"brittle" vs. non-ductile

jim beam said:

fracture energy absorption is what determines whether a material is
"brittle" or not. cfrp is not brittle like glass. it is not ductile,
[the apparent, er, "confusion"] but it is typically not brittle -
significant energy is absorbed as the fracture interface progresses
[depending on fabrication and constituent materials of course].

Every time we've seen a CFRP bike part that has been hit or loaded
hard enough to break it, it has been broken completely through. That
seems to indicate the the energy to complete a fracture isn't much, at
least for carbon-epoxy such as we see in bikes.

Chalo

Chalo said:

jim beam said:

fracture energy absorption is what determines whether a material is
"brittle" or not. cfrp is not brittle like glass. it is not ductile,
[the apparent, er, "confusion"] but it is typically not brittle -
significant energy is absorbed as the fracture interface progresses
[depending on fabrication and constituent materials of course].

Every time we've seen a CFRP bike part that has been hit or loaded
hard enough to break it, it has been broken completely through. That
seems to indicate the the energy to complete a fracture isn't much, at
least for carbon-epoxy such as we see in bikes.

Chalo

once metal starts to fracture, it usually breaks completely through as well.

jim beam said:

Chalo said:

Every time we've seen a CFRP bike part that has been hit or loaded
hard enough to break it, it has been broken completely through. That
seems to indicate the the energy to complete a fracture isn't much, at
least for carbon-epoxy such as we see in bikes.

once metal starts to fracture, it usually breaks completely through as well.

But the first-- and often the only-- sign of overload failure in a
metal frame is plastic deformation. The paint cracks, but the metal
remains intact. Not so with a CFRP frame. One second it's okay, the
next second you're skidding on your face/butt/elbows/etc.

Chalo

"jim beam" wrote: once metal starts to fracture, it usually breaks
completely through as well.
^^^^^^^^^^^^^^^^^^^
If that were true, the term "fatigue failure" would not exist. Consider
also the case where the load exceeds the yield point, but is below the
ultimate strength. There is deformation without fracture, which is a form
of failure that does not occur in brittle materials.

Leo Lichtman said:

"jim beam" wrote: once metal starts to fracture, it usually breaks
completely through as well.
^^^^^^^^^^^^^^^^^^^
If that were true, the term "fatigue failure" would not exist.

but fatigue is different! and energy absorption by a metal once a
fatigue crack has grown to critical is minimal. hence the layman's
belief that metal "becomes brittle" when it fails in this way.

Quoted message said:

Consider
also the case where the load exceeds the yield point, but is below the
ultimate strength. There is deformation without fracture, which is a form
of failure that does not occur in brittle materials.

that's called ductility. it's not fracture energy absorption.

Chalo said:

jim beam said:

Chalo said:

Every time we've seen a CFRP bike part that has been hit or loaded
hard enough to break it, it has been broken completely through. That
seems to indicate the the energy to complete a fracture isn't much, at
least for carbon-epoxy such as we see in bikes.

once metal starts to fracture, it usually breaks completely through as well.

But the first-- and often the only-- sign of overload failure in a
metal frame is plastic deformation. The paint cracks, but the metal
remains intact. Not so with a CFRP frame.

but in my experience, you usually have lots of audible warning.

Quoted message said:

One second it's okay, the
next second you're skidding on your face/butt/elbows/etc.

if you ignore the audible, yes. you don't stand on timber planking
that's making cracking noises do you?

In article <[email hidden]>, jim beam

Quoted message said:

Chalo said:

jim beam said:

Chalo wrote:
> Every time we've seen a CFRP bike part that has been hit or loaded
> hard enough to break it, it has been broken completely through. That
> seems to indicate the the energy to complete a fracture isn't much, at
> least for carbon-epoxy such as we see in bikes.
once metal starts to fracture, it usually breaks completely through as
well.

But the first-- and often the only-- sign of overload failure in a
metal frame is plastic deformation. The paint cracks, but the metal
remains intact. Not so with a CFRP frame.

but in my experience, you usually have lots of audible warning.

Just what is your experience? Is it statistically relevant?

Luke said:

In article <[email hidden]>, jim beam

Quoted message said:

Chalo said:

jim beam wrote:
> Chalo wrote:
>> Every time we've seen a CFRP bike part that has been hit or loaded
>> hard enough to break it, it has been broken completely through. That
>> seems to indicate the the energy to complete a fracture isn't much, at
>> least for carbon-epoxy such as we see in bikes.
> once metal starts to fracture, it usually breaks completely through as
> well.
But the first-- and often the only-- sign of overload failure in a
metal frame is plastic deformation. The paint cracks, but the metal
remains intact. Not so with a CFRP frame.

but in my experience, you usually have lots of audible warning.

