metallurgy question

zencycle said:

Aluminum has the characteristic of being able to be bent to a position
to hold its form, pretty much one time, and only to a certain extent
in that shape. When you bend it back, stress cracks occur, and even
breakage.

by absolutely no means is that unique to aluminum.

Quoted message said:

I'm only thinking of the context of cold-working, not forging or hot
working.

I seem to remember the reason for the crack and breakage upon
attempting a second manipulation was that the initial bend in cold
working sets up a crystalline structure, and subsequent working
essentially breaks the structure. Do I have that right?

no. there are many concepts above this, but basically, engineering
metal alloys are crystalline. when the crystals are deformed with work,
they accommodate that deformation by increasing defect density within
the crystal structure. and there is a limit to the amount of defect
density it can tolerate before rupture occurs. whether through cold
work or through the chemical changes that occur in aging, the ductility
limit is what is being experienced.

that some alloys appear to be more ductile than others in this case is a
function of initial state vs. final state. if the initial state were
further away from the rupture condition, you would indeed be able to
deform the component many times before failure. bendy foam tie-downs
for example have a highly ductile aluminum wire in them for instance -
and that wire is simply a long way off from its full work harness. many
bike components however are up near their ultimate limit and thus have
limited ability to accommodate more deformation. and if they did,
they'd be weaker.

Quoted message said:

I'm looking for the applicable terms:

Please correct me if I'm wrong. Is it the modulus of elasticity that
is the overall characteristic that I'm referring to?

no, "ductility" is the term you need.

Quoted message said:

What is the term used for the initial bend (if there is one)? and what
is the name of the stress factor that occurs after the second attempt
at working the part?

see above. many aluminum parts either naturally age and harden or are
artificially aged to harden them. this occurs after forming.
increasing hardness means decreasing ductility.

jim beam said:

zencycle said:

Aluminum has the characteristic of being able to be bent to a position
to hold its form, pretty much one time, and only to a certain extent
in that shape. When you bend it back, stress cracks occur, and even
breakage.

by absolutely no means is that unique to aluminum.

Quoted message said:

I'm only thinking of the context of cold-working, not forging or hot
working.

I seem to remember the reason for the crack and breakage upon
attempting a second manipulation was that the initial bend in cold
working sets up a crystalline structure, and subsequent working
essentially breaks the structure. Do I have that right?

no. there are many concepts above this, but basically, engineering
metal alloys are crystalline. when the crystals are deformed with work,
they accommodate that deformation by increasing defect density within
the crystal structure.

that effect is called "ductility".

Quoted message said:

and there is a limit to the amount of defect
density it can tolerate before rupture occurs. whether through cold
work or through the chemical changes that occur in aging, the ductility
limit is what is being experienced.

that some alloys appear to be more ductile than others in this case is a
function of initial state vs. final state. if the initial state were
further away from the rupture condition, you would indeed be able to
deform the component many times before failure. bendy foam tie-downs
for example have a highly ductile aluminum wire in them for instance -
and that wire is simply a long way off from its full work harness. many
bike components however are up near their ultimate limit and thus have
limited ability to accommodate more deformation. and if they did,
they'd be weaker.

Quoted message said:

I'm looking for the applicable terms:

Please correct me if I'm wrong. Is it the modulus of elasticity that
is the overall characteristic that I'm referring to?

no, "ductility" is the term you need.

Quoted message said:

What is the term used for the initial bend (if there is one)? and what
is the name of the stress factor that occurs after the second attempt
at working the part?

see above. many aluminum parts either naturally age and harden or are
artificially aged to harden them. this occurs after forming. increasing
hardness means decreasing ductility.

zencycle said:

Aluminum has the characteristic of being able to be bent to a position
to hold its form, pretty much one time, and only to a certain extent
in that shape. When you bend it back, stress cracks occur, and even
breakage.

I'm only thinking of the context of cold-working, not forging or hot
working.

I seem to remember the reason for the crack and breakage upon
attempting a second manipulation was that the initial bend in cold
working sets up a crystalline structure, and subsequent working
essentially breaks the structure. Do I have that right?

