I love forums. I use forums to solve HVAC issues, fix diesel engine problems, and learn how to change the brake pads on my car. When I moved to the SF/Bay Area, I actually used the local forum to learn about the wind conditions and kite beaches. I loved that forum so much I actually bought it from the founder. But that’s another story…
With a marketing budget of $0 for the last few years, forums are also a really good way to introduce the community to my product, and answer questions and get feedback. Even if I don’t sell a mast, I am happy educate the industry and consumers in the area of composites, and hope that the gear technology progresses as a result. Unfortunately, there are a lot of people out there who don’t want to be educated, and consider themselves the resident carbon fiber expert because they broke a mast or two (not a Project Cedrus, of course). So after getting fired up on forums, I thought I’d benefit my own site and do a blog entry of some of the basics of composite structures design… if you didn’t get enough of that in the previous posts documenting the entire process of Project Cedrus from conception to commercial ramp.
First off, and to set the record straight, carbon fiber is stronger than aluminum. To illustrate this, we are going to discuss the stress vs. strain curve and all the material properties that are derived from tensile tests and plotting the data on this curve. But in case you don’t get that far. I want to reiterate that carbon fiber is stronger than aluminum (and steel and even titanium). It’s also stiffer than *some of those materials. So if your carbon mast broke, or you find it too “flexy,” please do not blame the material. Your mast suffered from either poor design and/or manufacturing. I am not writing this to offend anyone, we are all learning as this sport evolves. But there is a very good reason all high performance airplanes, cars, America’s cup boats, and bicycles are all manufactured from carbon fiber. But the kite industry is much smaller and much younger, and foiling even more so! Everyone is still learning, and I am trying to help accelerate that process and improve our equipment whether it’s through a Project Cedrus mast or not.
This is a stress vs. strain curve. The data in this curve is derived from pulling on a small sample of material, and measuring the reaction force and the amount of strain (or “stretch”) of the material as it is loaded. Typically, these tests are run to failure, capturing the elastic modulus to yield, and plastic deformation to fracture. They are conducted using something like an Instron, and the data so valuable that I actually purchased my own used Instron from a medical device company that was going out of business. Unfortunately, I haven’t had a chance to get the machine up and running, so I will have to illustrate my points with conceptual data.
Stress is essentially pressure, or load distributed over an area with the area being the cross section of the coupon being tested. Stress is plotted along the Y axis of the graph, usually in units of psi (english) or Pa (metric). Strain is the amount of stretch, which is actually unit-less but typically measured in inches/inch or mm/mm of coupon length. In other words, a long coupon will stretch more than a short coupon for the same load because there is more material to elongate, but the % of stretch, or strain, is the same and based on the intrinsic mechanical properties of the material.
As soon as you start pulling on a test specimen, you obtain one of the most important mechanical properties of a material: Young’s Modulus, or Modulus of Elasticity (E). This value is derived from the slope of the linear elastic region of the stress vs. strain curve, and tells you how stiff the material is. Here are some examples of Young’s Modulus of various materials:
Aluminum (6061): 68GPa
Steel (316): 200GPa
Titanium (3/2): 110GPa
Intermediate Modulus Uniderectional Carbon Tape: 140GPa
Intermediate Modulus Woven Carbon Fabric: 80GPa
As you can see in the above table, carbon is about 2x stiffer than aluminum. It’s not quite as stiff as steel, so why aren’t foils steel? Because steel is 5x more dense than carbon fiber, and does not offer 5x more stiffness. What else is interesting in the above table? Notice that the pretty woven carbon fiber has half the stiffness of unidirectional carbon fiber, even though it’s often 2x the price! Consumers love the woven look, but the reality is it’s an inferior form of carbon fiber material because only half of the fibers are running in the direction of the applied load.
STRENGTH IS NOT STIFFNESS. These are two completely different mechanical properties, and while typically related, in the area of composites there can actually be an inverse relationship. You often read companies bragging about HIGH MODULUS CARBON… this means high stiffness, not strength. High modulus fibers actually have lower strength than standard or intermediate modulus carbon! So what’s the point of high modulus carbon? For stiffness critical applications like space telescopes that won’t really be exposed to structural loads in space, but need to keep perfect shape or alignment of solar panels or lenses. High modulus certainly has an application in the foil world where stiffness is critical… but at the expense of strength and cost? Up to you to decide.
As one continues to pull on the specimen, the curve begins to flatten at which point you have reached the “yield strength” of the material. This means you are now permanently deforming the coupon with increased load. Until this point, you could have let go and the specimen would have returned to original length. Now if you let go, the specimen will be longer. It hasn’t failed yet, but it is damaged. Table below contains the Yield Strength for the same materials as above:
Aluminum (6061): 270MPa
Steel (316): 290MPa
Titanium (3/2): 500MPa
Intermediate Modulus Unidirectional Carbon Tape: 2200MPa
Intermediate Modulus Unidirectional Carbon Tape (TRANSVERSE DIRECTION): 50MPa
Intermediate Modulus Woven Carbon Fabric (BOTH DIRECTIONS): 1000MPa
Yield Strength is probably the most important mechanical property of a material. This value determines how much of a material you will need to withstand applied loads without damage because the only way to reduce stress in a structure is to add more cross sectional area (material). Yield strength is a “strength” value of a material, but not the only one… we will get to that in a minute. But I want to take a second and look at the above table, where you will see that carbon fiber is much stronger than aluminum, steel, titanium, and even stainless steel (stainless is stronger than titanium, a common industry misconception. However, it is extremely important to note in the above table that the yield strength of carbon fiber in the direction perpendicular to the fibers is actually quite low. This is very unique to composites, it is called anisotropic behavior and means that their mechanical properties are different in each direction. The cause of this is simply the fact that perpendicular to the fiber, all you have is the “matrix,” typically epoxy resin, holding everything together. The benefit of this is that it allows you to tailor to the design and layup of carbon plies to give you the properties you want in each direction. For example, a mast needs to be very stiff in both torsion and bending, but doesn’t really need much strength from the leading edge to the trailing edge because there is no load applied in that direction. The easy way to get around this anisotropic behavior is to use woven fabrics, which is why so many carbon fiber structures you see have that checkerboard pattern. But the reality of using fabrics is that you completely eliminate the power and benefit of using carbon fiber in the first place by putting lower strength material where you don’t need it. However, one benefit to fabrics is a less brittle failure mode, which we will get to in the following paragraphs.
