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Author: Schneider, Andreas S. × End date: 2019 × Start date: 2000 × DDC: 530 × WGL Institute: INM ×

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Now showing 1 - 4 of 4
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    Temperature-dependent size effects on the strength of Ta and W micropillars
    (Amsterdam : Elsevier, 2016) Torrents Abad, Oscar; Wheeler, Jeffrey M.; Michler, Johann; Schneider, Andreas S.; Arzt, Eduard
    The strength of metals increases with decreasing sample size, a trend known as the size effect. In particular, focused ion beam-milled body-centered cubic (BCC) micropillars exhibit a size effect known to scale with the ratio of the test temperature to the critical temperature (Tc) of the BCC metal, a measure of how much the yield stress is governed by the lattice resistance. In this paper, this effect is systematically studied by performing high-temperature compression tests on focused ion beam-manufactured Ta and W single crystal pillars ranging in diameter from 500 nm to 5 Î¼m at temperatures up to 400 Â°C, and discussed in the context of bulk strength and size dependent stresses. Both metals show larger size effects at higher temperatures, reaching values that are in the range of FCC metals at temperatures near Tc. However, it is demonstrated that size effects can be considerably affected by material parameters such as dislocation density and lattice friction, as well as by the yield criterion used. Furthermore, for W, a change from uniform wavy deformation to localized deformation is observed with increasing temperature and pillar size, further indicating that the temperature ratio strongly influences the relative motion of screw and edge dislocations.
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    Adhesion behavior of polymer networks with tailored mechanical properties using spherical and flat contacts
    (Cambridge : Cambridge University Press, 2013) Lakhera, Nishant; Graucob, Annalena; Schneider, Andreas S.; Kroner, Elmar; Micciché, Maurizio; Arzt, Eduard; Frick, Carl P.
    Four acrylate-based networks were developed such that they possessed similar glass transition temperature (~-37 °C) but varied in material stiffness at room temperature by an order of magnitude (2-12 MPa). Thermo-mechanical and adhesion testing were performed to investigate the effect of elastic modulus on adhesion profiles of the developed samples. Adhesion experiments with a spherical probe revealed no dependency of the pull-off force on material modulus as predicted by the Johnson, Kendall, and Roberts theory. Results obtained using a flat probe showed that the pull-off force increases linearly with an increase in the material modulus, which matches very well with Kendall's theory.
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    In situ observation of contact mechanisms in bioinspired adhesives at high magnification
    (Cambridge : Cambridge University Press, 2011) Paretkar, Dadhichi; Schneider, Andreas S.; Kroner, Elmar; Arzt, Eduard
    We analyzed the contact mechanisms of bioinspired microfibrillar adhesives using in situ scanning electron microscopy. During adhesion tests we observed that (i) the superior adhesion of mushroom-shaped fibrils is assisted by the stochastic nature of detachment, (ii) the aspect ratio of microfibrils influences the bending/buckling behavior and the contact reformation, and (iii) the backing layer deformation causes the microfibrils to elastically interact with each other. These studies give new insights into the mechanisms responsible for adhesion of bioinspired fibrillar adhesives.
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    Temperature-induced switchable adhesion using nickel-titanium-polydimethylsiloxane hybrid surfaces
    (Hoboken, NJ : Wiley, 2015) Frensemeier, Mareike; Kaiser, Jessica S.; Frick, Carl P.; Schneider, Andreas S.; Arzt, Eduard; Fertig III, Ray S.; Kroner, Elmar
    A switchable dry adhesive based on a nickel–titanium (NiTi) shape-memory alloy with an adhesive silicone rubber surface has been developed. Although several studies investigate micropatterned, bioinspired adhesive surfaces, very few focus on reversible adhesion. The system here is based on the indentation-induced two-way shape-memory effect in NiTi alloys. NiTi is trained by mechanical deformation through indentation and grinding to elicit a temperature-induced switchable topography with protrusions at high temperature and a flat surface at low temperature. The trained surfaces are coated with either a smooth or a patterned adhesive polydimethylsiloxane (PDMS) layer, resulting in a temperature-induced switchable surface, used for dry adhesion. Adhesion tests show that the temperature-induced topographical change of the NiTi influences the adhesive performance of the hybrid system. For samples with a smooth PDMS layer the transition from flat to structured state reduces adhesion by 56%, and for samples with a micropatterned PDMS layer adhesion is switchable by nearly 100%. Both hybrid systems reveal strong reversibility related to the NiTi martensitic phase transformation, allowing repeated switching between an adhesive and a nonadhesive state. These effects have been discussed in terms of reversible changes in contact area and varying tilt angles of the pillars with respect to the substrate surface.