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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
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Breaking a pencil: tension v. compression v. bending moment
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 ****??, 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 |
#2
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Breaking a pencil: tension v. compression v. bending moment
On 5/22/2012 12:12 AM, Existential Angst wrote:
Awl -- As is proly intuitively clear to most, you break a pencil by applying a bending moment at the center, andsnap. 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 ****??, 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??? I don't mind breaking a few pencils. But I'm not sticking my finger in the door jamb. |
#3
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Breaking a pencil: tension v. compression v. bending moment
"Existential Angst" wrote in message ... 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 ****??, 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??? -- http://www.teachengineering.org/view...i_lesson_1.xml 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. |
#4
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Breaking a pencil: tension v. compression v. bending moment
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 ****??, 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??? Ah, not so good. Almost all of the action is on the top and bottom of 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 |
#5
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Breaking a pencil: tension v. compression v. bending moment
"Ed Huntress" wrote in message
... 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 ****??, 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??? Ah, not so good. Almost all of the action is on the top and bottom of 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! 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 ****.... WTF????? LOL -- EA -- Ed Huntress |
#6
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Breaking a pencil: tension v. compression v. bending moment
On Tue, 22 May 2012 02:00:05 -0400, "Existential Angst"
wrote: "Ed Huntress" wrote in message .. . 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 ****??, 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??? Ah, not so good. Almost all of the action is on the top and bottom of 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! Still, isn't this ultimately that the material unsuccessfully resisted an applied TORQUE?? 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 ****.... 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 |
#7
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Breaking a pencil: tension v. compression v. bending moment
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! |
#8
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Breaking a pencil: tension v. compression v. bending moment
"Existential Angst" wrote in message ... ... I don't know whazzup, but whenever I ax a Q, peeple keep axing me to READ ****.... WTF????? LOL -- EA 'Cause we can't draw pictures here. Good engineering explanations tend to be graphical, like the distribution of stress across a beam. jsw |
#9
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Breaking a pencil: tension v. compression v. bending moment
Existential Angst wrote: Still, isn't this ultimately that the material unsuccessfully resisted an applied TORQUE?? Yes, you are correct, if you are saying that leverage is the main reason a pencil is easy to break. A half inch long pencil will not be easy to break with your fingers. |
#10
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Breaking a pencil: tension v. compression v. bending moment
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 ****??, 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. 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 |
#11
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Breaking a pencil: tension v. compression v. bending moment
On Tue, 22 May 2012 02:09:02 -0400, Ed Huntress wrote:
On Tue, 22 May 2012 02:00:05 -0400, "Existential Angst" wrote: "Ed Huntress" wrote in message . .. 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 ****??, 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??? Ah, not so good. Almost all of the action is on the top and bottom of 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! Still, isn't this ultimately that the material unsuccessfully resisted an applied TORQUE?? 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 ****.... 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. The "Woodwright's Eclectic Workshop" has some asides on woodworking theory that I have found very useful -- but a whole book _about_ wood would be better. I should probably get that one. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com |
#12
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Breaking a pencil: tension v. compression v. bending moment
On Tue, 22 May 2012 09:59:51 -0500, Tim Wescott
wrote: On Tue, 22 May 2012 02:09:02 -0400, Ed Huntress wrote: On Tue, 22 May 2012 02:00:05 -0400, "Existential Angst" wrote: "Ed Huntress" wrote in message ... 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 ****??, 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??? Ah, not so good. Almost all of the action is on the top and bottom of 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! Still, isn't this ultimately that the material unsuccessfully resisted an applied TORQUE?? 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 ****.... 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. The "Woodwright's Eclectic Workshop" has some asides on woodworking theory that I have found very useful -- but a whole book _about_ wood would be better. I should probably get that one. If you work with wood, especially if you do anything even slightly 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/fp...chapter_10.pdf It's very good. Google "FPL Adhesives," without quotes, and you'll see papers on wood-to-metal, etc. -- Ed Huntress |
