On the topic of bending/force/geometry

samuraistuart

Well-Known Member
I am posting a quote from Kevin Cashen made on KnifeNetwork way back in 2011, was hoping Kevin or anyone can help to clarify. I am confused on the whole bending/force/geometry/HT thing. Terminology is extremely important especially in this discussion. I hope I don't bugger things up.

"How much force it takes to flex a blade is of course nothing more than a matter of its thickness or cross section regardless of heat treatment, but how much force it will take to bend it depends on the method of heat treatment. Where the bend happens, at 75 foot pounds or 500 foot pounds depends on how you heat treat."

OK, I've always heard that flex has nothing to do with the alloy or the HT, it is solely a function of geometry. What EXACTLY does that mean? As I take your quote, you say "how much force it takes to FLEX a blade is geometry", but then "how much force it takes to BEND depends on HT." Is there a difference between "bend" and "flex"? One is a function of geometry? but the other is a function of HT? I don't know the difference.

I think I understand, hoping I am saying this right, that HOW the failure occurs (tearing or clean snap as example) depends on HT. That if both blades were equal geometry, the one that was harder (more strength) would snap as opposed to tear, but both failures would occur at the same angle? but would occur at different ft/lbs? Thanks for any help, gentlemen.
 
I am posting a quote from Kevin Cashen made on KnifeNetwork way back in 2011, was hoping Kevin or anyone can help to clarify. I am confused on the whole bending/force/geometry/HT thing. Terminology is extremely important especially in this discussion. I hope I don't bugger things up.

"How much force it takes to flex a blade is of course nothing more than a matter of its thickness or cross section regardless of heat treatment, but how much force it will take to bend it depends on the method of heat treatment. Where the bend happens, at 75 foot pounds or 500 foot pounds depends on how you heat treat."

OK, I've always heard that flex has nothing to do with the alloy or the HT, it is solely a function of geometry. What EXACTLY does that mean? As I take your quote, you say "how much force it takes to FLEX a blade is geometry", but then "how much force it takes to BEND depends on HT." Is there a difference between "bend" and "flex"? One is a function of geometry? but the other is a function of HT? I don't know the difference.

I think I understand, hoping I am saying this right, that HOW the failure occurs (tearing or clean snap as example) depends on HT. That if both blades were equal geometry, the one that was harder (more strength) would snap as opposed to tear, but both failures would occur at the same angle? but would occur at different ft/lbs? Thanks for any help, gentlemen.

I must be honest that one of my pet peeves is when makers use the words “bend” and “flex” completely interchangeably when spinning a hyperbolic yarn. At its most innocent it merely adds to confusion on this topic, but I have not always been sure that the terms were swapped out innocently, especially when the issue is pressed and claims tend stand or fall based on the precision of the terminology and there is a strong resistance to being more clear with the description.

Flexing a piece of steel indicates that the deformation is elastic in nature and that when the load is removed the steel will return to its original shape before the load was applied.
Bending a piece of steel indicates that the deformation is plastic in nature and that then steel is permanently effected and will not return to its original shape when the load is removed.
The reason why it is so important to distinguish between these two concepts is because they correlate directly to specific, and separate, areas of the stress-strain curve that determine where, when and how heat treatment will affect the outcome of loading that piece of steel.

On the lower left range of a stress strain curve is what is called the “proportional range” or the elastic range of the steel and, as opposed to the rest of the curve, it is not curved but is instead a straight line climbing on angle as the load is applied. It is called the proportional range because all deformation (strain) is directly proportional to the load applied, i.e. stop adding load and deformation stops as well. This range involves the modulus or elasticity (also known as Young’s modulus) which is a function of cross section and not of heat treatment.

One can also view this area as how “stiff” the steel is. How much load it takes to deflect the steel a certain number of degrees and have it return to true when the load is removed is simply a result of how thick the steel is. What heat treatment does is increase the range of the elasticity of the steel before it bends, so a hardened and tempered piece of steel will “flex” much farther before it takes a permanent set and is “bent”. But so long as that point where “flex” turns to “bend” is not reached it is still just about the cross section of the steel and not heat treatment.

That point where this goes from “flex” to “bend” is the yield point and beyond it is a range of plastic type deformation that is no longer proportional, i.e. the steel will continue to deform even if no more load is added.
To help visualize this- picture a bar of steel held from one end, with the flats horizontal in a vise. You hang a 5 lb. weight on the end of it and watch it “flex” several degrees downward, it only flexes so far and if you want it to flex more you need to add more pounds. But when you remove the weight the steel goes entirely back to straight and true. This is the proportional range that is governed by Young’s modulus, this is “flexing” the steel, and here heat treatment is irrelevant, only the cross sections matters in determining how much weight you need to flex the steel a given number of degrees.

As you pile on more and more weight, the steel will eventually approach its yield point and when you reach that point the steel will begin to bend and you will be able to watch the steel continue to deform with the same amount of weight as it permanently bends towards the floor.

