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Breaking a pencil: tension v. compression v. bending moment
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Existential Angst
May 22, 2012, 1:12:32 AM5/22/12
to
Awl --
As is proly intuitively clear to most, you break a pencil by applying a
bending moment at the center, and <snap>.
But it would be REALLY difficult to PULL a pencil apart, or crush it.
Iow, the forces req'd to snap a pencil are small, the forces req'd to pull
it apart would be huge.
I was wondering what the explanation for this is.
And I think it may be as simple as this:
Ito of INTERNAL stresses in the pencil, when you simply pull on a pencil,
you generate an internal psi in the pencil, which is just total Force
divided by the cross sectional area, which if less than the material
whatever whatever (section modulus or some shit??, the pencil stays intact.
However, when you BEND a pencil, now you are generating torques of r x F,
and given the sizable r and F of bending, and the very small r of the pencil
for resisting sed Torque, the net F generated internally in the pencil
becomes very large, with very high resulting internal psi's, which then
exceed the intrinsic strength of the pencil.
Think finger in a door jamb... at the HINGE!!!! <ouch>
Sound good? Bad???
--
EA
As is proly intuitively clear to most, you break a pencil by applying a
bending moment at the center, and <snap>.
But it would be REALLY difficult to PULL a pencil apart, or crush it.
Iow, the forces req'd to snap a pencil are small, the forces req'd to pull
it apart would be huge.
I was wondering what the explanation for this is.
And I think it may be as simple as this:
Ito of INTERNAL stresses in the pencil, when you simply pull on a pencil,
you generate an internal psi in the pencil, which is just total Force
divided by the cross sectional area, which if less than the material
whatever whatever (section modulus or some shit??, the pencil stays intact.
However, when you BEND a pencil, now you are generating torques of r x F,
and given the sizable r and F of bending, and the very small r of the pencil
for resisting sed Torque, the net F generated internally in the pencil
becomes very large, with very high resulting internal psi's, which then
exceed the intrinsic strength of the pencil.
Think finger in a door jamb... at the HINGE!!!! <ouch>
Sound good? Bad???
--
EA
Richard
May 22, 2012, 1:19:52 AM5/22/12
to
But I'm not sticking my finger in the door jamb.
PrecisionmachinisT
May 22, 2012, 1:44:49 AM5/22/12
to
"Existential Angst" <fit...@optonline.net> wrote in message
news:4fbb204d$0$20894$607e...@cv.net...
When you bend something, part of it goes into compression and the other part
goes into tension, what happens next is basically determened by the
mechanical properties of the material being nent; wood is different from
metal in that wood doesn't tend to have much of an elastic limit, basically
it breaks off instead of taking on a permanent bend.
Ed Huntress
May 22, 2012, 1:48:24 AM5/22/12
to
the pencil -- the tension side and the compression side. The
compression strength of wood runs a little over half of its tensile
strength. The modulus also is higher in tension. So the compression
side collapses relatively easily when you bend it, compressing the
fibers and effectively moving the resulting neutral axis closer to the
tension side. Snap!
This is more or less readable:
http://www.fpl.fs.fed.us/documnts/pdf2001/green01d.pdf
If you really want to understand wood, the book _Understanding Wood_
by R. Bruce Hoadley is outstanding. Also, there are Forest Products
Laboratory PDF files on the Web, covering wood properties and wood
adhesives. These, too, are excellent, but you'll need some coffee to
stay awake when you read them.
--
Ed Huntress
Existential Angst
May 22, 2012, 2:00:05 AM5/22/12
to
"Ed Huntress" <hunt...@optonline.net> wrote in message
news:ea9mr75fcqhaphceo...@4ax.com...
Still, isn't this ultimately that the material unsuccessfully resisted an
applied TORQUE??
>
> This is more or less readable:
>
> http://www.fpl.fs.fed.us/documnts/pdf2001/green01d.pdf
>
> If you really want to understand wood, the book _Understanding Wood_
> by R. Bruce Hoadley is outstanding. Also, there are Forest Products
> Laboratory PDF files on the Web, covering wood properties and wood
> adhesives. These, too, are excellent, but you'll need some coffee to
> stay awake when you read them.
I don't know whazzup, but whenever I ax a Q, peeple keep axing me to READ
shit.... WTF?????
LOL
--
EA
>
> --
> Ed Huntress
news:ea9mr75fcqhaphceo...@4ax.com...
applied TORQUE??
