metallurgy question

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 wrote:
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.



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.


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.



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 wrote:
zencycle wrote:
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.



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".


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.


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.



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.

On Jun 6, 12:28 pm, zencycle wrote:
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

On Jun 6, 11:06 pm, jim beam wrote:
zencycle wrote:
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

no. there are many concepts above this, but basically, engineering
........
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, "
wrote:
On Jun 6, 12:28 pm, zencycle wrote:



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

On Jun 6, 3:28 pm, zencycle wrote:
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

On Jun 6, 3:28 pm, zencycle wrote:
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
,
" wrote:

On Jun 6, 12:28 pm, zencycle wrote:
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

wrote:
On Jun 6, 12:28 pm, zencycle wrote:
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's just elasticity, not modulus of. modulus is the slope of the
line in the hookes law region of a stress/strain graph.



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.

drop the "significant". /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.



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