Think of the yield point as a slider on the scale that starts with the steel being pristine and ends with the steel being in two pieces (the ultimate strength, or failure point), anything under the yield point is elastic and anything above it is plastic. With heat treatment we can move the yield point up and down the scale giving us either more elasticity in a blade before it snaps or more plasticity before it snaps.

If we try to remember that we are making knives then none of this matters all that much, but if we get into the business of using knives for pry-bars then things get all muddled up because a lot of folks don’t even take the key factor (Young’s modulus) into account. Instead, people who think they may need to pry with a knife will make it softer hoping it will bend rather than break, but how many pry-bars have you ever owned that would easily bend? A bendable pry-bar is rather useless. Instead what a good pry-bar does is rely on Young’s modulus to be stiff and neither bend nor break, regardless of its heat treatment, because it is a thick and stout piece of steel.
 
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Thank you so much, Kevin. Makes a LOT more sense to me now. That "flex" is different than "bend" is key to understanding this topic.
 
To expand on the second part of your question, failure would not necessarily be at the same angle. Take a bar made of unhardened A36 steel 1" wide and 1/4" thick and compare to a hardened bar of say 1095 or something. The A36 bar will "fail" by bending sooner (lower load and angle) than the hardened bar will fail by bending a little then breaking. Some steels have a pretty sharp yield point and you can feel it when it starts to bend. In the 1095 bar, you won't really be able to feel it, until just before it breaks, if then. However, once the A36 bar yields, it will keep bending with very small increases in load. The 1095 bar will still be in the elastic range and every degree of flex requires more force.

To try this out, take a 12" piece of you favorite steel, drill a hole in one end, then, still annealed, lock it in a vice and hook a fishing scale to it and pull. Repeat with a hardened piece and watch the difference in the scale.
 
Right.....I don't make pry bars, either. I really could care less about how my knives bend or don't bend. I want them to cut, and cut long. However, this concept/question creeps up now and again, and lots of folks simply refuse to accept the fact that HT has nothing to do with flex, that it is a factor of geometry. I just wanted to make sure I had my ducks in a row when discussing this with others.
 
Even when comparing blades that are the same in cross section, it's critical to remember just how sensitive lateral flexing is to small changes. Doubling the thickness gives an increase in stiffness of 8x. That doesn't happen often. However, going from 1/8" to 9/64" is still an increase in lateral stiffness of nearly 1.5x.
 
Man, this topic takes me back to my college days. Kevin, I wish you had been my strength of materials professor. I would have understood things much easier than I did with the middle eastern professor that I had teaching the course.

As a mechanical engineer, I have to worry about all of the above when designing mechanical systems. As a knife maker, I hardly even consider stress/strain, elastic/plastic deformation, or flexing/bending. I really only worry about edge geometry (profile design and grind angles) and edge retention (heat treatment process) and that's about as far as I worry about the mechanical properties of the blade steel. Indeed as Kevin said we are making knives which cut and not pry bars.
 
Man, this topic takes me back to my college days. Kevin, I wish you had been my strength of materials professor. I would have understood things much easier than I did with the middle eastern professor that I had teaching the course.

As a mechanical engineer, I have to worry about all of the above when designing mechanical systems. As a knife maker, I hardly even consider stress/strain, elastic/plastic deformation, or flexing/bending. I really only worry about edge geometry (profile design and grind angles) and edge retention (heat treatment process) and that's about as far as I worry about the mechanical properties of the blade steel. Indeed as Kevin said we are making knives which cut and not pry bars.

Thank you Brandant. The concepts really can be difficult to wrap your head around, there is something ingrained in us that says softer steel flexes easier, and often you need to actually hang some weights on a piece of steel for it to then click. But as a mechanical engineer you would know that Young’s modulus is used every day to design all kinds of structures and systems, it just doesn’t come into play much when making a cutting implement.

My real concern is when the pry-bar approach overcomes the knifemaking approach and I see people doing things to soften blades at the expense of edge holding in order to have it not break when prying. Once again, the irony is that a proper pry-bar does not bend, so I am not sure what is being made anymore. It is also worth noting that good springs don’t bend either. I often point out that if you want to stiffen your trucks springs you don’t re-heat treat them, you slip more leafs in the stack, and if you overload a truck spring it does indeed break instead of bend.

. knife \’nif\ 1a: A cutting instrument consisting of a sharp blade fastened to a handle (Webster’s Universal Dictionary)

Sometimes the obvious does indeed need to be revisited to refocus on what we are doing, I often begin some of my lectures with the excerpt above so that I have the basic foundation to work from without so much of the clutter of modern marketing. Since the first words of the very definition of what we are making is “A cutting instrument…” anything that interferes with, or threatens to supersede, the defining purpose is problematic and we have to re-examine what we are making.

I feel it is my responsibility as a knifemaker to educate the public about knife function and proper use rather than pander to abusive ideas that can make it harder for us to offer a quality product to the public without compromising the tool in order to cater to abuse.
 
Ok, I just looked on Mr. Cashen’s website, does he have books? I feel I need to read whole books on this stuff
 
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