Toughness?

[searcher]

Well to put this in more tangiable terms, toughness is the abilty of a material to absorb energy. In order to understand what that means we need to introduce two terms: yield strength and tensile strength. If you take a steel bar and bend it to a certain degree, it will snap back into its original position. The FORCE up to which this occurs is the yield strength. Meaning you can have two materials that behave very differently but have the same yield strength. One deforms very little because it is stronger, while the other one deforms (bends) by a large amount but still snaps back into its original position. This is called elastic deformation. If you stress it further than this point, the bar stays deformed. It has undergone plastic deformation. (For completeness sake, let's just mention that each of these stesses can be split up into shear, compression, tension and torsional stresses and strains).

Toughness is the ability to absorb energy, usually under a rapid impact. That means essentially that it is the measure of a material to yield (to undergo elastic deformation). But there is also a timing aspect. If the impact is so rapid that the material has no time to yield, it will shatter. So for high toughness a material must have both great yield strength and the ability to yield (rapid deformation in the elastic limit).

Ductility is the amount a bar can stretch during plastic deformation, before it breaks (tears). For large toughness, one usually prefers the material NOT to undergo plastic deformation, so usually ductility is not something that factors into toughness, but rather the ability to yield (which is essentially the "ductility", the amount a bar can stretch, during elastic deformation).

It is usually measure with one or several charpy impact tests.

Hope this helps. Please also keep in mind that this explanation is somewhat simplified.

Toughness ~ The ability of a material to absorb energy in the plastic range (associated with Tensile Strength). An indication of the amount of work (ie. energy applied) per unit volume which can be done on the material without causing it to rupture.

Resilience ~ The ability of a material to absorb energy when deformed elastically and to return it when unloaded (associated with Yield Strength).

Ref.: Mechanical Metallurgy 2nd ed. george dieter pp. 335-336

 

[searcher]

Well to put this in more tangiable terms, toughness is the abilty of a material to absorb energy. In order to understand what that means we need to introduce two terms: yield strength and tensile strength. If you take a steel bar and bend it to a certain degree, it will snap back into its original position. The FORCE up to which this occurs is the yield strength. Meaning you can have two materials that behave very differently but have the same yield strength. One deforms very little because it is stronger, while the other one deforms (bends) by a large amount but still snaps back into its original position. This is called elastic deformation. If you stress it further than this point, the bar stays deformed. It has undergone plastic deformation. (For completeness sake, let's just mention that each of these stesses can be split up into shear, compression, tension and torsional stresses and strains).

Toughness is the ability to absorb energy, usually under a rapid impact. That means essentially that it is the measure of a material to yield (to undergo elastic deformation). But there is also a timing aspect. If the impact is so rapid that the material has no time to yield, it will shatter. So for high toughness a material must have both great yield strength and the ability to yield (rapid deformation in the elastic limit).

Ductility is the amount a bar can stretch during plastic deformation, before it breaks (tears). For large toughness, one usually prefers the material NOT to undergo plastic deformation, so usually ductility is not something that factors into toughness, but rather the ability to yield (which is essentially the "ductility", the amount a bar can stretch, during elastic deformation).

It is usually measure with one or several charpy impact tests.

Hope this helps. Please also keep in mind that this explanation is somewhat simplified.

Toughness ~ The ability of a material to absorb energy in the plastic range (associated with Tensile Strength). An indication of the amount of work (ie. energy applied) per unit volume which can be done on the material without causing it to rupture.

Resilience ~ The ability of a material to absorb energy when deformed elastically and to return it when unloaded (associated with Yield Strength).

Ref.: Mechanical Metallurgy 2nd ed. George Dieter pp. 335-336

P.S. All steels deform about the same for the same stress (given the same shaped part). It doesn't matter if it is dead soft or rock hard steel. This is a standard trick question on engineering exams. The soft steel will permanently deform (reach yield point) at a lower stress, but up until then they will both deform the same amount for a given stress.

