Hexacor compared with other composites?

[mikereed100]

Hi Analyst,

There's no need to keep it short the next time, I appreciate everything you've posted and more. Please continue to post as much new and additional information as you wish -- the more detailed the better!



Here's where I was coming from when I started this thread ...

I suspect that 9mm thick Hexacor is not thick enough to replace 9mm plywood as the core in a composite sandwich if I want a sheer strength that's high enough for a particular application -- a small, extreme shallow water planing powerboat. I was thinking of using 25mm Hexacor instead, not only for light weight but also to eliminate wood as the core material so buyers won't be thinking of 'rot problems'.

Unfortunately I do not have the ability to do the calculations to determine whether or not 25mm Hexacor will be strong enough to be a good replacement for the 9mm plywood core, and this is why I've been trying to learn more from people who actually have more experience using this material than I do.

Ken,

You may already be aware of these folks as they are in your neighborhood and are distributors for Hexacor. They are building boats that sound similar to what you are planning and may be of help.

Mike

http://www.bakricono.com/

 

[Analyst]

Very interesting discussion! I have a simple question: Why not use balsa for decks and everything but hulls? As long as water doesn't get to it (easy to prevent) it has hi strength, low weight and is not affected by temperature. So why use anything else? I'm not being a smartass, just trying to understand more....

Naturewaterboy:

I agree that by the numbers, balsa is an exceptional core material with high transverse shear stiffness and strength values at low densities, however it is my opinion (only an opinion) that there are some inherent disadvantages that I have seen through comparison. The following are:

1. Resin absorption for wet layup application. I have seen balsa cross-sections from carefully manufactured structure that absorbed a considerable amount of resin. This is a major problem for two reasons. One: the obvious weight penalty for weight critical structure. Two: Print through due to resin shrinkage during cure from the access exotherm caused by the added resin volume. The second is avoided by staging the lay-up process to allow cure states or Barcol states, which will in return cost in time and more weight and money from the added secondary bond layer that may be required during each stage. The alternative is to apply more fiberglass to absorb the excess resin, At any rate, there is a pattern here in which more non-structural weight is added to produce a finish instead of a part.

2. Cost variability. The cost of balsa is very dependent on its availability and distribution location because it is a natural resource. The roller coaster of wood prices from a cost and demand standpoint in world markets is well-known to the construction industry and Balsa is no exception. Well seasoned boat manufacturers have been off and on the balsa price-coaster for a while. On the other hand, in the U.S. with oil barrel prices being much higher than they were a few years ago, the cost of gel-coat, resin and other oil-based products have driven through the roof. As far as PP Honeycomb, I am slightly unfamiliar with the details of this aspect, but a Nida-Core distributor said that the material base for their PP Honeycomb was not oil, but instead a material that I commonly associated with its gaseous form, but cannot remember exactly what it was so won't say what I am thinking. All I remember is that it was not oil based, which should stabilize its cost in todays marketplace.

3. Water absorption. Although it may be a solvable issue from a build standpoint, Balsa is inherently a thirsty material. It likes to drink while just sitting around a humid shop, getting weaker and heavier as it is waiting to be used. Expense and care must be taken with respect to storage and climate. Wate intrusion is also a factor. A colleague of mine has found that a tiny crack in one side of a boat can feed Balsa in the other side as long as their is a path - no matter how small, even though the balsa in the dry side is still "locked" out of the external environment. The water also reduces bond quality.

4. Processing. Balsa is not as easy to cut and taper as other materials. For example a box-knife can be used to cut PP Honeycomb.

NOTE ON WATER ABSORPTION:

It would not be fair to balsa if I did not mention the extensive use of aramid paper Honeycomb cores in aircraft, spacecraft/satellite structure and in racing marine craft. The lightest weight core with the highest stiffness and strength properties (for weight) that I am aware of is aramid paper honeycomb core. It is extremely attractive by numbers, processing (easily cut and bonded) and formability (overexpanded non-scrimmed core can wrap around a softball!) and cost. However, aramid (kevlar) is very hygroscopic and in paper form is like a sponge. The stiffness properties, as you would expect are reduced by this hygroscopic nature. But you still see it in these advanced applications. How do they get away with it? The answer is in their processing techniques. I would not recommend a structural component using aramid paper as a key structural component that did not already consist of prepreg facesheets with film adhesive, vacuum bagged and autoclave cured. In general I don't believe the majority of the marine industry has reached that standard or requirement yet.

Balsa seems to have the same negative problems as aramid honeycomb core, except no justification or promise can be made with respect to the processing methods common to the marine industry, that the material will not absorb resin, get wet (or already be wet), cost the same or less every time and machined or milled easily to the desired contour. It is like the aramid paper in its worst case scenario.

