What is it about?
The breakage strength of glasses and glass ceramics depends on their surface condition in practice. The depths of micro cracks determine how high a tensile stress at the surfaces may be before breakage occurs. Just as with many other materials strength degrades slowly with time. Such process is called corrosion. With glass and glass ceramics corrosion occurs, when tensile stress at surfaces reach a threshold value. Then micro cracks grow until they are deep enough to cause breakage. Over a wide range of speed growth is characterized by the so-called stress corrosion constant. For reliable calculations it is necessary to know its value with best accuracy. This constant depends on the environmental humidity because water assists micro crack growth of glass and glass ceramics. The stress corrosion constant has been determined for the ultra low expansion glass ceramic ZERODUR with an established method for normal humid and very dry environment.
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Why is it important?
The stress corrosion constant is now known accurate enough for life time calculation. The determination method covered short time loads and long lasting loads differing by factor of 10 000. This ensures a wide range of validity of the obtained values.
Perspectives
Before this work was done, published values for the stress corrosion constant existed, which differed so strongly, that it was not really clear, which one has to be used. This work confimed values obtained in two former studies. With the much wider range of stress increase rates used the stress corrosion constants for ZERODUR for normal humidity (31) and for extremely dry environment (79) (corresponding to use in vacuum) are now known unambiguously.
Dr Peter Hartmann
SCHOTT AG retired
Read the Original
This page is a summary of: ZERODUR®: new stress corrosion data improve strength fatigue prediction, September 2015, SPIE,
DOI: 10.1117/12.2188112.
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