Mechanical reliability of 8-m class ULE glass telescope blanks during manufacturing

Details Mechanical reliability of 8-m class ULE glass telescope blanks during manufacturing Mechanical reliability of 8-m class ULE glass telescope blanks during manufacturing

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995spie.2536..403g&link_type=abstract

Proc. SPIE Vol. 2536, p. 403-412, Optical Manufacturing and Testing, Victor J. Doherty; H. Philip Stahl; Eds.

Computer Science

Scientific paper

The manufacture of 8m class ULETM glass telescope mirror blanks involves multiple steps each of which can induce both stresses and flaws. To ensure the mechanical reliability of such large mirror blanks, both during and post-manufacturing, extreme care is required in the various process steps (including handling, grinding, finishing, and transportation) so as to limit the stress and flaw severity to below the threshold level for the specific surface finish. This paper examines the critical stress/time histories the blanks experience during manufacturing and their ability to initiate slow crack growth from grinding flaws. Two different surface finishes, namely 120 grit and 270/325 grit, with and without acid etching are characterized with respect to strength, flaw, and fatigue behavior. These data show that the 270/325 grit finish with acid etching is appropriate for the sagging step which imposes a static stress of 650 psi in the center region of the mirror blank over a two-week sag period. Similarly, the 120 grit surface finish is adequate for grinding, packaging, and transporting steps which impose a static stress of 435 psi in the support pad region for a three month period. These predictions, based on power law fatigue model, were verified by conducting static fatigue tests (at appropriate stress levels) on a large sample of 6 inch diameter ULETM discs (with appropriate surface finish) at 25 degrees C and 100% RH. A comparison of strength distribution of the discs, before versus after static fatigue tests, showed no changes in strength indicating absence of slow crack growth at stresses and environment representative of manufacturing process. Such a verification is imperative for selecting appropriate surface finishes for the mirror blanks to ensure their mechanical reliability during manufacturing.

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