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Author: deajohn5556
Time: 11/11/10 07:37 PST
This is a reply to message no. 21265 by battleship81
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Window for pressurized chamber made from laminated glass?

No problem, happy to respond any time.

Glass is VERY strong without flaws - glass fibers have been shown to exhibit 500,000 psi tensile strength.  But manufacturing processes produce flaws in windows.

Glass is brittle and fails without yielding and strength is based on initial flaw size. Fracture mechanics shows strength is proprtional to square root of flaw size.   With no flaws, the "perfect" glass "flaw" is limited by the atomic distance beween the molecules.  Theoretical strength is 1,000,000 psi.

Now in polishing the glass small flaws are produced, about 10 microns deep. Strength is reduced to about 40,000 psi.   For 100 um flaw,as might occur near a poorly prepared edge, strength is now reduced proptional to square root of depth, around 12,500 psi.

Now add moisture.  For glass and many brittle materials, water attacks the flaw when under stress and propagates it ( you can see this in car windshields under moisture and wind pressure where a small crack will continue to grow).  This is called slow crack growth, or stress corrosion.  After just ine second in water, strength is cut in half, to 6250 psi in our 100 um flaw example.  Now after a long exposure to moisture, say several months or even years, and accounting for statistics in knowledge of flaw depth, strength continues to degrade,  to about 1000 psi.  Most codes use this as a safe allowable.

So the poor glass started out good for 1,000,000 psi and now is only good for 1000 psi.

This phenomenon occurs in the presence of water, ammonia, and other liquids; although amines have not been tested they are derivatives of ammonia so we can assume this as a corrosive enviromnment like water.

To answer your question about why two pieces, tests show that glass will break into 2 pieces for every 1000 psi stress, approximately; at 10,000psi this is 20 pieces, etc.  It is a function of energy; at 1000 psi energy is low, so it breaks into only two pieces.   For example, if you see a single crack grown across your window it was under 1000 psi stress; if a rock is thrown at it it will break into many pieces,due the high impact and high tensile stress produced.   Cracks will only grow in tension.

In order to calculate effects of moisture, every glass is different, but if stresses are below 1000 psi I think you are very safe for a lifetime, provided edges are properly prepared and attach to chamber is done with  resilient material like silicone to prevent thermal stresses and high edge forces.

Hope this helps; feel free to respond

 

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