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Michael Edelson




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PostPosted: Mon 01 Sep, 2008 8:56 pm    Post subject: Sword failure from bending...or not?         Reply with quote

My group, NYHFA, makes extensive use of training blunts, primarily those by A&A, but also some by Albion, Alchem and other makes. We use these swords hard, anything from full power drilling to free play. They get smashed into each other with full force, stepped on, bent in thrusts against pells, etc. None of the heavily used ones are perfectly straight (anymore). One particular A&A fechterspiel was a bit on the soft side and has bent back and forth a few times. This one gets used particularly hard...it's a wonderful training tool, perfect balance, weight, etc.

I've always heard that flex testing a sword causes microfractures, and that when a sword takes a set and you straighten it, the steel is irreperably damaged, etc. When someone bends a western sword in cutting, it is often assumed that the sword is no good, and that it must be replaced, lest it fail at any minute, killing everyone in a 10 mile radius. Happy

Well, my question is....where's this damage? These swords, particularly the most heavily abused, have been going strong for at least a couple of years now and show absolutely no sign of failure. I can't even remember the number of times a couple of these were straightened. If it was true that bending a sword causes significant damage to the structure, then these swords should have fallen a part by now.

Can anyone shed some light on this for me?

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Jean Thibodeau




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PostPosted: Mon 01 Sep, 2008 9:40 pm    Post subject:         Reply with quote

Michael;

I don't know if you saw this Topic I started a while back about sword testing and a reply I posted from Mark at OlliN where he goes into some detail about how he tests his swords before they go out the door to be sure that if they fail it will be still in their hands rather than fail in the customers hands: The Topic didn't generate much input and I am surprised as I thought it would bring in some debate and information from some makers or metallurgists. Surprised Question

Here is the link as I think it is relevant to your question and I don't mind if the discussion continues on your Topic rather than mine. Wink Cool
http://www.myArmoury.com/talk/viewtopic.php?t...ight=ollin

Maybe if a sword takes a slight set it might be less damaging to avoid bending it back as it is the repeated bending and unbending that causes the damage if overdone ?

The risk of leaving it bent may be that it will tend to bend even more if overstressed again ?

By the way I find your Topic very interesting and encouraging because those of us who don't use our swords much or very lightly may seriously underestimate what they can take before becoming unfunctional: Cosmetic damage does preoccupy the collector's mindset and what might seem " ruined " by a collector is just normal minor wear to a user, maybe ?

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Michael Edelson




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PostPosted: Tue 02 Sep, 2008 2:43 am    Post subject:         Reply with quote

Hi Jean,

After reading your topic I wanted to clarify my use of the word "bend". By bend I mean set. I use the word flex to describe deflection off true that returns to true on its own. These blunt swords have taken a set several times and been straightened without any noticeable weakness.

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Michael Edelson




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PostPosted: Tue 02 Sep, 2008 3:39 am    Post subject:         Reply with quote

Something else from your topic. You said:

Quote:
This effectively means a destroyed sword that ceases to be useful. ( Except that a bent sword can be straitened but becomes much more prone to bend at the same point or break the next time it is stressed ).


I don't know how true that is. I push swords hard, and I've had two types of bent sharps (I don't pay attention to the blunts enough to notice these things):

1. Stubborn swords....these swords take a set and cannot be straightened over the knee...they need a vice and/or heat. These swords stubbornly take the same exact set if the circumstances of the first set are repeated (and maybe a tad less stress, but in my experience not much less)....almost like they found their true calling in that set and want to return there at all costs.

2. Softer swords....these swords will take a set and be easily straightened to dead true over the knee (unless they are poorly heat treated, then they can never be made perfectly straight) and from what I can tell are either not prone to the same set or only slightly more prone than they were to begin with.

Naturally, I much prefer the softer swords, but only those with good heat treatments (read expensive swords) that return to dead true.

I'd really like to hear from some experts about this and especially my blunts and why there seems to be no real damage from bending (setting) taking place that I can see. How many sets can a sword take before it becomes seriously weakened?

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Jean Thibodeau




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PostPosted: Tue 02 Sep, 2008 5:00 am    Post subject:         Reply with quote

Michael Edelson wrote:
Hi Jean,

After reading your topic I wanted to clarify my use of the word "bend". By bend I mean set. I use the word flex to describe deflection off true that returns to true on its own. These blunt swords have taken a set several times and been straightened without any noticeable weakness.