I'm looking for the applicable terms:

Please correct me if I'm wrong. Is it the modulus of elasticity that
is the overall characteristic that I'm referring to?

What is the term used for the initial bend (if there is one)? and what
is the name of the stress factor that occurs after the second attempt
at working the part?

The modulus of elasticity is how much the material
bends or elongates per applied force. That is not
quite what you are talking about.

A material that can take a significant plastic
deformation without cracking or breaking is called
ductile; the opposite is brittle.

The decrease in ductility after the first bend is
due to "work hardening." Work hardening increases
yield strength and decreases ductility.

http://en.wikipedia.org/wiki/Work_hardening

Ben

jim beam said:

zencycle said:

Aluminum has the characteristic of being able to be bent to a position

by absolutely no means is that unique to aluminum.

I didn't write or imply that it was

Quoted message said:

no. there are many concepts above this, but basically, engineering

........

Quoted message said:

limited ability to accommodate more deformation. and if they did,
they'd be weaker.

Thank you for the rest, it was helpful.

Thanks ben, this was _most_ helpful

On Jun 7, 12:07 am, "[email hidden]" <[email hidden]>

Quoted message said:

zencycle said:

Aluminum has the characteristic of being able to be bent to a position
to hold its form, pretty much one time, and only to a certain extent
in that shape. When you bend it back, stress cracks occur, and even
breakage.

Quoted message said:

I'm only thinking of the context of cold-working, not forging or hot
working.

Quoted message said:

I seem to remember the reason for the crack and breakage upon
attempting a second manipulation was that the initial bend in cold
working sets up a crystalline structure, and subsequent working
essentially breaks the structure. Do I have that right?

Quoted message said:

I'm looking for the applicable terms:

Quoted message said:

Please correct me if I'm wrong. Is it the modulus of elasticity that
is the overall characteristic that I'm referring to?

Quoted message said:

What is the term used for the initial bend (if there is one)? and what
is the name of the stress factor that occurs after the second attempt
at working the part?

The modulus of elasticity is how much the material
bends or elongates per applied force. That is not
quite what you are talking about.

A material that can take a significant plastic
deformation without cracking or breaking is called
ductile; the opposite is brittle.

The decrease in ductility after the first bend is
due to "work hardening." Work hardening increases
yield strength and decreases ductility.

http://en.wikipedia.org/wiki/Work_hardening

Ben

zencycle said:

Aluminum has the characteristic of being able to be bent to a position
to hold its form, pretty much one time, and only to a certain extent
in that shape. When you bend it back, stress cracks occur, and even
breakage.

I'm only thinking of the context of cold-working, not forging or hot
working.

I seem to remember the reason for the crack and breakage upon
attempting a second manipulation was that the initial bend in cold
working sets up a crystalline structure, and subsequent working
essentially breaks the structure. Do I have that right?

I'm looking for the applicable terms:

Please correct me if I'm wrong. Is it the modulus of elasticity that
is the overall characteristic that I'm referring to?

What is the term used for the initial bend (if there is one)? and what
is the name of the stress factor that occurs after the second attempt
at working the part?

Zencycle :

Hope this helps. Devote sometime to reading this :
http://spokesmanbicycles.com/page.cfm?pageID=330

Ron
http://cozybeehive.blogspot.com

zencycle said:

Aluminum has the characteristic of being able to be bent to a position
to hold its form, pretty much one time, and only to a certain extent
in that shape. When you bend it back, stress cracks occur, and even
breakage.

You've gotten some good explanations. Let me chip in a couple
practical examples.

Don't make too much of the idea that "when you bend it back, stress
cracks occur, and even breakage." They certainly may not.

I've straightened a bent (integral, not removable) derailleur hanger
on my wife's Cannondale, and a bent alloy crank arm on one of my son's
bikes. Both are doing fine, long after the re-bending. (Of course,
the hanger is a very low stress part.)

