Is this progress in composite construction?

In the spirit of an excellant thread in the boat design forum, Is there great progress taking place in composite methods ? I have followed construction of foam and glass composite structures, and I was wondering has there been any great success with either parabeam www.parabeam.nl. or structiso http://home.nordnet.fr/~jmaquet/index.htm. ?
These products seem to provide a strong support structure to both the outer and inner fiberglass skins in excess of that provided by foams. Originaly boats were built with stringers and ribs to stiffen the shell. I have read that with foam the transport of compressional forces to tension serves this role in current composite designs. These new products would seem to provide for that force transport with a stronger member than a foam one. In the failures I have witnessed forces overcome the properties of foam and fractured the shells. If more strength would transport forces from the outer to inner shells are we making progress. Would just stregthening the shells be as good a solution ? Or has structures like ribs and stringers been incorporated into the foam, is it worth the effort? I relize that honeycomb cores often fail because of adhesion problems and larger members might adhere better. Has any one tried these two products, do you know of others I should examine - What are your favorite discoveries regarding the future of composite construction ?

 

tja

I used parabeam some ten years ago or something very simular and was not impressed. I definately wouldn't use it as a core in a boat. The other product i have never seen before. Personally I prefer to use solid glass where possible. Core materials have there place in a boat but maybe not the whole boat, Tom

 

I have used parabeam in boat construction, but I was not impressed also. It tends to be quite brittle, and I do not like the idea of having a hollow core.

Structiso is based upon either PU foam, which I do not like for boat construction because it is open to watervapour, which then condenses in the foam, and will never get out again. The other option is PE foam. I have no experience with this in boat construction. It is great for closed moulding, at it can be compressed in tight areas, thus not needing to chamfer foams exactly to shape.

Just before Christmas I will infuse a large panel with Structiso under vacuumfilm. I am curious how that turns out to end up.


However, I do not expect this material to replace the normally used materials. But maybe I prove myself to be wrong within the next years.

 

I have been looking in to nida honeycomb. I believe it's cheif attraction is the flexibility it imparts to a composite construction. This improves the ultimate failure mode to be "non catastrophic" The downside seams to be the low shear strength. They currently are developing a froam filled high compression form that might be very intereasting since it would be stiffer without being brittle. Hopefully they will chose a better foam than Structiso, I don't knnow why the didn't chose a PVC.
Has anyone seen a simular product or had experience with nida. There is a story of the strength of hulls built that way - drop tested from 50ft.! on their web site - http://www.nida-core.com/sandwich.asp

 

I have read the testimonials of the strength of nida-care and I am also impressed to a certain degree on the product. But I still have a problem with the shear strength.
I am also finding that information outside of the manufacture's Data is hard to come by. I have seen threads posted with very low response's on nida-core. None the less I like the data.There is a thread related to ??'s on using nida -core with balsa for added
shear strength but unconfirmed that any one has done this. The gentleman suggested
3/8'' nida-core on ext 1 layer mat 1/4'' balsa , int. then lay-up. in a build class of about 40' loa. cruiser / cabin. This would definitly add shear value and also lighter than
a heavy lay-up of glass and resin . It appears that the only response was that the bonding of 2 materials seemed to be in question But with mat in between it should not cause a problem . they do it with 2 layers of balsa in the same method to achieve good impact and added shear. As far as cost effective, it all makes sense. You have the best of 2 superior products for core materials. Very Interesting . Let me know if you come up with hands on experience I have had enough Tech Data and I'm still waiting for the real guys to open up with some info.
Good luck .....

 

tja

I use Nida Core exstensively in the boats that I build. However I'm only building boats from 10 to 20 ft. I don't use it to core the bottoms. I use 10mm Nida Core in the floors, decks, and transoms. When it comes to compression i've found that the more glass that you have on each side of the core the better it works at distrbuting the load of a fastener. In the transoms I use multiple layers of nida core with 32oz. biaxial between each layer as well as doubleing up biaxial on the inside and out side of the transom. This may seem excessive but I like to over build my boats and the transoms are still lighter then wood. If you call Nida Cora they have a fairly good tec support team that might be able to answer in more detail then there literature. I have one boat that is being used by the U S Border Patrol with 225 HO Evenrude that tops out at 85mph if you have the ball to go that fast. They have a couple of guys that will. Seventy is about my limit. Hope This helps, Tom

 

TJA,
Intereasting - long term have you seen a down side to nida ? I have read that it is less stiff than other cores and would not make a good deck core. I also wondered why you did not choose to use it in the bottoms, and what did you use - Thank you for your real world experience.
I have not built a core hull yet and I will probably not want to experiment without real hard evidence - In contacting nida support their response was that a tri-ply of two cores with a glass panel between has been used to great sucess in bottoms, stiffening the structure.
Has anyone heard of this approach ?