>
> This is more or less readable:
>
> http://www.fpl.fs.fed.us/documnts/pdf2001/green01d.pdf
>
> If you really want to understand wood, the book _Understanding Wood_
> by R. Bruce Hoadley is outstanding. Also, there are Forest Products
> Laboratory PDF files on the Web, covering wood properties and wood
> adhesives. These, too, are excellent, but you'll need some coffee to
> stay awake when you read them.
shit.... WTF?????
LOL
--
EA
>
> --
> Ed Huntress
Ed Huntress
May 22, 2012, 2:09:02 AM5/22/12
to
On Tue, 22 May 2012 02:00:05 -0400, "Existential Angst"
The wood only cares about tension and compression. (Shear is involved,
but you don't have to worry about it in your example.) You have to
transform the torque into tension and compression loads if you're
going to start with torque.
>
>>
>> This is more or less readable:
>>
>> http://www.fpl.fs.fed.us/documnts/pdf2001/green01d.pdf
>>
>> If you really want to understand wood, the book _Understanding Wood_
>> by R. Bruce Hoadley is outstanding. Also, there are Forest Products
>> Laboratory PDF files on the Web, covering wood properties and wood
>> adhesives. These, too, are excellent, but you'll need some coffee to
>> stay awake when you read them.
>
>I don't know whazzup, but whenever I ax a Q, peeple keep axing me to READ
>shit.... WTF?????
>LOL
<g> Hey, it's up to you. There is one enjoyable book to read on the
subject, IMO, and it's _Understanding Wood_. It reads like it was
written by an actual human being. It even has lots of pretty pictures
and drawings. He writes for people who want to *make* things out of
wood. Most decent libraries have it.
Hoadley used to write for _Fine Woodworking_. He has a nice style.
--
Ed Huntress
but you don't have to worry about it in your example.) You have to
transform the torque into tension and compression loads if you're
going to start with torque.
>
>>
>> This is more or less readable:
>>
>> http://www.fpl.fs.fed.us/documnts/pdf2001/green01d.pdf
>>
>> If you really want to understand wood, the book _Understanding Wood_
>> by R. Bruce Hoadley is outstanding. Also, there are Forest Products
>> Laboratory PDF files on the Web, covering wood properties and wood
>> adhesives. These, too, are excellent, but you'll need some coffee to
>> stay awake when you read them.
>
>I don't know whazzup, but whenever I ax a Q, peeple keep axing me to READ
>shit.... WTF?????
>LOL
subject, IMO, and it's _Understanding Wood_. It reads like it was
written by an actual human being. It even has lots of pretty pictures
and drawings. He writes for people who want to *make* things out of
wood. Most decent libraries have it.
Hoadley used to write for _Fine Woodworking_. He has a nice style.
--
Ed Huntress
Richard
May 22, 2012, 2:39:05 AM5/22/12
to
On 5/22/2012 1:09 AM, Ed Huntress wrote:
>
> <g> Hey, it's up to you. There is one enjoyable book to read on the
> subject, IMO, and it's _Understanding Wood_. It reads like it was
> written by an actual human being. It even has lots of pretty pictures
> and drawings. He writes for people who want to *make* things out of
> wood. Most decent libraries have it.
>
> Hoadley used to write for _Fine Woodworking_. He has a nice style.
>
Yes he does!
>
> <g> Hey, it's up to you. There is one enjoyable book to read on the
> subject, IMO, and it's _Understanding Wood_. It reads like it was
> written by an actual human being. It even has lots of pretty pictures
> and drawings. He writes for people who want to *make* things out of
> wood. Most decent libraries have it.
>
> Hoadley used to write for _Fine Woodworking_. He has a nice style.
>
Jim Wilkins
May 22, 2012, 7:34:25 AM5/22/12
to
news:4fbb2b73$0$14812$607e...@cv.net...
> ...> I don't know whazzup, but whenever I ax a Q, peeple keep axing
to be graphical, like the distribution of stress across a beam.
jsw
> ...> I don't know whazzup, but whenever I ax a Q, peeple keep axing
> me to READ shit.... WTF?????
> LOL
> --
> EA
'Cause we can't draw pictures here. Good engineering explanations tend
> LOL
> --
> EA
to be graphical, like the distribution of stress across a beam.
jsw
jim
May 22, 2012, 8:38:38 AM5/22/12
to
Existential Angst wrote:
>
> Still, isn't this ultimately that the material unsuccessfully resisted an
> applied TORQUE??
the main reason a pencil is easy to break.
A half inch long pencil will not be easy to break with
your fingers.