Yes I understood it as being a set and if very small and subtle it may be better to leave it alone as in your example of stubborn swords like you said are maybe defaulting to where they want to be ?

As to bending, taking a set and then being straitened again it probably is only a problem if repeated hundreds of time, maybe?

I'm mostly guessing and my opinion about this has been influenced by your first post that seems to indicate that good swords can take a lot more abuse than I thought !

Just as an example: Take a common paperclip and straiten it out. Now bend it one way and then bend it the other and repeat. It seems to take extreme bending over dozens if not hundred of cycles before the paperclip finally gives up the ghost.

A well heat treated sword should bend and not take a set unless bent to an extreme degree but once it takes a set it will stubbornly want to stay that way. A softer sword will take a set more easily, will be easier to straiten and will resist a larger number of cycles of setting and straitening before eventually failing. Oh, and any bending that doesn't cause a set probably causes little or no cumulative damage ? ( Again guessing: Took machine shop courses decades ago as well as strength of material courses that probably covered some of this, but it was to long ago for me to be sure about the details but I think I am right in basic principle ).

At least that is what I believe but we really need input from industry professionals to confirm this guessing.

Quantifying the degree of deformation and the number of cycles of deformation is something that is related: A few degrees of set does much less permanent damage to the metal than extreme bending. Back to the paperclip example: If you only bend it a little it might take thousands or millions of cycles for it to fail/break. If you bend it severely one way and back the other a very small number of cycles will cause failure.

Airplanes eventually must be retired from service as the constant vibration will cause metal fatigue but some very old airplanes subject to low stress can last for decades and still be safe while high performance jet fighters become dangerous to fly much earlier.

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Craig Johnson
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PostPosted: Tue 02 Sep, 2008 6:44 am    Post subject: Stress and the sword         Reply with quote

Hi Guys,

Interesting discussion. This is an area of sword mechanics that has seen many declarations of what is good and bad over the years and something that is probably closer to a balance of issues than any other area of sword attributes if you are after the truth of the matter. The ability of a sword to take deformation and return to its original shape has many contributing factors. The quality and style of heat treatment, attributes of the particular steel, design of the blade, stress and type of deformation of use can all contribute to what will be the functional attributes of the finished sword.

It is important to realize that a training blade and a sharp will react differently by design. The reason period training swords are the way they are is they found these designs to be the best for longevity balanced with the right "feel" to replicate use of sharps in a fight. The set of a blade can be from induced stress from use or a "memory" in the material to take a certain shape when loads are applied towards that shape of memory.

The ability of a training sword to take some abuse and return to shape, even if encouragement is needed, allows the tool to use its shape and design attributes to increase durability. This is also why one would not expect a rebated sharp to last as long as a training sword designed along the principles of the originals. If you look at the type of swords used in blunted tournament play you will also see they are more than just rebated sharps.

The material chosen to make the pieces today is an important factor. The period practice pieces where not as hard as the swords we make today in most cases. The materials we are able to chose for the blades allow us to achieve a hardness that most modern customers are looking for but at the same time give the ductile qualities that where present in the period blades. The result is we can achieve a durable piece at a harder heat treatment level than they would probably have been able to consistently produce in say the 15th or 16th C.

Best
Craig
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Thom R.




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PostPosted: Tue 02 Sep, 2008 9:46 am    Post subject:         Reply with quote

I run a materials testing lab that does a few million dollars a year and one of the things we do is test steel rods (for rock bolts) for bending, work hardening and fatigue (which will lead to failure). I don't want to belabor this though because I am not in the mood for another internet shouting match this week

In a sense, its a case of good and bad. You're right in the sense that just because the blade takes a set, it doesn't mean that it is on the edge of failing and is now substantially more dangerous than before. ..... however, repeated bending and re-setting will cause work hardening in the zone, which will eventually lead to grain breakage, a loss of ductility, fatigue, and eventual failure in tension. Just how many times this can occur before the fatigue in the metal becomes a concern is the main issue and for a properly heat treated sword that might be many 100s of times (I don't know not having tested swords). But eventually yes, repeated sets taken in the same zone will cause the steel to fail. But that might take years and years, it all depends on the steel, the heat treat, the blade cross section, the amount of bending. just my $0.02 tr

ps my comments are specifically geared to the heat treated poriton of the blade taking a set, not the tang.
pps be careful with the "no sign of damage". the damage is not going to be visually obvious and steel even when properly heat treated, is still relatively brittle and sword blades are relatively thin so the potential for a "omg!" moment is real
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Craig Johnson
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PostPosted: Tue 02 Sep, 2008 10:34 am    Post subject: Fatigue         Reply with quote