 

I've used NIDACORE mainly in decks and topsides with no problem. I think it could be used in hulls with no particular problems if well engineered.

First advantage it won't rot or suck tons of water if a skin is broken.
Second with the system of polyester mat heat-welded over the honeycomb the bonding of the skins becomes a breeze (I've worked extensively with high tech composites and used all kinds of honeycombs; Nidacore is not the very best which is the NOMEX but Nidacore's far the easiest honeycomb to work and get good results).
Third NIDACORE has good resilience values, and seems to age very well.

About foams, the lone ones that seem to age correctly are the good PVC like Airex (there are now 20 years highly abused multihull racers and they remained structurally sound) which are very expensive.

The shear values are not really an issue if some engineering calculations are done before to keep the max shear stresses within the limits, or by adding reinforcements like "mini" stringers. There are several solutions.

As with all foams and honeycombs, you have to change the core to a more rigid material at the places you plan to install hardware and subsequent screws. A putty of epox resin/silica/glass fiber is very effective and simple. The putty may be taylored to get the best strenght at the lower weight.

The technical support from the company is very good.

The approach of using two layers of Nidacore with a glass panel is very effective. There are several reasons for this approach:

1/ economical reason;
You have not to buy a full box of thicker core for just a few square meters of transoms or flat bottom, as the Nidacore is not detailed.
2/ mechanic's reasons of the simplest level:
- The rigidity (or inertia) a plane varies at the third power of the thickness. So if the thickness is double, the rigidity is 2 power 3 = 8 times higher.
- If you consider the honeycomb wall as a column under compression you divided the height of the wall by 2 (a bit like a spreader on a mast) with an intermediate support, so getting 2 power 2 more compression strenght.
-Again on torsional or shear stresses, the height of the wall is divided by 2, so the amount of rotation allowed is divided.

As the gluing interface between the 2 layers of core is on the neutral fiber of the plane, there is not need of heavy scantlings for this layer. Just the minimal quantity of glas and resin to insure a good bond and to withstand the (small) stresses at this place. In most cases a simple layer of mat.

I'm not very fond of using two dissimilar cores as balsa and nidacore glued together. Gluing is not so easy sometimes, it takes work so it's expensive, and the behaviour of 2 dissimilar cores may induce mechanical problems. It's easier to calculate a panel with a "strong enough" outside skin and sufficient overall thickness.

 

tja

I don't use core material in the bottoms. I use a solid laminate alternating CSM, 32oz. 0-90 biaxial, and or 1708 + - 45 biaxial. Stringer grid is also solid glass biaxial and glassed in 100% with CSM and biaxial . Personally I don't think that I would use Nida Core below the water line if your using any type of resin other then ISO or Vinyl Ester as water migration is posible. Also unless you vacume bag I think that it would dificult to ensure that there are no air voids, Tom.

 

I used Nida-Core 10mm (3/8") in the complete hull of 4 meter sailing boats (Flying Junior)

What I liked:
-easy to use in compound curves, the whole boat could be made with 6 large pieces of Nida, instead of 28 pieces as with PVC (I used vacuum)
-easy to vacuum without the need to punch the Nida (as with PVC)
-high absorption of shock loads
-price

What I didn't like:
-relatively heavy, 80 kg/m3, whereas for PVC on many places I would be able to use 60 kg/m3
-not stiff. I needed to use 10mm, insted of 6mm with foam. So again more weight
-some cells would fill (halfway) with resin
-I had trouble with a flooded boat. The cells had filled with water. When travelling by car to Italy, I took the boat with me, to have the heat and the sun to dry out the boat and empty the cells again.
-difficult to taper the edges for a smooth transition

The reason Structiso is not using PVC foam at the moment is price, and application. Structiso is widely used in VARTM (closed molding with semi-rigid top mold), mainly for "general construction" where the properties of PVC are not needed, and PU is sufficient. The PE variant is great: No need for tailoring the foam to exact shape, as it can be compressed to shape by the semi-rigid top mold.

When doing calculations for price of Structiso, please remember that you get a layer of fiber on both sides with it.

