A method is described to make multiple measurements of the strength of a single
hemispherical ceramic dome with a ring-on-ring flexure test of disklike sections removed
from the dome by a core drill. Preliminary results show that dome cores exhibit lower
strength than flat disks, because the domes have larger critical surface flaws than those
present on disks. Careful attention to polishing of domes is required to realize maximum
strength from the domes. The fracture toughness of optical quality yttria disks and
hemispherical domes was found to be approximately 0.7 MPa'I m.
The strengths of sapphire disks of two different crystallographic orientations and
bars of three different orientations were measured as a function of temperature in ring-on-
ring flexure or 4-point bending. One set of disks (OO cut) had the crystallographic
C -axis normal to the flat surface which contained the crystallographic a- and m -axes. The
average strength of these disks dropped from 154 ksi at 20°C to 21 ksi at 800°C. In another
set of disks (900 cut), the crystallographic c- and m -axes were parallel to the flat surface.
The average strength of these disks dropped from 84 ksi at 20°C to 48 ksi at 800°C. The
strength of sapphire bars whose tensile axis was the crystallographic m axis dropped from
103 ksi at 20°C to 86 ksi at 1400°C. The strength of sapphire bars whose tensile axis was the
crystallographic a-axis dropped from 113 ksi at 20°C to 74 ksi at 1400°C. The strength of
sapphire bars whose tensile axis was the crystallographic c-axis dropped from 153 ksi at
20°C to 35 ksi at 1400°C.
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