Rehashing "flex"

[Kevin R. Cashen]

Since it is common place for the same basic questions to be asked every 5 to 7 days by folks new to the topics and this board (once again, no problem that is how we learn), I thought it was only fair that I get to bring up and repeat a topic that I feel is very important to discuss again for the benefit of those who may have not heard it before.

As I sit here (all darned day) working on my presentations for an upcoming class, listening to music and enjoying a Montecristo platinum and a good bourbon, I am taking a break as I contemplate a recent conversation dealing with elasticity in metals and thinking that it is worth chatting about again. So I decided to put my side of the conversation here has a refresher thread on a topic that is VERY important to the bladesmith in 2008…

There are two modes of deformation, plastic and elastic. This takes us back to the discussion about planes of atoms slipping half way to the next space and then returning to their original position or slipping all the way to the next slot over. Halfway = flex, all the way to next slot = bend. Anything in the elastic category is governed by Young's modulus which is the same regardless of heat treatment at room temperature. A fully hardened blade will take exactly the same amount of force to deflect it as an identical blade that is fully annealed. The only way to change "stiffness" is to make it thinner or thicker. Of course as soon as the steel takes a set at all you have moved from elastic to plastic deformation and the modulus of elasticity no longer applies as you have reached the yield point. What heat treatment does is moves the yield point up and down within the elastic range (also called proportional range), a softer steel will bend much quicker in the elastic range but then has a much greater plastic range before failure, while a fully hardened piece of steel will resist any deformation and continue to flex much farther but will have much less of a plastic range before failing. It is much more difficult to bend the harder steel but the harder steel will not deform near as much before breaking. But both samples will take the exact same force to flex and return to true. Think of the laws of conservation of energy and matter, the universe will never give us a free lunch, all it will allow us to do is make trade offs.

Trying to measure our heat treatment by applying numbers or testing how much force it take to flex a blade does nothing but expose our lack of understanding of very basic principles that engineers and metallurgist use every day in designing beams in bridges buildings and any other structure that uses steel.

The simplest definition of “strength” in materials is resistance to deformation. That deformation could be plastic or brittle. However resistance to brittle failure is more accurately called toughness. But I like to use the term “strength” in one area- impact. I like to think of my blades as having impact “strength” as opposed to toughness, since toughness can be ever increasing with softness (a dead soft piece of 1018 will have superior toughness but I don’t want a blade made from it ) while some steel can be heat treated to take greater impacts without going to softer extremes.

Common sense according to what our eyes see tells us that heat treat can affect stiffness, after all why do we have the term “spring temper”. However when you look closer, and I don’t mean with microscopes or lab equipment but with the naked eye and pay close attention to how identical hard and soft steel pieces behave we see that what looks so simple and straight forward is not at all real. All of us have been lead to believe something by our culture that is not the case.

For more detailed information you can also see the following thread here:
http://www.bladeforums.com/forums/showthread.php?t=440355&highlight=curve+discussion

 

[Karl B. Andersen]

I do my thinking laying in the bath tub, (since I don't drink or smoke) and had this thought the other day.
On my JS Performance blade, I could only flex the knife in the vise about 20 degrees. Then I had to slide a three foot cheater bar over the handle to get it beyond that and all the way to 90.
Be that what it may, Kevin.
My concern here is that I noticed the other day that it returned from the 90 degrees back to almost the exact same point at which I could no longer bend it by hand.
Is there any relation between the severe resistance and the return to the same point?

 

[Keith_H]

Thanks! You make bending things in a vise so interesting!

 

[Kevin R. Cashen]

...Is there any relation between the severe resistance and the return to the same point?

Cool! Actually there is a direct correlation due to strain or "work" hardening. It is cool when the stuff in books coincides with actual hands on.

 

[Matthew Gregory]

... listening to music and enjoying a Montecristo platinum

SAVE ME ONE!!!!!!!

A fully hardened blade will take exactly the same amount of force to deflect it as an identical blade that is fully annealed. The only way to change "stiffness" is to make it thinner or thicker.

Pardon my use of the font style editor on your words here, Kevin -but these two sentences should be reinforced, my friend.

 

[Matthew Gregory]

.
My concern here is that I noticed the other day that it returned from the 90 degrees back to almost the exact same point at which I could no longer bend it by hand.
Is there any relation between the severe resistance and the return to the same point?

I think I'm being a dolt on this one, Karl, but I'm having trouble following you. Do you mean that the blade came back after the 90 degree bend to the point (the "20 degree mark") where you needed to apply the cheater bar?

 

[Karl B. Andersen]

I think I'm being a dolt on this one, Karl, but I'm having trouble following you. Do you mean that the blade came back after the 90 degree bend to the point (the "20 degree mark") where you needed to apply the cheater bar?

Correct.
It did that immediately after the test.
That magic marker line you see on the top blade is where the vise jaws were.
Both blades performed identically.