Tim Wescott
May 22, 2012, 10:56:57 AM5/22/12
to
Although, if you need short pencils I would recommend sawing, or just
finding out where country clubs get their supplies of pencil stubs.
Breaking them is definitely messy.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Tim Wescott
May 22, 2012, 10:59:51 AM5/22/12
to
theory that I have found very useful -- but a whole book _about_ wood
would be better. I should probably get that one.
Ed Huntress
May 22, 2012, 12:02:21 PM5/22/12
to
structural, it's a good read that will give you a lot of insights.
You can get the same technical info from the FPL white papers, but
they're not as enjoyable to read, nor as well illustrated. However,
get the FPL papers on adhesives -- particularly this one:
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_10.pdf
It's very good. Google "FPL Adhesives," without quotes, and you'll see
papers on wood-to-metal, etc.
--
Ed Huntress
Ed Huntress
May 22, 2012, 12:18:28 PM5/22/12
to
On Tue, 22 May 2012 09:56:57 -0500, Tim Wescott
<t...@seemywebsite.please> wrote:
>On Tue, 22 May 2012 01:12:32 -0400, Existential Angst wrote:
>
>> Awl --
>>
>> As is proly intuitively clear to most, you break a pencil by applying a
>> bending moment at the center, and <snap>. But it would be REALLY
>> difficult to PULL a pencil apart, or crush it.
>>
>> Iow, the forces req'd to snap a pencil are small, the forces req'd to
>> pull it apart would be huge.
>>
>> I was wondering what the explanation for this is. And I think it may be
>> as simple as this:
>>
>> Ito of INTERNAL stresses in the pencil, when you simply pull on a
>> pencil, you generate an internal psi in the pencil, which is just total
>> Force divided by the cross sectional area, which if less than the
>> material whatever whatever (section modulus or some shit??, the pencil
>> stays intact.
>>
>> However, when you BEND a pencil, now you are generating torques of r x
>> F, and given the sizable r and F of bending, and the very small r of the
>> pencil for resisting sed Torque, the net F generated internally in the
>> pencil becomes very large, with very high resulting internal psi's,
>> which then exceed the intrinsic strength of the pencil. Think finger in
>> a door jamb... at the HINGE!!!! <ouch>
>
>You've hit the nail on the head.
Just be careful about how you conceive of things like "very high
<t...@seemywebsite.please> wrote:
>On Tue, 22 May 2012 01:12:32 -0400, Existential Angst wrote:
>
>> Awl --
>>
>> As is proly intuitively clear to most, you break a pencil by applying a
>> bending moment at the center, and <snap>. But it would be REALLY
>> difficult to PULL a pencil apart, or crush it.
>>
>> Iow, the forces req'd to snap a pencil are small, the forces req'd to
>> pull it apart would be huge.
>>
>> I was wondering what the explanation for this is. And I think it may be
>> as simple as this:
>>
>> Ito of INTERNAL stresses in the pencil, when you simply pull on a
>> pencil, you generate an internal psi in the pencil, which is just total
>> Force divided by the cross sectional area, which if less than the
>> material whatever whatever (section modulus or some shit??, the pencil
>> stays intact.
>>
>> However, when you BEND a pencil, now you are generating torques of r x
>> F, and given the sizable r and F of bending, and the very small r of the
>> pencil for resisting sed Torque, the net F generated internally in the
>> pencil becomes very large, with very high resulting internal psi's,
>> which then exceed the intrinsic strength of the pencil. Think finger in
>> a door jamb... at the HINGE!!!! <ouch>
>
>You've hit the nail on the head.
internal psi's." Nothing "internal" matters much in this case. The
core resistance to compression is very low in the cross-grain
direction with wood. That's why balsa-cored laminates arrange the
balsa with the end grain perpendicular to the surfaces.
This is a basic static-bending matter, and the terms and concepts from
statics are the best way to think about it. Aside from keeping the
compression and tension sides apart at low loads, the wood inside has
little to do with anythng. And EA talks about "resisting said torque."
It doesn't resist torque. It resists tension and compression, almost
all of which is going on at the surface fibers on the tension and
compression sides. You have to convert the torque to values of tension
and compression to get anything meaningful out of that.
The idea of bending strength, which is useful in isotropic materials
like metal, really is a derived concept, which combines the tensile
and compressive strengths of the material with its elastic modulus. It
just makes a confusing mess of equations with wood, because, as the
paper I linked to explains, the strength and stiffness values of wood
vary widely in each direction. And wood is a composite material. The
static strength-of-materials equations are similarly complicated to
those of fiber/resin composites. Cellulose is the fiber; lignin is the
resin.
