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Questions for OReely on Lamination,Materials and Strength
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Questions for OReely on Lamination,Materials and Strength
I didn't want to hijack the stringer, foam versus wood thread.
Please disregard cost when considering my inquiry.
Part - infused FRP stringer grid with epoxy matrix, bonded to hull with with methacrylate
Hat section - layer of uni carbon tape
Grid sidewall - offset fiberglass layers with the appropriate layers of biaxial (+/-45) carbon knit
I realize you gain stiffness (thickness = stiffness cubed) by increasing thickness, but is this not at the cost of significant weight gains? Would the gains in physicals & weight savings justify the use of carbon in this application? Do you think we will see a move in the marine industry to fiber placement, similar in application to what is seen in aerospace?
This is not a personal or customer project, it is only my curiousity.
Not to hijack your thread, but I’d add a question of my own. How thick is the Plexus adhesive under the stringer grid? The applications I’ve seen are quite thick, like ½” or more. Would you be concerned about the adhesive layer being ( a lot) less stiff than the CF layup? Can the structural rigidity of the CF grid be translated to the hull effectively through a material like Plexus?
Relentless, I was going to reply to your thread, but I thought it best to get some info on your background.
Are you an engineer?
Trey
After I posted I thought about the thread's title...I'd like to have everyone's input, as it makes for a good discussion. Please, Trey, do add to this discussion!
My degree is not in structural engineering. My background is as lamination manager/supervisor for a handful of OEM marine manufacturers. I have also been on the supplier side producing preformed stringers for use in OEM applications, this was in a project management/account manager role. Currently, I'm north american account manager for our aerospace business; with a focus on the tooling side. I do, however, assist our outside sales team & distributors - in all industries - with technical service throughout the country.
My role is unique as I get to see different processes through the gambit of industry. One week I'll be working with an aircraft engine manufacturer & the next I may be in a boat shop infusion a hull, as was the case last week.
Not to hijack your thread, but I’d add a question of my own. How thick is the Plexus adhesive under the stringer grid? The applications I’ve seen are quite thick, like ½” or more. Would you be concerned about the adhesive layer being ( a lot) less stiff than the CF layup? Can the structural rigidity of the CF grid be translated to the hull effectively through a material like Plexus?
I posed this scenario strictly as a hypothetical. I've seen MMA bondlines from .25" - .375", I'm sure there are those with significantly thicker bondlines.
Good question regarding the ability of MMA to transfer energy to the hull...or is it the hull transfering energy to the stringer? I don't know to be honest hopefully someone who does will add to the discussion...
I don’t want to try to answer a question intended for someone else, but I thought I might add a bit to help in the understanding of the subject.
First, it is very true that you gain strength and stiffness with a thicker panel. But it’s not exactly that simple since the stringers will work as a unit with the hull to create a box section that will resist axial, bending, and torsion forces. An in depth discussion of structural analysis would take forever so I’ll limit my remarks to bending in a longitudinal stringer.
You have a hull with stringers bonded to it that have sidewalls and the hat. When the hull tries to flex upward, there is a point somewhere between the hull and hat called the neutral axis where there is no force. From this axis, tension increases in the sidewalls to a point where the maximum tension in this simplified system is in the hat. Compression forces increase toward the hull until the maximum compression is in the hull skin. The forces reverse when the hull flexes downward so that the hat is in compression and the hull skin is in tension.
Possible failure modes include:
Tension failure in the hat or hull skin.
Buckling of the hat or hull skin due to compression.
Shearing of the sidewalls.
Buckling of the sidewalls.
Shearing of the sidewall to hull bond.
Assuming a hollow rectangular shape with an aspect ratio of h=2b, the prime failure mode would likely be a buckling of the sidewalls and the next likely would be buckling in the hat. This is because the local section (the sidewall or hat) is relatively thin and wide (high length, low radius of gyration).