Just what is your experience? Is it statistically relevant?

eh? the privilege of arguing against me lies with /you/ big guy, not me
- so feel free to go ahead and present your own information. whenever
you're ready...

"jim beam" wrote: (clip) but fatigue is different! and energy absorption
by a metal once a fatigue crack has grown to critical is minimal. (clip)
^^^^^^^^^^^^^^^^^^
Jim, you're answering a question different from the one I raised. Fatigure
failure begins with a microscopic crack, which acts as a stress raiser, and
grows gradually, until the remaining sound metal can no longer carry the
load. This is the interval I was thinking of in my response to:
"once metal starts to fracture, it usually breaks completely through as
well."

Leo Lichtman said:

"jim beam" wrote: (clip) but fatigue is different! and energy absorption
by a metal once a fatigue crack has grown to critical is minimal. (clip)
^^^^^^^^^^^^^^^^^^
Jim, you're answering a question different from the one I raised. Fatigure
failure begins with a microscopic crack, which acts as a stress raiser, and
grows gradually, until the remaining sound metal can no longer carry the
load. This is the interval I was thinking of in my response to:
"once metal starts to fracture, it usually breaks completely through as
well."

if you're considering the rupture that follows beyond the point of
ductility, that statement stands. you're right that fatigue is a
different matter, but fatigue is not the failure mode being considered.

In article <[email hidden]>,

Leo Lichtman said:

"jim beam" wrote: (clip) but fatigue is different! and energy absorption
by a metal once a fatigue crack has grown to critical is minimal. (clip)
^^^^^^^^^^^^^^^^^^

Jim, you're answering a question different from the one I raised.
Fatigure failure begins with a microscopic crack, which acts as a
stress raiser, and grows gradually, until the remaining sound metal
can no longer carry the load. This is the interval I was thinking of
in my response to: "once metal starts to fracture, it usually breaks
completely through as well."

Here are some photos of metal failure, in this case an aluminum stem.
It would be interesting to discuss the failure mode of aluminum versus
CF in this type of application, since CF stems are available. This
might provide a concrete example to compare the qualities of the
material. How would CF versus Al endure this situation?

The cracks developed over an unknown period of time, but two days before
it was discovered the owner and I did a 104 mile ride that included
gravel roads, steep hills and rough pavement. The owner had noticed
some creaking for a couple of rides prior to the 104 miler. This stem
survived at least 150 miles of riding approaching this condition. It
was in service for five years seeing about 4000 miles of use a year.
The owner is 6 feet tall and about 160 pounds. The stem is a 3ttt
"Forge Ahead." The handlebar was a 3ttt road bar, 25.8 mm diameter at
the clamp area according to my calipers.

The stem was oriented so that the extension was flat (parallel to the
top tube). The major cracking is along the sides of the stem and
extending underneath, with another less-displaces crack extending around
the top. There is about 1 cm of intact metal separating the long cracks
on the side and the crack across the underside of the stem.

http://www2.bitstream.net/~timmcn/stemphotos/stem_right.png

http://www2.bitstream.net/~timmcn/stemphotos/stem_bottom.png

http://www2.bitstream.net/~timmcn/stemphotos/stem_clamp.png

http://www2.bitstream.net/~timmcn/stemphotos/stem_left.png

http://www2.bitstream.net/~timmcn/stemphotos/stem_left-under.png

Tim McNamara said:

In article <[email hidden]>,

Leo Lichtman said:

"jim beam" wrote: (clip) but fatigue is different! and energy absorption
by a metal once a fatigue crack has grown to critical is minimal. (clip)
^^^^^^^^^^^^^^^^^^

Jim, you're answering a question different from the one I raised.
Fatigure failure begins with a microscopic crack, which acts as a
stress raiser, and grows gradually, until the remaining sound metal
can no longer carry the load. This is the interval I was thinking of
in my response to: "once metal starts to fracture, it usually breaks
completely through as well."

Here are some photos of metal failure, in this case an aluminum stem.
It would be interesting to discuss the failure mode of aluminum versus
CF in this type of application, since CF stems are available. This
might provide a concrete example to compare the qualities of the
material. How would CF versus Al endure this situation?

that depends. if it's single-piece, there may be advantage, but the
cheapo "carbon" stems are aluminum ends with a bit of cfrp tube glued in
between. the aluminum ends will still fail.