Shear plays a minute role in this with metal. With wood, it can be an
issue in terms of the shear-delamination of the wood fibers. The
result can be like the difference between a piece of plywood and a
stack of veneers of the same material and thickness. The glued plywood
is much stronger and stiffer. But in simple bending, shear
delamination hardly enters the equation at all. You've typically
exceeded the compression strength of the wood before internal fibers
begin to delaminate.
Nothing is ever simple when you start picking it apart, even a pencil.
d8-)
--
Ed Huntress
Tim Wescott
May 22, 2012, 12:30:44 PM5/22/12
to
> Nothing is ever simple when you start picking it apart, even a pencil.
> d8-)
fails the failure isn't in "bending" (because that doesn't exist) -- it's
in compression, followed by tension.
So if your goal is to understand that "things that break in bending
really break in tension, compression or shear" and in your lack of
wariness you chose a pencil as your example instead of a glass or
polycarbonate rod, then what I said is true.
If your goal is to understand that "you shouldn't use a wooden example
object when you ask strength of materials questions" -- well, I guess EA
chose the right question.
(Now I want to see a glass rod, a wood rod and a carbon fiber rod after
breaking -- or better, during breaking in slow-mo. Maybe with a couple
of tubes tossed in. Is there anything that breaks in shear before it
breaks in tension or compression?)
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
Ed Huntress
May 22, 2012, 12:45:48 PM5/22/12
to
On Tue, 22 May 2012 11:30:44 -0500, Tim Wescott <t...@seemywebsite.com>
Uh, yeah. I think that's exactly what I said. d8-)
>
>So if your goal is to understand that "things that break in bending
>really break in tension, compression or shear" and in your lack of
>wariness you chose a pencil as your example instead of a glass or
>polycarbonate rod, then what I said is true.
I'm getting confused here. I think that's what I was cautioning about.
At least, I was trying.
>
>If your goal is to understand that "you shouldn't use a wooden example
>object when you ask strength of materials questions" -- well, I guess EA
>chose the right question.
?? It sounds to me like he chose a complicated one that sounds simple,
but isn't.
>
>(Now I want to see a glass rod, a wood rod and a carbon fiber rod after
>breaking -- or better, during breaking in slow-mo. Maybe with a couple
>of tubes tossed in. Is there anything that breaks in shear before it
>breaks in tension or compression?)
Laminated rubber sheets, glued together with a brittle adhesive. d8-)
Soft, green wood can be similar. If it gets softer as you bend it back
and forth, without actually breaking the stick, it's because you're
delaminating the wood fibers. It's a composite, and you're breaking or
shearing one component of the composite free of the fibers -- the
bonding adhesive -- before the fibers let go.
--
Ed Huntress
>
>So if your goal is to understand that "things that break in bending
>really break in tension, compression or shear" and in your lack of
>wariness you chose a pencil as your example instead of a glass or
>polycarbonate rod, then what I said is true.
At least, I was trying.
>
>If your goal is to understand that "you shouldn't use a wooden example
>object when you ask strength of materials questions" -- well, I guess EA
>chose the right question.
but isn't.
>
>(Now I want to see a glass rod, a wood rod and a carbon fiber rod after
>breaking -- or better, during breaking in slow-mo. Maybe with a couple
>of tubes tossed in. Is there anything that breaks in shear before it
>breaks in tension or compression?)
Soft, green wood can be similar. If it gets softer as you bend it back
and forth, without actually breaking the stick, it's because you're
delaminating the wood fibers. It's a composite, and you're breaking or
shearing one component of the composite free of the fibers -- the
bonding adhesive -- before the fibers let go.
--
Ed Huntress
Tim Wescott
May 22, 2012, 12:49:34 PM5/22/12
to
On Tue, 22 May 2012 12:45:48 -0400, Ed Huntress wrote:
> On Tue, 22 May 2012 11:30:44 -0500, Tim Wescott <t...@seemywebsite.com>
> wrote:
>
>>On Tue, 22 May 2012 12:18:28 -0400, Ed Huntress wrote:
>>
>>> On Tue, 22 May 2012 09:56:57 -0500, Tim Wescott
>>> <t...@seemywebsite.please> wrote:
>>>
>>>>On Tue, 22 May 2012 01:12:32 -0400, Existential Angst wrote:
>>>>
>>If your goal is to understand that "you shouldn't use a wooden example
>>object when you ask strength of materials questions" -- well, I guess EA
>>chose the right question.
>
> ?? It sounds to me like he chose a complicated one that sounds simple,
> but isn't.
