Lifetime of structures made from the glass ceramic ZERODUR under stress loads

What is it about?

With brittle glass or glass ceramic items there is a general reluctance against loading them mechanically. The proverbial tendency to breakage, its sudden occurrence and the total damage in many cases quite often prevent even considering the application of glass. Concerns might increase further when key elements of high value projects are to be made from glassy materials. The design model used in the past was based on a statistical theory, which predicted breakage even for very low stress loads. In its practical use several choices for safety factors had to be made. As a rule in each case the choices were made conservatively. In the end allowable stresses resulted, which were so small, that in many cases the use of glassy materials was excluded. However, waiving the use of glassy materials providing unique properties might be equivalent to waiving the total project. This asked for a closer look on their breakage behavior. The glass ceramic ZERODUR provides the unique property extremely low and homogeneous thermal expansion for large items such as mirrors or carrier structures, which have to maintain their size and shape with utmost precision even under considerable temperature changes. Applications are huge astronomical telescope mirrors or extremely accurate positioning frames in microlithography for example. The article reviews research on the bending strength of ZERODUR based on much larger statistical samples than were available before. The experimental results reveal the existence of a minimum strength for items with surfaces prepared with defined grinding, lapping, polishing and etching processes. This removes uncertainties of breakage risks in the low stress range. The threshold breakage stresses lie considerably higher than found before with the old method. This holds especially for surfaces etched for strength increase. Furthermore, it is possible to calculate the minimum design strength for long-term loaded items including fatigue introduced by humid environment. The other way around it is possible to calculate the minimum lifetime for a given constant load. The validity of the calculation method could be verified with long-term tests. With the findings outlined in the article it is possible to allow much higher mechanical loads on ZERODUR with considerably improved reliability. The method is not restricted to ZERODUR. It also holds for optical glass and most probably also for other technical glass types if their special properties are sufficiently known

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Why is it important?

The method allows much higher stress loads on temperature insensitive ZERODUR structures with high reliability. This enables applications relevant for scientific and technical progress, which were excluded before.

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