A new type of core?

Designers and builders of fiber reinforced plastic (FRP) structures appear to have the paradigm that the only way to build a light, stiff, robust structure, e.g. a boat hull, is to use a low-density material that is dissimilar to FRP to separate and to transfer loads between FRP skins. FRP cores include balsa, various foams and honeycombs, and the typical debate, for example the one that has occurred in the thread started last year titled “Vinylester Question”, is about "which is the ideal low density material”. A particular concern in such debates should be core properties under dynamic loads since transients may have forces of ill-defined magnitude and direction and fatigue can and ultimately does occurs. Use of dissimilar materials has the inherent problems of bond interfaces and different physical properties such as strengths, moduli, thermal expansion and other environmental characteristics (e.g. balsa absorbs water, foams can change dimensionally with aging). These cores, unlike the fibers in skins, either cannot be aligned because of essentially isotropic properties, as in foams, or alignment of the anisotropic properties is not easily varied, as in balsa and honeycombs, to optimize strength-to-weight for specific loadings.

Twenty-two years ago, my desire for a light, stiff 30‘ sailboat outweighed concerns about dissimilar materials, and in 21 years of enjoyable hard use, I had to repaired several “dissimilar materials” failures. Starting a search for a next boat, I found that core technology had really not advanced. Hence, I questioned the apparent paradigm. What if the core could be essentially the same material as the skins? Obviously FRP cannot be foamed to achieve low density. However, what if already formed FRP “rods” or fibers that wet with resin and turn into FRP “rods” during molding could be used to separate and transfer loads between skins? The composition, alignment, size and distribution of the rods could be varied as a function of location to determine static and dynamic structure properties. Very low-density foam that meets handling and manufacturing requirements could be used to hold and align the rods or fibers during molding. The use of FRP rods either reduces or eliminates the issue of dissimilar materials depending on if the foam provides a load transfer function in the final product (another degree of freedom for the designer). Wetting fibers to form rods as part of the molding process eliminates many of the concerns of secondary bonds in the structure.

Details of this core concept are contained in the published US patent application 20030170441, which I wrote and submitted essentially on a dare. (It became a double-dare when a patent attorney said I could not get a patent without considerable legal expense.)

My purpose here is to change the focus of the debate about “which is the ideal low density material for a core” by asking the following three questions:
1) Does a core paradigm exist or are there practical reasons, including cost, for dissimilar materials being used even in high-cost, light, stiff FRP structures?
2) If cost were no object, as a function of location in a boat, what are ideal static and dynamic core properties and can these be met by current core materials?, and
3) Would an improved core ultimately allow for a significant improvement in boat design/performance?


-Fred Boyle

 

Since we are on the subject of using fiberglass as a core, why not a corrugated core similar to corrugated cardboard. I realize that strength and rigidity will be different depending on the orientation of the corrugation but I think it could produce a strong, light structure. One big advantage is that if water gets into the core, it will only fill a small chamber and it can easily be drained out without damaging the core.

 

Details of this core concept are contained in the published US patent application 20030170441, which I wrote and submitted essentially on a dare. (It became a double-dare when a patent attorney said I could not get a patent without considerable legal expense.)

From your description of your idea, I think some other people in the aerospace and infrastructure markets might have had the same thoughts (www.aztex-z-fiber.com X-Cor product and www.webcoreonline.com Tycor product).

1) Does a core paradigm exist or are there practical reasons, including cost, for dissimilar materials being used even in high-cost, light, stiff FRP structures?

In core materials the rules are "light as air stronger than steel, and cheap as dirt". One thing you must also remember is that fiberglass is inherently a combination of dissimilar materials (fibers, resin, fillers, etc.). I don't think the failures seen in sandwich construction are attributed to using dissimilar materials, but by people just not building them correctly.

2) If cost were no object, as a function of location in a boat, what are ideal static and dynamic core properties and can these be met by current core materials?, and

Ideal properties for core materials in certain locations on a boat depend on what conditions you put the boat through. In my opinion, there are core materials out there (high density foams and balsa) that can take any load seen on any commercial FRP boat, its the people on board these boats that might not be able to withstand the loads...

3) Would an improved core ultimately allow for a significant improvement in boat design/performance?

Only if it was significantly lighter, stronger, and cheaper than what is currently out there.


-Fred Boyle[/QUOTE]

 

Thanks for the response, and I am will to accept that my impression of the current state of core technology is wrong, but I offer the following.