 

[Cliff Stamp]

Toughness ~ The ability of a material to absorb energy in the plastic range (associated with Tensile Strength). An indication of the amount of work (ie. energy applied) per unit volume which can be done on the material without causing it to rupture. .

There are several definations of toughness and several materials tests used to measure them, the above defination you quoted for example would have M2 at 65 HRC being much tougher than S7 at 57 HRC. In general it is more useful to seperate strength and toughness based on ability to handle loads of different speeds as HoB noted, this is how it is treated in a lot of materials works and tested by tensile/torsional deformation (strength) as opposed to charpy/izod/torsional impact (toughness). For large knives used for chopping you are definately not looking at toughness simply by the tensile strength, that would lead to a lot of broken blades, you need resistance to impact mainly and decent ductility in extremes.

-Cliff

 

[HoB]

Actually, toughness is the ability to absorb energy in both the elastic and plastic range combined, but that is a bit besides the point.

Just as Cliff said, toughness is a term that is frequently used in the knife-world to describe a particular property. Usually, when looking for a tough knife you would like one that you can pound on without it being harmed in any way. If you baton a knife hard and the blade doesn't fracture but the spine mushrooms like a penny on railroad tracks, you wouldn't really call that a tough knife either. However, if I am suddenly going to use the term "resilience" for property that we are all looking for, it is quite likely that few on this forum will know, what I am talking about. I am a scientist but I have learned that it can be beneficial scarificing accuracy in terminology if it promotes understanding.

Also, as an experimentalist I would also have to wonder from what little I know about the c-notch charpy test for example (a test for toughness), whether it doesn't really measures toughness mainly in the elastic region, but I may very well be wrong there.

Finally, the ductiliy for most bladesteels hardened to the common hardness, is usually not very high (meaning, it will soon brake after deflection exceeds the elastic range), especially not for the better stainless steels. Meaning, the toughness value is domiated in this case probably so much by resilience, that it is probably acceptable to set them equal but I am happy to be convinced otherwise, if I am wrong.

 

[ghost squire]

I don't think the charpy tests very accurately measure much of anything for our purposes here in knifeland.

 

[HoB]

Well, the c-notch charpy test measures a material property. The challange I think is more to relate that property to one that we in knife-land are interested in.

 

[Cliff Stamp]

The problem with charpy/izod tests is that they are heavily grain biased and they often don't for example show regions of embrittlement that do show up on torsional tests and are regions which were known from actual tool use (500F embrittlement in low alloy steel). Alvin was the first person I saw address this in regards to knives, Crucible notes the difference in parallel/perpendicular charpy behavior on some steels, but doesn't give enough information, you need the temper spread, ideally for multiple austenization points.

The problem with definations of toughness is that they vary from one book to the next, I have seen them defined for example as the area under the stress/strain graph, or ductility, or impact, all of these are reasonable definations based on toughness meaning "hard to break". It seems reasonable to me to seperate strength and toughness based on speed of impact as this makes a huge difference in what you want in a knife.

In regards to what they mean, often times you see impact toughness = chip resistance, which is just as bad as wear resistance = edge retention. It depends on what you are cutting, steels can chip for many reasons, edges can chip out on cutting cardboard for example and there is little to no impact, it is more an issue of strength/ductility and of course wear.

-Cliff

 

[me2]

I'm just happy someone has found a use for Dieter's Mechanical Metallurgy book. Searcher, you didnt happen to plow through that with Dr. Murty at NCSU in Raleigh did you?

 

[searcher]

I'm just happy someone has found a use for Dieter's Mechanical Metallurgy book. Searcher, you didnt happen to plow through that with Dr. Murty at NCSU in Raleigh did you?

No, Technion: Israel Institute of Technology.

 

[me2]

Wow, outclassed again. Do you find it useful at all? I didnt find it very useful for my job.