The best way prevent this buckling is to reduce the unbraced length of the local section. An example would be to take a plastic 6” ruler and compress it axially as if it were a column. It will tend to buckle about the weak axis. If you hold your fingers at the midpoint as to prevent the buckling, you’ll notice it will take a lot more force to make it buckle, and it’ll buckle in an ‘S’ shape instead of the ‘C’ seen in the unbraced example. What you’ve done is to increase the axial capacity of the ruler by altering the properties of the shape. You made its effective length shorter.
If you fill the hollow area in the stringers with foam or wood, it would help to stabilize the hat and sidewalls and prevent them from buckling. The stronger the filler material, the increased bracing action it would provide. Once you get to something as strong as wood, the filler will also resist some of the bending force, and if you constructed the stringer over a solid (single or multiple plies) wood core, the core may actually resist more of the bending moment than the balance of the system.
All of this leads back to your original question as I read it – Would using less of a more expensive, stronger material for the hat and sidewalls result in a more efficient section?
I would think that it would not. The reason is that the physical properties of the shape have as much influence on the strength and rigidity of a section as the physical properties of the materials that make up that section. In the subject case, the difference in the material strengths would not yield a significant increase in the overall member properties because the section’s shape would be the limiting factor.
If you have more of an understanding of mechanics of materials than I have assumed, I apologize for the over simplification. Also, in reality, many other factors come into play including the hull sides, deck, and cap among others, and the entire sum of components act as a system.
Regarding the bonding layer, I think a lot of the methacrylate based adhesives have a high elongation at break. I don’t think it would come into play in the scenario of dynamic short term loading as long as the adhesive had adequate strength to resist the shear loads between the two components. I would be more concerned about tension creep due to sustained loads. It wouldn’t be a deal breaker, you’d just have to evaluate the joint for static loads and choose your adhesive accordingly.
Also, this link: http://oneoceankayaks.com/Sandcore.htm gives a good layman’s explanation of the mechanics of bending. It deals with cored panels, but the theory is the same with a box section, only the sidewalls in a box section perform the role of the core in a cored panel.
Very good input, Trey, you've precisely answered my question. Personally, I've never seen benefit of using CF uni tape along the hat...stringer failure, that I've been witness to, has typically been from side wall buckling or hard 90 degree angles for rigging & such.
I've also been involved in projects where we utilized geometry to gain structural advantage by designing what are basically corrigates...preformed 2.2lbs nominal PU encapsulated in a single layer of 43oz triaxial glass for the side walls & 36oz 0/90 for the hat, 6" thick by 20" @ the tallest point that runs about a third of a 25' stringer. In the corrigated areas we would nest a single 1" thick with a 4"top & 6" bottom trapazoid PU & 24oz 0/90 glass preform.
So i completely understand incorporating geometry into the design of your stringer. In addition the need to calculate hull skins in the design or the structure.
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Trey, a nice description of structural properties of beam sections, thanks for your post.
Would it not seem regardless of strain rate (within reason) that an external pressure load applied to the outside of the hull must first go through the (Plexus) adhesive layer in order to complete the load path to the structural grid? If this layer deflects at a magnitude substantially greater than the ultra stiff CF hat sections, the hull skins will not be properly coupled to the stringer grid and one would expect a compromise in torsional and simple bending performance.
In plain English, have we not put a lot of time, money and expense into a high tech laminate only to give up the benefits through the structural connection to the hull?
I would submit that if the use of CF in the stringers is desired or necessary, it might be better to tab these components directly to the hull skins using a high stiffness connection.
Trey, a nice description of structural properties of beam sections, thanks for your post.
Would it not seem regardless of strain rate (within reason) that an external pressure load applied to the outside of the hull must first go through the (Plexus) adhesive layer in order to complete the load path to the structural grid? If this layer deflects at a magnitude substantially greater than the ultra stiff CF hat sections, the hull skins will not be properly coupled to the stringer grid and one would expect a compromise in torsional and simple bending performance.
In plain English, have we not put a lot of time, money and expense into a high tech laminate only to give up the benefits through the structural connection to the hull?
