How To Measure Thermal Shock in Thick Armor Glass? 3

[pavlovin]

A thick (several inches), multi-layered piece of armor glass is cracking due to suspected thermal shock from the defrost heater. The glass has a heating film for defrosting in between two of the glass layers. This heating film only covers part of the glass area. Cracks originate at the edge of the top layer of glass near the line where the defrost film ends.

To investigate this, the glass was instrumented with strain gauges and thermocouples. The defrost heat causes the temperature to rise to about 230F steady state in the area with defrost film and about 130F measured about 6" away in an area without defrost film, but the glass cracks before steady state is reached. Strain gauges suggest strains of over 400microstrain in the transition area, but the glass data is not available to determine what the stress is.

The objective is to show whether the cracking is exclusively due to thermal stress, or due to something else (installation stresses, etc.) as well.

My questions:
1. Can strain due to thermal shock be measured with strain gauges, despite the temperature change? If not, where can I find standard practices for this?
2. How can strain gauge and temperature data be used to estimate stress due to thermal shock, despite unknown glass properties?

Thank you

 

[unclesyd]

I don't know how you could quantify but something we routinely done on laboratory glassware was use polarized light and a viewer to look for residual strains especially on cooling prior to annealing. The glass was Pyrex or Vycor and at the most a total thickness of 1/2". I don't have any experience with glass of your type or thickness but
with sufficient power you may be able to see the beginning of stress concentration at the edge of the heater.

Another thing we did use on some layer sight glass to look at the tress and location was the using a pressure sensitive film. At the time we used a Fugi Film.
here is another good one.

http://www.sensorexpert.com/

Our research group took this a step further and on development work on films for glass plates would use the Polarized light and well as some other optical measurement on the test specimens.

Acoustic Emission would work but I think the glass will already be cleaving like a rock. You might contact the people below, I don;t know if Hal Dunega is still around but he would be the one to talk to.

http://www.deci.com/AboutDeci.htm

Not part of the answer but could you ramp up the heater so as not to get huge differential thermal shock?

 

[Compositepro]

It is important to remember that measurements will not fix your problem. Granted, it may help, but it could just tell you the obvious that the design needs to change. A defroster should not be heating glass to 230F. The heater needs to be closer to the surface which is being heated. In most applications I've seen the heater is a film or printed pattern right on the surface.

A finite element analysis could give you a lot of clues on temperature and stress distributions.

 

[Ceramicguy]

I agree with unclesyd's crossed polarizer qualitative approach. Looking at the glass prior to heating will give an indication of the initial stress state of the glass. The neat thing is, you can apply the heat while watching the glass in real time to see the changes and relative magnitudes of the stress fields. A careful examination of the fracture surfaces correlated with your direct observations of the stress should yield signicant information. You should be able to put strain gages on the glass for direct measurement if needed but I think the information you need can be obtained with a low cost qualitative series of tests and observations.

Bruce

http://www.accuratus.com

 

[flyingmelon]

Armor glass is not easy to be cracked by defroster. Should be the problem of the outer stress...

Glass material are very useful.
Quartz Substrates, Quartz Plates

http://www.quartzlab.com