 

[HoB]

Sorry, that I am warming this up once more and sorry for the length post that is about to follow.

I have been thinking about this for a little while and it seems that what I have written earlier to this topic didn’t really help that much and seem to be more confusing than anything else and for that I would like to apologize.

The source of the problem really lies with the colloquial use of the word “toughness” we have gotten used to (which is: capable of enduring strain, hardship, or severe labor). I had to remind myself that toughness is used in material science used for other materials than steel as well and is closer to the original meaning of the word, which can also mean “glutinous or sticky”. In German the word for toughness can even be used for liquids in a sense similar to “very viscous”: Syrup, for example, is a “tough” liquid.

Although the difference is quite literally semantics, the definitions that searcher gave are simply describing something different then what we usually mean when we talk about toughness in a blade. We usually mean one form of impact-toughness or another. I am trying to explain the difference below:

Copper for example is very tough. Since energy is force*distance, toughness is the force it takes to stretch it times the distance you have elongated or bend it. It has practically no resilience: If you bend it, it stays bend, it has practically no elasticity. Glass or a rubber band on the other hand has practically no toughness but very high resilience: either it springs back to its original form or it breaks. Resilient materials seem to break rather than tear, while tough materials seem to rather tear than break.

However, this definition is very different to the way we use toughness in the knife world which is really a form of impact-toughness which is tested quite differently than toughness. It also seems that tests for impact toughness do not seem to distinguish between toughness and resilience. Usually, a material is notched, subjected to shock and tested whether that material fails or not, disregarding whether the material elastically or plastically deformed to absorb the shock. The unit of measurement is still the same but the method is quite different. Here the shock is induced by a mass at a certain velocity and the energy absorbed is E=1/2 m*v^2. This is really what we mean when we talk about the toughness of a blade. In most blade steels the absorbed energy is obviously mostly due to the resilience of the material and not to its toughness, yet it is still tested by a toughness test. So what we are really after when we talk about toughness is actually “impact-resilience” but don’t know of a test that claims to measure that. Which is probably also the reason why there are different definitions of toughness.

The reason for the distinction between toughness and impact toughness is in the limitation of the definitions searcher posted: When closely examined the definition of resilience I plain wrong since energy is never absorbed in an elastic deformation but only stored (it can be recovered upon unloading, while the energy that went into deformation can not be recovered). But I agree that this is obviously unimportant as the definition are descriptive enough. However, both definitions fail at high speed impacts. A tough material may stop a car by deformation, but a bullet with the same or lower energy will tear right through it. Same with resilience. While a leaf spring may absorb shock of a car going over a pothole and shatter/fracture upon impact of bullet. You can try that for yourself with putty. You can easily tear it apart in one quick motion, while you can stretch it out forever if you go slowly. Same thing with something that has more resiliance than toughness: you can take a glass plate and load it carefully with some weight and you can clearly see how it bows. You just have to tap it softly but quickly with a hard object and it breaks.

After all that it is probably pretty clear why there is a confusion about toughness. However, when bladesteels are tested for toughness, it usually refers to impact toughness most commonly a charpy test. As ghostsquire pointed out, the real question is, how exactly that value relates to performance in a bladesteel. I would disagree with him that it has no relation at all, but I would also see it more as an indication or a trend than a hard and fast correlation. I would find it unlikely that a steel (at a given hardness) with a charpy value twice as high as another steel (also at a given hardness) would turn out to be in practice less tough in practice (in a blade that is) than the one with the lower charpy value.

By the way in materials that have decent amount of both resilience and toughness, it is quite difficult to separate them. Usually, the force required drops when entering the plastic region. You can experience when bending a nail for example. It is much easier to keep it moving once you have reached the point where the nail doesn’t spring back.

 

[searcher]

Wow, outclassed again. Do you find it useful at all? I didnt find it very useful for my job.

