Abstract
Increasing demand for using the glass ceramic ZERODUR® for optical elements with high mechanical loads called for strength data based on statistical samples larger than 20 specimens. The data now available for a variety of practical surface conditions (ground, lapped, and etched) allow stresses by factors 4 to 10 times higher than before. The larger samples revealed that breakage stresses of ground surfaces follow the three-parameter Weibull distribution. The threshold parameter of this distribution reflects the existence of an upper limit for the microcracks depth within such surfaces. It is equivalent to a minimum strength below which breakage probability is zero. Its use in the well-established crack growth theory allows calculating minimum lifetimes including fatigue using the stress corrosion constant for the prevailing environmental humidity. Long-term loading tests have confirmed the validity of the model. For fully etched surfaces, the Weibull statistics fails because in such cases failure mechanism is not unique anymore. Nevertheless, ZERODUR® with fully etched surfaces that are free from other damages still exhibit minimum breakage stress above 100-MPa tensile stress. The successful satellite mission LISA Pathfinder has confirmed the possibility to apply ZERODUR® for utmost precision experiments together with high mechanical loads.
Highlights
The key property of the glass ceramic ZERODUR® is its extremely low-thermal expansion.[1]
The target is to find the dependence of breakage failure probability on surface tensile stress for a given surface condition
It contains the two samples of the only ground surfaces D151 and D25, which define the range of ground surface breakage distributions for ZERODUR®
Summary
The key property of the glass ceramic ZERODUR® is its extremely low-thermal expansion.[1] and sometimes even more important is the very high homogeneity of this property throughout the total volume of very large items.[2] Both together are the essential criteria deciding on the application of the material. Supports and frames serve to hold just the optical element’s own weight In such conditions, the tensile stress loads on the surfaces are fairly low. The specific cause for starting extended investigations in 2007 was the satellite project LISA Pathfinder This space mission combined the requirement of utmost length precision with high mechanical strength.[3] ZERODUR® is used as the optical bench for laser interferometry and as a clamping frame keeping all elements of the experimental setup together, the optical bench, and two inertial mass containers.
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