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  1. #1
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    carbon frame stiffness after 5 years or more

    does anyone know if carbon fiber frames lose stiffness or rigidity after years of riding? I tried researching it but came up empty. thanks.

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    Randomhead
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    stiffness is a function of the fiber, which shouldn't age. There have been questions raised about the longevity of the matrix, but there is carbon fiber out there that is decades old by this time

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    thanks

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    Senior Member squirtdad's Avatar
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    Quote Originally Posted by unterhausen View Post
    stiffness is a function of the fiber, which shouldn't age. There have been questions raised about the longevity of the matrix, but there is carbon fiber out there that is decades old by this time
    isn't stiffness the combination of fiber, the matrix and the alignment and amount of fiber? Carbon fibers individually are stiffer (and more brittle from the little bit I have seen) than fiberglass, but without the epoxy resin holding it together the fibers have no reall functional stiffness.
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    Randomhead
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    the fibers are really only stiff in tension, but in tension their stiffness dwarfs that of the matrix. The matrix adds very little stiffness, but obviously it is required for structural stability.

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    Quote Originally Posted by unterhausen View Post
    the fibers are really only stiff in tension, but in tension their stiffness dwarfs that of the matrix. The matrix adds very little stiffness, but obviously it is required for structural stability.
    I can't really agree with that. The matrix glue has to complement the fiber choice.
    The highest modulus Carbon with a soft curing epoxy isn't going to be a stiff frame.
    Different types of epoxy have wildly different strength ratios. West epoxy is very highly thought of as a binding resin but something like Poly Epoxy has double the strength.
    If the carbon is really only providing tensile strength then it has to be bracing itself against something with good compression resistance to show that strength.
    West epoxy has about 15,000 psi of compressive strength while Poly Epoxy is 32,000.

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    Randomhead
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    I think I'll have to disagree and say that we agree

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    As squirtdad said, the alignment is key. Since we are talking longevity, the issue is change. Stuff that is most likely to change is the resin, the fiber can last a long time. Alignment determines how the resin is loaded, the extreme example is a tube that is wound mostly in the 90, this actually does exist one fairly well known example is the ugly stick fishing rod, but it is very common on any tube designed mostly for durability. Some braid in 45/45 is pretty common, these structures will have the resin as a large part of the stiffness, of course one can use fiber in all kinds of directions to create optimal structures. In practice well designed carbon should outlast the user as far as fatigue is concerned on bikes. Particularly if the tubes are virtually not deflected in use. Think fishing rods, golf shafts, or arrows without good fiber alignment and you would have structures likely to fatigue, but they also last a long time.

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    Quote Originally Posted by Canaboo View Post
    I can't really agree with that. The matrix glue has to complement the fiber choice.
    The highest modulus Carbon with a soft curing epoxy isn't going to be a stiff frame.
    Different types of epoxy have wildly different strength ratios. West epoxy is very highly thought of as a binding resin but something like Poly Epoxy has double the strength.
    If the carbon is really only providing tensile strength then it has to be bracing itself against something with good compression resistance to show that strength.
    West epoxy has about 15,000 psi of compressive strength while Poly Epoxy is 32,000.
    I think you are a bit confused about stiffness and strength.

    Since the material as a whole must equalise strain for a given stress, the load taken by the different materials depends on their ratios of strain to stress, which is their stiffness.

    CF is approximately 100 times as stiff as the epoxy matrix, the exact number is obviously dependent on fibre type and resin but epoxies are around 3.5 GPa and CF ranges from roughly 200 to 400 GPa*. The law of volumes applies**, so for a standard 60% content of 300 GPa fibre the load carried by the fibre is 0.6 x 300 / (0.6 x 300 + 0.4 x 3.5) which is 99.3%.

    The function of the matrix is to bind the fibres together so they share the load. Problems arise when the soft matrix allows the fibres to bend so they are no longer loaded optimally. This is what makes CFRP weaker in compression than in tension: a tension load pulls the fibres straight so the material will hold until the fibres snap which takes a huge load. In compression the applied load tends to bend the fibres so the material will hold until the fibres start to buckle which takes a smaller load, how much smaller being dependent on a number of factors such as layup.

    Problems also arise arise when the matrix can't transfer the load between layers properly, typically because of interlaminar shear. When this occurs the various layers can move against each other and are thus independently loaded and the overall properties suffer.

