• Finite Element Modeling Of Photoplastic Process In The Cold Drawing Of Polycarbonate Bars

      Choi, Dong Ju; Lee, Jonah H. (2001)
      The numerical simulation of the mechanical behavior and the photoplasticity method of polycarbonate have been studied by simulating an existing photoplastic experiment of a cold drawing process. This study will allow understanding the mechanical and optical behavior of polymers and also assessing of the adequacy of existing numerical models and the photoplastic experimental method such that both can be validated and improved in the future. We proposed several optical-mechanical relationships for the photoplasticiy method and also adopted two existing viscoplastic models for polymers. Among the studied optical-mechanical relationships, the Non-Gaussian optical-mechanical relationship for photoplasticity of large deformation of polycarbonate is found to agree well with existing photoplastic calibration tests. Two types of viscoplastic numerical models adopted in this study are the physically based constitutive model and the phenomenological constitutive model with J2 flow theory. The model with the physically based constitutive model and a non-Gaussian optical-mechanical relationship for the photoplastic method could simulate the cold drawing experiment better than the phenomenological viscoplastic model. This study included a parametric study of the physically based constitutive model with the imperfection geometry and several material property variances to understand the mechanical and optical behavior of polycarbonate.
    • The role of the interface for perpendicular magnetic anisotropy of iron/terbium and iron/gadolinium multilayers

      Freitag, Andrea Else (1998)
      Ferromagnetic multilayers showing perpendicular magnetic anisotropy (PMA) can be used as information storage materials with significantly increased storage density compared to materials currently used. A detailed knowledge of the mechanism that creates PMA and the impact of different parameters on the magnetic properties in such multilayers is essential. 26 Fe/Tb and 11 Fe/Gd multilayer samples with a wide range of thicknesses were fabricated by planar magnetron sputtering. The other fabrication parameters besides thickness were left constant as far as possible. The magnetic and structural properties of these samples were examined using x-ray diffraction, Mossbauer spectroscopy and torque magnetometry. This study attempts to explain the PMA in multilayers of Fe and Gd or Tb. It has been found that the magnetic ordering in such multilayers is induced by a mixed interface very close in composition to that of amorphous thin alloy films of the constituent elements that show maximum PMA. The other important factor determining the magnetic properties of these multilayers is an exchange interaction between adjacent Fe layers. The thickness of the interface and the thickness of the particular layers of the constituent elements determine the alignment of the magnetic moments in such multilayers. No significant difference in the magnetic and structural properties between Fe/Tb and Fe/Gd multilayers could be measured.