I don't know that I'd agree with everything you said there about the load limit being 2X because of the failure mode of composites but the 2X is a good number due to their fatigue life.  One of the projects I worked on was a fiberglass/epoxy submarine that was designed to descend to 1000 meters (Deep flight 1 & 2). These are subs you fly by the way, but at any rate, the subs have a couple of strikes against them that need to be contended with.  The first is that composites are sensitive to fiber alignment.  Failure is almost always at at area of fiber crimp.  The other is that for ABS certification (American Bureau of Shipping) which requires a pressure test to 2X the operational depth of the vessel.  This works great for metal which in compression pretty much is unaffected if it survives the test.  So for certification, we'd have to demonstrate by submerging it to 2000 meters.  The problem with composites is that they are not tolerant of getting it near their ultimate load as is metal.  So we didn't certify the sub since it would really require a design depth of 4000 meters to be able to be tested to 2000 meters so that it wasn't fatigued to operate at 1000 meters.  So we would have to design a part with a safety factor of 4X.  So what we found in our pressure tests of the models was that a part taken to near ultimate may come out looking flawless and pristine but when tested again, would fail at 20% of ultimate.  So the lesson here was that if designed to have a safety factor of 2, you could pretty much have an unlimited fatigue life.

Of course the resin and base fiber are also very important as is the weave.  Carbon generally fatigues at about 1/5 to 1/10 the rate of fiberglass or aluminum.  And toughened resin fatigue at a slower rate than more brittle non-toughened systems.  And interestingly, the race cars built from them seem to be safer than their metal predecessors.  This seems to be due to the core's ability to dissipate energy by disbonding to the face and crumpling the cells like little beer cans being stepped on.

The subs were very cool though and used in a Nat'l Geographic show on diving with Manta rays off of Mexico.

Dan Newland
ES #61