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.
