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  • 1.
    Eman, Jesper
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Brottöjningar vid hål i ultrahöghållfast stål för olika håltagningsmetoder2007In: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, p. 96-Conference paper (Other academic)
  • 2.
    Eman, Jesper
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Study and characterization of localization and failure behaviour of ultra high strength steel2007Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the vehicle industry there is a constant struggle to develop cars with high passive safety without increasing the fuel consumption. High passive safety requires a very rigid behaviour of the crash protecting components. Accomplishing this often leads to an increase in the weight of the components. An increase in weight results in a higher fuel consumption which is bad for the environment as well as for the economy of the car owner, therefore the manufacturers turn to new materials. One of these new materials is ultra high strength steel which is the material in focus for the present thesis. To be able to utilize all the advantages of ultra high strength steel the material behaviour must be investigated in detail. In this thesis, sheets of ultra high strength steel, which are produced by press-hardening, are investigated using a method called digital speckle photography. When using the method of digital speckle photography (DSP) a series of photographs are taken of a deforming specimen. Prior to the experiment a random pattern (speckles) has been applied to the specimen and by studying the deforming speckle pattern on the images, the deformation fields through time can be established. Within the present thesis the deformation fields up to the point of fracture have been investigated on a length scale of the order of 10e-4 meters. With length scales of this magnitude the deformation inside a localized neck can be investigated. This is done, both for a specimen shape that induces a fracture initiation at an inner point of the specimen and a specimen shape where fracture starts from the edge of a hole. These investigations show that there is a strong localization of the strain before fracture is initiated. The local strain values inside a localized neck are significantly higher than the strain values that can be observed with conventional experimental techniques involving extensometers. It is also noticed that the method used to make holes play an important role for the onset of fracture. Some methods hardly affect the material at all while others can decrease the level of local strain at the onset of fracture down to about a third of the value for unaffected material. Furthermore, a method for characterizing the material based on full-field measurements is presented. The method is a fast and simple alternative to previously used inverse modelling procedures where the material model of a finite element simulation is updated iteratively to make the simulation produce the same results as the experiment.

  • 3. Eman, Jesper
    et al.
    Sundin, Karl-Gustaf
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Oldenburg, Mats
    Spatially resolved observations of strain fields at necking and fracture of anisotropic hardened steel sheet material2009In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 46, no 13, p. 2750-2756Article in journal (Refereed)
    Abstract [en]

    In this work plastic strain localization, also referred to as necking, of press-hardened ultra-high strength steel is observed using digital speckle correlation. The region of the neck is studied during tensile tests of specimens specially designed to facilitate strain localization at an inner point of the material, thus avoiding edge effects on localization and fracture. By using measurements with a length scale small enough to properly resolve the neck, its growth and shape can be studied. Furthermore, the anisotropy of the material is investigated by examining specimens cut out at different angles to the rolling direction. It is seen that the local fracture strain of specimens cut out along the rolling direction is approximately twice as high as it is for specimens cut out perpendicular to the rolling direction.

  • 4.
    Sundin, Karl-Gustaf
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Eman, Jesper
    Fracture strains at holes in high-strength steel: a comparison of techniques for hole-cutting2007In: Experimental analysis of nano and engineering materials and structures: proceedings of the 13th International Conference on Experimental Mechanics, Alexandroupolis, Greece, July 1-6, 2007 / [ed] Emmanuel E. Gdoutos, Dordrecht: Encyclopedia of Global Archaeology/Springer Verlag, 2007Conference paper (Refereed)
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