I've also reshaped the aluminum hanger for the handlebar bag I made.
The hanger is one of those that loops under the stem, over the bars,
and cantilevers forward. This is the largest bar bag I've ever seen,
and it's been heavily loaded many times since 1978. The hanger's 3/8"
diameter 2024 aluminum, heat treated T4 after fabrication. After
many, many bends and twists, it's never cracked.

Loosely speaking, the closer a metal gets to it's bleeding edge of
maximum possible strength, the less the ductility it has, and the more
chance of cracks if you do deform it. But many parts aren't very
close to that edge, and can easily stand some moderate deformation.

- Frank Krygowski

In article
<[email hidden]>,

Quoted post said:

zencycle said:

Aluminum has the characteristic of being able to be bent to a position
to hold its form, pretty much one time, and only to a certain extent
in that shape. When you bend it back, stress cracks occur, and even
breakage.

I'm only thinking of the context of cold-working, not forging or hot
working.

I seem to remember the reason for the crack and breakage upon
attempting a second manipulation was that the initial bend in cold
working sets up a crystalline structure, and subsequent working
essentially breaks the structure. Do I have that right?

I'm looking for the applicable terms:

Please correct me if I'm wrong. Is it the modulus of elasticity that
is the overall characteristic that I'm referring to?

What is the term used for the initial bend (if there is one)? and what
is the name of the stress factor that occurs after the second attempt
at working the part?

The modulus of elasticity is how much the material
bends or elongates per applied force.

Not quite.
It is elongation per applied force per area of applied force.
Modulus of elasticity is stress/strain.
Strain is force applied per area.
Stress is amount of deformation.

Elastic modulus is a 2-tensor of dimension 3.
9 components.
The diagonal components give deformation normal to
a coordinate plane given force applied normal to
the coordinate planes.
The six off diagonal components give shear deformation
for force applied parallel to coordinate planes.
The 9 components could all be different from each other.

--
Michael Press

Michael Press said:

In article
<[email hidden]>,

Quoted post said:

zencycle said:

Aluminum has the characteristic of being able to be bent to a position
to hold its form, pretty much one time, and only to a certain extent
in that shape. When you bend it back, stress cracks occur, and even
breakage.

I'm only thinking of the context of cold-working, not forging or hot
working.

I seem to remember the reason for the crack and breakage upon
attempting a second manipulation was that the initial bend in cold
working sets up a crystalline structure, and subsequent working
essentially breaks the structure. Do I have that right?

I'm looking for the applicable terms:

Please correct me if I'm wrong. Is it the modulus of elasticity that
is the overall characteristic that I'm referring to?

What is the term used for the initial bend (if there is one)? and what
is the name of the stress factor that occurs after the second attempt
at working the part?

The modulus of elasticity is how much the material
bends or elongates per applied force.

Not quite.
It is elongation per applied force per area of applied force.
Modulus of elasticity is stress/strain.

the slope of the line, yes.

Quoted message said:

Strain is force applied per area.
Stress is amount of deformation.

no.

stress is force per area.
strain is elongation per length.

Quoted message said:

Elastic modulus is a 2-tensor of dimension 3.
9 components.
The diagonal components give deformation normal to
a coordinate plane given force applied normal to
the coordinate planes.
The six off diagonal components give shear deformation
for force applied parallel to coordinate planes.
The 9 components could all be different from each other.

that's mental masturbation if you can't define stress and strain correctly.

On 2008-06-08, Michael Press <[email hidden]> wrote:
[...]

Quoted message said:

Elastic modulus is a 2-tensor of dimension 3.
9 components.
The diagonal components give deformation normal to
a coordinate plane given force applied normal to
the coordinate planes.
The six off diagonal components give shear deformation
for force applied parallel to coordinate planes.
The 9 components could all be different from each other.

So for a lump of steel, am I right in thinking the tensor looks like
this:

E 0 0
0 E 0
0 0 E

where E is about 200GPa.

But for CF or something anisotropic, I would have different values all
over the place.

