Please tell us what you mean by, "designed to 50% of it's design limit", in
your second paragraph .
----- Original Message -----
Sent: Monday, March 29, 2004 11:11
PM
Subject: [LML] Re: e-glass vs carbon
fiber
Hi Rob
Yes, that's true for materials like steel that have a yield so I should
have stated that in the first place. And yes, it is a simplistic
example. Carbon and glass both have pretty much a straight strain to
failure so my example was only valid for that sort of material. The
elastic limits are also pretty much linear and close to 100% except at the
extreme strain where some fibers start to fail individually or the
matrix resin starts failing. The only non-linearity is on the
initial strain but once under load, the stress/strain is virtually a straight
line. One thing you can observe is that laminates with a high void
content have a lot more little "burbs" along the plot since they tend to have
microfracturing which can lead to failures particularly in cycling. But
those tend to be at the high end also.
One rule that we had on a submarine I worked on was that the sub should
be designed to 50% of it's design limit since compression is the worst case
for composites since it is virtually unavoidable to prevent kinks. And
virtually all compression failures start at a kink in the fiber. If we
designed it too close, it would fail on the next dive even though outwardly
there were no signs of failure. We found that a microphone could pick up
the sound of fiber/resin fracturing which turned out to be a good way to tell
if you submarine was toast. Since almost all resins shrink, it can't be
helped (note there are some resins that have zero shrink but they are poor
structurally).
You can find "Deep Flight" on a websearch if you're interested.
Dan