Wood/Composite Fusion

Wood Boatbuilding Fusion
Some time ago I was invited to give a lecture to the Seattle Central Community College school of wooden boat building.
I advisd them that their futures were not in steamed knees and nuances of refastening a hull. I told them that the future of wood boatbuilding was in fusing wood/epoxy technology with composite technology. Together they offered advantages that neither can separately
I don’t know if they were impressed, but some of my fusion samples never made it back to me. I guess that is good.
Wood and plywood have many advantages that many composites don’t have. Woods, for example, have a fatigue strength that is only matched by carbon fiber and is more than double that of E glass. After some 10 million use cycles, plywood and carbon have about 50% of the strength they started with. E-glass has about 22% of the strength that it had initially.
Plywood has an industrial smooth surface finish that composites can only achieve after hours of finsh work or a large bagging table. It comes that way right out of the factory. One should try to use those advantages given by the economy of mass production wherever possible.
A fusion composite part probably will not need a mold. The plywood appearance face can form the part. When I built the composite lunar lander, if I would have had a big budget, I might have infused the parts with triaxial roving and pvc or sen core. I would have needed molds or at least a bagging table. I did not have the budget so I vacuum bagged my own SIP panels by vacuum bagging ply faces onto foam core.
By itself wood or plywood is too heavy to be efficient against out of plane loads or to be restrained against global loads. Composite cores like pvc, sen, or balsa can make a very strong panel by bagging the core to the plywood and in turn, bagging more plywood or composite fabric onto the opposite face. The result can resist out of plane loads and or, restrain a structure against global loads at much much less weight than a solid plywood or even plywood and stringers can do.

All wood used as an engineering material must be encapsulated in epoxy. That not only improves the wood’s mechanical properties, but it protects it against moisture and rot. The epoxy also becomes the bond with the core and or fabric that is used. Epoxy is also much easier to get right, especially if the temperature of the workspace changes during the day. Polyester or vinylester initiators and catylists must be mixed accurately to often fractions of a per cent. And that measure must change as a function of temperature. I always say that you can be almost brain dead and still get epoxy right.
Plywood has extra benefit from combining with carbon fiber. Plywood and carbon fiber both have a stretch to failure of about 1%. They work very well together. In comparison, E-glass has a stretch to failure of about 5% to 6%. So if the combined part of plywood and E-glass is loaded to failure, the plywood will fail while the glass is onIy contributing about 20% of its strength. I have seen plans of catamarans by famous multi designers, who charge much more than I do, who design beams with plywood and E-glass combined. With enough glass, it works, but is very inefficient. And I remember one set of plans I saw with something like 20 layers of e-glass uni was called for. The schedule not only had no cross laminates, but the contact surface with the plywood was minimal. Those each are a failure waiting to happen. Combined? Wow.
E-glass does have a place with fusion products, but not against global loads. Glass cloth is great at providing damage protection and a vapor barrier for plywood. Glass cloth or biaxial roving are ideal for joining parts where global loads are not involved. The extra give that glass has can do very well to prevent the energy of crack initiation on a plywood free edge. Like a ripstop.
Ideally the carbon fiber is vacuum bagged to the plywood with the warp in the same direction as the plywood 0 degree. In general you can replace about 10 thicknesses of plywood by one thickness of carbon fiber, for bending strength. Note that many other factors enter in, but in general that’s true.
Plywood, lumber and composites combined can create products that are synergistically better than the different parts that were combined. Fusion.

14 thoughts on “Wood/Composite Fusion”

  1. Kurt- I think you have to be very careful in making statements in regard to fatigue or endurance limit ratios for materials. If the stress ratio lose were as you say then composite aircraft could not fly as the material properties would become less then the required service properties at some time in the aircraft’s life. Plus you relate strength to the fibres. Its not the fibres that fatigue its the stuff that holds it together or the interface. It you use good processing and high toughness resins for instance the strength loss is minimal in service conditions. I have read reports on comparing end of life composite service aircraft parts to same age spare parts (with no service stress) and the difference in tested strength has been statistically indeterminate. Its preferable to discuss structures as endurance strength and materials as fatigue strength as the failure mechanisms involved are quite different. For instance if you make a plane fuselarge out of GLARE which is a composite of aluminium and glass it does not need to be inspected in its entire service life for various failures that are common in monolithic metal. Regards Peter S

  2. I refer you to Gougeons extensive fatigue testing on woods and Ronnal Reichard’s extensive materials fatigue testing. If resin were the weak link, then carbon and e-glass would have similar strength percentage after a million cycles. They don’t.

  3. I refer you to Gougeons extensive fatigue testing on woods and Ronnal Reichard’s extensive materials fatigue testing. If resin were the weak link, then carbon and e-glass would have similar strength percentage after a million cycles. They don’t. If I recall correctly, the load was kept at some 20% of ultimate. Maybe the aircraft are not loaded near that.