Quoted message said:

The cracks developed over an unknown period of time, but two days before
it was discovered the owner and I did a 104 mile ride that included
gravel roads, steep hills and rough pavement. The owner had noticed
some creaking for a couple of rides prior to the 104 miler. This stem
survived at least 150 miles of riding approaching this condition. It
was in service for five years seeing about 4000 miles of use a year.
The owner is 6 feet tall and about 160 pounds. The stem is a 3ttt
"Forge Ahead." The handlebar was a 3ttt road bar, 25.8 mm diameter at
the clamp area according to my calipers.

The stem was oriented so that the extension was flat (parallel to the
top tube). The major cracking is along the sides of the stem and
extending underneath, with another less-displaces crack extending around
the top. There is about 1 cm of intact metal separating the long cracks
on the side and the crack across the underside of the stem.

http://www2.bitstream.net/~timmcn/stemphotos/stem_right.png

http://www2.bitstream.net/~timmcn/stemphotos/stem_bottom.png

http://www2.bitstream.net/~timmcn/stemphotos/stem_clamp.png

http://www2.bitstream.net/~timmcn/stemphotos/stem_left.png

http://www2.bitstream.net/~timmcn/stemphotos/stem_left-under.png

it's possible the stem is defective in that the longitudinal cracking is
clearly taking advantage of an anisotropy weakness. other than that,
i'd question the stem clamp tightening torque, question handlebar
diameter and stick to 3t's user's manual/warranty guideline - in it, the
statement: "Replace the stem after 15/20000km intensive amateur use." is
quite unambiguous.

Quoted message said:

Quoted message said:

fracture energy absorption is what determines whether a material is
"brittle" or not. cfrp is not brittle like glass. it is not ductile,
[the apparent, er, "confusion"] but it is typically not brittle -
significant energy is absorbed as the fracture interface progresses
[depending on fabrication and constituent materials of course].

Every time we've seen a CFRP bike part that has been hit or loaded
hard enough to break it, it has been broken completely through. That
seems to indicate the the energy to complete a fracture isn't much, at
least for carbon-epoxy such as we see in bikes.

This is both true and incredibly misleading. There are a lot of people out
there riding on damaged (broken) carbon frames & forks that aren't aware of
it. Few know what to look for, and even fewer *want* to look for the damage,
preferring instead to think "phew, looks like it came out OK, I don't have
to shell out a ton of money!"

So yes, it's true that few have "seen" a carbon fiber product that's broken
and not yet completely failed, but that doesn't mean they're not out there.
It just means that people don't know what to look for, or prefer not to. We
find damage to bikes frequently that the owner missed. There's this
common-sense thing that's sometimes lacking. If an impact was nasty enough
to have likely caused serious damage, it probably did. You just didn't look
hard enough, or in the right places, to find it. A huge number of "just
riding along" failures happen due to damage from a prior incident, and that
small pothole just happened to be the final straw.

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

"jim beam" wrote: it's possible the stem is defective in that the
longitudinal cracking is clearly taking advantage of an anisotropy
weakness.(clip)
^^^^^^^^^^^^^^^^^
Jim, what leads you to bring up anisotropy--is it that you need it to hold
up your end of the argument? If you hear hoofbeats, look for horses, not
zebras.

Mike Jacoubowsky said:

So yes, it's true that few have "seen" a carbon fiber product that's
broken and not yet completely failed, but that doesn't mean they're not
out there. It just means that people don't know what to look for, or
prefer not to. We find damage to bikes frequently that the owner missed.
There's this common-sense thing that's sometimes lacking. If an impact was
nasty enough to have likely caused serious damage, it probably did. You
just didn't look hard enough, or in the right places, to find it. A huge
number of "just riding along" failures happen due to damage from a prior
incident, and that small pothole just happened to be the final straw.

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

Ain't that the truth. I was hit by a car last month and my Reynolds Ouzo
Pro fork only had scratches on it and otherwise seemed fine. I am well
aware that carbon products can hide damage that will cause a future crash so
I replaced the fork. I still kept thinking "but it looks fine". The
thought of fork failure during a high speed descent allowed me to produce my
credit card. The LBS used the Park tool to remove the King baseplate so I
could move it to the new fork but it would not budge. They ended up bending
the plate edges and still it would not come off the fork. So, perhaps the
fork deformed a bit in that area. Anyway, I' glad I don't have to keep
wondering it my old fork is going to fail. Better save, and slightly
poorer, than sorry.

