|
The mechanical reliability of 8 m class ULETM glass telescope mirror blanks is a key requirement for both the blank manufacturer and end-user. The massive monolithic blank experiences stresses during the various manufacturing steps including handling, grinding, sagging, and finishing. Likewise, it is subjected to vibrational loads and road shocks during transportation to end-user. And finally, it experiences transient stresses during installation and steady-state static stresses throughout its useful life in service. The large blank, which weighs nearly 35 tons, must have appropriate surface finish to sustain manufacturing, transportation, installation and service stresses with low probability of flaw initiation and propagation. 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 750 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' diameter ULETM discs (with appropriate surface finish) at 25 degree(s)C and 100% RH. Such a verification is imperative for selecting appropriate surface finishes for the mirror blanks to promote mechanical reliability during manufacturing and service.
|
