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Why do spokes bend differently when heated?

  1. Oct 14, 2006 #1
    Cheap (or very old) bicycle spokes are made of carbon steel. Most modern high-quality spokes are made of stainless steel.

    When bent at roughly 90 degrees and heated with a propane torch, a carbon spoke bends further, curling up slightly, but a stainless steel spoke unbends, uncurling slightly.

    This bend _| becomes _\ with carbon steel.

    This bend _| becomes _/ with stainless steel.

    The idea was to heat spoke elbows and use the change in angle to show the presence or absence of residual stresses after various processes that are supposed to stress relieve the spoke elbows, but since the carbon steel and stainless steel spokes bend in opposite directions when heated, something else must be going on.

    Here are two sample bent spokes clamped in a vise with their free ends pointing upward || against a convenient background:

    http://server5.theimagehosting.com/image.php?img=256acarbonstainlessbent.jpg
    or http://tinyurl.com/y8lodt

    And here they are after heating, splaying apart \ / to show their different behavior:

    http://server5.theimagehosting.com/image.php?img=257acarbonstainlessheated.jpg
    or http://tinyurl.com/tnrhw

    Carbon is on the left, stainless on the right.

    I'm guessing that one material, probably carbon, is changing internally to a different configuration that overwhelms the stress relief effect seen in the other material.

    There's a current thread on rec.bicycles.tech, but so far no one has any suggestions:

    http://groups.google.com/group/rec....22e33c12f19/3d8392f040513563#3d8392f040513563

    I doubt that the slightly different diameters (2 mm carbon versus 1.8 mm stainless steel) or the possible plating on the carbon or anything except the different materials will explain the reaction to heating.

    More spokes did the same thing when bent and laid flat on boards to eliminate any effect from gravity or the grip of the vise. In these pictures, the carbon spoke is the lower spoke and has a nipple on it:

    http://server5.theimagehosting.com/image.php?img=260acarbonlower.jpg
    or http://tinyurl.com/ygl3nw

    The spokes bend in opposite directions when heated:

    http://server5.theimagehosting.com/image.php?img=261acarbonlower.jpg
    or http://tinyurl.com/ykd3vs

    Any suggestions would be appreciated.

    Cheers,

    Carl Fogel
     
  2. jcsd
  3. Oct 15, 2006 #2

    Astronuc

    User Avatar

    Staff: Mentor

    The higher carbon, changes the elastic-plastic behavior of steel, and it will effect the phase transformations. Similarly additions of Ni and Cr, will affect the phase behavior and elastic-plastic behavior.

    If the carbon steel unbends, then it has residual compressive stress on the inner side of the bend and residual tensile on the outside. The stainless steel should flow more readily than the carbon steel, so I imagine there is less residual stress in the stainless steel than the carbon steel.
     
  4. Oct 15, 2006 #3
    It's the stainless spoke that unbends (straightens). The carbon spoke bends even further.

    It isn't a question of which spoke bends more when heated, but rather why the two spokes bend in opposite directions.

    Bent at the same 90 degree angle _|, the carbon and the stainless spokes bend in different directions when heated: _\ versus _/

    Thanks for the idea that the stainless spoke may have less residual stress than the carbon spoke. Spoke failure is a topic often beaten to death on rec.bicycles.tech, with lots of theory, but little data--yet I don't recall anyone ever mentioning this point. Anecdotal claims are often made for considerable improvements in spoke durability between 1980 and 1990, when carbon spokes were pretty much replaced by stainless steel.
     
  5. Oct 15, 2006 #4
    A bit more information would be helpful. For example, what type of stainless? Austenitic? What temperature/color did the steel reach? However, you are most likely dealing with a transformational effect in the carbon steel.
     
  6. Oct 15, 2006 #5
    Virtually all stainless steel bicycle spokes are supposed to be 304 or 18/8, according to the manufacturers.

    I ended up heating spokes to a cheery orange glow, but that was just being thorough--the bending motion is noticeable almost as soon as the flame from a propane torch hits the surface and long before the metal begins to turn red.

    A blue propane flame is usually expected to be around 1300C, 2500F.

    As far as I can see, the carbon spoke starts to curl up further (_\) and the stainless steel spoke starts to uncurl (_/) right away.

    A transformational effect seems likely, but it's odd that the effect appears to start almost immediately. And when such a simple experiment gives such a strange result, I can't help wondering if I'm missing something that should be embarrassingly obvious.
     
  7. Oct 16, 2006 #6

    Astronuc

    User Avatar

    Staff: Mentor

    My guess would be some kind of phase transition in the carbon steel (depends on which phases present (bainite, pearlite, austenite), which could be different than the tranformation in austenitic 304 (possibly martensitic), in addition to stress relief. The unbending outward is stress relief, with the compressive stress pushing it outward, while the tensile stress pulls it outward. What is the carbon content of the carbon steel, or what type of carbon steel?

    The other factor is the diameters - 1.8 mm vs 2.0 mm (10%) - which could result in a different residual stress level/distribution for the same bending. If the different steels are bent somewhat differently, because of geometry or differences in Elastic Modulus, Yield Strength, UTS, that also causes a difference in behavior during annealing (in addition to phase transformation).

    Also, it's possible that the carbon steel is processed differently than austenitic steel, i.e. differences in thermo-mechanical processing.


    IIRC, carbon steels have lower fatigue threshold, and even susceptible to low amplitude, high frequency fatigue. The other matter is stress corrosion cracking, especially in areas where salt is applied (assuming one rides in winter in these areas, or lives near the coast).

    304 is also susceptible to chloride attack, with 316 less so, and 6% Mo steels even lesser.
     
  8. Dec 1, 2006 #7
    Cautionary note.

    I remember the introduction of stainless steel spokes. Prior to this time most spokes were plated with cadmium. This has excellent corrosion resistence. Cadmium plating is a gray color and looks alot like galvanizing but smoother. Unfortunately it is very toxic and would likely evaporate when the spoke is heated. As such be very wary of any fumes from your tests.

    Re the bending anomaly. It would be helpful to know the manufacturing methods used for both spoke types. The carbon may have been swaged and bent hot and the stainless cold formed. This could account for the difference.
     
  9. Dec 9, 2006 #8
    Stainless steal according to one source I have read, actually exhibits the shape memory effect, although I have not seen carbon steal listed. If you are curious, look up SME or shape memory effect on google, or just ask a question.

    -scott
     
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