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Jeroen Zuiderwijk
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PostPosted: Thu 21 Sep, 2006 5:08 am    Post subject: Fatigue in swords?         Reply with quote

In the Mythbusters thread I saw fatigue mentioned as a potential cause of failure in swords, and I've seen it being mentioned several times before. Now metal fatigue is not my speciality, but as an engineer in the aerospace industry, I do have some basic knowledge regarding the subject. Metal fatique is a type of failure where the material starts to form cracks under repeated cycles of stress on the material, which grow until the material strength is reduced so far that it will break. The amount of cycles the material will be able to handle depend on the level of the stress. The closer the stress to the ultimate stress (breaking point) of the material, the fewer the cycles. However, I wonder if anyone has ever observed cracks appearing in their swords due to fatigue? These cracks can be anywhere from microscopic hairline cracks, to cracks running through the metal large enough to be visible by eye. The microscopic cracks do weaken the material locally, but won't result in fatigue failure until they've grown large enough to severely weaken the material. They can however form the weakest spot where the blade will fail for example due to impact in relatively hard and brittle metal. I'd personally suspect that what's been thought of as fatigue is actually an entirely different type of failure, as nobody would work with swords that are covered in clearly visible cracks.
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Kjell Magnusson

Location: Sweden
Joined: 10 Jun 2004

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PostPosted: Thu 21 Sep, 2006 6:00 am    Post subject:         Reply with quote

Hm, well, as per the other thread here I just posted in, trying to remember my textbooks here... I guess it might be more of a fatigue-weakening of the blade, combining microfractures and work hardening (embrittlement) to produce a weak spot which then fails not as much through fatigue as through sudden chock instead? So, not pure fatigue failure, but rather a combination cause of fatigue and sudden impact. Also worth noticing would be that I guess much of the "fatigue" might come not as much from proper use of the blade, but from bad cuts (such as slamming it into another sword in an attempt to destroy either, or both, blades). This could then be made worse by normal use (crack growth due to the normal stress-cycles), to the point where even a normal blow will break the blade (another bad blow being able to snap it much earlier of course). If this is to be considered a fatigue failure or not then, well, I'll leave that one I think.
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Shane Allee
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Location: South Bend, IN
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PostPosted: Thu 21 Sep, 2006 6:01 am    Post subject:         Reply with quote

Not to get too far off topic here.... *G*

Metal fatigue is one of the problems I always had with certain Roman's comments about Celtic swords. Most of us know the old myth about the Celts having to straighten their swords with their feet after every couple swings. We know now from enough samples that have been studied to know that their swords were not like that. It really seems unlikely that you could do that much bending without it quickly leading to the sword breaking. So if that was truly the case that particular Roman would have been writing about how their swords were breaking rather than just bending. Plus we would find a heck of a lot more broken swords.

To answer you question though... The only times I have seen metal fatigue on swords, be it cracking or just loosing the ability to spring back from repeated bending has been during destructive testing. Sometimes you could see visible cracking prior to them breaking, other times you couldn't. I don't really know if most swords would have taken the kind of abuse needed to make metal fatigue a problem in historical use. Maybe if some knucklehead is trying to use it as a shovel or something, but normal use. Problems with the metal and/or the heat treatment I would imagine would have been the source of many more problems.

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Torsten F.H. Wilke

Location: Irvine Spectrum, CA
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PostPosted: Thu 21 Sep, 2006 11:25 am    Post subject:         Reply with quote

One point that should be brought up here is the homogenuity of the blade material. Local areas of incorrectly achieved metallurgy would also have an important effect on crack propagation, toughness, and yield-point / ultimate strength, all resulting in possible blade failure at that locale. How well could historical bladesmiths achieve an equivalent of a similar homogeneous modern alloy? Were their techniques susceptible to metallurgical mess-up, or were the results fairly fool-proof? Something as simple as a slight impurity can really mess things up for those in the modern aerospace industry, where the needs exacted on materials are closer to their design limits, though, than were necessary on a medievil battlefield. How pure could historical methods get the working material, and how pure would the forging processes keep the material? I guess the purity of local ore would even come into play. Some impurities are even very beneficial. I wonder if some weapons making centres gained notoriety, good or bad, just for this reason?

ps; A culprit as simple as hydrogen can ruin a metal, especially iron. It squeezes into the atomic lattice structure, just like sugar dissolving into water, causing it to tighten. Makes it very brittle. They call it hydrogen embrittlement, and it was a serious problem with some of the steel manufacturing processes at the turn of the century. That, along with the wrong alloy being used sunk the Titanic! Hydrogen embrittlement was actually only a small contributing factor. I believe that soaking iron alloy for extended periods of time in an acid, such as Vinegar, results in hydrogen embrittlement, so, word to the wise when aging your mail, etc.! (acids are wonderful contributors of rampant H ions) But, I think that needs to be verified, it's been a long time since I dealt with that.
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Doug Gardner

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PostPosted: Thu 21 Sep, 2006 7:07 pm    Post subject: Re: Fatigue in swords?         Reply with quote

Jeroen Zuiderwijk wrote:
In the Mythbusters thread I saw fatigue mentioned as a potential cause of failure in swords, and I've seen it being mentioned several times before.

Hi Jeroen,

I can't speak for the other folks who mentioned metal fatigue, but I really did mean metal fatigue. Basically what they did was to keep hitting the blades, causing extreme flexing, until they broke. I think the potential exists that what they saw at failure was partly the result of the accumulated damage from the previous flexing. This isn't true of all of the blades, of course. The rapier pretty clearly was just doubled back on itself, and the whipping motion simply snapped it.

One thing that wasn't mentioned was that before they built their special blade swinging machine, they had clamped one of the swords in a vice, and had two of their guys beating on it for all they were worth, until the two guys just couldn't do it any more. Who knows what kind of fatigue was introduced in those two blades from that exercise? They never did x-ray the blades to look at the damage.

One of my criticisms of their method was that it didn't control for accumulated damage. of course, they could come back at me and say that combat swords weren't just used for a single hit, so accumulated damage is historically plausible. However, they weren't trying to bust the myth that swords broke. They were trying to find out if a sword could cut through another sword, and they concluded that it couldn't. Frankly, I think cutting a hardened sword is unlikely, so I guess I agree with that part of their conclusion.

Doug Gardner
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