Browsing by Author "Meinhard, Holger"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Hardness and flow behaviour of glass in the nanometre range - An Interpretation of the load dependence of the hardness
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Meinhard, Holger; Grau, Peter; Berg, Gunnar; Mosch, Susan
    The flow behaviour of glass was investigated by Vickers hardness indentation technique with Variation of the deformation rate by six Orders of magnitude. The hardness tests were carried out on commercial sheet glass with three different loading regimes. A general flow equation in form of a power law was used to describe the deformation behaviour of the materials. The influences of loading mode and deformation rate on the parameters of this law were investigated. An essential result of this work is that the load and Penetration depth dependence of the hardness, respectively, the so-called indentation size effect, is an inevitability of the special methodology of the indentation technique during deformation of strain rate-sensitive materials.
  • Thumbnail Image
    Item
    Mechanical properties of sheet glass at high pressure during indentation experiments
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Meinhard, Holger; Fränzel, Wolfgang; Grau, Peter
    Viscous flow is responsible for permanent deformation of glasses above the transformation temperature (Tg). The results of macroscopic indentation experiments (maximum loading forces Fmax ≥ 2 N) with sharp indenters (Vickers or Berkovich pyramids) demonstrate this property by crack-free indents at T ≥ Tg. But it is also possible to make crack-free indents at room temperature (RT), well below Tg, if the load, the contact area and the deformation rate, respectively, are small enough, which is consistent with glass as a supercooled liquid but inconsistent with glass as a well-known ideally brittle material at RT The mechanisms of this permanent deformation of glasses by indentation near RT have not been understood completely up to now. For the analysis of the macroscopic indentation experiments with Vickers pyramids and spherical indenters, respectively, a viscoelastic deformation behaviour is assumed for the complete temperature range from RT to Tg. Therefore the rheological analysis of all experiments were performed with the help of a simple Maxwell model. The results of Vickers indentation experiments were compared with cylinder compression experiments and ball indentation experiments which were analysed by the same rheological principles.
  • Thumbnail Image
    Item
    Viscosity of glass below the transformation temperature
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Meinhard, Holger; Fränzel, Wolfgang; Grau, Peter
    Glass is an ideally brittle material at room temperature. The results of hardness indentation tests demonstrate this property by crack formation at the corners of the indent. But it is possible to avoid the crack formation if the load, the contact area and the deformation rate, respectively, are small enough. The mechanisms of this permanent deformation of brittle glasses by indentation close to room temperature have not been understood completely up to now. Viscous flow is responsible for permanent deformation of glasses above the transformation temperature (Tg). The same mechanism of deformation during indentation experiments is assumed in this work hypothetically, taking into account the very high stresses (GPa range) in the local volume around the indenter tip. This hypothesis of deformation at room temperature will be verified by independent experiments that work like viscometers in principle (e.g. cylinder compression experiments, ball indentation experiments). All experiments were analysed with the help of simple rheological models. Additionally some ball indents were analysed by topographical investigations to get first hints of viscoelastic deformation behaviour below Tg of glass.