For years I was a huge fan of Nidacore, a polypropylene honeycomb product. 

    https://www.merrittsupply.com/product/nida-core-structural-honeycomb-plain-h8pp/

  There's nothing to rot, any water that does get in there is less likely to cause failures than it would in foam, and you can beat the freakin' pogies out of it and still have a useful sandwich as long gas the skins hadn't pulled off.  I currently have a Nomex honeycomb-cored boat, which is definitely light and strong, but it's nowhere near as resilient as the polypropylene.

  Some of this information is from observation of what local fisherman have done to abuse their boat decks; the rest is from destructive tests done at Practical Sailor in an article on core materials for their October 2007 issue.  While impact testing could produce long internal cracks in the various foam core products (rendering a whole section useless in shear), the honeycombs had very localized damage. 

  Two photos follow my signature (also useful when pondering impact resistance in general).

  However, I started wondering about nidacore after seeing a photo of nidacore cells that somehow had water inside even though the hull section had not been damaged.

  Foam also has the advantage of being easily milled with a CNC machine for things like rounded edges, rabbets for better joins, and other things that would leave open cells in a honeycomb core.  Milling everything ahead of time, before the infusion, is a big part of the new HP designs.

  So for now I'm sold on foams proven to be resilient to impact, both by testing and by navies.

  And, of course, I fully support others' desires to find less expensive materials without track records, that could end up proving to be superior over the long haul.

        - Mike

 

That paper is discussing honeycomb cores, which behave a little differently to closed cell foam.  The wrinkling they refer to could be viewed as  micro-buckling of the skin with the skin still adhered to some part of the cells but the cells have collapsed near the plane of the where they attach to the core.  They distinguish wrinkling from buckling as they refer to buckling of the cells between skins due to perpendicular compressive stress. 

When I refer to skin buckling with a foam core it is the failure of the skin in compression in the plane of the skin due to buckling resulting from core or adhesion failure enabling the skin to become unstable.  It is more a gross failure than wrinkling.  Both are the result of the in-plane compressive stress on the skin under compression.

A wrinkled skin on honeycomb core may be still serviceable but a buckled skin on foam core will rapidly deteriorate.  The composite panel has been reduced to essentially a single skin on a foam panel so has lost most of its rigidity.  If the outer skin is exposed to water pressure it will soon delaminate in the failed area. The honeycomb may leak and take water into the core but it will survive for longer.  

In the oceanskayaks link there were a couple of images showing buckling failure of the skins on foam core.  

Rick

On 16 Nov 2018, at 11:13 pm, Björn bjornmail@gmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:

Rick, about the buckling, is it what the paper calls  "Core crushing"/"Indentation" or "Face wrinkling"?

See Chapter 2, and especially Figure 2..9 and Table 2.3

Design of Sandwich Structures

Achilles Petras