Yes, that's what I meant. And I was crossing my fingers when I suggested
> On Tue, 22 May 2012 11:30:44 -0500, Tim Wescott <t...@seemywebsite.com>
> wrote:
>
>>On Tue, 22 May 2012 12:18:28 -0400, Ed Huntress wrote:
>>
>>> On Tue, 22 May 2012 09:56:57 -0500, Tim Wescott
>>> <t...@seemywebsite.please> wrote:
>>>
>>>>On Tue, 22 May 2012 01:12:32 -0400, Existential Angst wrote:
>>>>
>>If your goal is to understand that "you shouldn't use a wooden example
>>object when you ask strength of materials questions" -- well, I guess EA
>>chose the right question.
>
> ?? It sounds to me like he chose a complicated one that sounds simple,
> but isn't.
a glass rod as an example, hoping that there's no unexpected fracture
modes that would make a materials expert say "well, yes, but...". (Other
than nicks and scratches -- I'll overlook those _on purpose_).
Ed Huntress
May 22, 2012, 12:52:45 PM5/22/12
to
On Tue, 22 May 2012 11:49:34 -0500, Tim Wescott <t...@seemywebsite.com>
other discontinuity. Without them, the theoretical tensile strength of
glass approaches 400,000 psi. In fact, S-glass fiber achieves around
300,000 psi. Not bad for glass, eh? d8-)
I thought we'd better not go there.
--
Ed Huntress
wrote:
>On Tue, 22 May 2012 12:45:48 -0400, Ed Huntress wrote:
>
>> On Tue, 22 May 2012 11:30:44 -0500, Tim Wescott <t...@seemywebsite.com>
>> wrote:
>>
>>>On Tue, 22 May 2012 12:18:28 -0400, Ed Huntress wrote:
>>>
>>>> On Tue, 22 May 2012 09:56:57 -0500, Tim Wescott
>>>> <t...@seemywebsite.please> wrote:
>>>>
>>>>>On Tue, 22 May 2012 01:12:32 -0400, Existential Angst wrote:
>>>>>
>
>>>If your goal is to understand that "you shouldn't use a wooden example
>>>object when you ask strength of materials questions" -- well, I guess EA
>>>chose the right question.
>>
>> ?? It sounds to me like he chose a complicated one that sounds simple,
>> but isn't.
>
>Yes, that's what I meant. And I was crossing my fingers when I suggested
>a glass rod as an example, hoping that there's no unexpected fracture
>modes that would make a materials expert say "well, yes, but...". (Other
>than nicks and scratches -- I'll overlook those _on purpose_).
Glass ALWAYS breaks as the result of nicks and scratchs -- or any
>On Tue, 22 May 2012 12:45:48 -0400, Ed Huntress wrote:
>
>> On Tue, 22 May 2012 11:30:44 -0500, Tim Wescott <t...@seemywebsite.com>
>> wrote:
>>
>>>On Tue, 22 May 2012 12:18:28 -0400, Ed Huntress wrote:
>>>
>>>> On Tue, 22 May 2012 09:56:57 -0500, Tim Wescott
>>>> <t...@seemywebsite.please> wrote:
>>>>
>>>>>On Tue, 22 May 2012 01:12:32 -0400, Existential Angst wrote:
>>>>>
>
>>>If your goal is to understand that "you shouldn't use a wooden example
>>>object when you ask strength of materials questions" -- well, I guess EA
>>>chose the right question.
>>
>> ?? It sounds to me like he chose a complicated one that sounds simple,
>> but isn't.
>
>Yes, that's what I meant. And I was crossing my fingers when I suggested
>a glass rod as an example, hoping that there's no unexpected fracture
>modes that would make a materials expert say "well, yes, but...". (Other
>than nicks and scratches -- I'll overlook those _on purpose_).
other discontinuity. Without them, the theoretical tensile strength of
glass approaches 400,000 psi. In fact, S-glass fiber achieves around
300,000 psi. Not bad for glass, eh? d8-)
I thought we'd better not go there.
--
Ed Huntress
Artemus
May 22, 2012, 4:36:11 PM5/22/12
to
"Existential Angst" <fit...@optonline.net> wrote in message
news:4fbb2b73$0$14812$607e...@cv.net...
>
> Still, isn't this ultimately that the material unsuccessfully resisted an applied
> TORQUE??
>
I think torque would be when the wood is twisted along the axis
> TORQUE??
>
of the fibers.