Sometimes I use Dieter's Mechanical Metallurgy. For instance, I never understood why roller levelling works to flatten steel strip until I read about the Bauschinger (sp?) effect and how a stress-strain hysteresis loop is formed by bending a strip back and forth thru rolls that actually serves to dissipate internal stresses in the strip.

Not too much to do with knives except knives made from premium steel strip under 1/2" thickness has probably undergone this process.

Dieter also gives the standard toughness definition of the area under the stress-strain curve to fracture (Ultimate Tensile Stength or UTS).

The notched-bar impact tests are hard to use in practice because the triaxial stresses in a notch are tough to describe and correllate to other size/types of notches. Where they are really usefull is in finding the ductile to brittle transition temperature of different steels. I'm sure the guys who designed the WWII Liberty Ships wish they were aware of that. Again, not too usefull for blades (unless you get into a low temperature sword fight).

 

[harrymole]

Whoa! You guys gave me more than I could have imagined on this subject. Although I understand most some of it is very technical and has caused me to do some research on my own. Thanks for that I am a Systems administrator at my job and I love to learn new things. I appreciate all of the input and hope to take this and use it. Does toughness apply to small blades? What I mean buy that I have a WOO in A2 although this is considered a "tough" steel I will never use it to chop with and beacuse of its' 1/8" thickness there will be no prying. SO, what practical use is there to use a "tough " tool steel in a knife less than 4"? Don't get me wrong I love this little knife and it is razor sharp and holds its' edge very well, but so does my BM in D2 and 154cm. So why use it if it will never be used to its full potential? Enough I have stirred the pot to long on this subject. Thank you everyone for there input.

 

[Cliff Stamp]

I would find it unlikely that a steel (at a given hardness) with a charpy value twice as high as another steel (also at a given hardness) would turn out to be in practice less tough in practice (in a blade that is) than the one with the lower charpy value..

This was noticed in use (charpy/izod doesn't correlate well to fracture failures well in tool steels) which is why torsional impact tests were often used instead, details are given in ASM's "Tool Steels" by Roberts and Cary, 4th Edition. Of course if all you have are charpy/izod and if you are looking for impact it is a lot more relevant than a slow load.

-Cliff

 

[Joe Talmadge]

Whoa! You guys gave me more than I could have imagined on this subject. Although I understand most some of it is very technical and has caused me to do some research on my own. Thanks for that I am a Systems administrator at my job and I love to learn new things. I appreciate all of the input and hope to take this and use it. Does toughness apply to small blades? What I mean buy that I have a WOO in A2 although this is considered a "tough" steel I will never use it to chop with and beacuse of its' 1/8" thickness there will be no prying. SO, what practical use is there to use a "tough " tool steel in a knife less than 4"?

Great thread!

If we mean "impact toughness" when we say the word "toughness" (which I agree with HoB, that's what we do mean), then of course it follows that as you go to smaller, thinner, lighter knives, you can't generate as much force so you can and should sacrifice some toughness in order to get the other properties you're looking for in that smaller, thinner knife. But saying "I'll tradeoff some toughness" doesn't necessarily mean you need to change steels. For example, you're looking at a 4" slicer, and your choices are M-2 or ATS-34. Go with the ATS-34 since toughness doesn't matter so much, and ATS-34's stainless and ggood wear resistance might be just the ticket, right? Well, not necessarily ... M-2 at 60 Rc is a tougher steel than ATS-34 at 60 Rc. But I can take my M-2 up to 64+ Rc, where its greater strength makes it a better choice for any number of applications.

In short, as you go to smaller knives and applications which require less toughness, you can trade off some toughness for other properties. But don't automatically think you need to change steels to get that tradeoff ... you can also get the same tradeoff by using the same steel, but heat-treating it differently to bring out the properties you need. Pick the overall package of [steel + heat treat] that meets your requirements, rather than just picking less-tough steels as you go to smaller knives.

Joe

 

[HoB]

Dieter also gives the standard toughness definition of the area under the stress-strain curve to fracture (Ultimate Tensile Stength or UTS).