This is, however, as stated before, only an opinion.

 

[naturewaterboy]

Analyst,
Thanks for the info. I did cut open my 1978 flybridge floor,and was amazed to find some of the balsa was completely rotted into mud, but some was good as new. I did notice that the original scrimmed balsa did not have any resin between the blocks - i guess that this would have slowed the water down, but would have added some weight. My guess is that Silverton just took the easy, least expensive way out, as it appears they relied soley on caulking to seal the deck mounted hardware.

 

[AndrewK]

Analyst

I tried to correlate your 3 point bending equation with measured deflections for some foam cored panel off cuts. Measurements did not correlate, can you get in touch with me off line via email at ansu_ko@bigpond.net.au ?

Cheers Andrew

 

[brian eiland]

I'm trying to relearn/review some sandwich core technologies.

I am VERY IMPRESSED with the postings by ANALYST in this subject thread. Regrettably, It appears he was only on the forum for a short month.

You owe yourself the time to go back and review his postings !

 

[brian eiland]

I found this posting of his particularly interesting,...

Yes, I have heard (through a PP HC core supplier) that DNV has created a restriction on PP materials on decks and sides.

To that, I ask you, what is the motivation of a certification and standards organization in restricting a material type for any reason other than environmental factors, human health factors, and/or processing hazards?
PP has none of the problems mentioned above.

Unless the material has one of the three problems stated above, there is no intelligent means of restricting a material type.

Otherwise, the motivation for the restriction could only be 1. political (ties with companies that make competitive materials intended for the same application), 2. empirical (equations in standards and testing methods are insufficient in defining the behavior of the material (common problem)) 3. Prejudice (individuals creating the standard have a personal preference in what kind of materials that they want to see for that application and apply a God-like rule over the design for structures requiring their certification).

As stated before, material selection must be done on an application basis. Good certification organizations and self-checking companies apply specifications on the mechanical or physical properties of the material (not the material type itself) on an application based method (hull/liner/door/spar/wing skin/etc.). To go even further, especially with composite structures, laminate and/or sandwich properties are a means of specification, ignoring completely (within the spec) the properties of the plies/or core making up the laminate, although they must be known to create the laminate/sandwich properties.

(i.e. for a hull bottom panel of a ship of X length and Y beam with a panel aspect ratio of a/b constrained by BC1, BC2, ... BCn, at a max speed of ZZ knots, in sea conditions ABC or D which creates a panel pressure zone of C psi or N/mm^2 at that station location, the cross section of the structure/panel must have an overall flexural stiffness of XXXX to maintain a deflection of W.WWW inches or mm and critical buckling load higher than XXXX lb/in or N/mm and must have a stress/strain allowables of YYYYY psi or N/mm^2 or must meet a factor of safety of EEE.)

Note that in the above example, the material type would never enter the calculation.

Also additional constraints that may be more material dependent could be peel stress/strain of a bond, or energy release rate of an cut-out, thermal/moisture expansion rate, thermal/moisture degradation rate, impact energy absorption, surface hardness for debree, degree of chemical resistance, etc. These are all still defined by their performance in the specific application, not by the name of the material.

Material type just does not come into the picture. The requirement is only that the mechanical properties of the materials within the structure meet these requirements. Hence, Foam versus PP versus Balsa are be examined on an equal playing field.

If the motivation for the restriction is test based, what may be likely is that the DNV performed tests on panels with identical core thickness and did not account for the specific properties. You cannot expect a core material of the same thickness but with a lower transverse shear modulus (Gc or Gxz, Gyz or G13, G23 or GL, Gw) to achieve the same deflection as a material with a higher transverse shear modulus. This still cannot be grounds for material-type restriction. It should be designer’s preference. (If the designer wants to save money and eliminate weight and wood by replacing the 0.5 inch Balsa with 0.75 inch PP Honeycomb, it should be allowed to do so. If a designer wants to use 10 inches of HDPE on the sides and bottom of his/her boat and can prove that structural integrity and stability is achieved then this should be his/her choice to do so. It may not be a good idea, but if it works, it works. A certification company cannot limit the ingenuity of the marketplace to materials that they see fit.)

It is also likely (and common) that the wrong test for Honeycomb sandwich panels was performed. C393-62 is acceptable, whereas the common 1 inch wide strip 3-point bending method is inaccurate for HC because of cell size.

Nevertheless, material selection MUST BE APPLICATION BASED NOT MATERIAL TYPE BASED.

Sorry about the length. I will try and keep it short next time.