What is it about?
Glass and glass ceramics are brittle materials. Their breakage behavior has been described traditionally using the two parameter Weibull statistical distribution. This approach renders finite breakage probability even for very low mechanical loads. In a series of measurements the existence of a threshold breakage stress could be proven. This corresponds to the existence of micro cracks with limited maximum crack depth in glass items surfaces. In combination with the well established crack growth law of stress corrosion this allows calculating a minimum lifetime for given applications before breakage occurs.
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Why is it important?
The new approach removes statistical uncertainty from breakage analysis of brittle materials as long as mechanical loads remain under a calculable limit value. This limit value is considerably higher than design strength values derived with the traditional two parameter Weibull distribution. Now higher mechanical load applications become possible with high reliability for glass and glass ceramic items.
Perspectives
The threshold strength approach is justified for all glass and glass ceramic applications, where the items' surfaces possess a maximum micro crack depth. This is the case for most technically prepared glass and glass ceramic items. For given applications the threshold stress can be determined as well as the stress corrosion constant of the material / environment combination. The initially increased effort with a higher number of specimens needed will be rewarded with higher possible loads and reliability. This has been shown for the extremely low expansion glass ceramics. However, it can be extended to all glass and glass ceramics, whose breakage strength are determined by the presence of micro cracks in the surface.
Dr Peter Hartmann
SCHOTT AG retired
Read the Original
This page is a summary of: ZERODUR - bending strength: review of achievements, August 2017, SPIE,
DOI: 10.1117/12.2272562.
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