Re: CF Bike Shatters Top Tube and Down Tube after hitting a Road Divot

jim beam wrote:
Peter Cole wrote:
Derk wrote:
Tim McNamara <tim...@xxxxxxxxxxxxx> wrote:
I remain convinced that CF is an
inappropriate material for bike frames, even though saying so usually
nets me a few comments to the effect that I am a knuckle dragging mouth
breather who should go back to the trees.
I agree with Tim: it wouldn't be my first choice either.

Derk

Composite failure modes is a very complicated subject which has been researched heavily in recent years due to the importance of composites in aircraft and, perhaps more importantly, the growing interest in the automotive market.

Composites can provide extremely high levels of "specific energy absorption" (the amount of energy absorbed per unit weight), in best case scenarios higher than typical metals.

Metal structures absorb energy through deformation, while composite absorption is much more complicated, involving interaction between the reinforcing fiber and the matrix (epoxy typically). Much of the composite absorption comes from friction as the (broken) fibers are pulled from the matrix and/or the matrix deforms around/between the plies.

In worst case scenarios, when the fiber/matrix interaction is compromised, the bulk material can exhibit the characteristics of the underlying fiber, which is very brittle. This can cause "catastrophic" as opposed to "progressive" failure. The amount of energy absorbed is proportional to the area under the stress-strain curve. Plain fibers (carbon) don't elongate much, so the area under the curve is small.

careful peter - you're presuming anybody would actually /use/ "brittle".

and carbon fibers don't elongate in any mode other than elastic. they have no ductile deformation mechanism like metal.


The actual performance of composite structures under failure loads is subject to a number of parameters, such as part geometry, fabric weave and layup angles, load axis, strain rate, layup order and fiber to matrix ratios. From a geometric perspective, long thin walled tubes are among the most likely shapes to have catastrophic failure tendencies.

While carbon fiber composites are still gaining experience in new applications, a great deal is known about similar fiberglass composites which have been used for many decades in consumer level products (boat hulls, surfboards, snow skis, etc.). Historically, in those products, structural problems (particularly from delamination) have been long recognized.

While fiberglass is a much less attractive material for performance applications (planes & bikes) than CF, it's a bit safer from a catastrophic failure for at least a couple of reasons. First, the fiber is less brittle (will elongate more),

absolutely not. neither glass not carbon fibers are ductile. the only "elongation" you'll ever get out of them is elasticity.

Who said anything about ductility? That's your red herring.

Glass fiber typically has 3x the elongation at failure vs CF.




and second, it is often used isotropically, with things like chopped fiber mats providing similar strength (and stiffness) in all directions.

like campy carbon. like trek carbon.

I think you misunderstand.



To get the high performance you want (strength to weight) out of CF you really want to exploit it in anisotropic modes. This, unfortunately, increases the tendency to catastrophic failure (lowers toughness).

massive over-generalization. any anisotropic material has different properties in different directions. lower strength perpendicular to the load axis is seldom a problem.

That's the opposite/inverse of what I said.


I remember a group canoe trip years ago with several fiberglass, one kevlar and one aluminum canoes used in white water. All the fiberglass canoes were destroyed (smashed), the aluminum was badly dented, while the kevlar showed hardly a mark. CF can have excellent toughness, but it has to be designed for it.

but that wasn't a function of the carbon - that was s function of the composition.

Just an illustration of toughness.



On the whole, I think CF has been around long enough in bike frames to make it reasonably trustworthy, although I'd still avoid cheap CF. I'd rather crash a steel fork than a CF one, too.


then use the product of a manufacturer that worries about their reputation and manufactures in a country where people know what they're doing and aren't cutting corners. i've had a wheel-smashing impact on my look carbon fork and it hasn't blinked.

I think this is just a windy restatement.
.

Relevant Pages

  • Re: CF Bike Shatters Top Tube and Down Tube after hitting a Road Divot
    ... Composite failure modes is a very complicated subject which has been researched heavily in recent years due to the importance of composites in aircraft and, perhaps more importantly, the growing interest in the automotive market. ... Metal structures absorb energy through deformation, while composite absorption is much more complicated, involving interaction between the reinforcing fiber and the matrix. ... Plain fibers (carbon) don't elongate much, so the area under the curve is small. ...
    (rec.bicycles.tech)
  • Re: CF Bike Shatters Top Tube and Down Tube after hitting a Road Divot
    ... Composite failure modes is a very complicated subject which has been researched heavily in recent years due to the importance of composites in aircraft and, perhaps more importantly, the growing interest in the automotive market. ... Metal structures absorb energy through deformation, while composite absorption is much more complicated, involving interaction between the reinforcing fiber and the matrix. ... Plain fibers (carbon) don't elongate much, so the area under the curve is small. ...
    (rec.bicycles.tech)
  • Re: CF Bike Shatters Top Tube and Down Tube after hitting a Road Divot
    ... Composite failure modes is a very complicated subject which has been researched heavily in recent years due to the importance of composites in aircraft and, perhaps more importantly, the growing interest in the automotive market. ... Metal structures absorb energy through deformation, while composite absorption is much more complicated, involving interaction between the reinforcing fiber and the matrix. ... and carbon fibers don't elongate in any mode other than elastic. ... While fiberglass is a much less attractive material for performance applications than CF, it's a bit safer from a catastrophic failure for at least a couple of reasons. ...
    (rec.bicycles.tech)
  • Re: joing metal with carbon fibers
    ... You can't weld normal plastic-matrix composites (like carbon fiber). ... At the temperature you need to melt the ...
    (sci.engr.joining.misc)
  • Re: CF Bike Shatters Top Tube and Down Tube after hitting a Road Divot
    ... Composite failure modes is a very complicated subject which has been researched heavily in recent years due to the importance of composites in aircraft and, perhaps more importantly, the growing interest in the automotive market. ... Metal structures absorb energy through deformation, while composite absorption is much more complicated, involving interaction between the reinforcing fiber and the matrix. ... and carbon fibers don't elongate in any mode other than elastic. ...
    (rec.bicycles.tech)
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