There don't seem to be any "shear components" in my matrix for steel,
but I don't really understand that: coordinate planes are usually
orthogonal, which means force normal to one plane is parallel to the
other two. So I don't see how you can divide forces into two sets of
those normal to coordinate planes and those parallel to them.

On 2008-06-09, jim beam <[email hidden]> wrote:
[...]

Quoted message said:

/any/ material that plastically deforms is ductile to some degree.
the question is, "how much". to be clear, the o.p. is describing low
ductility, not brittleness.

What's the difference between low ductility and brittleness?

"Ben C" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

On 2008-06-09, jim beam <[email hidden]> wrote:
[...]

Quoted message said:

/any/ material that plastically deforms is ductile to some degree.
the question is, "how much". to be clear, the o.p. is describing low
ductility, not brittleness.

What's the difference between low ductility and brittleness?

It means that beam can beam.

Ben C said:

On 2008-06-09, jim beam <[email hidden]> wrote:
[...]

Quoted message said:

/any/ material that plastically deforms is ductile to some degree.
the question is, "how much". to be clear, the o.p. is describing low
ductility, not brittleness.

What's the difference between low ductility and brittleness?

energy absorption on fracture for one. it's all about the propagation
mechanism. but you're on the right track that the two are related.

In article <[email hidden]>,

Ben C said:

On 2008-06-08, Michael Press <[email hidden]> wrote:
[...]

Quoted message said:

Elastic modulus is a 2-tensor of dimension 3.
9 components.
The diagonal components give deformation normal to
a coordinate plane given force applied normal to
the coordinate planes.
The six off diagonal components give shear deformation
for force applied parallel to coordinate planes.
The 9 components could all be different from each other.

So for a lump of steel, am I right in thinking the tensor looks like
this:

E 0 0
0 E 0
0 0 E

where E is about 200GPa.

But for CF or something anisotropic, I would have different values all
over the place.

There don't seem to be any "shear components" in my matrix for steel,
but I don't really understand that: coordinate planes are usually
orthogonal, which means force normal to one plane is parallel to the
other two. So I don't see how you can divide forces into two sets of
those normal to coordinate planes and those parallel to them.

Epoxy resin and carbon fiber lay ups have anisotropic elastic properties,
as do various crystals.

<http://books.google.com/books?id=90_ORVHeNkIC&pg=PT237&lpg=PT237&dq=anisotropic+crystal+%22elastic+modulus%22&source=web&ots=Zg1nRkq41y&sig=39g5dHxh5jGudvEd3_N-aug5sFc&hl=en>

--
Michael Press

Michael Press said:

In article <[email hidden]>,

Ben C said:

On 2008-06-08, Michael Press <[email hidden]> wrote:
[...]

Quoted message said:

Elastic modulus is a 2-tensor of dimension 3.
9 components.
The diagonal components give deformation normal to
a coordinate plane given force applied normal to
the coordinate planes.
The six off diagonal components give shear deformation
for force applied parallel to coordinate planes.
The 9 components could all be different from each other.

So for a lump of steel, am I right in thinking the tensor looks like
this:

E 0 0
0 E 0
0 0 E

where E is about 200GPa.

But for CF or something anisotropic, I would have different values all
over the place.

There don't seem to be any "shear components" in my matrix for steel,
but I don't really understand that: coordinate planes are usually
orthogonal, which means force normal to one plane is parallel to the
other two. So I don't see how you can divide forces into two sets of
those normal to coordinate planes and those parallel to them.

Epoxy resin and carbon fiber lay ups have anisotropic elastic properties,
as do various crystals.

<http://books.google.com/books?id=90_ORVHeNkIC&pg=PT237&lpg=PT237&dq=anisotropic+crystal+%22elastic+modulus%22&source=web&ots=Zg1nRkq41y&sig=39g5dHxh5jGudvEd3_N-aug5sFc&hl=en>

so? that doesn't address his question in the slightest. perhaps you
shouldn't try to [censored] outside your area of expertise?

ah, but i remember now, you're the guy that thinks anodizing crack
orientation has no bearing on fatigue initiation! now your confusion
becomes clear!