The key requirements for an optical mirror material include low density, high Young's modulus, low coefficient of thermal expansion, high thermal conductivity, and high diffusivity. Not included among these are fracture toughness and stress corrosion constant, which control slow crack growth and long-term reliability under static or dynamic loads during manufacturing and in-service. The reliability requirement becomes crucial as the mirror size increases and/or its mission takes on strategic importance. This paper compares the critical properties of three ultralow expansion materials, namely ULE$+TM$/, Zerodur and Astrositall$+TM$/. It demonstrates how these properties affect the bending rigidity and safe allowable stress for the mirror subjected to different types of loading, namely: (i) its own weight and (ii) external load. An analysis of bending rigidity, bending stress, and safe allowable stress shows that mirror blanks of two different materials can be designed to be equivalent in terms of their rigidity without any weight penalty. The lower modulus and lightweight material like ULE glass requires about 10 percent higher thickness which reduces the bending stresses 20 percent compared to those in Zerodur or Astrositall mirrors of identical size. The lower stress, according to Power law fatigue model, is highly beneficial in that it improves the mechanical reliability of ULE mirror during manufacturing, transportation, installation and in-service by two orders of magnitude over that of Zerodur and Astrositall mirrors. The fatigue and fracture data for the three materials are used to estimate the safe allowable stress for facilitating mirror design from mechanical reliability point of view.
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