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
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
. . .
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.
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
If I had to choose between studying
unglamorous joints and jumping from the
100th floor I'd probably choose studying
the unglamorous joints.
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.
And usually bonded to the skin via an extensive patch to spread the
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
Core materials usually aren't very strong at all so direct tapping
doesn't provide a useful connection.