....Of course, raw corecell is also not as stiff and about three times the cost and not finished. But I could glue headliner to corecell and not add much weight if I can resolve the stiffness. That could be done with a one sided lamination or two sides light....
Well, at the heart of your question is really the basics of structural theory and understanding. Since which material structural arrangement is better?...only a simple analysis will tell you that…
For example, take a flat bat, let’s say it is 50 x 5mm.
If you have this FB orientated vertically (long side is 50mm) the inertia is 5.208cm4, with a modulus of 2.08cm3.
If this same flat bar is rotated 90 degrees, so the width is 50mm and its depth is 5mm, the values become 0.052cm4 and 0.21cm3.
By orientating the same structural element you can change the stiffness, its inertia/modulus, by a factor of 100 and 10 respectively.
So, in your case all you are doing, or beginning to realise, is that by adding skins, moving them apart with a material that weights nothing (the purpose of a core), you increase the stiffness.
So, taking that same flat bar which is now 50mm wide and 5mm thick, if you split this into 2 skins of 2.5mm each and separate them by 10mm, the inertia becomes 0.9896cm4 which doesn’t sound much, but has increased its stiffness by a factor of 19 – which is massive! And the modulus becomes 1.32cm3, an increase of over 6 times.
This then leads into real structural understanding and how to use and manipulate materials and structural arrangements to achieve an objective. When designing a structure, especially with low modulus materials, the design driver is the deflection, not the stress. So, the metric of measure becomes what is called Flexural Rigidity or Flexural Stiffness – which is pretty much where you started this thread. It is the “EI” in the bending moment equation which is integrated to become the deflection.
The E = Young’s modulus and the I = 2nd moment of inertia.
So, now back to the 2 skins that are 2.5mm and 10mm apart.
If the E, the young’s modulus increases, in simple words, if you select a much stronger material for the skins, the value of EI increases; the higher the EI the less deflection. And the by-product of this is a lowering of the stress too.
So you can select a material, cheap that has a low E. But to satisfy a design criterion results in having either a very thick single skin which ends up heavy, or 2 skins separated at a large distance apart – which may be too thick to sue effectively.
Or
If you select a material that is very expensive but very strong, you could end up with a thin single skin, that is to say thinner than the low E version and be lighter, or 2 skins that are also thinner with much less distance between them, but ends up way beyond your budget for the whole project.
How you decide which you want and which arrangement is simply dictated by your budget and the geometric dimensional constraints that you may or may not have.
That’s it….
