Ditmar on sat 18 dec 04
To understand glass better, there are 3 principles / concepts to =
familiarize yourself with.
Softening point, strain point, annealing point. =20
Basically said, there is a temperature below which strain introduced =
during cooling will not relieve itself and become permanent. If the =
strain is beyond the physical capabilities, the glass fractures. It's =
the idea behind tempered glass. It's in an induced permanent stress =
during manufacture. The balance between tension and compression within =
the glass gives it it's strength.
If glass flowed / moved, it would be impossible to make tempered items. =
Permanent strain would eliminate itself through flow. You can see the =
strain ( if present) with a polarizing filter.
Don't forget, there is no single definitive item called "glass". There =
are thousands.( indeed, glazes are modified glasses) Even the same batch =
from the front of the furnace is slightly different from the back.=20
Ditmar
Ivor and Olive Lewis on sun 19 dec 04
Dear Ditmar,
When you speak of "Strain" do you mean "Stress" ?
I ask because as I understand things, Young's Modulus of Elasticity
tells us that up to the Elastic Limit, "Strain" is proportional to
"Stress", meaning that the amount of movement produced is related by a
linear equation to the degree of force applied.
Beyond the Elastic LImit strain is premanentand the sample will not
return to its original dimensions
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ditmar on sun 19 dec 04
Strain and stress in this sense synonymous. ....to a degree. Strain
point is a definitive term in glass. ( Stress/strain being produced in the
body of the glass. )
( don't extrapolate too much between metals and glasses, as far as terms and
properties)
Crystal structure and compression / tension values make glass quite unique
vs. metals.
Ditmar
Though printed a considerable time ago, Modern Glass Practice by Scholes and
Greene
is an excellent resource book for glass and it's properties.
----- Original Message -----
From: "Ivor and Olive Lewis"
To:
Sent: Saturday, December 18, 2004 2:57 PM
Subject: Re: Can glass move ......last thoughts on subject
> Dear Ditmar,
> When you speak of "Strain" do you mean "Stress" ?
> I ask because as I understand things, Young's Modulus of Elasticity
> tells us that up to the Elastic Limit, "Strain" is proportional to
> "Stress", meaning that the amount of movement produced is related by a
> linear equation to the degree of force applied.
> Beyond the Elastic LImit strain is premanentand the sample will not
> return to its original dimensions
> Best regards,
> Ivor Lewis.
> Redhill,
> S. Australia.
>
>
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Ivor and Olive Lewis on mon 20 dec 04
Dear Ditmar,
Since it will be almost impossible for me to acquire a copy of Modern
Glass Practice by Scholes and Greene within the next six weeks because
our visiting library staff are having their annual leave perhaps you
would be so kind as to provide definitions of "Stress" and "Strain"
that you are using, in precise and concise words.
It seems as though you are saying that, in terms of substances with a
glass structures,"Strain" is the motion, or change in dimension due to
internal stresses developed during cooling or heating which would
result if the material were not rigid . Hence it is an immeasurable
quality. Would this distinguish it from CoE differences between fired
clay and mature glaze which causes crazing and shivering ?
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
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