Browsing by Author "Grau, Peter"

Now showing 1 - 5 of 5
  • 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
    Load dependence of the hardness of silicate glasses - Not just due to indenter tip defects
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1993) Grau, Peter; Berg, Gunnar; Fränzel, Wolfgang; Schinker, Martin
    The real contact area of Berkovich indenters with typical indenter tip defects is calculated as a function of the penetration depth in a general form and compared with literature results for Vickers indenters. The influence of the tip defects on the load dependence of hardness is treated for both types of indenters, taking into account possible purely elastic deformation at the beginning of contact. Using the example of recording microhardness measurements on glass made using a Berkovich indenter, it is shown that a linear dependence exists between load and depth of penetration in the millinewton load region which can be explained by a rounded indenter tip. However, the resulting corrections for the real contact area are not sufficient to compensate for the load dependence of the hardness in the entire load region. A method is proposed for showing definitely that the load dependence of the hardness is caused not just by indenter tip defects but also by material properties. This method is tested on extensive measurements of the authors, as well as on results of Frischat and the Oak Ridge group.
  • 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
    Nonlinear Maxwell behavior of sodium silicate glass in the surroundings of glass transition temperature
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2005) Fränzel, Wolfgang; Grau, Peter
    Presented are results from uniaxial cylinder upsetting tests of sodium silicate glass at different constant deformation rates each combined with relaxation experiments in the temperature range 480 to 580°C. Due to the complete stress relaxation to final load zero (vanished carrying capacity) also below glass transition temperature Tg, it follows Maxwell behavior in principle. But the rheological analyses of loading and unloading experiments together show that only nonlinear Maxwell model approaches can adequately describe the deformation behavior of the glass in the surroundings of Tg.
  • 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.