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3V flex test pic

Ed, how think is that stock and whats the grind? How long was the bar? That kind of flex and return to true is pretty impressive. Can you compare that behavior to some of the other high-carbon steels you have worked with?

-Cliff
 
Collin, basically (for a first approximation) the thinner the grind the weaker the blade. A deep hollow grind for example will not be near as strong as a convex one.

-Cliff
 
Makes sense Mr. Stamp and I appreciate your answer but I was wondering if the thicker blade would take a set at a lesser degree than a thinner blade. It would seem to me the thinner blade would be more flexible and so set at a greater degree.

thanks and take care
collin
 
From what little I remember from materials class, thickness doesn't matter in yield damage. A thicker blade will just have a larger radius in the deformation. Is that right?

Harv
 
Well, I am glad I asked because that is certainly not the answwer I expected. Thank you both very much for enlightening me.

thanks and take care
collin
 
Somewhere we're using wildly different views of how a bend test is performed. From most points of view thickness is extremely important in the angle to which an item can be bent. For example old .02" thick alarm clock springs could be wound into 1 inch diameter circles while .1" thick springs I've made for a radio telescopes were used in 5 foot diameter applications.

A metal exceeds its elastic limit (takes a set) when part of the metal exceeds a certain PSI of stress. This happens when it is locally stretched more than a certain percentage. When you bend something the most highly stretched region is on the outside of your bending curve. The thicker the material the more the outer surface is stressed (given a fixed bending radius).

The angle of bend is meaningless if you don't control your radius of curvature or the length of your sample. For example I could take a piece of window glass that was 10 feet long and bend it a few degrees without breaking. If you work with a 6" piece of glass you would say that it couldn't be bent at all.

I say that if you take 6" samples of any steel and put 1" in a vice and apply pending moment sideways you will find that the blade takes a set or fails at a smaller angle for a thick blade than a thin blade. The thick blade will take much higher pressure to get there, but the angles of the failures will be lower for thicker materials. (All this assumes same alloys and heat treatments).

Most of the time we are not concerned about the angles achieved before blade failure, only force. If I'm prying with a blade with 100 pounds of force on the handle I won't notice whether it deflects 3 degrees or 5 degrees, just whether the blade ends up bent or broken. That is the reason that thicker is almost universally better for prying type forces. If you make a filleting knife you want thin blades to easily flex to follow bones. Even here you aren't as concerned about the angle of deflection before failure, just the ease of getting a small deflection. Here thin is better because it gives you a lower spring constant (fewer pounds required per inch of deflection).

Anyway, you can bend a thin blade at a tighter angle than a thick blade, but you probably don't care. If you want to do apples-to-apples bending comparisons use the same thickness of material and the same lengths.
 
My questions are how far can you bend the blade before it takes a set (doesn't spring back by itself)? And what is the rockwell hardness of the blade?
 
That first question is a good one -- how far will it bend before it fails? That's useful to know if you own the knife so you know when to stop. If you don't own the knife yet and you're considering buying it you really want to know how much force can it take before it fails?

Rockwell hardness does not correspond with edge-holding when comparing different alloys. The Rockwell test measures one kind of hardness but it's not the kind of hardness that determines edge-holding. The Rockwell test is only meaningful for knives when comparing two blades of the same alloy heat-treated different ways.

-Cougar Allen :{)
 
I hope all our nitpicking questions don't give the impression we're not impressed. Speaking for myself, I never thought for a moment this is anything like the Cold Steel video (it can pierce a car door! Yes, it can even pierce a sheet of typewriter paper!!!). I am impressed -- not sure just how much I'm impressed yet, but I am impressed.

-Cougar Allen :{)
 
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