Question regarding flexibility

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by R.C.Reichert, Feb 5, 2020.

  1. R.C.Reichert

    R.C.Reichert

    976
    Jul 26, 2008
    Is it realistic that a fully hardened and tempered blade (stock removal, 1/8" 1075 or 1084 for example) should be able to flex 90° without snapping, or is that something only to be expected of blades that have been forged, selectively hardened, or given a soft back draw? My fully hardened and tempered 1075 blades have a fine grain, hold a good edge and they are tough as nails, but if put in a vise they are very hard to flex past even maybe °40. If pushed too far, they don't bend, they snap. So is that to be expected?
     
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  2. SBuzek

    SBuzek KnifeMaker Knifemaker / Craftsman / Service Provider

    Dec 7, 2006
    Yes it is. Knives ment to bend 90* are for the ABS test, hard edge and soft spine.
     
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  3. FunkCoaster

    FunkCoaster

    112
    Nov 2, 2010
    My understanding as a wannabe material scientist. Take with a grain of salt.

    If you have a sword (say 48"), the handle can be bent 90 degrees relative to the tip and it can return to true (elastic deformation).
    The stress/force is distributed over the length. As it isn't a sharp 90 degree but a radius, it can 'add up' to a 90 degree bend without too much localized stress.

    With a razor (4" length), there's pretty much no way it will be able to do the same. It will encounter the yield strength and inelastic deformation will occur.

    If we isolate the thickness as another variable, a piece of 4" length .0001" thickness foil can bend 180 degrees and return to true.

    So length and thickness both influence what is possible, since the inside and outside of the bend are encountering opposite tensile forces of compression and tension. Thinner means less tensile force.

    The hardness of the blade influences what happens when the yield strength is encountered. A largely soft or unhardened blade is likely to bend and take a set. A largely hardened blade is more likely to snap. But something needs to give to deal with that force.
     
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  4. Larrin

    Larrin Gold Member Gold Member

    Jan 17, 2004
    A knife 1/8" which is fully hardened and tempered is likely not going to make it 90°. That doesn't change if it's been forged.
     
  5. i4Marc

    i4Marc

    Oct 19, 2011
    From what I have read I believe the ability to bend is more a function of geometry/cross section/taper rather than hardness. Think fillet knife @ 63 HRC.
     
  6. john april

    john april KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Feb 27, 2006
    if its fully hardened, i think you did well with the near 40 degrees !
     
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  7. Rick Marchand

    Rick Marchand Donkey on the Edge Moderator

    Jan 6, 2005
    The ABS flex/bend test is to show you have control of heat(soft spine, hard edge). You can make a knife that will pass the 90deg test and return straight but why risk it if it's not a requirement? IMO, it is not a measure of a knife's quality to flex/bend 90degs.
     
  8. R.C.Reichert

    R.C.Reichert

    976
    Jul 26, 2008
    Well that's a relief. Thanks guys. :)

    I was just thinking to myself..."Well shit, my knives can't do THAT, am I'm doing something wrong?"

    I have this bad habit of second guessing myself and my technique. I need to learn to just trust the process and do what works....follow the recipe.
     
    Last edited: Feb 6, 2020
  9. Nathan the Machinist

    Nathan the Machinist KnifeMaker / Machinist / Evil Genius Moderator Knifemaker / Craftsman / Service Provider

    Feb 13, 2007
    The length to thickness ratio is pretty important too. In my experience some 10 inch choppers that are 3/16 thick (fully hardened HRC 60-61) will bend 90 degrees and spring most of the way back straight but thicker or shorter knives can't quite bend that far.
     
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  10. Larrin

    Larrin Gold Member Gold Member

    Jan 17, 2004
    You can see the equations for calculating surface stress for a simple rectangular cross section here: https://knifesteelnerds.com/2018/03/13/why-doesnt-heat-treating-affect-steel-flex/

    If you solve for deflection instead of force you can see that the stress is divided by the length squared for a given deflection. So length is important. The stress increases linearly with thickness for a given deflection. The knife breaks when the stress has exceeded the ultimate strength of the steel.
     
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