-potential based cohesive laws are considered. It is shown that the initial stiffnesses of the cohesive law must be matched to the elastic moduli of the orthotropic solid in order to achieve a constant ratio of the cohesive stress components ahead of the crack tip. A simple condition for this is provided. For non potential based laws an additional apparently sufficient condition on the non-linear part of the cohesive law is identified: The traction vector must not change direction if the directionof the separation vector is constant. Fulfillment of this condition provides a uniform local mode mix in the cohesive zone with a value equal to the global mode mix. It is demonstrated that potential based cohesive laws display a varying local mode mix at the crack tip for cases with a mode dependent work of separation. This is identified as acomplicating feature in terms of calibrating the parameters of a cohesive law to experimental data.

  • 7.
    Svensson, Daniel
    et al.
    University of Skövde, School of Technology and Society. University of Skövde, The Virtual Systems Research Centre.
    Alfredsson, K. Svante
    University of Skövde, School of Technology and Society. University of Skövde, The Virtual Systems Research Centre.
    Stigh, Ulf
    University of Skövde, School of Technology and Society. University of Skövde, The Virtual Systems Research Centre.
    Jansson, N. E.
    GKN Aerospace Engine Systems Sweden, Trollhättan, Sweden.
    An experimental method to determine the critical energy release rate associated with longitudinal compressive failure in CFRP2013In: Proceeding of the 19th international conference on composite materials (ICCM 19), 2013Conference paper (Refereed)
    Abstract [en]

    A test specimen for measurement of the critical energy release rate associated with longitudinal compressive failure is proposed. High strains are localized by decreasing the out-of-plane thickness towards the anticipated damage region which consists of a unidirectional (UD) laminate. Thus, the compressive fibre failure mode is isolated. Microscopic studies show that the UD-material fails in a kinking mode. A method based on a generalized form of the J-integral and full-field measurements of the strain field is developed to extract the fracture energy. The fracture energy in four experiments is measured to be 20-40 kN/m. Finite element simulations  are  performed  to  validate  the experimental results. The essential features of the response are captured by modelling the damage region with cohesive elements.

  • 8.
    Svensson, Daniel
    et al.
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Alfredsson, K. Svante
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Stigh, Ulf
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Jansson, Nicklas E.
    GKN Aerospace Engine Systems, Trollhättan, Sweden.
    Measurement of cohesive law for kink-band formation in unidirectional composite2016In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 151, p. 1-10Article in journal (Refereed)
    Abstract [en]

    Kink-band formation is an important mechanism limiting the compressive strength of high strength composites. A cohesive zone model is used to model the material in a unidirectional carbon fibre composite that forms the kink-band. Equilibrium of configurational forces is used to design and evaluate the experiments. Microscopic studies show that a kink-band is formed with the height of about 200 μm. The corresponding cohesive law has a peak stress of about 1.5 GPa, a compression at failure of about 50 μm and a fracture energy of about 25 kJ/m2.

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