Art
Artemus
May 22, 2012, 4:38:19 PM5/22/12
to
"Tim Wescott" <t...@seemywebsite.com> wrote in message
news:-7idnT_1xKupIibS...@web-ster.com...
>
> (Now I want to see a glass rod, a wood rod and a carbon fiber rod after
> breaking -- or better, during breaking in slow-mo. Maybe with a couple
> of tubes tossed in. Is there anything that breaks in shear before it
> breaks in tension or compression?)
>
direction of the fibers.
Art
Jim Wilkins
May 22, 2012, 4:58:55 PM5/22/12
to
"Artemus" <bo...@invalid.org> wrote in message
news:jpgtg1$t1i$2...@dont-email.me...
>> ...Is there anything that breaks in shear before it
>> breaks in tension or compression?)
>>
> Wood will break in shear when the force is applied along the
> direction of the fibers.
> Art
Green tree branches often deform that way in storms.
>>
> Wood will break in shear when the force is applied along the
> direction of the fibers.
> Art
jsw
Existential Angst
May 22, 2012, 8:25:11 PM5/22/12
to
"Tim Wescott" <t...@seemywebsite.please> wrote in message
news:3b-dnUpeO_CkNCbS...@web-ster.com...
Also, altho I chose a pencil, really it was just a shorthand for ANY thin
material. A pencil seemed good, because you can break it with 4 fingers,
yet absolutely cannot pull it apart.... I tried today.
I agree that the micromechanisms of failure would be different for different
materials, BUT in my li'l torque analysis, the door-hinge torque analogy
would seem to apply to ALL (thin) materials being more bendable than they
are extensible or compressible.
Ultimately all thin materials will be subject to the compression/tension
thing at opposite faces, just that how they ultimately succumb to that
failure will differ. Or so's Moi thinks...
The compressibility thing can also be misleading, as well, as long thin
members appear "compressible", when in fact that is just an instability that
leads to *bending* failure.
To see this, put a 12 ft long pc of 1/8 x 1 aluminum on a 2x4 with a cupla
screws, and that alum will NOT be doing any compressing any time soon --
above and beyond the shear resistance of the screws. In fact, you can
eliminate screw strength interference by just "banding" the alum in place.
--
EA
news:3b-dnUpeO_CkNCbS...@web-ster.com...
> On Tue, 22 May 2012 01:12:32 -0400, Existential Angst wrote:
>
>> Awl --
>>
>> As is proly intuitively clear to most, you break a pencil by applying a
>> bending moment at the center, and <snap>. But it would be REALLY
>> difficult to PULL a pencil apart, or crush it.
>>
>> Iow, the forces req'd to snap a pencil are small, the forces req'd to
>> pull it apart would be huge.
>>
>> I was wondering what the explanation for this is. And I think it may be
>> as simple as this:
>>
>> Ito of INTERNAL stresses in the pencil, when you simply pull on a
>> pencil, you generate an internal psi in the pencil, which is just total
>> Force divided by the cross sectional area, which if less than the
>> material whatever whatever (section modulus or some shit??, the pencil
>> stays intact.
>>
>> However, when you BEND a pencil, now you are generating torques of r x
>> F, and given the sizable r and F of bending, and the very small r of the
>> pencil for resisting sed Torque, the net F generated internally in the
>> pencil becomes very large, with very high resulting internal psi's,
>> which then exceed the intrinsic strength of the pencil. Think finger in
>> a door jamb... at the HINGE!!!! <ouch>
>
> You've hit the nail on the head.
Others don't seem to think so!!
>
>> Awl --
>>
>> As is proly intuitively clear to most, you break a pencil by applying a
>> bending moment at the center, and <snap>. But it would be REALLY
>> difficult to PULL a pencil apart, or crush it.
>>
>> Iow, the forces req'd to snap a pencil are small, the forces req'd to
>> pull it apart would be huge.
>>
>> I was wondering what the explanation for this is. And I think it may be
>> as simple as this:
>>
>> Ito of INTERNAL stresses in the pencil, when you simply pull on a
>> pencil, you generate an internal psi in the pencil, which is just total
>> Force divided by the cross sectional area, which if less than the
>> material whatever whatever (section modulus or some shit??, the pencil
>> stays intact.
>>
>> However, when you BEND a pencil, now you are generating torques of r x
>> F, and given the sizable r and F of bending, and the very small r of the
>> pencil for resisting sed Torque, the net F generated internally in the
>> pencil becomes very large, with very high resulting internal psi's,
>> which then exceed the intrinsic strength of the pencil. Think finger in
>> a door jamb... at the HINGE!!!! <ouch>
>
> You've hit the nail on the head.