 

[Nathan the Machinist]

Just to reiterate again again, in the off chance that anybody didn't follow everything Kevin said (I have a hard time wrapping my head around everything he says). There is no heat treatment that will effect the stiffness of a steel blade. The flex mod is just under 30000 KSI. Period. Nothing you can ever do to change that. A softer blade (or back spring) is not more flexible than a harder blade. They are always the same.

This is also pretty much true of steel alloy. Practically no difference.

I've gotten into more arguments about these two with people because it is counter intuitive. For example, people thinking a 4130 chromo race car frame is stiffer than a mild steel one. Stronger, yes, but it will handle the same because it is the same stiffness (if the same dimensions).

Disclaimer: this is all within the elastic limits of the steel.

 

[Karl B. Andersen]

.......A softer blade (or back spring) is not more flexible than a harder blade. They are always the same.

Disclaimer: this is all within the elastic limits of the steel.

Understood.
That's not what I was illustrating above.
My point was simply that the blade returned from 90 degrees to the same point at which I could no longer flex it by hand.
There's something "special" about that point!

 

[Fred.Rowe]

Originally Posted by Kevin R. Cashen
A fully hardened blade will take exactly the same amount of force to deflect it as an identical blade that is fully annealed. The only way to change "stiffness" is to make it thinner or thicker.

This is a tempered blade? Its not a rite out of the quench, blade is it?

 

[Don Hanson III]

A fully hardened blade will take exactly the same amount of force to deflect it as an identical blade that is fully annealed. The only way to change "stiffness" is to make it thinner or thicker.

We had a good talk about this on Fogg's forum a while back. Some folks had a very hard time accepting it. I learned a lot about this, making fillet knives, years ago.

Good stuff, Kevin.

Karl, your blades did very well :thumbup:

 

[Karl B. Andersen]

Yes, Don, they did. I'm just still looking for that answer to that "point" of return.
Remember when I had to switch to the cheater pipe? The blade returned to that point from 90 degress.
Two years ago I got to watch Tim Zowada do his demo at Ashokan showing the indentical resistance to flex between a hardened and an unhardened blade.
He used a gizmo that registered the poundage of resistance.
They were exactly the same.

 

[Nathan the Machinist]

Understood.
That's not what I was illustrating above.
My point was simply that the blade returned from 90 degrees to the same point at which I could no longer flex it by hand.
There's something "special" about that point!

Karl,

Sometimes a thorough explanation makes it harder to grasp a point. I was trying to reiterate and boil down what Kevin said because it is hard to grasp. I wasn't commenting on anything you'd said.

 

[MeDoctor]

Understood.
That's not what I was illustrating above.
My point was simply that the blade returned from 90 degrees to the same point at which I could no longer flex it by hand.
There's something "special" about that point!

And I thought you were just jerkin' our chains with that first reply!

 

[Sam Salvati]

Interesting, VERY! So flex has almost nothing to do with the steel being hard or soft, it has to do with the geometry?

 

[Kevin R. Cashen]

I just cut this part of the old thread to reintroduce it here:

Does the amount of people who believe something to be true, have any bearing upon whether it is a fact? All it can do is make the reality of the matter very difficult to accept or even be heard. This is the sad case of the belief that soft steel, or edge hardened blades, being stronger, or ever as strong as, a fully hardened blade. And even more tragic is the widespread belief that heat treatment can have any effect at all on how much force it requires to deflect a blade in a vice. That is right, it may turn your world upside down, but it is one of the biggest myths to be perpetrated on knifemaking for centuries, and one of the most surprising since it is so easy to discredit.

Now right from the outset I must point out and heavily stress that the following curve is not based on hard numbers from a single test, it is an approximation that I drew up to demonstrate these concepts, so those who already know this stuff have been made aware of this disclaimer and we can focus on the issues, not the accuracy of Kevin’s drawings!

The Stress/Strain curve:

The very first thing I would like to point out on the chart is the shaded area to the left. This is called the proportional range and corresponds with elasticity. The “proportional” come from the fact that any deformation is directly proportional to the force applied- you stop applying force, the blade stops moving. If you remove the load the blade goes back to the shape it was before the load. This the basis of “flexing” steel, heck it is flexing steel.

I am starting here because one horrible misnomer that entirely burns my bacon is calling a “bend” test a “flex” test. If the blade doesn’t return entirely to its original shape it cannot be called a “flex test” because the proportional limit was exceeded and the thing bent! Confusing “bending” with “flexing” may leave the consumer with the impression that your blades can return to true after 90 degrees, and whether that communication is intentional or not, it is less than honest, and it is for that reason I will not relent on this point.

The curve above is what is generated when steel is subjected to a tensile type of test. When load is applied there will initially be elastic deformation that will be able to entirely reverse itself when the load is removed. This range is where we want to stay in with our knives if we wish them to remain undamaged by loads. You will notice that the right hand side of that range is a straight line; this is because it is constant for the steel regardless of its heat treatment. This is governed by a formula know as Young’s modulus or the Modulus of Elasticity.

