Thumb rule for honeycomb,corrugated structures.

[kdaenerys]

What is a thumb rule for equivalent yield strength (improvement or reduction) for perforated, honeycomb and corrugated structures as compared to solid structures? Is there a thumb rule for light weighting for a similar case?
The parameters for thumb rule could be weight being reduced/increased, strength, cost, complexity in manufacturing/design/analysis etc.

 

[SwinnyGG]

There isn't one.

 

[rb1957]

succinct !

you've asked about a whole range of materials ...

honeycomb panels ... there's a whole extensive set of analysis to verify if a panel is acceptable. On a very simplistic level you could look at the bending stiffness (or the axial area) of the two faces and compare to a solid plate.

corrugated panels ... again a whole set of analysis, but generally corrugated panels increase the shear stiffness/allowable of the flat panels, similar to lightening holes.

perforated panels ... probably consider the area lost (to the holes) for a reduction of strength ? Shear and axial behaviour probably different.

so, no "rules of thumb".

another day in paradise, or is paradise one day closer ?

 

[GregLocock]

On our first solar car somebody had the great idea of using a hole saw to remove material from the vertical faces of our built-up box sections. This gave me the heebie jeebies. So I grabbed a piece of the material we had (glass fibre skins over a 13mm foam core), and cut a 50mm disc out. Then weighed it. I can't remember how much it weighed, but it was pretty obvious that to get a 1 kg weight saving, barely detectable in the scheme of things, we'd have to drill an awful lot of holes.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Sparweb]

If you do come up with a rule of thumb, whatever it is for Yield in tension will be wrong for compression, and vice-versa. And then there's bending, and shear, and so on, and the way the load is applied...
But that doesn't mean that a well-designed and suitably-chosen sandwich panel cannot be lighter than a construction of aluminum sheets and stiffeners of equivalent strength and stiffness, if you could find one case of each where both accomplish the same goal efficiently. However, your design goals are probably very different if you are choosing the sandwich over the sheet structure, and very often making comparisons is completely unfair. Sandwich is excellent for floor boards in commercial aircraft (distributed support and out-of-plane applied loads). Less ideal for the frames that support the floor (discrete supports and in-plane applied loads).

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[RPstress]

It's usually best to take an example H/C panel and assess its performance. Use one of the usual stressing guides. Hexcel's are free and quite good (

https://www.hexcel.com/Resources/Technology-Manuals

- a bit of googling will yield many more). You can calculate properties such as weight vs. bending stiffness, bending strength, verious buckling failures, etc. quite simply with hand calcs and Excel or Mathcad (or pencil and paper if you've got a little time). Honeycomb panels are the lightest way to make flat or gently curved structures. Tightly curved structures will need a little extra mass but you are still looking at roughly halving mass or (quite a lot) better compared with monolithic 2D structures, and also an improvement over foam sandwich 2D structures.

 

[ESPcomposites]

Regarding the Hexcel honeycomb manual, it is OK for giving you an overview and introduction to some of the failure modes. But there are a couple things to be aware of:

- The solutions are for metals and many are not appropriate for composites. This is noteworthy because Hexcel sells composite materials and you may have thought the solutions would work for composites (they don't say the solutions are only applicable to metals). They also provide some composite properties in the document, which could seemingly be used with the solutions, but it would be incorrect to do so.

- Some of the solutions use empirical correction factors (but they don't mention when this is done). Just be aware that some solutions may not be "pure" as you may have expected. The correction factors vary depending on the source so if you see a different solution elsewhere, this may be the reason.

 

[WKTaylor]

kdaenerys...

Awhile back I realized that many/various 'the rules of thumb' we look for come the hard way... after first-hand design, analysis, testing and service EXPERIENCE You develop an 'internal sense' as to which direction the answer's lay.

Short-cuts by 'rules of thumb' are almost always intended for use by 'the EXPERIENCED'... those who can appreciate the nuances and difficulties embedded-in, or absent-from, the simplified approach. There is no substitute for EXPERIENCE.

EXPERIENCE, combined with back-of-the-envelope sketches [active thinking-thru the problem] helps integrate to, obvious and not-so-obvious [real-world] elements that may come to bare on the problem that significantly affect the outcome. Then, and only-then, can simplified analytical models [CAD+FEM] point in the direction of the most efficient answer... which can then be worked/defined in-detail.

In my line of work I have developed numerous internal rules-of-thumb to quick-answers/effective based on many personal EXPERIENCEs... which are relatable to others only when I have the time to discuss the experience/permutations that 'fed-into' deriving my answer.

“EXPERIENCE is a cruel teacher. First she gives you the test; then she teaches you the lesson.” – version of Vern Law’s quote

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]