I would submit that if the use of CF in the stringers is desired or necessary, it might be better to tab these components directly to the hull skins using a high stiffness connection.
To your point about bonding with MMA, are you saying the hull skins are performing most of the work juxtaposed to a grid that has been tabbed to the hull, which would be a more effecient way to transfer energy to the stringer? That's an interesting question, especially when you consider all of the builders who bond their grids in with MMA.
I didn't want to hijack the stringer, foam versus wood thread.
Please disregard cost when considering my inquiry.
Part - infused FRP stringer grid with epoxy matrix, bonded to hull with with methacrylate
Hat section - layer of uni carbon tape
Grid sidewall - offset fiberglass layers with the appropriate layers of biaxial (+/-45) carbon knit
I realize you gain stiffness (thickness = stiffness cubed) by increasing thickness, but is this not at the cost of significant weight gains? Would the gains in physicals & weight savings justify the use of carbon in this application? Do you think we will see a move in the marine industry to fiber placement, similar in application to what is seen in aerospace?
This is not a personal or customer project, it is only my curiousity.
It is very hard for me to decouple mfg cost from material selection and construction methodology but every once in a while you might run across an unlimited budget so here goes.
If we are talking about a purely performance oriented mindset, then yes, the physicals and weight savings would justify the use of carbon fiber. This is, of course, on the ragged edge of performance. Consider a record breaking hydro or an America's Cup challenger. These guys are hanging it all out there, they expect things to break (and they have) every once in a while.
These highly engineered vessels are purpose built and that purpose is speed. It's the only venue that I think justifies the ultimate use of exotic materials and construction. Your more mundane consumer product, the daily driver, so to speak, doesn't require carbon tapes to produce a successful product.
Carbon spars, while not commonplace, are making inroads in the sailboat industry. From what I've read, other than the pure racing products, they are increasing performance but they are not really on that borderline between standing upright and breaking. They are built with longevity in mind. Better performance but not really on the ragged edge.
As for your question about fiber placement, I haven't priced out any of the equipment but I would guess it would amount to a pretty hefty sum. Capital expenditures are probably being looked at very closely right now, even by the big guys. I doubt any boat company is going in that direction any time soon. But then again, ten years ago, if you told me that ten year old kids would be walking around with cell phones today, I probably would have said no way. Half the kids that live on my street are mobile and texting each other so what do I know.
On that other thread, I was just trying to point out to the rebel without a clue that there was more than one way to build a boat. I don't necessarily think that high tech is always the answer. Appropriate use of technology is a balancing act. Using technology just because it's out there is sort of like cooking a steak in a microwave. Sure, you can do it, but isn't it better on the grill?
Trey and darbikrash both make excellent points and obviously have a great grasp on the subject.
On the subject of MMA adhesives, as Trey pointed out, a common grid is linked to the rest of the boat to create a box. I don't think the minimal amount of movement due to the elongation of the MMA would significantly impact the vessel's overall performance. Obviously, the thicker the bond line, the more impact. Even at 1/2" I would think movement would be minimal although I would strive for a thinner bond line.
I would agree with darbikrash that taping the grid to the hull might be the ultimate in structural stiffness but how much better will it make the boat? I would think that only those extreme performance boats would see any tangible benefit from the additional effort. Even boats that many of us mere mortals consider to be out there on the performance scale (Homesite's Fountain as an example) aren't really out there. Standard laminations and technologies like MMA bonded grids will work fine for them.
Come now, don't be so hard on yourselves. I'm sure you guys aren't dumb, you just haven't had the benefit of training or years of experience that some of us have had.
Consider a record breaking hydro or an America's Cup challenger. These guys are hanging it all out there, they expect things to break (and they have) every once in a while.
Great discussion. And so far no name calling, it’s amazing.