All materials have a number that corresponds to this and that number represents the amount of force required to elastically stretch a material a given amount based upon its dimensions. For steel the formula is E= 30 x 10 6 psi. These are huge numbers so for our use we can scale them down and say something more reasonable, like, it would require 30,000 pound per square inch to elastically stretch a piece of steel 1/1000th of an inch. If you have data sheets on your steel look for it, it is often included… really .

The main point that we really need to take from this discussion is that on this number heat treatment will have no effect at all.

So why is this important to flexing and bending knife blades? Because it determines the “stiffness” of the blade in actual flexing, and some very knowledgeable people making knives today have gotten this completely wrong. “Stiffness”, or the amount of force it takes to pull a blade over in a vice, is based upon the cross section and has nothing to do with heat treatment! If you flex a 1/8” thick blade 20 degrees while it is fully hard it will take the exact same foot pounds as a blade that is dead soft. This can be best demonstrated in a classic example used for this subject. Take two identical pieces of the same bar of steel, only one is annealed and the other is hardened, and clamp them both horizontally to a bench. Now hang identical weights from them, and they will deflect exactly the same amount. You can continue to add weights to both and have the same results until the proportional range is exceed and then the soft steel will begin to bend, but the hard steel will continue to flex. Once bending begins the deformation will continue even though no more weight is added, thus it has exceed the proportional limit and reached its yield point. But the hardened piece of steel will continue to take vastly more weight before it gives, it won’t bend much but will instead break.

So one can have a blade that bends under a few foot pounds, but does not break, or they can have a blade that requires a grown man really leaning on a cheater bar to deform but that deformation will be fracture. What we want is up to us, but we cannot say that heat treatment does anything more than move the yield points around, and we cannot call soft steel “strong”.

Measuring the force required to flex a piece of steel while in the proportional range is a waste of time! Any sharp engineer could crunch some numbers and give you the actual force without ever holding the blade as long as he had the dimensional cross section measurements. Which brings up the point that if one wants to change the amount of force to flex, we have established that they can forget about the heat treatment, but they can simply change the thickness versus length and get instant results. It takes little force to take a fillet knife to 90 degrees, and it may even avoid taking a permanent bend. On the other hand a blade that is over ¼ “ thick will require a cheater bar and will be more prone to a permanent set because of it. If you want a decided advantage in the bend test without even knowing how to heat treat, make a blade that is wide and thin.

This is why when person tells me they made a blade that “flexed” to a certain degree and returned to true, and then insist on feeding me a line about their heat treating- I don’t care! It isn’t relevant, and the only thing they have given me information on is how they grind their blades, and how little they really know about basic material science.

 

[mete]

I knew it all the time !! Want a flexible filet knife ? Make it skinny. If you were normal kids you would have taken apart a clock [and couldn't put it back together].Do you remember the coil spring and how tightly coiled it was ? That's because it was thin and they often were made of 1095.

 

[sunshadow]

Tim Zowada's demo of flex being the same in fully hardened steel and fully speroidized steel at ashokan was absolotely mindblowing! It is absolutely the same until the point of failure or plastic deformation, the hardened steel the curve just continued a whole lot further. Sure disproved a whole lot of "just temper it to make it more flexible bunk that I grew up with

-Page

 

[Kevin R. Cashen]

...I've gotten into more arguments about these two with people because it is counter intuitive. For example, people thinking a 4130 chromo race car frame is stiffer than a mild steel one. Stronger, yes, but it will handle the same because it is the same stiffness (if the same dimensions)...

I feel your pain Nathan, I have had that same argument more times than you can count, especially in a field like bladesmithing where everybody has been mislead with endless nonsense about bendable blades. A bent blade is just as worthless as a broken one, and even more so when one considers that it will take around 1/10 the force for one to bend than it will for it to break (I know I have heard them all, believe me "but Kevin a bent blade can at least be bent back and still used" What the *$@! is the point if all it will do is bend again?) How many jokers can broadcast how their heat treating will allow a blade to "flex" to a supernatural ammount and have so many in the knife world fall for it, where any other field that understands basic material properties would laugh at such a profound lack of knowledge.

I will be more than happy to rehash this topic again and again until the facts are understood by more people than have bought the load of crap that has been perpetuated about this. It is easy to misunderstand flex, bend and Young's modulus, but it is almost unavoidable when folks are mislead with so much bad information.

 

[Kevin R. Cashen]

I knew it all the time !! Want a flexible filet knife ? Make it skinny...

Now here is where I am ignorant and need some education since I am a catch and release kind of guy (I don't really like fish and don't need to clean them!). What is the purpose of "flex" in fileting a fish? I understand the desire for a very thin blade, but it isn't like there are magic forces that will cause a compound flex in the blade in order to conform to the little rib cage I have always assumed that flex was just a side effect of having a blade thin enough to do the job and folks somehow started to focus on the flex instead of the proper blade proportions. What am I missing here?