Tim McTeague

Leo Lichtman said:

"jim beam" wrote: it's possible the stem is defective in that the
longitudinal cracking is clearly taking advantage of an anisotropy
weakness.(clip)
^^^^^^^^^^^^^^^^^
Jim, what leads you to bring up anisotropy--is it that you need it to hold
up your end of the argument? If you hear hoofbeats, look for horses, not
zebras.

look closely at the pictures - those surface marks you see running the
length of the stem are the result of the anisotropic substrate. if
you've never looked down a metallurgical microscope you may have
problems visualizing what i'm talking about.

http://www.mee-inc.com/gmet11.jpg

this is a stainless steel, but the highly anisotropic [directional]
nature of the material is evident. with the stem, you're simply seeing
crack growth propagate along the weakest path, along the grains.

jim beam said:

following peter cole's confusion on this subject, it seems we need a
little clarification of what constitutes "brittle" fracture.

"brittle" is where there is very little energy absorbed as the fracture
interface propagates through a material. glass is the classic example
of this - once a crack is present, it takes almost nothing to continue
the crack's progress to complete failure.

"ductile" however is a totally different animal. significant energy is
absorbed during ductile deformation. but ductile deformation is not
fracture, it's another process entirely! once ductile deformation has
occurred, fracture requires /further/ propagation energy.

fracture energy absorption is what determines whether a material is
"brittle" or not. cfrp is not brittle like glass. it is not ductile,
[the apparent, er, "confusion"] but it is typically not brittle -
significant energy is absorbed as the fracture interface progresses
[depending on fabrication and constituent materials of course].

indeed, fracture energy absorption in non-ductile composites can be so
high, they're actually used in applications /specifically/ for this
reason. anyone familiar with motorcycle helmet testing will be aware of
this. same for bullet-protective military helmets. both are
non-ductile composites with very high fracture energy absorption. energy
absorbed depends on constitution, but the principle applies, and is
worth repeating - it's energy absorbed on fracture interface progression
that determines toughness, not whether the material is ductile.

What's this the 4th thread? You can keep going but it seems like you
haven't convinced anybody yet. It might help if you cited *any* source
supporting your unique view.

You're confusing fatigue with brittle failure and CF composites with
Kevlar composites/hybrids (among other things).

Yes, CF is sometimes mixed with other materials in applications like
body armor, but the purpose is not to absorb energy, but to stiffen the
assembly to avoid blunt force trauma. Again, you're confusing force with
energy, which you have done consistently through these threads.

CF composites can be used to design energy absorbing *structures* (F1
nose & tail boxes), but that involves crushable geometries that absorb
energy in a very specific manner. CF is not usually used in a direct
energy absorption role but as in a supporting strengthening/stiffening
role (as in hoop strength reinforcement in fiberglass vault poles)
That's not how bike parts are designed, so comparisons are misleading.

As everyone in the industry seems to know, low velocity impacts lead to
microcracks in the matrix, which eventually coalesce into macro cracks
and sudden failure. This is cumulative and not easily detected.

If a parts designer simply optimizes for CF specific strength and
modulus, they're stuck with low impact resistance and brittle failure.
That's how CF bike parts are generally designed.

Peter Cole said:

jim beam said:

following peter cole's confusion on this subject, it seems we need a
little clarification of what constitutes "brittle" fracture.

"brittle" is where there is very little energy absorbed as the
fracture interface propagates through a material. glass is the
classic example of this - once a crack is present, it takes almost
nothing to continue the crack's progress to complete failure.

"ductile" however is a totally different animal. significant energy
is absorbed during ductile deformation. but ductile deformation is
not fracture, it's another process entirely! once ductile deformation
has occurred, fracture requires /further/ propagation energy.

fracture energy absorption is what determines whether a material is
"brittle" or not. cfrp is not brittle like glass. it is not ductile,
[the apparent, er, "confusion"] but it is typically not brittle -
significant energy is absorbed as the fracture interface progresses
[depending on fabrication and constituent materials of course].

indeed, fracture energy absorption in non-ductile composites can be so
high, they're actually used in applications /specifically/ for this
reason. anyone familiar with motorcycle helmet testing will be aware
of this. same for bullet-protective military helmets. both are
non-ductile composites with very high fracture energy absorption.
energy absorbed depends on constitution, but the principle applies,
and is worth repeating - it's energy absorbed on fracture interface
progression that determines toughness, not whether the material is
ductile.