#13
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Breaking a pencil: tension v. compression v. bending moment
On Tue, 22 May 2012 09:56:57 -0500, Tim Wescott
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 ****??, 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 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 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. |
#14
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Breaking a pencil: tension v. compression v. bending moment
On Tue, 22 May 2012 12:18:28 -0400, Ed Huntress wrote:
On Tue, 22 May 2012 09:56:57 -0500, Tim Wescott 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 ****??, 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 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. (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?) -- 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 |
#15
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Breaking a pencil: tension v. compression v. bending moment
On Tue, 22 May 2012 11:30:44 -0500, Tim Wescott
wrote: On Tue, 22 May 2012 12:18:28 -0400, Ed Huntress wrote: On Tue, 22 May 2012 09:56:57 -0500, Tim Wescott 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 ****??, 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 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. (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. 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 |
#16
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Breaking a pencil: tension v. compression v. bending moment
On Tue, 22 May 2012 12:45:48 -0400, Ed Huntress wrote:
On Tue, 22 May 2012 11:30:44 -0500, Tim Wescott wrote: On Tue, 22 May 2012 12:18:28 -0400, Ed Huntress wrote: On Tue, 22 May 2012 09:56:57 -0500, Tim Wescott 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_). -- 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 |
#17
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Breaking a pencil: tension v. compression v. bending moment
On Tue, 22 May 2012 11:49:34 -0500, Tim Wescott
wrote: On Tue, 22 May 2012 12:45:48 -0400, Ed Huntress wrote: On Tue, 22 May 2012 11:30:44 -0500, Tim Wescott wrote: On Tue, 22 May 2012 12:18:28 -0400, Ed Huntress wrote: On Tue, 22 May 2012 09:56:57 -0500, Tim Wescott 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 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 |
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Breaking a pencil: tension v. compression v. bending moment
"Existential Angst" wrote in message ... 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 of the fibers. Art |
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Breaking a pencil: tension v. compression v. bending moment
"Tim Wescott" wrote in message ... (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?) Wood will break in shear when the force is applied along the direction of the fibers. Art |
#20
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Breaking a pencil: tension v. compression v. bending moment
"Artemus" wrote in message ... "Tim Wescott" wrote in message ...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. jsw |
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Breaking a pencil: tension v. compression v. bending moment
"Tim Wescott" wrote in message
... 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 ****??, 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!! 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 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 |
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Breaking a pencil: tension v. compression v. bending moment
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... Paper. Paper shear. Oh, sorry! Bad example - wrong group. Sheet metal - sheet metal shear. BECAUSE - the force is applied - in shear - not bending. |
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Breaking a pencil: tension v. compression v. bending moment
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. Maybe you are ready for my favorite material structures book. http://www.amazon.com/Structures-Thi.../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. |
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Breaking a pencil: tension v. compression v. bending moment
On Tue, 22 May 2012 20:25:11 -0400, "Existential Angst"
wrote: "Tim Wescott" wrote in message ... 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 ****??, 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!! 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. FWIW, I think you're going to confuse yourself thinking of strength of 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. That is taken up in the calculations of Euler bucking and other instability. Look up "Euler buckling." 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. |
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Breaking a pencil: tension v. compression v. bending moment
"Richard" wrote in message
... 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. Maybe you are ready for my favorite material structures book. http://www.amazon.com/Structures-Thi.../dp/0306801515 More reading..... sigh Sounds good tho.... can I get it in Audible Kindle?? -- EA 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. |
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Breaking a pencil: tension v. compression v. bending moment
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-Thi.../dp/0306801515 More reading.....sigh Sounds good tho.... can I get it in Audible Kindle?? It has pictures too... |
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Breaking a pencil: tension v. compression v. bending moment
"Richard" wrote in message
m... 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-Thi.../dp/0306801515 More reading.....sigh Sounds good tho.... can I get it in Audible Kindle?? It has pictures too... Thank gawd! -- EA |
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