Also, altho I chose a pencil, really it was just a shorthand for ANY thin
material. A pencil seemed good, because you can break it with 4 fingers,
yet absolutely cannot pull it apart.... I tried today.
I agree that the micromechanisms of failure would be different for different
materials, BUT in my li'l torque analysis, the door-hinge torque analogy
would seem to apply to ALL (thin) materials being more bendable than they
are extensible or compressible.
Ultimately all thin materials will be subject to the compression/tension
thing at opposite faces, just that how they ultimately succumb to that
failure will differ. Or so's Moi thinks...
The compressibility thing can also be misleading, as well, as long thin
members appear "compressible", when in fact that is just an instability that
leads to *bending* failure.
To see this, put a 12 ft long pc of 1/8 x 1 aluminum on a 2x4 with a cupla
screws, and that alum will NOT be doing any compressing any time soon --
above and beyond the shear resistance of the screws. In fact, you can
eliminate screw strength interference by just "banding" the alum in place.
--
EA
Richard
May 23, 2012, 1:06:13 AM5/23/12
to
On 5/22/2012 11:30 AM, Tim Wescott wrote:
>
> (good but lengthly discussion snipped)
>
>> Nothing is ever simple when you start picking it apart, even a pencil.
>> d8-)
>
> Well, yes. But from a very-first-order approximation, when the pencil
> fails the failure isn't in "bending" (because that doesn't exist) -- it's
> in compression, followed by tension.
>
> So if your goal is to understand that "things that break in bending
> really break in tension, compression or shear" and in your lack of
> wariness you chose a pencil as your example instead of a glass or
> polycarbonate rod, then what I said is true.
>
> If your goal is to understand that "you shouldn't use a wooden example
> object when you ask strength of materials questions" -- well, I guess EA
> chose the right question.
>
> (Now I want to see a glass rod, a wood rod and a carbon fiber rod after
> breaking -- or better, during breaking in slow-mo. Maybe with a couple
> of tubes tossed in. Is there anything that breaks in shear before it
> breaks in tension or compression?)
>
Sure. But not by bending...
>
> (good but lengthly discussion snipped)
>
>> Nothing is ever simple when you start picking it apart, even a pencil.
>> d8-)
>
> Well, yes. But from a very-first-order approximation, when the pencil
> fails the failure isn't in "bending" (because that doesn't exist) -- it's
> in compression, followed by tension.
>
> So if your goal is to understand that "things that break in bending
> really break in tension, compression or shear" and in your lack of
> wariness you chose a pencil as your example instead of a glass or
> polycarbonate rod, then what I said is true.
>
> If your goal is to understand that "you shouldn't use a wooden example
> object when you ask strength of materials questions" -- well, I guess EA
> chose the right question.
>
> (Now I want to see a glass rod, a wood rod and a carbon fiber rod after
> breaking -- or better, during breaking in slow-mo. Maybe with a couple
> of tubes tossed in. Is there anything that breaks in shear before it
> breaks in tension or compression?)
>
Paper. Paper shear.
Oh, sorry! Bad example - wrong group.
Sheet metal - sheet metal shear.
BECAUSE - the force is applied - in shear - not bending.
Richard
May 23, 2012, 1:10:57 AM5/23/12
to
On 5/22/2012 7:25 PM, Existential Angst wrote:
>
>> You've hit the nail on the head.
>
> Others don't seem to think so!!
>
> Also, altho I chose a pencil, really it was just a shorthand for ANY thin
> material. A pencil seemed good, because you can break it with 4 fingers,
> yet absolutely cannot pull it apart.... I tried today.
>
> I agree that the micromechanisms of failure would be different for different
> materials, BUT in my li'l torque analysis, the door-hinge torque analogy
> would seem to apply to ALL (thin) materials being more bendable than they
> are extensible or compressible.
> Ultimately all thin materials will be subject to the compression/tension
> thing at opposite faces, just that how they ultimately succumb to that
> failure will differ. Or so's Moi thinks...
>
> The compressibility thing can also be misleading, as well, as long thin
> members appear "compressible", when in fact that is just an instability that
> leads to *bending* failure.
> To see this, put a 12 ft long pc of 1/8 x 1 aluminum on a 2x4 with a cupla
> screws, and that alum will NOT be doing any compressing any time soon --
> above and beyond the shear resistance of the screws. In fact, you can
> eliminate screw strength interference by just "banding" the alum in place.
You've come a long ways, pilgrim.
>
>> You've hit the nail on the head.