That doesn't really seem to be a very useful property for knifes, unless you are prying with it to fracture (or rupture). Well, or you are torquing on the blade. I tend to forget about that (see below ).

The notched-bar impact tests are hard to use in practice because the triaxial stresses in a notch are tough to describe and correllate to other size/types of notches.

True, but maybe I am picturing this wrong: I kind of figured that chopping with a chipped blade (maybe you just hit a nail) is essentially a poorly controlled notched-bar impact test? So I figured there should be some correlation at least.

The problem with impact test, as far as I can see, is the dependence on the velocity of the impact. Obviously, once the impact approaches the speed of sound in the steel, everything changes, but there are probably several other transitions at lower speeds as well.

Well, just toughness as searcher defined or rather resiliance will have some bearing on the performance of a small knife. Just yesterday I used a cheap Buck (420HC) to make a hole into a can (to pour out the milk inside). Punching the hole wasn't the problem, but when I carefully enlarged the hole by twisting and turning the blade in the hole a large chip broke out. A more resilient steel would have been fine and a tougher steel would have simply deformed. But then, of course you might argue, I shouldn't have done that in the first place...and I would probably agree .

 

[Cliff Stamp]

True, but maybe I am picturing this wrong: I kind of figured that chopping with a chipped blade (maybe you just hit a nail) is essentially a poorly controlled notched-bar impact test?

Notches are much bigger, standard izod v-notches are 2 mm deep on 10 mm stock, charpy can be 5 mm deep on 10 cm stock for c-notches. They are also stressed so as to focus the impact on the notch, either right behind it, or just above it. In use of blades I have never seen an impact actually fracture at a chip, even the big ones. You are looking more at unnotched results.

...when I carefully enlarged the hole by twisting and turning the blade in the hole a large chip broke out.

Torsional strength.

-Cliff

 

[harrymole]

I see what you mean Joe. I and many others look at tool steel or carbon steel and think which is the toughest for the knife I need, but never consider the HRC of said steel. I had a Buck w/ ATS-34 heat treated by Paul Bos, the knife was harder to sharpen but would hold a killer edge to the other ATS-34 blades I had at the time. Well, this has opened my eyes to not just looking to get the "best" steel in my knives but also see what the Manufacturer or maker is heat treating their blades at. I also saw this in CS vs. my Ontario Rat folder. Both had AUS-8 steel but the Ontario was treated a point or two higher and you could really tell the differance in edge taking and holding. Thanks to all!

 

[ghost squire]

That doesn't really seem to be a very useful property for knifes, unless you are prying with it to fracture (or rupture). Well, or you are torquing on the blade.

Oh, I think it means a lot more then that (area under stress strain curve). When you baton on the spine of a knife with a framing hammer, these things apply. Think about it, the area under a piece of glass's stress strain curve would be almost nonexistent, and if you batoned on that it would shatter instantly. When you chop into a piece of wood, the knife's toughness is being tested with every chop you take. A glass axe and a lead axe are both extremes of the balance between ductility and strength, and therefore not very tough in our practical sense.

I like measuring a piece of steel's toughness by calculating the area under the stress strain curve. It's easy and simple, and just looking at it and calculating the area, you can roughly see how tough you can expect the piece of steel to be.

 

[HoB]

But that is exactly my point. Batoning (impact) has nothing to do with a semi-static stress-strain experiment. Besides, you wouldn't really want permanent deformation so I fail to see, why a large area from the yield point to fracture would be of much interest, or we would be using brass or copper for knives. It seems to me that what is really of practical importance is the area under the stress-strain curve up to the yield strength (and maybe a bit beyond). That area is, by the way, far from non-existing in glass. Glass is able to hold only small amounts of strain to fracture, but it does so at huge stesses. It is an extremely steep rise in stess-strain curve and it is practically ideally linear in the first part. Especially if thin glass fibers are tested.