Browsing by Author "Varner, James R."

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    Einfluß von Oberflächenbeschädigungen auf die Festigkeit von Glasstäben
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1975) Varner, James R.; Oel, Heribert J.
    Oberflächenschäden von Glasgegenständen vermindern deren Festigkeit. Typische Schadensformen wurden auf Glasstäben erzeugt, wodurch Festigkeitswerte erhalten wurden, die je nach Beschädigung zwischen 6 und 84 kp/mm² lagen. Eine stereografische Auswertung von Rasterelektronenmikroskop-Bildpaaren ermöglichte ein Vermessen des Bruchursprungs. Der Zusammenhang zwischen Bruchursprung und Oberflächenverletzung konnte aufgezeigt werden. Es läßt sich eine Bruchursprungskonstante bestimmen, deren Wert ähnlich wie bei der Bruchspiegelkonstante festigkeitsunabhängig ist. Influence of surface defects on the strength of glass rods Surface defects of glass articles reduce their strength. Typical defect forms were produced on glass rods, whereby strength values were obtained depending on the type of damage, which lay between 6 and 84 kp/mm² . Stereographic evaluation of scanning-electron-micrograph pairs allowed the measurement of fracture origins. The relation between fracture origin and surface damage could be shown. A fracture origin constant can be determined, whose value, similarly as with the fracture-mirror constant, is independent of strength. Influence de détériorations superficielles sur la résistance de baguettes de verre Les détériorations superficielles diminuent la résistance des objets en verre. Des lésions typiques sont produites sur des baguettes de verre, de manire à obtenir des résistances se situant entre 6 et 84 kp /mm² selon la détérioration. Une exploitation stéréographique des paires d'images obtenues au microscope électronique à balayage permet de mesurer l'origine de la fracture. On peut mettre en évidence la relation existant entre l'origine de la fracture et la lésion superficielle. On arrive ä déterminer une constante d'origine de fracture dont la valeur est indépendante de la résistance, comme c'est le cas pour la constante de miroir de fracture.
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    Impact damage characteristics of Space Shuttle Orbiter thermal protection tiles
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1985) Varner, James R.; Goldstein, Howard E.; Katvala, Victor E.
    Space Shuttle Orbiter thermal protection tiles were subjected to impacts under various conditions, ranging from low-velocity impacts with a large, high-mass object to high-velocity impacts with a small, low-mass projectile. Fractography showed that failure of the glaze was due to bending (with the origin at the glaze/tile interface), or impact (with the origin on the surface being struck), or a combination of these two. Tiles made using experimental compositions of fibrous refractory composite insulation were more resistant to impact damage than those made from vitreous silica fibers only. The primary reason for this difference was the fact that the glaze was in compression on the experimental tiles, while it was in tension on the standard tiles. Impacts with large objects tended to produce bending failures, offen with extensive surface cracking. Impacts with small projectiles, especially when at higher velocities, tended to produce impact or mixed bending/impact failures; damage was localized, with holes offen being punched in the coating.
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    Thermoelastic impact damage on glass
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1991) Akcakaya, Reha; Varner, James R.
    Etched, as-received and abraded float glass, and etched low-expansion borosilicate glass were impacted at a velocity of 5.62 m/s by a spherical steel impactor of 10.3 mm diameter. The temperature of the glass specimens and that of the impactor were varied, and the effect of thermal shock on the resulting impact damage was investigated. The number, density and the radial spread of inner ring cracks that formed inside the primary ring crack, and the probability of cone crack formation were observed to increase significantly with increasing temperature difference between the impacting bodies, and with the abrasion of the glass surfaces prior to impact. Impacts on soda-lime-silica float glass at specimen temperatures of 700 and 750 °C revealed inner ring cracks without cone cracks, suggesting that significant strength degradation may result in glasses under these impact conditions. Borosilicate glass was found to be much more resistant to inner ring crack formation than soda-lime-silica glass.