What's this the 4th thread? You can keep going but it seems like you
haven't convinced anybody yet. It might help if you cited *any* source
supporting your unique view.

eh? i'm starting a new thread because your [typical] obfuscation is so
convoluted, i really can't be bothered to untangle it. and you keep
getting away from the central point - you don't understand materials.

Quoted message said:

You're confusing fatigue with brittle failure

no i'm not - you're [wrongly] asserting that low ductility means brittle.

Quoted message said:

and CF composites with
Kevlar composites/hybrids (among other things).

no, but you'd love to put those words of deliberate deceit into my mouth.

Quoted message said:

Yes, CF is sometimes mixed with other materials in applications like
body armor, but the purpose is not to absorb energy, but to stiffen the
assembly to avoid blunt force trauma.

er, that is somewhat "confused".

Quoted message said:

Again, you're confusing force with
energy,

no - that's a statement of deliberate deceit.

Quoted message said:

which you have done consistently through these threads.

see above.

Quoted message said:

CF composites can be used to design energy absorbing *structures* (F1
nose & tail boxes), but that involves crushable geometries that absorb
energy in a very specific manner.

no amount of "crushable geometries" would matter if the material were in
fact "brittle". truth is, the /material/ absorbs energy on fracture,
hence their use. "crushable geometry" would mean nothing if the parts
were made of glass.

Quoted message said:

CF is not usually used in a direct
energy absorption role but as in a supporting strengthening/stiffening
role (as in hoop strength reinforcement in fiberglass vault poles)
That's not how bike parts are designed, so comparisons are misleading.

no, you're trying to deceive again.

Quoted message said:

As everyone in the industry seems to know, low velocity impacts

without numbers, "impact" is an utterly meaningless word - it's the
ephemeral straw-clutch by one desperately seeking to stir fear,
uncertainty and doubt.

Quoted message said:

lead to
microcracks in the matrix,

again, without numbers, that is a meaningless statement. [handily
deceitful though.] with composites, the discontinuity between fiber and
matrix accounts for a good deal of short range "problem". there are
ways of addressing but not overcoming this. different matrix materials,
even carbon nanotube in the matrix resin, has significant benefits -
hence easton's use for instance.

Quoted message said:

which eventually coalesce into macro cracks
and sudden failure.

and not if they don't. just like metal fatigue and dislocation migration.

Quoted message said:

This is cumulative and not easily detected.

just like metal fatigue and dislocation migration.

interestingly, i don't see you ringing the bells of armageddon about
metal fatigue and how everyone needs x-ray and ultrasound on their metal
frames. but you're trying to deceive, so that's to be expected.

Quoted message said:

If a parts designer simply optimizes for CF specific strength and
modulus, they're stuck with low impact resistance and brittle failure.

[censored]. you don't know what "brittle" means and you're trying to
deceive by misuse of that word.

Quoted message said:

That's how CF bike parts are generally designed.

that's suppositional [censored].

Mike Jacoubowsky said:

Quoted message said:

Quoted message said:

fracture energy absorption is what determines whether a material is
"brittle" or not. cfrp is not brittle like glass. it is not ductile,
[the apparent, er, "confusion"] but it is typically not brittle -
significant energy is absorbed as the fracture interface progresses
[depending on fabrication and constituent materials of course].

Every time we've seen a CFRP bike part that has been hit or loaded
hard enough to break it, it has been broken completely through. That
seems to indicate the the energy to complete a fracture isn't much, at
least for carbon-epoxy such as we see in bikes.

This is both true and incredibly misleading. There are a lot of people out
there riding on damaged (broken) carbon frames & forks that aren't aware of
it. Few know what to look for, and even fewer *want* to look for the damage,
preferring instead to think "phew, looks like it came out OK, I don't have
to shell out a ton of money!"

indeed.

Quoted message said:

So yes, it's true that few have "seen" a carbon fiber product that's broken
and not yet completely failed, but that doesn't mean they're not out there.

absolutely - the warning signs are there but are frequently ignored.

Quoted message said:

It just means that people don't know what to look for, or prefer not to.

indeed.

Quoted message said:

We
find damage to bikes frequently that the owner missed. There's this
common-sense thing that's sometimes lacking. If an impact was nasty enough
to have likely caused serious damage, it probably did. You just didn't look
hard enough, or in the right places, to find it. A huge number of "just
riding along" failures happen due to damage from a prior incident, and that
small pothole just happened to be the final straw.

well said.