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Reading about composite materials in the 787 on Wikipedia, I came across this:

Carbon fiber, unlike metal, does not visibly show cracks and fatigue, prompting concerns about the safety risks of widespread use of the material; the rival Airbus A350 uses composite panels on a frame, a more conventional approach, which its contractors regarded as less risky.

I bolded the curious part. What does "composite panels on a frame" mean exactly?

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Traditionally, aircraft are built of frames (analogous to a person's skeleton), as shown below, and the smooth outer skin of the aircraft consists of panels fixed to these frames. There are many frames and, for example, the small size of the windows is because they have to fit between the frames.

Boeing 747 frames Image source: Wikipedia

The A350 uses this traditional construction, except the frames and panels are built of composite materials (carbon fibre reinforced plastic), rather than aluminium. However, the fuselage of the Boeing 787 seems to consist of a composite "tube", with reinforcing ribs running the length of the plane. There seem to be many fewer "ribs" running around the fuselage in this design than in traditional ones.

Boeing 787 fuselage section
Image source: Wikipedia

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    $\begingroup$ There seems to be a large spider inside that SSC... $\endgroup$ – FreeMan Jul 27 '15 at 14:11
  • $\begingroup$ That's a good picture of the 787 structure, but it only shows two frames for some reason. The spacing of frames and stringers is larger than planes like 777/A350 but not dramatically so. $\endgroup$ – fooot Jul 27 '15 at 14:58
  • $\begingroup$ Why is there a spider on your bulkhead? $\endgroup$ – DrZ214 Jul 27 '15 at 21:08
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    $\begingroup$ @DrZ214, why shouldn't there be? $\endgroup$ – Mark Jul 28 '15 at 0:09
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Composite material are built in panels. The idea is to built a metal structure (spars and ribs for example), and to cover it with composite sheets, instead than aluminium.

Why not to do a structure in composite?

Composite materials are very complex and still not completely known: we have been using Aluminium in the last century and we know EVERYTHING about it, also very unusual or peculiar behavior.

Composites are still to be completely understood, that's one of the reason to use a metal structure. Moreover composites are not suitable for isotropic stress and they will never completely replace the metal.

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    $\begingroup$ Saying we know everything there is to know about AL just seems to be asking for trouble... $\endgroup$ – FreeMan Jul 27 '15 at 14:09
  • $\begingroup$ @FreeMan indeed it's some decades without structural failures in aircraft due to the design. Because we know everything about the behavior of Aluminium, in every conditions of temperature, humidity and fatigue in daily use. $\endgroup$ – Gianni Alessandro Jul 27 '15 at 14:36

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