Quote:
Originally Posted by relentlesspursuit
To your point about bonding with MMA, are you saying the hull skins are performing most of the work juxtaposed to a grid that has been tabbed to the hull, which would be a more effecient way to transfer energy to the stringer? That's an interesting question, especially when you consider all of the builders who bond their grids in with MMA.
Not saying this at all. Many builders, as you point out, successfully bond stringers, hull/deck joints, all manner of structural parts with great success using Plexus. It’s good stuff.
My comments apply to the use of carbon fiber in such an installation.
I think this may be a good time to bring up the concept of laminate balance, wherein layers of high stiffness materials are matched to other layers with lower material stiffness all with varying thicknesses. This is important because some high stiffness materials, like CF cannot tolerate high deflections.
Consider an example of a piece of ¼” plywood, maybe a 4’x8’ sheet. If you stand it on edge restraining the ends, you can push the center inwards some considerable distance, say 6”, without damaging the material. Now bond a thin reinforcing layer of carbon fiber on one side, and do the same test. You’ll likely crack the carbon and not hurt the plywood at all. This is because the carbon/plywood laminate is not balanced, e.g. the thin plywood tolerates a large deflection, the thin high modulus CF will tolerate very little deflection. In a properly designed laminate the plys work together, not against each other.
So to continue with this example, to correct this imbalance, you might use one of two strategies:
1.) Increase the plywood thickness so that it’s bending deflection at a given load is very near to the CF layer max deflection rating;
2.) Or, get rid of the CF and use a lower modulus material, such as E glass with a max elongation at break that is more closely matched to the plywood.
When designing composite structures with different materials, and/or cores, it is important to match the elongation at break numbers between plys (as well as the adhesives). This is especially important when using CF as a reinforcing material. In such a case, the existing structure already has some baseline deflection, if this deflection is greater than the CF will tolerate, you’ll just crack the CF layer.
Boat builders sometimes refer to CF as a “stress magnet”. This comment comes from using CF as a reinforcing material, and boat builders are often frustrated when they try and use a high tech material for reinforcement, and these products just end up breaking anyway while the lower modulus (E-glass) ply layers suffer no damage. The nice thing about conventional E glass solid laminates is that when something is not stiff enough, you just add more layers until it is stiff enough. Boat builders love this benefit, because it’s foolproof and really does not require any engineering. This all changes when you start using Kevlar or CF in a laminate, and it also changes with the use of core materials. An engineered laminate is specified with the designer knowing exactly which layer (or adhesive) will fail first, and how it will fail. Additionally, the plys will be balanced, and all work together to yield the lowest cost, most efficient structure.
I understand the desire to mold the grid structure in a separate mold, this will be much easier to vacuum infuse as a separate piece. I also understand the attraction of adding CF reinforcing materials to make things stronger. So far so good. I also “get it” that attaching this separate piece with a something less than perfect match to inner mold line of the hull is best accomplished with a conformal adhesive layer.
I just wonder how much of the stringer grid stiffness, as a standalone part, is transferred to the hull through the Plexus adhesive layer. I’d also wonder how much deflection there is in the resulting box structure, given the Plexus connection, and how this would impact the CF layers.
Maybe if the use of CF is desired, this should be done as an integral part of the hull layup, wherein the stringers and CF reinforcements are laid up all at once with the hull.
Maybe if the idea of using a separate vacuum infused stringer grid is more attractive, this could be done using only E glass and Vinyl ester resin on the stringers, and then bonded to the hull with Plexus.
Wow, a failure in "light winds". That's not something I would expect. I'm sure they probably have some stress monitoring equipment embedded in the mast so they can analyse the thing and figure out what went wrong. Good thing they've got spares, unless they were built the same way.
I guess Larry will just stroke another check.
This mast failure dovetails nicely with darbikrash's latest post. Carbon fiber often suffers catastrophic failure. Rather than giving signs (i.e. stress cracks in glass) that it is being overstressed, it just lets go when stressed beyond it's limits.
Questions for OReely on Lamination,Materials and Strength
hopefully someone who does will add to the discussion...