 

Further core research

After contacting several manufacturers I think I'm even more confused. Nomex core has a very strong compressive strength. But the shear numbers seem far below Balsa. I am surprised that all this high tech stuff is still lower in shear than old balsa wood. I really don't like balsa as it absorbs water and is a great fire problem. If it soaks fuel or oil it has the worst of both situations.
I also do not see how if the nida core is glassed on both sides, that it would migrate water across the cells. It should seem to do this only localy and then it should be a better material than foam, which may never dry out. And why doesn't this problem occur in Nomex ?
Also if you were to try to stiffen with additional cores and mini stringers, which to chose. I would think Nomex might be a good choice but I understand it is hard to bond reliably to. As far as PVC foam, cross linked should be stiffer but linear appears to have a greater impact resistance. As well as resistance to crack propigation. It seems that the ministringers may be a more direct solution as there will not be different core properties, but how to transport the forces between the stringers, I would think the composite of two different cores would be great for that.
Finally, could anyone give me a crude guess as to the comparitive costs - it seems like trying to compare apples to oranges from distributors. Though Nida seems the most cost effective solution.
:?:

 

Here is a sight that has an extensive price list on Balsa core and divinycell.
www.fiberglass supply Inc .com
Also another site that I have found for Nida-core .
www.boatbuilderscentral.com Both sites have plenty of information and other products that may help you. I'll let you do the math .sense you know more of what your looking for.
In addition the price of balsa and Nida are close so you must be the judge.
Balsa is still my bet . it has the strength, shear values, and compression better than nida-core . see there web sight for comparison charts .They stand by there product
and also don't hide information .
In my opinion , As an amature boat builder , I think alot of us put to much effort in the High tech evolution of boat building Materials. Don't get me wrong, There is nothing greater than
having the motivation to educate our selves .but as amatures, we should have a boat or 2 under our belts built simply the old fashion way and evolve with our experience to bigger and better thing educating ourselves along the way. There are a bazillion boats out there built with balsa and most are fine boats. Check out some sights on Yacht surveyors , There articles on this subject are wide ,Just pick one and you'll find articles on this matter. I know in past years I have become information overloaded.Hard to make good sound decisions ! And come to find in the long run for an amature .Its all methodical...........the basics are simply better in the long run.

 

These links did not come up for me ..........(ABOVE ) try typing them .....sorry

 

Final thoughts on composite construction

After extensive research there seem to be at least three schools of thoughts on compsite construction.
1st - solid - no core. It does have the advantage of great strength with a weight penalty. But it also finds great strength with flexibility providing impact resistance. Additionaly there is very little fautige problems and certianly little water take up and delamination concerns. The ultimate limitation is that the strength of the member reaches a limit based upon the charactristics of the FRG.
2nd. A cored hull that presents a lighter weight option and provides a geometric advantage with a thinker hull providing a mechanical advatange between the skins. The drawback is that the thicker member provides increased shearing and greater stiffness. The shearing can delaminate the core from the skin as the hull deflects and the difference in movement between the skins shear the core. If the core is very strong the hull will be stiff and will not absorbe shocks well. To make maters worse the impacts that force a small deflection will probably produce a catastrophic failure unknown in a solid - no core hull.
3ird. A cored hull with great adhesion and flexibility. This kind of core might be represented by nidacore, and to a lesser extent corecell as a compromise core. These cores have lower stiffness and shear strength but resonably strong compression. Hulls built with these cores produce lower stiffness and strength but higher impact resistance. As the hull flexes these cores remain laminated and do not fail in shear. But the hull is more likely to deflect as they are not as strong. This gives great concern to a simple cored structure and require additional structures, additional sandwiches or stringers, to recover their stiffness - raising weight.

I would claim that the greatest advantage to the core is the geometric advantage of the greater thickness. But it is a little unrealistic to expect both a flexible and stiff quality of the core material. While additional structures can recover stiffness, additional structures cannot make a core more flexible. From the website of a surveyor with a great dislike of cored hulls, the greatest concern seemed to be impact strength, and the greatest compliment was offered to a corecell hull with a double sandwich construction. While this may be more complex I think a double sandwich with stringers and flexible cores would provide the best hull compromise for cruisers today. There would certainly be a weight and cost penalty, but with a kevlar sheild, a better case could be made for fiberglass in a contest with steel for toughness. At least in the weight constrained cruisers, such as a cat.

What do you guys think ? - Mark :idea:

 

I have to agree with you Mark. There are trade-offs. and most of the time it is in the weight factor. All obviously work well. As you mentioned in a previous post, Adhesion
has caused problems with foam cores, Although I have not worked with Nida-core,
Due to its poly skin I think that the adhesion would be good. You seem to be on the right track in your research. 3 schools of thought pretty much sums it up.
With 2 more , Steel and Aluminum.or Alloys.
As far as Surveyors, They are (The Boat Builder Crime forensics) To what stands up and what don't. If you read enough of them you'll start to see a pattern.
None the less, You have your research in order. Sounds like decision time.