I do understand that fiber reinforce polymers are a composite of dissimilar materials and that failures do occur in an “all FRP” structures. However, FRP has a very good history of being able to handle relatively large dynamic loads and deflections for long periods of time without failure (the FRP leaf spring in a Corvette is an example). Maybe many boats are being used beyond their design intent, but there appears to be a far greater number of failure modes that can and do occur in a cored structure.

I have often hear the comment that “sandwich construction” problems are caused “by people just not building them correctly”; usually from people associated with a core supplier or boat builder/dealer. Does this imply that all core materials are equal; picking the right boat builder is what is important. Well maybe I have talked to the wrong people at boat yards, but the conversations I’ve had indicate that core related problems are not just limited to a few manufacturers or construction methods; although there are certainly those that are better than others.

I agree that commercial FRP boats can take any load for which they are designed, which can be greater than what people can withstand. The advantage that people have, however, is that if they survive an ordeal, they can heal and in many cases become stronger because of it. This is not the case with cored FRP structures. Fatigue, i.e. the degradation of physical properties with use, is a concern when deciding the best core material, and this is often not discussed in “core” literature of debates.

If a new core must be all three – lighter, stronger, and cheaper – than current products, this is a major hurdle to a new product. There are, however, many core suppliers who win new business without having all three. All I was hoping to find was a core that is more robust than the others. One that can provide long life in a light-weight boat under reasonably hard use while allowing for some imperfections that may occur in the construction and/or maintenance of the boat. Maybe that already exists and the remaining debate is only about relatively insignificant details.


-Fred Boyle

 

Core Materials

With regard to core materials used inthe boating industry I think it is imperative that we find alternatives. I think this article I found on the web will put things in perspective: (I am building a yacht so if anyone has found any environmentally friendly core materials other than water absorbing balsa I would be very interested)

WHY DO WE CARE ABOUT PVC use in boatbuilding?

Health Risks
PVC is a major precursor to dioxin formation and therefore poses many serious health risks to humans throughout the life cycle of the product. Dioxin was listed as one of the twelve priority pollutants slated for worldwide reduction and/or elimination (UNEP, 1995) at the UN global convention on persistent organic pollutants. Additionally, the EPA has stated that dioxin is 300,000 times more potent a carcinogen than DDT (Greenpeace, 1998).

Manufacture
The manufacturing of PVC produces highly toxic byproducts including dioxin, hydrochloric acid, and vinyl chloride. These three toxins have been linked to severe health problems including cancer, diabetes, neurological damage, reproductive and birth defects. In fact, cancer blooms have been observed in areas where PVC production facilities exist. For example, four workers at the same PVC production plant in Kentucky were diagnosed with an extremely rare liver sarcoma that typically shows up in only 25 people each year (CDC, 1997).

Lifetime
During the manufacturing process of PVC various toxic chemicals, such as phthalates, are added to aid in its durability and versatility. Subsequently, during the lifetime of these products some of these chemicals can leach out into the environment. In fact, government studies have shown that some of these chemicals can be ingested by children from toys containing PVC during normal use (Greenpeace Web Site). Additionally, because the construction industry accounts for 60% of all PVC use (Greenpeace Web Site), accidental fires can significantly contribute to dioxin formation and release.

Disposal
Another major problem with PVC is that its many additives make it difficult to recycle. In fact, less than 1% of post-consumer PVC is currently being recycled (Greenpeace website). Therefore, waste PVC products are usually landfilled or incinerated. Problems arise in the incineration of PVC because PVC contains chlorine (Greenpeace website). Burning chlorinated plastic leads to the formation and release of dioxins and other toxic chemicals. Additionally, incineration of PVC leaves behind toxic ash that must also be disposed of. Landfilling is a preferable but not perfect alternative. PVC, like all plastics, is not biodegradable and therefore remains in landfills indefinitely. Additionally, it has been speculated that toxic chemicals leach out of PVC wastes and can potentially contaminate soil and ground water. However, because landfilling does not involve incineration, dioxin release is not an immediate problem (except in the case of accidental fire).

 

Honeycomb cores are widely available in many materials, have been for a long time.

Bill H.

 

Crinion, you anonymous, tree-hugging, Greenpeace-quoting rascal, floating this boatload of official-sounding envirnspeak down the friendly waters of discussion. I'm more concerned about this psuedo-scientific stuff leaching into my children's education than what they ingest when they eat their toys. Go chain yourself to the bottled water industry. I hear they're using un-paid gay whales in their research.

 

I have a couple samples of the Webcore product that we plan to test (infuse). When I get a chance to do it, I will post something. Looks interesting though.