Toughness?

[Gringogunsmith]

Hey Cliff.
This may be an inappropriate question so if you choose not to answer I fully understand.

Name your 5-10 favorite production( readily available) fixed blade knives in the 5 1/2 to 8 inch range.. in no particular order, of course.. If you would

 

[Gringogunsmith]

Allow me to elaborate a bit..
Not necessarily the best due to your testing.. Moreso just what YOU like..

 

[searcher]

That depends on how you define it and there are many definations. For many applications the fact that the thicker knife will snap earlier is a problem, especially if by thickening it you have reduced the angle so much that it won't bend at all now and/or everything is focused at one spot. The angles to set and then crack are increased when the cross section is lower, the force needed and thus strength is much higher and this is usually a common tradeoff in alloy as well as geometry.

Speaking of how people use knives, a blade which can bend a lot more would be considered tougher by many simply because it was harder to break when prying and is far less likely to be accidently overstressed and broken, though far more easily bent. You can see the same thing in edges, edge angles which are very thin and acute can pass "toughness" tests that thicker edges fail, the brass rod test for example.

In knives the application is further compounded beyond the basic materials data because as you thicken a knife to get strength it loses cutting ability and often general control and thus it sees much greater and uncontrolled forces in use. Often times it is readily possible for an edge to become stronger and tougher *in use* by reducing the edge angle, but any materials data would show differently. But you have to consider the use of the tool, not simply the raw ability of the steel.

-Cliff

I must not have been clear. If you pry till the blade breaks it will take 4 times the force to break the "double thick" blade

 

[Cliff Stamp]

I must not have been clear. If you pry till the blade breaks it will take 4 times the force to break the "double thick" blade

Yes, it will be much stronger, I have posted the cross sectional dependance on strength many times in the past. However it will also break at a lower angle showing less toughness. This is why as I noted several makers will adjust geometry and/or hardening to decrease strength but gain toughness because from a user perspective it can keep the blade from breaking if the blade bends to a larger angle, see Fowler's work for a radical shift of this perspective. Also as noted there isn't uniform agreement on that perspective, some makers will maximize strength and end up with knives which have no ability to deform, it is similar to prying with a file, very stiff and then *SNAP*.

You also have to consider the effect as noted on the cutting ability because this will effec the functional toughness because knives which don't cut as well will end up having far greater stress on them in use at the same tasks. I have seen for example edges on axes take less deformation when the angle was reduced for several reasons. This directly contradicts the above raw materials perspective which implies the edges are stronger, they of course are, but in use they will be weaker if you consider them vs task and this is how any user will of course evaluate them.

Name your 5-10 favorite production( readily available) fixed blade knives in the 5 1/2 to 8 inch range.. in no particular order, of course.

Ragnar offers a bunch of Leuku's in that price range which make decent wood working knives, they start off really inexpensive, to about $50 or so for the ones with the traditional wood grips.

For a heavier blade for significant chopping, I like the Ratweiler design, the RD series has a similar pattern, the CU/7 and Combat Bowie are lighter more machete and less axe like blades, also solid.

-Cliff

 

[searcher]

Yes, it will be much stronger, I have posted the cross sectional dependance on strength many times in the past. However it will also break at a lower angle showing less toughness. This is why as I noted several makers will adjust geometry and/or hardening to decrease strength but gain toughness because from a user perspective it can keep the blade from breaking if the blade bends to a larger angle, see Fowler's work for a radical shift of this perspective. Also as noted there isn't uniform agreement on that perspective, some makers will maximize strength and end up with knives which have no ability to deform, it is similar to prying with a file, very stiff and then *SNAP*.

You also have to consider the effect as noted on the cutting ability because this will effec the functional toughness because knives which don't cut as well will end up having far greater stress on them in use at the same tasks. I have seen for example edges on axes take less deformation when the angle was reduced for several reasons. This directly contradicts the above raw materials perspective which implies the edges are stronger, they of course are, but in use they will be weaker if you consider them vs task and this is how any user will of course evaluate them....


-Cliff

If I have a blade that is 4 times as strong why would I want it to bend as far as a thin steak knife? I can pry with it with 4 times the force. Who cares what angle it breaks at?

Knives by their nature are brittle (break by "brittle fracture" mechanism which is by cleavage at grain boundaries). If they weren't they would not be hard enough to hold an edge worth a darn. You could make a knife out of a car leaf spring (upper 40s Rockwell C hardness) but it wouldn't hold much of an edge (unless you annealed it and requenched and tempered it to be harder). Softening knives so they are more ductile is a compromise on edge holding ability. It is tough to find a material that fails in a ductile manner at 60 Rockwell C hardness

 

[Cliff Stamp]

If I have a blade that is 4 times as strong why would I want it to bend as far as a thin steak knife? I can pry with it with 4 times the force. Who cares what angle it breaks at?

Because you usually don't know the break point until after it breaks so you can end up with a set or a break the fact that it took more force might not always be relevant. To get specific I used a H1 for some wood digging and it was very stiff due to the cross section and then broke and lost a significant part of the tip. With the Cara Cara it was obvious it didn't have the strength and an attempt at prying just left the tip bent, so you staighten it and move on.

This is why makers like Fowler will heat treat their knives so they are much weaker than if they are full hard, but they bend to much greater angles and will not fail by brittle fracture. In extremes makers like that want the knives to stay in one piece and they will argue they are tougher and they are in several respects. Then you have guys like Strider who prefer maximum strength, and disregard impacts or bending, and when the knives break they can go in multiple pieces, even shard.

I am not arguing that there is no point to increasing thickness, however just that you have to consider what you lose, with knives it is cutting ability, ability to bend, and of course the knife is heavier, generally there are also sharpening concerns, then higher cost, etc. . As with most factors there is some sort of "sweet spot" where the balance of properties gives the best compromise. Your viewpoint doesn't bound anything so why stop at 4x, this indicates an obvious problem with the arguement as you have no constraint.

Knives by their nature are brittle (break by "brittle fracture" mechanism which is by cleavage at grain boundaries). If they weren't they would not be hard enough to hold an edge worth a darn.

For steel knives, not all of it needs to be as hard as the edge, you have laminates, sam/mai, pattern steels, differential hardening/tempering, there is a fairly large amount of knives that will break by ductile failure I have handled and seen dozens including bending them to the point where they tear apart. Mission's Beta-Ti also won't see brittle failure, they also actually have to be torn apart, similar when subjected to impacts it will actually be cold worked and deform readily. Plus as noted, even when knives are very hard if they are thin they will have a large plastic region, Phil Wilson can bend his S90V knife to the point that they set simply because they are ground so thin. I have full hard (65/66) hrc knives and the edges are so thin and acute that when they are overstressed they fail by plastic deformation and not fracture.

-Cliff

 

[HoB]

Deleted
Never mind.....

After spending the entire day hosting a guy who grows crystals for a living. I should have know better and should have stayed away from this thread. As they say: You shouldn't take your work home....