This MedLibrary.org supplementary page on Strain point is provided directly from the open source Wikipedia as a service to our readers. Please see the note below on authorship of this content, as well as the Wikipedia usage guidelines. To search for other content from our encyclopedia supplement, please use the form below:
Related Sponsors
Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a Lehr.1 Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses caused by quenching and significantly decrease the overall strength of the glass.
The glass is heated until the temperature reaches a stress-relief point, that is, the annealing temperature (also called annealing point) at a viscosity, η, of 1013 Poise = 1012 Pa·s, at which the glass is still too hard to deform, but is soft enough for the stresses to relax. The piece is then allowed to heat-soak until its temperature is even throughout. The time necessary for this step varies depending on the type of glass and its maximum thickness. The glass is then slowly cooled at a predetermined rate until its temperature is below the strain point (η = 1014.5 Poise). Following this, the temperature can safely be dropped to room temperature at a rate limited by the heat capacity, thickness, thermal conductivity, and thermal expansion coefficient of the glass. After the annealing process the material can be cut to size, drilled or polished.
At the annealing point (η = 1013 Poise) stresses relax within several minutes, while at the strain point (η = 1014.5 Poise) stresses relax within several hours.2 Stresses that are still present below the strain point are permanent.
See also
References
- ^ E. F. Collins (1921) Electrically heated glass annealing lehr. Journal of the American Ceramic Society 4 (5), 335–349.
- ^ Werner Vogel: "Glass Chemistry"; Springer-Verlag Berlin and Heidelberg GmbH & Co. K; 2nd revised edition (November 1994), ISBN 3540575723
External links
|
|||||||||||||||||||||||
 |
This glass engineering or glass science related article is a stub. You can help Wikipedia by expanding it. |
Wikipedia content modification information:
- This page was last modified on 19 September 2008, at 23:04.
Wikipedia Authorship and Review
Wikipedia content provided here is not reviewed directly by MedLibrary.org. Wikipedia content is authored by an open community of volunteers and is not produced by or in any way affiliated with MedLibrary.org.
Wikipedia Usage Guidelines
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article on "Strain point".
The URL for this specific entry is:
All Wikipedia text is available under the terms of the GNU Free Documentation License. (See Copyrights for details). Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc.