Metal tube bending physics

Discussion in 'General Science & Technology' started by dsdsds, Sep 4, 2008.

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  1. dsdsds Valued Senior Member

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    What exactly happens when a metal tube is bent. I'm interested primarly on the x-section on the Inner diameter (ID).

    Does the x-sectional inner area at the bend point decrease? Even in a slight (but plastically deformed) bend?

    If so, does the reduction in area dependant on material (steel, copper, ect..) or just geometry (ID, wall, bend radius) -- for example bending a copper tube and a steel tube (both tubes identical design) to the same curve, which one will have a more restrictive x-section? (should I assume the material with a lower modulus of elasicity?)

    OK, a little more complicated now.. How about a tube made of superelastic NiTi material. Bend it to a point where it is still in the elastic range, then heat it up to set it permanently to that shape. Does it retain its original x-sectional area?

    Where can I get more info on the physics of tube bending?
     
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  3. CheskiChips Banned Banned

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    I don't believe material transcends shape. Perhaps certain metals can condense more and copper might even flat out break. But I think generally the more strong the material, the less space you'll have inside (it will compress less). All of which neglect sheer stress.
     
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  5. Stryder Keeper of "good" ideas. Valued Senior Member

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    Usually the wall closet of bend compresses on itself, it also usually ends up bowed slightly into the pipe causing a decrease in area. The external wall is stretched and obviously weakened through it's reduction in thickness.

    Obviously depending on the angle of the bend and how much pipe was used to generate the bend defines how steep these effects occur. Materials are also a factor, some are obviously more brittle potentially generating faults where as others allow for stretching slightly.

    If you want a bended piece of pipe to maintain it's structure under say high pressures, you are better off dealing with a "Casting" of the bend you want rather than attempt to stretch a pipe since obviously stretched pipe is weakened.

    As for more info on pipe bending, you could look at pipefitting considering their education often includes geometry. (If of course the Chewing bakky foreman isn't preoccupied chewing out their arse)
     
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  7. Oli Heute der Enteteich... Registered Senior Member

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    http://www.google.co.uk/search?hl=en&safe=off&q=pipe bending formula&btnG=Search&meta=
    Might be a good start.

    It's a common "problem" in engineering and is mostly, for all practical purposes, an empirical thing rather than an exact science.
    In other words charts are used which indicate minimum bend radius for a given tube size.
    Another place to look might be something like Machinery's Handbook - I believe there are free downloadable versions floating around the net.
    It's a handy reference (despite being two volumes each about 2 inches thick!) and not only usually gives quick ready-reference data but normally goes quite deeply into the theory of the subject.
     
  8. dsdsds Valued Senior Member

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    Thanks for all your responses.

    I'm involved in a project which consists of designing a system to deliver and control high pressure liquid through a long but small ID metal tube. This tube will be designed with many bends. To start development, I need to answer this question:

    Assuming you have 2 metal tubes of DIFFERENT materials but
    IDENTICAL in length, ID, OD, surface roughness and finish, and bent to the exact same shape,

    Will you have the same pressure loss and flow characteristics in both tubes? (you will if the inside geometry is identical --- will it be?)

    ok -- also assuming adiabatic conditions.
     
  9. buckybeam Registered Member

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    some good info would be fluid dynamics. ive studied it a bit but really lack the knowledge to help you much. but yes in most instances if all aspects are the same then behavior should be the same. with some limitations. fluid interactions with the material and such if you get my drift. keep in mind only if all specs are the same, more than likely that will not be the case. much will depend on the accuracy and precision needed for the tests.
     
  10. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I assume that you want to make the pressure loss as small as posible.

    There is a comercial product, called "cerabend" if memory serves me correctly. It is eutectic complex of low melting temperature metals with melting point less than 100C. Plumbers us it to bend pipes with out them "buckling" -I.e. they fill the pipe solid with cerobend and then bend it as a solid rod. After it is bent as desired gentile heating from one open end first (for safety reasons) will recover most of the cerabend. Any that is still adhering to the ID walls, can be washed out with boiling water.

    I think a copper tube warmed with a hot air gun and initially inclinded slightly in liquid cerabend could be solidly filled by a little gentile "sucking" on the elivated end. (Use a rubber hose to keep warm metal out of your mouth.) The let it cool to solidfy (I do not know if it will shrink away from the ID wall, but suspect it may in spots but only slightly. If that is a problem, you could later re-heat locally with the air gun to move the voids to one end and cut them off - much like the way silicon impurities are removed by a migrating zone of liquid.) Probably these voids will not make any sigificant difference in the bends you can achieve with a now "solid" rod -It certainly will not buckle anywhere to significantly reduce the x-section ID.

    PS if you are interested in the math of bending hollow tubes, I hope either you are good at searching the internet OR can write some fine grid scale (in the region of the bend) computer simulation code. I am 99.99% sure that there is no analytic approach to this problem.
     
