Re: more cfrp bleating

jim beam wrote:
Peter Cole wrote:

Brittle failure is a well known characteristic of CF composites.

what? that it shatters like glass??? bullshit.


That said, it is possible to design energy absorbent *structures* with CF composites. Such structures usually rely on the tearing of fibers from the composite rather than simple fiber tensile failure. I posted an example of this in the Audi design for a tubular crash structure. The example of the steerer tube tearing through the head tube is similar,

which contradicts your earlier statement. which one do you want peter? you can't have it both ways.

It contradicts nothing I've said.

<http://seattletimes.nwsource.com/html/boeingaerospace/2003849110_boeingtest23.html>

"Ed Fasanella, a senior NASA engineer and crash expert, worked on the two earlier 737 drop tests and is involved with extensive crash testing at NASA's Langley Research Center in Virginia."

"Since it is composite, it's harder to analyze," Fasanella said, "You almost have to do a physical test."

While metal bends and crumples rather than breaking under high loads, he said, carbon-fiber plastic composites are typically "brittle."



The specific topic is crashworthiness of composite fuselages. This is a new application.

it's not "new". and it's not inferior - your implication.

From the FAA
<http://www.thefederalregister.com/d.p/2007-06-11-E7-11153>

"The 787 fuselage will be fabricated with carbon fiber reinforced plastic (CFRP) semimonocoque construction, consisting of skins with cocured longitudinal stringers and mechanically fastened
circumferential frames. This is a novel and unusual design feature for a large transport category airplane certificated under 14 CFR part 25. Structure fabricated from CFRP may behave differently than metallic structure because of differences in material ductility, stiffness, failure modes, and energy absorption characteristics. Therefore, impact response characteristics of the 787 must be evaluated to ensure that its survivable crashworthiness characteristics provide approximately the same level of safety as those of a similarly sized airplane fabricated from traditionally used metallic materials."


> and please
please please /try/ to get the basic science right - you have no basis for criticism if your fundamentals are wrong.

NASA Langley did a (single) drop test of a all-composite fuselage:

<http://www.sae.org/technical/papers/951168>

"As part of NASA's composite structures crash dynamics research, a general aviation aircraft with composite wing, fuselage and empennage (but with metal subfloor structure) was crash tested at the NASA Langley Research Center Impact Dynamics Research Facility. The test was conducted to determine composite aircraft structural behavior for crash loading conditions and to provide a baseline for a similar aircraft test with a modified subfloor. Structural integrity and cabin volume were maintained. Lumbar loads for dummy occupants in energy absorbing seats were substantially lower than those in standard aircraft seats; however, loads in the standard seats were much higher than those recorded under similar conditions for an all-metallic aircraft"

The bottom line is that a composite fuselage will have less energy absorption in a crash than a similar metal one. This can be mediated with special energy absorbent structures. The Boeing engineer feels that certification is being rushed on the 787. The current process is to perform a single drop test to validate computer models which will be used to predict overall crash dynamics. He feels this is not conservative enough given the incomplete understanding of the behavior of composites in these circumstances.
.

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