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Metal fatigue

(8 posts)
  • Started 8 years ago by Darkerside
  • Latest reply from neddie
  • This topic is not resolved

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  1. Darkerside
    Member

    Dear material scientists of this parish.

    400km ago I made a bracket to support the front of the rear mudguard on the recumbent. This consisted of a 20cm-ish piece of aluminium 2x16mm strut (from a B&Q rummage box...), with a 70-ish degree bend near one end.

    Today, I noticed that it had snapped at the bend. There's no massive force going through the strut as the guard is supported by two more pairs of stays, but it did have to flex a touch with the bounce of the guard.

    Thoughts? Is aluminium particularly vulnerable to back and forth flexing (would I be better with steel?), or should I get something slightly thicker? Or go back to the inner-tube rubber bungee I used for 000s of happy miles before.

    Ta much.

    Posted 8 years ago #

  2. chdot
    Admin

    go back to the inner-tube rubber bungee I used for 000s of happy miles before

    Posted 8 years ago #

  3. I were right about that saddle
    Member

    One of us will maybe extemporise on the crystal structures involved but I'll just say that thin alloy isn't really strong enough for brackets, especially once it's been bent. Retain bits of stainless steel in your 'oh god I've turned into my dad' coffee tin of odd screws and useful things.

    Posted 8 years ago #

  4. steveo
    Member

    I had a (diy) lamp go like that. I consulted a mechanical engineer (colleague with misapplied education) he suggested a rubber washer at the mounting points for the bolt to mate with cuts out most of the vibration. My bracket was fine after that.

    Posted 8 years ago #

  5. le_soigneur
    Member

    Steel/ferrous metals & titanium- you need to bend them more than that (repeatedly) and they will fail. You can keep bending them less than that as many times as you like and they will not fail.
    Aluminium, copper etc do not have this fatigue threshold, you just have to bend them a small amount & do it more often to achieve failure.

    Posted 8 years ago #

  6. wingpig
    Member

    My old sparebike's front porteur rack was held to the fork crown with an aluminium bracket which snapped at one of its bends. The bends came with it and were not an aftermarket application by me. I noted that a few of the equally-light and non-ferrous brackets in my tub of bits of brackets I've fished out of the Bike Station have wee extra perpendicular buttressy sort of bits on the inside of some of their bends.

    Posted 8 years ago #

  7. Darkerside
    Member

    As adding extra supports is far beyond my humble skills, I instead returned to the B&Q rummage bin and now have a suitable bit of steel. I shall report back...

    Posted 8 years ago #

  8. neddie
    Member

    When a metal is bent backward and forward e.g. under vibration, it gets harder and therefore more brittle - this is known as work-hardening.

    This happens very quickly in certain metals. A copper pipe can really only be bent once - if you try to bend it back it will break. Aluminium is poor in this respect too. Steel is much better - see how many times you have to bend a paper clip back and forth until it breaks.

    The shape of the bracket can also have an effect. Internal corners that are "too square" will increase the stress, whereas more rounded corners will cope better. The case of the DeHavilland Comet fuselage break-ups was attributed to the windows being "too square" and being fixed with rivets instead of glue - both working to increase the stress concentrations. The aircraft was redesigned with rounder windows after several crashes, but by then it was too late...

    https://en.wikipedia.org/wiki/BOAC_Flight_781

    https://en.wikipedia.org/wiki/Stress_concentration

    Posted 8 years ago #

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