    * I've seen some references to UHM carbon at up to 800 GPa but I don't know if anyone gets to use it.

    ** If the fibre isn't completely wet by the matrix, a modified law of volumes applies where a diminution factor is applied to the fibre volume. One good example of this is flax fibre composites where the diminution factor can be 1/3 or less so flax fibre composites aren't very strong or stiff at all: Although flax itself is about 70 - 80 GPa so it's about 1/3 as stiff as a medium modulus carbon, a 50% Flax composite struggles to reach 15 GPa which is about 1/10 what you could achieve with said medium modulus carbon.
    Last edited by Mark Kelly; 07-16-12 at 01:17 AM.

  10. #10
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    Quote Originally Posted by Mark Kelly View Post
    I think you are a bit confused about stiffness and strength.

    Since the material as a whole must equalise strain for a given stress, the load taken by the different materials depends on their ratios of strain to stress, which is their stiffness.

    CF is approximately 100 times as stiff as the epoxy matrix, the exact number is obviously dependent on fibre type and resin but epoxies are around 3.5 GPa and CF ranges from roughly 200 to 400 GPa*. The law of volumes applies**, so for a standard 60% content of 300 GPa fibre the load carried by the fibre is 0.6 x 300 / (0.6 x 300 + 0.4 x 3.5) which is 99.3%.

    The function of the matrix is to bind the fibres together so they share the load. Problems arise when the soft matrix allows the fibres to bend so they are no longer loaded optimally. This is what makes CFRP weaker in compression than in tension: a tension load pulls the fibres straight so the material will hold until the fibres snap which takes a huge load. In compression the applied load tends to bend the fibres so the material will hold until the fibres start to buckle which takes a smaller load, how much smaller being dependent on a number of factors such as layup.

    Problems also arise arise when the matrix can't transfer the load between layers properly, typically because of interlaminar shear. When this occurs the various layers can move against each other and are thus independently loaded and the overall properties suffer.

    * I've seen some references to UHM carbon at up to 800 GPa but I don't know if anyone gets to use it.

    ** If the fibre isn't completely wet by the matrix, a modified law of volumes applies where a diminution factor is applied to the fibre volume. One good example of this is flax fibre composites where the diminution factor can be 1/3 or less so flax fibre composites aren't very strong or stiff at all: Although flax itself is about 70 - 80 GPa so it's about 1/3 as stiff as a medium modulus carbon, a 50% Flax composite struggles to reach 15 GPa which is about 1/10 what you could achieve with said medium modulus carbon.
    No, I may have generalized but I'm not confusing the two terms. In building bike frames I would think that with the materials used the two terms are less separate than you are saying. We're not talking about Silk for example.

  11. #11
    Randomhead
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    my point is that as long as the epoxy is of sufficient strength for structural stability, it doesn't affect the overall stiffness of the aggregate structure that much. I suppose it would take fea to really show that. I'm sure it can be seen in an analysis, particularly if the frame is extremely lightweight.

  12. #12
    coprolite fietsbob's Avatar
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    Plus Lots of frame designs get past round tube and lug style shapes..
    and make the shape serve the stresses presented to those portions.

    Even Hydroformed Aluminum are using tube Re-shaping to beef up joints.

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    Quote Originally Posted by unterhausen View Post
    my point is that as long as the epoxy is of sufficient strength for structural stability, it doesn't affect the overall stiffness of the aggregate structure that much. I suppose it would take fea to really show that. I'm sure it can be seen in an analysis, particularly if the frame is extremely lightweight.
    That was my point: if you replaced the resin in my example with something which was four times softer (0.9 GPa) the overall stiffness of the aggregate sructure changes from (0.6 x 300 + 0.4 x 3.5) = 181.4 to (0.6 x 300 + 0.4 x 0.9) = 180.4, a change of less than 0.6%.

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    Is this a purely engineering "on paper" discussion or real world tests where an actual structure was made which may illuminate some overlooked points?

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    Randomhead
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    Quote Originally Posted by Canaboo View Post
    Is this a purely engineering "on paper" discussion or real world tests where an actual structure was made which may illuminate some overlooked points?
    well, as I said in my initial reply to you, I think we agree that the matrix is very important to the overall strength of the frame. Where we seem to be diverging is over how much it contributes to the stiffness. Obviously, if it wasn't holding the fiber in the proper location, the fiber wouldn't be contributing as much stiffness as it might. However, that would represent a failed condition which doesn't seem to happen under normal use.