>
> Others don't seem to think so!!
>
> Also, altho I chose a pencil, really it was just a shorthand for ANY thin
> material. A pencil seemed good, because you can break it with 4 fingers,
> yet absolutely cannot pull it apart.... I tried today.
>
> I agree that the micromechanisms of failure would be different for different
> materials, BUT in my li'l torque analysis, the door-hinge torque analogy
> would seem to apply to ALL (thin) materials being more bendable than they
> are extensible or compressible.
> Ultimately all thin materials will be subject to the compression/tension
> thing at opposite faces, just that how they ultimately succumb to that
> failure will differ. Or so's Moi thinks...
>
> The compressibility thing can also be misleading, as well, as long thin
> members appear "compressible", when in fact that is just an instability that
> leads to *bending* failure.
> To see this, put a 12 ft long pc of 1/8 x 1 aluminum on a 2x4 with a cupla
> screws, and that alum will NOT be doing any compressing any time soon --
> above and beyond the shear resistance of the screws. In fact, you can
> eliminate screw strength interference by just "banding" the alum in place.
Maybe you are ready for my favorite material structures book.
http://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/0306801515
For anyone who has ever wondered why suspension bridges don’t collapse
under eight lanes of traffic, how dams hold back—or give way
under—thousands of gallons of water, or what principles guide the design
of a skyscraper, a nightgown, or a kangaroo, this book will ease your
anxiety and answer your questions. Structures: Or Why Things Don’t Fall
Down is an informal explanation of the basic forces that hold together
the ordinary and essential things of this world—from buildings and
bodies to flying aircraft and eggshells. In a style that combines wit, a
masterful command of his subject, and an encyclopedic range of
reference, J. E. Gordon strips engineering of its technical mathematics
and communicates the theory behind the structures of a wide variety of
materials.Chapters on ”How to Design a Worm” and ”The Advantage of Being
a Beam” offer humorous insights into human and natural creation. For
architects and engineers there are cogent explanations of the concepts
of stress, shear, torsion, fracture, and compression, and chapters on
safety design and the relationship of efficiency to aesthetics. If you
are building a house, a sailboat, or a catapult, here is a handy tool
for understanding the mechanics of joinery, floors, ceilings, hulls,
masts—or flying buttresses. Without jargon or over-simplification,
Structures surveys the nature of materials and gives sophisticated
answers to the most naive questions, opening up the marvels of
technology to anyone interested in the foundations of our everyday lives.
Ed Huntress
May 23, 2012, 8:18:25 AM5/23/12
to
materials in terms of torque. Torque is good for figuring out statics
and dynamics. It is just a complication when you're dealing with
material *strength*.
When you're dealing with the strength of materials, practically
EVERYTHING resolves into tension and/or compression. Even shear is
analyzed in terms of a combination of tension and compression, with
the help of a little calculus to evaluate the distribution of forces.
>Ultimately all thin materials will be subject to the compression/tension
>thing at opposite faces, just that how they ultimately succumb to that
>failure will differ. Or so's Moi thinks...
>
>The compressibility thing can also be misleading, as well, as long thin
>members appear "compressible", when in fact that is just an instability that
>leads to *bending* failure.
instability. Look up "Euler buckling."
Existential Angst
May 23, 2012, 7:00:24 PM5/23/12
to
"Richard" <cave...@earthlink.net> wrote in message
news:nZCdnTwlw-rv7CHS...@earthlink.com...
More reading..... <sigh>
Sounds good tho.... can I get it in Audible Kindle??
--
EA
news:nZCdnTwlw-rv7CHS...@earthlink.com...
Sounds good tho.... can I get it in Audible Kindle??
--
EA
Richard
May 23, 2012, 9:35:17 PM5/23/12
to
On 5/23/2012 6:00 PM, Existential Angst wrote:
>
>> You've come a long ways, pilgrim.
>>
>> Maybe you are ready for my favorite material structures book.
>>
>> http://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/0306801515
>>
>
> More reading.....<sigh>
> Sounds good tho.... can I get it in Audible Kindle??
It has pictures too...
>
>> You've come a long ways, pilgrim.
>>
>> Maybe you are ready for my favorite material structures book.
>>
>> http://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/0306801515
>>
>
> More reading.....<sigh>
> Sounds good tho.... can I get it in Audible Kindle??
Existential Angst
May 24, 2012, 1:02:43 PM5/24/12
to
"Richard" <cave...@earthlink.net> wrote in message
news:4JydnXpGmPb1DSDS...@earthlink.com...
--
EA
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