  4. Hi Kurt – When you make a general reference like see this or see that please make a specific reference so it can be followed up. I’ve had a look and can’t find these works on the net. Your conjecture about the differences in the fibres/resin strength is incorrect due to the fact that the strain to failure of the fibres relative to the resins is different and the way the fibres are coupled to the resin is different as well. Every researcher gets different results because they use different methods, different materials and different processes. Unfortunately most of the work I have done over the last 30 years is commercial and propriety so can’t be published but its clear much better results than what you describe can be achieved. Peter

  5. Being a design office I don’t really have time to putter on these things, but look for Structural Composites testing lab and anything that you can find from Marine Applications of Composite Materials published proceedings. I assume everybody does a D-790 test about the same way. This is not new stuff and I have never heard of it being refuted.

  6. I am interested in books and papers on this as well. I did not find much at “Structural Composites Testing Lab”. Peter, if you find anything could you share them. I’m looking through some of the papers under “Marine Applications of Composite Materials” now.

  7. Trouble is I have almost all of the procedings, but in paper. They never really did get on the net. Am sure Gougeons have a lot. Try Ocean Engineering department at Florida Institute of Technology. Anything by Ron Reichard. Contact Structural Composites directly. They would know what has been put online.

  8. Hi Jak- I will. Look up glulam there are many standards and specifications for this around. Then you use this as a basis and extend it to cover the long fibre additions. The combination of timber and glass or carbon fibres for boat building is a good thing. The methods suit boat builders, you don’t need to build moulds and the fibres enhance the timbers weaknesses such as its low strength. Its a good step up from pure timber. But it won’t get you the same properties as good quality glass or carbon so don’t wish for too much. Cheers Peter S

  9. microlam lumber is called for in my construction manual as it has much better strenght properties. Don’t discount good quality plywood too much. We had to test plywood for every USCG certified ply/epoxy cat. Many tests had bending strength at or close to 20,000 psi. Compare to the default fiberglass strength in the ABS ORY Guide of 25,000 psi.

  10. microlam lumber is called for in my construction manual as it has much better strenght properties. Don’t discount good quality plywood too much. We had to test plywood for every USCG certified ply/epoxy cat. Many tests had bending strength at or close to 20,000 psi. Compare to the default fiberglass strength in the ABS ORY Guide of 25,000 psi. And recall that both carbon and plywood have very similar stretch to failure percentage, where e-glass has much more stretch so will not help as much in global loads.

  11. Hi Kurt – I’m quite unclear why you put forward the argument that because glass stretches more its not valuable in this application. The fibre may stretch but a glass laminate still fails at 1-3% elongation. The default ABS maybe 25ksi (172MPa) but we regularly test glass laminates at 900MPa flexure and 30GPa which is very worthwhile for the cost of E glass vs carbon. Plus with high modulus glass becoming available (100GPa vs 70Gpa) the gains are even better as we build 40GPa laminates vs 30Gpa with std eglass. Its not a matter of ultimate elongation its a matter of relative stiffness of the components and putting the right stiffness in the right place. Cheers Peter S

  12. all of my data has e-glass elongation to fail at 5% to 6%. Plywood and carbon are both at around 1%. So in a plywood and eglass layup, even a typical triaxial hand layup of 60,000 psi bending would only contribute 10 or 12 thousand psi to the job. No doubt aerospace materials are much higher quality than the usual backyard builder can do.

  13. Hi Kurt – I just looked thru some manufacturers data on eglass and can’t find any that quote the fibre at more then 5%. Some Sglass is at 6%. A typical carbon fibre is T700 and its e=2.1% I’m not trying to be picky just trying to understand your figures and your logic. If your are proportioning the strength contribution from the materials tensile strength via its elongation this is incorrect thinking. The laminate distributes strain proportional to its axial rigidity (AE) if in tension or compression. If the composite is a beam in bending then the strain is proportional to the components rigidity (IE). Which component fails first needs to be calculated and it may not be the one with the lowest strain at failure. To go a couple of notes back if we placed a steel strap on the top of a timber beam using your logic it would not contribute to the strength because its elongation at failure is 35% plus. Or if you look at its elongation at yield this would depend on the grade selected. I work in the boat building area not aerospace and I do survey builds which require testing and the builders I work with get the figures I have mentioned before. Even the back yarders I work with using infusion get the numbers I’ve quoted before.

  14. none of my data in the proceedings nor mfg data has e-glass elongation at less than 5%. Thats waht I use. I assume they use same axial rigidity as I have not seen it noted in any testing I have.
    Steel is so much stronger that it would take most of the load most of the time. I find timber and rubber band a more useful example. I’m not a data maker. I just use what data I have. Take it up with Eric Greene or Prof Reichard.

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