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    It would be interesting to see the epoxy choices used for most bikes.
    As an aside I wonder if the noodly reputation older carbon bikes seem to have is similar to the "work hardening" that steel riders can sense every time they need a new bike.
    Most carbon riders are convinced the bike breaks down over time.

  17. #17
    Andrew R Stewart Andrew R Stewart's Avatar
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    "Most carbon riders are convinced the bike breaks down over time."


    This is the perceptional crux. Lab tests are one thing, rider testimonals are another. Guess which sells more bikes...

    "Racers" (and their wanabies) are always looking to blame/explain with simple statements. Andy.

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    "That was my point: if you replaced the resin in my example with something which was four times softer (0.9 GPa) the overall stiffness of the aggregate sructure changes from (0.6 x 300 + 0.4 x 3.5) = 181.4 to (0.6 x 300 + 0.4 x 0.9) = 180.4, a change of less than 0.6%."

    That doesn't pass the laugh test, but I assume you know what you are talking about. Having made a lot of carbon structures, I don't know what it is I am missing. People work with this stuff quibble over minute differences in resin hardness compared to what you are taking about. And in regulated venues like aviation you have specifications. Hardness is the ability to resist deformation and moving to one fourth the hardness would be moving from a Shore value of 80 ish for hard epoxies, to one of 20 which is sub elastic bands. Since the resin is a major carrier of compressive loads, I don't see how we escape a substantial reduction in stiffness.

    With composites strength or stiffness is defined differently than with steel. With composites it is defined in relation to the pathway for the design load. So would talk about the stiffness of the bow limb, or airplane spar, even though there is potentially some other direction in which the desired properties would be much lower than the main load path.

    Imagine you built a frame only out of epoxy, forgot the carbon, and sucked in the commensurate resin. Would there be any difference in that case? Because in the real world that is what you have with fiber alignment. You either have some angle in which the fiber is not aligned, and the resin is doing/some portion of the stiffness work, or you have a lot of carbon going in all kinds of directions with heavier weight. So normally once structures get light, unless they are very simple as far as alignment goes, say bow limbs, then they tend to have some kind of problems with fiber alignment.
    Last edited by MassiveD; 07-21-12 at 11:49 AM.

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    Randomhead
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    point taken. Is there any indication that the resin becomes more compliant with age?

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    I don't know about sitting around time, but it looses capacity due to environmental factors, it is a plastic, and it looses it through cycles of loading. But not all structures are equal as regards that. I have fly rods that go back to the dark ages (70s), they are as good as ever, probably because the fiber alignment is near optimal, and the cycles to failure budget is way in excess of requirements. But people notice changes in arrows over relatively short periods. This is interesting because people with arrows often have spine testers for rating wooden shafts, and those are well distributed, so this could be valid info. High end shafts have very optimal fiber alignment due to weight issues, and less impact issues, and they should last until one looses or breaks them, which could be a long time.

    I don't know much about carbon frames, but for the most part they look like they ought to last a long time. There is very little visible bending, constraints on proper sizing of elements seem minimal. Weight issue is pretty massive, that would be the one issue. It can take a while to work out the balance with carbon. Carbon usually gains a difficult to overcome reputation for structural failure, and then it takes over. I remember seeing the whole bow of an America's Cup boat rip off in comp, but carbon is ubiquitous in well funded craft today. I have seen this failure to success thing with boats ( whole fleet of open 60s basically fell apart about 10 years ago during a bad storm), cross country skis, rods, archery gear, forks, and so on. Carbon pretty much always wins. Only big exception is golf where it really sucks but is commercially successful anyway.

  21. #21
    Bicycle Lifestyle AsanaCycles's Avatar
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    I guess the person to ask would be Craig Calfee.

  22. #22
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    Quote Originally Posted by unterhausen View Post
    point taken. Is there any indication that the resin becomes more compliant with age?
    In transferring what has happened in the marine industry possibly. Not more compliant but not all epoxys are mixed completely, where there is not full crosslinking it could be potentially a problem. I have always wondered about electrolysis too. Carbon is a great conductor, various metals along the galvanic scale may have been attached to the frame, people sweat, that has salt, bikes can get wet. It has been documented in the marine industry of epoxy degrading from these similar interactions along the current path.

    In the aircraft and marine industries it is typical to isolate metallic fittings from carbon often, such as with an E glass layer to prevent this.

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