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[Richard Majka <richard.majka AT yale.edu>]

Hi Tom;

  I don't know if you took this into account, but the elasticity of copper clad Kapton is quite different from bare kapton ("spring constant" is quite a bit higher with copper).  Those of use who played a bit with "copperless" GEM foils realized this.  I think this argues even more strongly for your point.

Dick Majka

On Fri, Oct 26, 2018 at 3:54 PM Thomas K Hemmick <tkhemmick AT gmail.com> wrote:

Hi Bill

We've attached a single slide on the math of comparing the amount by which kapton elongates under force as compared to the amount by which the frames bend (here approximated as straight bars).  The calculation for Kapton is a freshman physics (if your prof covers stress/strain :) ) calculation for which I have re-written the result to be proportional to the force/width.  The bar bending is standard engineering for something supported at both ends (e.g. wikipedia) and I've calculated the maximum deflection under uniform load.  This is also written as proportional to force/width for easy comparison to the kapton stretching.  The results are quite telling.

Under any given force/width, the kapton stretching is TINY compared to the frame bend under the same condition.  Thus, we are lead to conclude that the frame will yield to the kapton until the kapton is basically under no internal stress.

When people "stretch" kapton foils, they are really only straightening them and not spring-loading them.  The "stretch" should really be simply pulling on the kapton until it is flat and no actual internal stress.  So the TRUE parameter is to make sure that the tension remains low enough the the full stretch of the kapton is very small.  If we decide on 100 microns of kapton stretch as an upper limit for Delta-L, then we are limited to 1 Newton/cm (100 grams/cm) as the absolute top tension.

------------------------

Short answer:  YES STRETCH, YES OPTIGUARD, but stay WELL(!!!) below 100 grams/cm when pulling.

Tom & Klaus

PS--Typical on older techniques was to pull with scotch tape against the table (e.g. COMPASS).  Again, pull to flatten, not to generate internal stresses since you simply cannot hold those back.

On Fri, Oct 26, 2018 at 1:47 PM William Llope <wjllope AT wayne.edu> wrote:

Hi Ed,
Yes - comments from Tom H and all the other GEM experts are sought.
I also sent it to the list as the other framing sites might be interested
in the discussion. Thanks
    bill

___________________________
W.J. Llope       Ph.D., Assoc. Prof.
http://wjllope.physics.wayne.edu/
Wayne State University, Physics     
666 W. Hancock, Room 347
Detroit, MI  48201
313-577-9805




> On Oct 26, 2018, at 12:46 PM, EdwardOBrien <eobrien AT bnl.gov> wrote:
>
>     Hi Bill,
>      This is, of course, a question for Tom to answer. Right?
>
>     Ed
>
>
> On 10/26/2018 11:56 AM, William Llope wrote:
>> Hi all,
>> we have a question and need a yes or no...
>> http://wjllope.physics.wayne.edu/sPHENIX/sphenix_stretching.pdf
>>
>> thanks a lot, cheers
>> bill
>>
>> ___________________________
>> W.J. Llope       Ph.D., Assoc. Prof.
>> http://wjllope.physics.wayne.edu/
>> Wayne State University, Physics
>> 666 W. Hancock, Room 347
>> Detroit, MI  48201
>> 313-577-9805
>>
>>
>>
>>
>> _______________________________________________
>> Sphenix-tpc-l mailing list
>> Sphenix-tpc-l AT lists.bnl.gov
>> https://lists.bnl.gov/mailman/listinfo/sphenix-tpc-l
>