Tapping Carbon Fiber

I have been working with a product that has fasteners (M8) connected to into tapped holes in carbon fiber. My first instinct is that this
seems to be a terrible idea. Has anyone worked with this? Any ideas for ballpark torque specs? Anyone know of any threaded inserts for this material?
Thanks.
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Dear rhybec:

URL:http://www.machinedesign.com/BDE/FASTENING/bdefj1/bdefj1_6.html ... agrees with you, that this is a bad idea.
Rather than concentrating on threading the composite matrix, maybe looking at the design of the joint would be a good idea.
I would recommend starting with Google Advanced, requiring all the words: fastener carbon fiber
Composite materials are *engineered systems*, and have NO bulk material forgiveness for bad (or ad hoc) designs. If you need a threaded hole at a certain location, there are imbedments available that can be woven in on location. They will be blind tapped holes in some chemically compatible material, that has ways of distributing force (more or less evenly) into the composite matrix.
David A. Smith
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"N:dlzc D:aol T:com \(dlzc\)" N:dlzc1 D:cox T: snipped-for-privacy@nospam.com in
. . .

Design around the composite structure, not vice versa.
The plane that crashed in New York City shortly after 9/11 failed due to a repair more suitable for a fiberglass boat hull.
Apparently someone just slapped on a patch of cloth and resin and sanded it down like a surfboard ding.
Bret Cahill
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rhybec wrote:

Tapping sounds very bad. Look into composite inserts.
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According to a MIT study the WTC had poorly designed joints and would not have pancaked down, at least not as quite as quickly without this defect.
Fasteners, joints, inserts, fittings, pins etc. are not considered very glamorous and are not promoted enough in many formal education programs, probably because a full structural analysis could get pretty complicated very quickly when a little cost effective common sense is often enough to overcome most problems.
A lot of inventors make a lot of money off of fastener improvements.
If I had to choose between studying unglamorous joints and jumping from the 100th floor I'd probably choose studying the unglamorous joints.
Bret Cahill
Bret Cahill
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Jeff Finlayson wrote:

Also.. Composite inserts are Not tapped in like metal inserts. They are adhesively bonded to the composite.
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And usually bonded to the skin via an extensive patch to spread the load.
ASCII graphic:
========| |=======------------====| |====---------------- XXXXXXXXXXXX////| |////XXXXXXXXXXXXXXXX XXXXXXXXXXXX////| |////XXXXXXXXXXXXXXXX ---------------------------------------
The above shows the original skin (fibre) as -------- the patch fibre as ========== and the original core as XXXXX
The new reinforcing "plug" indicated by //////// has been bonded "blind" on one side leaving the original skin intact, but drilled out from the other side to allow insertion of the plug. The plug is bonded to the skin on the blind side as well as the core material before one or more layers of fibre "fabric" are laid over the top of the plug to also allow it to transfer loads to the other skin.
A stronger connection can be made by additional reinforcing layers of fabric on the blind side if that is accessible and doesn't have to be smooth. Washers and bolt-through provide even greater strength.
Core materials usually aren't very strong at all so direct tapping doesn't provide a useful connection.
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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Remember that threaded inserts aren't any stronger than their outer threaded connection. Threading an insert into a material is no better than threading a screw of that size into it. The insert is larger in diameter than the screw, but why not just use the bigger screw?
I have seen young engineers who see steel threaded inserts like Helicoil or Keensert installed into aluminum plates and assume that this was done to strengthen the threaded connection. If this were true, the designer would have merely used the larger, outer thread size for the actual screw. These inserts are generally used in aluminum parts that have to be assembled and disassembled often, since repeated tightening of a screw directly into the aluminum causes galling.
If you're worried about the integrity of the threaded material, then I would try to design it with through-holes and use a bolt, washer and nut instead. If the material is relatively soft, you might use a wood screw.
Don Kansas City
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