How do you judge toughness?

Carl_First_Timer

Carl_First_Timer

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Dec 6, 2010
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I'm trying to figure a good way to tell how "tough" a knife blank is. By tough I mean how hard it is to break. What ways if any do you all use to test toughness? I was thinking of putting a hardened knife blank in a press with 3 pins, and go until breakage. I would use the amount of pressure to determine the "toughness".

Any insight, as always, is greatly appreciated.
 
Carl, there are actually two ideas of toughness to consider, the gradual loading or bending that knifemakers most often mistakenly think of as toughness, and the actual definition of toughness based on sudden loading recognized by science and industry.

gradually bending a blade in a vice to see how far it will go is actually an exercise in observing various degrees of tensile, compressive and shear strengths of the steel and not toughness. The steel will behave very differently under sudden loading than with gradual pressure. With gradual bending the steel has time to compensate with internal mechanisms of deformation to allow its tensile, compressive and shear strengths to handle the load, but when the load is applied suddenly the steel will behave in a more brittle manner than one would expect because all it has to work with is its impact toughness.

You can use the gradual bend thing and be in the company of many knifemakers but be aware that you will actually be observing all kinds of things that have little to do with toughness, or even the steel or heat treating. For example how far the blade will flex before taking any sort of a set will be mostly affected by how thick or thin it is regardless of steel type or heat treatment.

I like the actual application of impact toughness, not only is it more accurate, it more realistically simulates the forces a large knife would see in proper knife use. Knives make lousy pry bars, and pry bars make even worse knives, but larger blades will often be called upon to chop things, i.e sudden loading on the very thin edge. Chopping harder materials with the finished knife is one of my favorites. Also impacting the edge with a 1/2" brass round hard enough to make nice cuts, straight on is good but if you really want to push the limits at an angle to the edge is really hard on it.

Of course if you really want to compare the actual measured toughness of a given steel find a spec sheet on your alloy that includes data on "impact toughness", and compare it to other steels you are considering. The data sheets I used to get from Crucible on my steels included all of this and much more in the way of very useful and highly accurate information.
 
The only thing I can add is a few of my personal observations and opinions...

I have knives that see regular "abuse" by most folks standards. I keep a fine cutting knife (for knife tasks... lol) and carry a "do-all" beater blade. The latter sees chopping, batoning, prying, digging, scraping, hammering and use with traditional flint and steel. It gets scratched, rolled and rrrrrrrreally dirty. I need it to be tough enough not to chip out at the edge but strong enough not to be subjected to the amount of flex that would push the steel to it's breaking point. I keep a fair amount of spine thickness and fully harden the blade, then temper the entire blade back to the point where it won't chip. I used to draw the spine back even further but have since found that leaving the hardness gave it the strength to resist flexing in the first place. Besides, if I am prying a stump open, I want as much energy transfering into that task as I can... To have the knife flex 45degs and spring back to straight, wont get me those yummy mealworms! Oddly enough, I find that I can get by without fine cutting tasks but would have to do some serious improvising to take the place of my sharpened crowbar.


I guess what I'm saying is aside from impact toughness, decide whether you want the knife to flex or be strong enough to not flex.

Rick
 
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I have some sheet metal shear blades that I made knives out of. I did it in the hardened state, so they still have the factory heat treat. I guess I could just use the heck out of it, and see how it holds up.
 
Carl_First_Timer said:
...I guess I could just use the heck out of it, and see how it holds up.
Click to expand...

That is by far is the best sort of testing, IMO... use it for it's intended purpose (+ a bit, for good measure). You have to have a constant from which to base performance, though. If you have worked these blades from a hardened state, make sure that you didn't mess with the HT somewhere down the line by overheating. Do you have access to more of these blades? Do you know the manufacturer? Once you change the properties, your testing is all over the place.

Rick
 
Rick Marchand said:
That is by far is the best sort of testing, IMO... use it for it's intended purpose (+ a bit, for good measure). You have to have a constant from which to base performance, though. If you have worked these blades from a hardened state, make sure that you didn't mess with the HT somewhere down the line by overheating. Do you have access to more of these blades? Do you know the manufacturer? Once you change the properties, your testing is all over the place.

Rick
Click to expand...

I profiled and beveled it in a Wire EDM. No thermal changes more than a few tenths deep. I was whittling a .5" radius into the side of an aluminum bar last night to see how well the blade would hold up to sideways stress. No break out or chipping.

Thanks guys. I was just making sure I didn't re-invent the wheel if there was some sort of process used for testing out a knife.
 
I think it depends on what type of testing you want to do. Do you want to know if the knife performs well for a given task, or do you want to know the specific properties of the knife?

If you just want to know how well it will perform, just use it, and compare it with a control knife to give you some baseline (as mentioned by Rick).

If you really want to know the properties of the knife however, that is a very different thing. You have to use tests that only test a single attribute of the knife's performance.

As long as you don't confuse the two you should be fine.
 
I test edge toughness using the edge flex on a steel rod. 4 flexes and the blade should not chip in actual use.

Strength is an easy one, clamp on a torque wrench and flex the blade as far as you want to, the torque wrench will evaluate the strength of you knife and rapidly identify any stress raisers that weaken your blade.

Then use the heck out of it, providing it is still in one piece.
Good luck and thanks for wanting to know.
 
One last post...

I figure, if I can't break it under my own power, it'll take just about whatever I can throw at it in the field. Vices, pipes, torque wrenches are useful in destroying a blade to fully test it's attributes (like Barnet and Ed touched upon) and I suggest (especially if you are doing your own heat treat) to do Rockwell/flex/destruction testing. I do use the edge flex test to zone in on my temper but everything else is done in the field.

Rick
 
Another very useful test is the rope slice, while Rockwell is an empirical test, it is ot always predictive of cutting performance.
 
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