    Last edited by a moderator: Sep 4, 2008
  11. buckybeam Registered Member

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    ive used cerrobend. fun stuff. you do get deformity. great stuff for thin walled tubing. sand works well with pvc and other plastics.
     
  12. dsdsds Valued Senior Member

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    1,678
    Cool stuff. I did not know of theses low temp melt alloys. But would not be practical in bending a tube with an ID of .010" and length of 80".
     
  13. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I think that IS possible, but just guessing, if you switch from the hot air gun I suggested to an electric current heating the tube as you "suck" * the cerrobend up; but be careful not to heat too much and breath any vapors - I bet they are about as harmful as it gets to your health. (Cerrobend has a lot of lead and other toxic metals in it.) Also after you have the tube full, I suggest you keep one end still in the liquid cerrobend bath and decrease the current slowly to allow "plastic flow" to keep the tube nearly full when the cerrobend in it solidifies. Certainly it is easy to try and the minium amount of cerrobend you can buy (a pound?) is vastly more than you need.

    Note that during the filling process, most of the electrical heating will be in the still unfilled part of the tube, so with a little experimentation, I think you may even be able to make the voids with contraction to solid migrate out as you fill. - Sort of a natural combination of the "zone heating" I suggested earlier for void removal with the filling process. If your tube were a little bigger, I bet it could be filled with a liquid/solid slurry of cerrobend if you had optimium heating conditions. That would reduce the void volume produced with final solidification; but the electrical anneal I suggested above (slowly decrease current) will do this in your tube, I think. I.e. when the cerrobend is in a slurry state, surely plastic flow will help you keep it full.

    Can you tell us what the project is?

    ------------------
    *I would not be at all surprised if you do not even need to "suck" when the tube is nearly horizontal. You are in a range where capillary forces (assuming cerrobend "wets" the metal of your chosen tube material.) may drive it up the tube. Your tube is enormous on a molecular scale, and if "wet" by liquid cerrobend, I think that is what matters most. --- Don't think like a human - think like a physicist!!!

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    Last edited by a moderator: Sep 5, 2008
  14. rajendra.jain Registered Member

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    chemical formula to made the pipes and tube

    I got chemical formulae information from one of the company name RegentSteel to related pipes and tube.

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    Range :
    15 NB UP TO 1200 NB IN SCH 5S , 10S ,10, 20 , 40S , 40 , STD , 60 , 80S , 80 , XS , 100 , 120 , 140 , 160 & XXS
    Form : SEAMLESS / ERW / WELDED IN ROUND & SQUARE .
    Stainless Steel :
    ASTM / ASME SA 312 GR. TP 304 , 304L , 304H, 309S ,309H , 310S, 310H , 316 , 316TI , 316H , 316 LN , 317 , 317L , 321 , 321H , 347 , 347 H , 904L .

    ASTM / ASME SA 358 CL 1 & CL 3 GR . 304 , 304L , 304H, 309S ,309H , 310S, 310H , 316 , 316H , 321 , 321H , 347 .
    Duplex Steel : ASTM / ASME SA 790 UNS NO S 31803 , S 32205 , S 32550 , S 32750 , S 32760 .
    Nickel Alloy :
    ASTM / ASME SB 163 UNS 2200 ( NICKEL 200 ), ASTM / ASME SB 163 UNS 2201 (NICKEL 201 ), ASTM / ASME SB 163 / 165 UNS 4400 (MONEL 400 ), ASTM / ASME SB 464 UNS 8020 ( ALLOY 20 / 20 CB 3 ), ASTM / ASME SB 704/705 UNS 8825 INCONEL (825), ASTM / ASME SB 167 / 517 UNS 6600 (INCONEL 600 ), ASTM / ASME SB 167 UNS 6601 ( INCONEL 601 ), ASTM / ASME SB 704 /705 UNS 6625 (INCONEL 625), ASTM / ASME SB 619/622/626 UNS 10276 ( HASTELLOY C 276 ).
    Copper Alloy :
    ASTM / ASME SB 111 UNS NO. C 10100 , 10200 , 10300 , 10800 , 12000,12200, 70600 , 71500 . ASTM / ASME SB 466 UNS NO. C 70600 ( CU -NI- 90/10) , C 71500 ( CU -NI- 70/30)
    Carbon Steel :
    ASTM / ASME A 53 GR. A & B , ASTM A 106 GR. A , B & C . API 5L GR. B , API 5L X 42 , X 46 , X 52 , X 60 ,X 65 & X 70 . ASTM / ASME A 691 GRA , B & C
    Alloy Steel :
    ASTM / ASME A 335 GRP 1 , P 5 , P 9 , P 11 , P 12 , P 22 , P 23 , P 91

    ASTM / ASME A 691 GR 1 CR , 1 1/4 CR , 2 1/4 CR , 5 CR , 9CR , 91
     
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