Next up, we have ultimate strength, which is the highest stress level achieved by the specimen before failure or fracture. The strain between yield and fracture is called plasticity, and many ductile metals can have ultimate strength levels quite a bit higher than their yield strength with lots of plastic strain. Composites on the other hand, tend to exhibit much more linear-elastic behavior to failure… which means their yield strength is very close or equal to their ultimate strength and there is not much plasticity. This is where carbon fiber gets a bad rep. People think that because it doesn’t bend like aluminum before it fails, it is not as strong. But that’s not true at all. Carbon fiber has much higher ultimate strength than aluminum, you just don’t know when you’re close to reaching it until it’s too late. This is known as a brittle failure mode. When a material continues to carry load beyond yield strength and strain significantly before failure, this is known as ductile failure.
FAILURE MODE HAS NOTHING TO DO WITH FAILURE LOAD. Mode is how something fails, load is at which point it fails. Again, despite composites having a brittle failure mode, they typically fail at a much higher load (stress) than ductile metals, with steel being far more ductile than aluminum. So yes maybe your carbon mast exploded when you hit the reef, but it actually took a lot more stress than your aluminum mast did before it bent. There are some structures designed around ductility and energy dissipation, with automotive crash structures being the most common example. But I’m not going to get into energy absorption and crush structures. Below are ultimate strengths for the same materials discussed above.
Aluminum (6061): 310MPa
Steel (316): 580MPa
Titanium (3/2): 600MPa
Intermediate Modulus Uniderectional Carbon Tape: 2200MPa
Intermediate Modulus Woven Carbon Fabric: 1100MPa
I just painted a pretty picture for carbon fiber. It’s stiffer and stronger than aluminum, and we didn’t even touch on the fact that it’s 30-40% less dense than aluminum. So why are riders breaking their carbon masts or complaining that they are not as stiff as aluminum? Not only that, but you’re paying twice as much for the privilege to ride an inferior product. The answer is design, and I could write a whole book on this, but I’m going to only briefly touch on a few of the design decisions being made by the industry that are resulting in carbon fiber masts not living up to their potential.
- Industry is making carbon masts too thin. Bending stiffness of a structure varies with the thickness^3 (thickness * thickness * thickness). This has a much bigger influence than young’s modulus of the material. As an example, a mast that is 18mm thick will have twice the stiffness as one that is 14mm, assuming the same material. However if the mast is solid, it will also have more than twice the material, which means twice the weight, and a lot higher cost. Industry is making carbon masts thin because they can’t make them hollow, but in doing so are eliminating the stiffness. This is why Project Cedrus is hollow. It gets the material out towards the surface where it can be more effective at reacting bending and torsion loads, while eliminating useless material on the inside of the mast.
- The tapered top mount helps at the board attachment, but you are still primarily stressing the epoxy resin at the mount, NOT the carbon fibers. I have had a surprising number of people ask me to make a carbon board mount. I have tried to explain many times that there is a very good reason for my aluminum board mount: It would take so much more carbon fiber than metal to properly size this joint to keep stress low, that there would be no weight benefit to using carbon. The reason stress must stay low is because of a type of stress called inter laminar tension, which occurs when you try to straighten fibers that are bent or curved. You are basically relying on the strength of epoxy to hold the mast together, not the strength of the carbon fibers. As you can see in the photo above, all you have in the middle of that flared structure is a ball of resin and fibers in random directions. It’s a nightmare to manufacture, and has extremely variable strength because who knows what’s going on inside there…
- Dynamic loads and board width have the biggest impact on stress. Contrary to popular belief, the biggest stress on a foil is not the wing size, but the board size. The board is where you place your feet and determines how much of a bending moment or twist you can put on the mast. The bigger the board, the more your input is amplified. Next up are dynamic loads, which result primarily from jumping. Taking off from a jump, and landing, are an easy way to put 2-3x+ your body weight on the mast. A 75kg guy jumping a mast is putting 2-3x as much force on the structure as a 100kg rider mowing the lawn. Wings do have an effect, but not as much, because the center of pressure of wings does not go outward as quickly as wingspan.
In closing, if a carbon fiber structure does not weigh 30-40% less than an aluminum version serving the same purpose, then the structure/component is not properly designed. This is a simple fact, based on years of experience and well known within the aerospace industry. My goal from day 1 with Project Cedrus was to be 30-40% lighter than aluminum with equivalent or higher stiffness, and I succeeded. This was also 3 years ago at this point, and I’m selling far more masts now than I was back then. I still get comments from perspective customers about drag, and the fact that it’s 19mm thick, to which I respond that mast drag is a small component of total system drag and not one of my riders has ever complained about it, let alone noticed it. It’s still not enough to convince some people, but that’s OK. I think within the next few years all masts will be closer to 19mm thick, because the drag from these large surface area wings and lower speeds of wing-foiling make the mast drag insignificant. Someone please buy a me a beer if I’m right;) Until then, please don’t blame carbon fiber on forums for the lack of stiffness and strength in these expensive masts. It’s not the material, it’s the design.