Thank you for your comments Thom, I would not want it to get to a shouting match either Happy I am interested in your direct experience and knowledge in this area. I find it is always best to go to the science and check out the basics in discussions of this sort as it almost always covers the points in question very well. More often than not this leads to understandings of the originals that are more nuanced and insightful of the good design and material choices they were making for the pieces being used than we give them credit for. I think it is vital to use the material sciences when looking at these topics. The knowledge of how steel and iron work and opperate and our understanding of swords and how they where used need to be intergrated more to give us good understanding of what these items where like. To often in the past the scientist and the historian have not communicated very well and it has lead to many misunderstandings and assumptions.

If it lies in your field of knowledge would you mind commenting on the degree of the set that is taken being a factor to fatigue forming? By this I mean the sharpness of the angle that occurs in the bend. I have heard that a gradual bow is less stressful than a sharp bend like your elbow bending, but I do not know the degree to which they would be different. I am assuming this would be generally the same for most long steel objects not specificly swords alone.

Best
Craig
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Thom R.




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PostPosted: Tue 02 Sep, 2008 11:15 am    Post subject: Re: Fatigue         Reply with quote

Well generally the sharper and bigger the bend the worse it is, although its not a linear function. i am not sure I can answer your question more specifically in terms of angles of bending, say via a chart, what typically we do is work off an ASTM standard. I am sure there have been experiments and there is data out there in the technical literature though. what will cause a lot of fatigue fast is bending in opposite directions, that is, to one side then the other and repeat. although its a bit more complex, essentially with bending, part of the beam goes into tension and the other into compression, if you alternate that back and forth you get work hardening quite quickly. The salient point is that in engineering we call it "work hardening" because with repeated bending you will lose ductility and the steel will become more brittle eventually. with properly heat treated swords though - with a 25 degree set, you might find you can do this many 100s of times before the fatigue in the metal becomes critical. I just don't know. During Victorian times swords were regularly tested in bending to make sure they did not take a set, however I don't know of any test data where a sword has been repeatedly bent to examine fatigue.....

the other comment I have with respect to fatigue and bending is that the cross section is also critical, that is a 20 degree set for one type of sword might not be a big deal whereas for another type of sword it is far more serious as to get that degree of bend requires far more internal structural damage. e.g. a thin type X vs a diamond section type XVIII.....

tr
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Jean Thibodeau




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PostPosted: Tue 02 Sep, 2008 3:42 pm    Post subject: Re: Fatigue         Reply with quote

Thom R. wrote:


the other comment I have with respect to fatigue and bending is that the cross section is also critical, that is a 20 degree set for one type of sword might not be a big deal whereas for another type of sword it is far more serious as to get that degree of bend requires far more internal structural damage. e.g. a thin type X vs a diamond section type XVIII.....

tr


Thank for your reply as it does seem to confirm what I wrote or at least that what I wrote is in the ballpark of being true. Wink Cool

Oh, with a thick sword bending is much more difficult and takes more force to bend than a very thin sword but if one gives both a 30 bend ( Not a set, just a bend ) the surfaces of the thick sword with be under much greater tension and compression just because of the geometry. At an extreme a paper thin sword properly heat treated is going to resist bending very much less but the grain structure will be very much less stressed being stretched or compressed much less for any specific degree of bend.

A mathematically zero thick blade should show zero resistance to bending and zero stress in the material as none of it would be in tension or compression. ( Thom: I'm assuming that you know this, also assuming that I am correct, and I am just adding to your comments. Big Grin Cool ).

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Michael Edelson




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PostPosted: Wed 03 Sep, 2008 1:20 pm    Post subject:         Reply with quote

Thank you for your input, everyone, a lot of really good information here.

Crair or Thom....do either of you have any theories to explain why some swords take a set and stay there while others want to return to true so much it takes minimal pressure over your knee to straighten them? I've had one I hung on a wall and it straightened overnight to almost true.

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PostPosted: Wed 03 Sep, 2008 3:36 pm    Post subject: Memory         Reply with quote

Hi Michael

The material will be in a state of tension from heat treatment. The perfect blade would have these tensions completely equal from side to side and back and front. Not necesssarily equal tension every where but that it is all balanced. Perfection is never achieved. There fore most every blade will have some latent tendencies in its internal tension. It may have a desire to saber or warp or, my favorite, twist. When a blade decides to move from stress or heat it will have a tendency to follow what it would like to do. The stronger this tendency the more work to get it back. The less work the more even the tension (the heat treat being even and consistent) a softer blade will be more durable but easier to move around.

Hope that explains it clearly. The science of this can get complicated with molecular bonds, different states of carbon formation and such.

Best
Craig
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Jared Smith




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PostPosted: Wed 03 Sep, 2008 4:52 pm    Post subject:         Reply with quote

I was wondering if the consummate craftsman-bladesmith might even sometimes have his creation fall victim to imperfect grain structure within bar stock (cases of milled sword blanks)?

In aerospace machining of alloys we sometimes encounter a moderately thick block with very peculiar grain structure deep within. This does not become evident until large amounts of milling or turning (such as hollowing out a bowl shape) are performed. Then an observant machinist often recognizes it with the naked eye, and points it out to the engineer or production manager. This really takes a full anneal temperature (hotter than the non-magnetic initial austenitizing-quench noze temperature by several hundred degrees in many cases), plus some extended time to correct. Even after this, it may still exhibit problems and warp in heat treat. In retrospect, it has often turned out more cost effective to discard such a milled piece, rather than try to anneal it and realign the grain structure. I have been through the experience roughly 4 times, and ended up scrapping the piece after it exhibited persistent problems in holding tolerances through heat treat, post heat treat final machining-polishing, or simple passage of time afterward.

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Thom R.




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PostPosted: Wed 03 Sep, 2008 5:25 pm    Post subject:         Reply with quote

I agree with Craig and Jared, it really depends on the alloy and the heat treatment and subsequently the internal structure of the steel........ the great thing about steel is the broad range of engineering properties which can be created via very small changes in chemistry and heat treatment. this is due to the complex way that the internal structures develop relative to the carbon/iron bonding. i am not a big fan of wikipedia in general, but the first page of the wiki page on "Steel" is pretty good and outlines the basics of the phase transitions between ferrite, austenite, martensite. however, i don't mean to belabor this point either, but cross sectional geometry of the sword also has a lot to do with the "springiness" that you are talking about. for another example, you have the bundled plates that make the leaf springs on a car - same concept
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Michael Edelson




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PostPosted: Wed 03 Sep, 2008 5:38 pm    Post subject:         Reply with quote

Thank you guys, I really appreciate your input. It is because of people like you that this forum is the tremendous resource that it is.
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Thom R.




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PostPosted: Wed 03 Sep, 2008 5:48 pm    Post subject:         Reply with quote

here is one thing that has always interested me. having looked at lots of antiques from the 17th -early 19th c., one more often sees swords where the tip has been broken off than say, other places on the blade. obviously this would be primarily related to less force being required to create a radical bend (say greater than 30 degrees) because of the thin cross section but I also wonder if it has to do with the steel structure being slightly different "out there" on the tip. tr
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Michael Edelson




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PostPosted: Wed 03 Sep, 2008 7:01 pm    Post subject:         Reply with quote

Well, from what I know of the practical use of swords, I'd say that could be for several reasons. The way swords are designed (taper, etc) the area near the tip typically has the greatest flexibility and the thinnest cross section. Another factor could be that the tip gets lodged in stuff and is subject to stress when that stuff collapses to the floor, torquing the blade.
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Craig Johnson
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PostPosted: Wed 03 Sep, 2008 7:58 pm    Post subject: Sword material         Reply with quote

Hi Jared

Not only imperfect grain structure but major lamination and structural issues. Several years back now we switched to 6150 from 1075. The main reason was a whole series of problems with the material splitting in heat treat. Splitting in the plane of the material. I had the pieces peel away from each other like bananas. It was not a happy time to be around me Happy

Over the years I have seen pockets and voids, strange colored areas, small seams and lots of specs and little pips in the middle of a piece of material. That being said this is far less than one sees evidence of in the sword blades from period.

Thom I could not agree with you more on the cross sectional geometry. I would say it is a major element in how they approached the subject of sword blade construction and the attributes they were trying to achieve in stiffness of blade and action in combat.

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Craig
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Tim Harris
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PostPosted: Wed 03 Sep, 2008 8:18 pm    Post subject:         Reply with quote

Michael,

I've encountered similar issues, dealt with in these threads:

http://www.myArmoury.com/talk/viewtopic.php?t...highlight=
http://www.myArmoury.com/talk/viewtopic.php?t...highlight=

There may be something of interest there.
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