A rough Finite Element model shows that the Cho can reduce stress in a blade

H

Hepkat

Joined
Jul 7, 2011
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14
Hi Everyone,

I don't know if this has been discussed before, but from reading material on the HI website I got the impression that the Cho cutout just above the handle of the khukuri is traditional and it's purpose is more or less lost to history.

I had a hunch that it could actually have a structural benefit, as the extremely narrow cross-section of the blade edge can cause stress concentration. The cho can move the stress to the thicker areas of the blade, reducing the maximum stress in the blade.

This morning I ran some Finite Element Simulations and saw that the Cho can reduce the maximum stress in the blade by ONE THIRD.

I did not use real values for the geometry. I used generic structural steel material properties and did not account for the variation in modulus from the differential heat treatment or hardening. I did not check for mesh convergence, or verify my results were in accordance with the beam equations. I did not apply realistic loading conditions. In other words, please take these results with a grain of salt!

I do think that the ratio of the maximum stresses is likely a good result. (although I would not publish this without doing a LOT more refinement and verification). At the very least it indicates that the strength of the blade can be improved by adding this feature.

Anyhow, I think it's cool that a traditional feature like this could have a significant positive effect on the strength of the blade.

If anyone is interested to help me to post it, I have a .pdf that shows some pictures of the stress concentrations.

--Justin
 
Hepkat said:
I used generic structural steel material properties and did not account for the variation in modulus from the differential heat treatment or hardening. I did not check for mesh convergence, or verify my results were in accordance with the beam equations. I did not apply realistic loading conditions. In other words, please take these results with a grain of salt!Anyhow, I think it's cool that a traditional feature like this could have a significant positive effect on the strength of the blade.--Justin
Click to expand...

Justin,

That is just what I was going to say! :eek:
 
Interesting. It would be good if you could share your results.

Have you tried varying the depth of the cho (perhaps as a function of thickness and height of the spine)? That should make a big difference in stress relief. And, of course, one wonders if your mesh is appropriate for the analysis ... but that's a standard worry for mechanical or electromagnetic models of this sort.

We old lab physicists often stop cracks (in metals and some ceramics) in their tracks by making a small bore hole (drilled -- mechanically or optically) just at the apex of a crack. We've always believed this relieved the local stress.
 
I believe what Justin is trying to say is - Cho is Good! Thanks for the very interesting post. I've never run across a story about a khuk breaking at the cho, which would seem a logical weak point. Very cool info. Thanks again Justin.
 
That is an extremely interesting observation. What about that hollow characteristic reduces stresses within the blade?
 
Hepkat I've never thought about the Cho possibly having that kind of effect, and even if it turned out not to be the case that is still sharp thinking and for that I tip hat my hat sir.
 
Hey Guys,

So, I've been thinking about this more this afternoon and maybe I can explain my reasoning.

If I take a beam and bend it, the stresses are maximum at the top and bottom. One in tension, the other in compression (technically compression is a minimum). This is why I-beams are so useful, because they are thickest where the stresses are maximum.

A knife is the opposite of an I beam. The edge (where it will have maximum tension) is very thin. Because it is thin, it doesn't take as much force to deform it and overall isn't a problem. (kinda like how a heavy rock and a light rock fall at the same rate. There's twice as much gravity force, but it's got twice as much inertia holding it back). As the blade deforms, the thicker (i.e. stiffer) parts hold more of the force. So the weakness from the edge being thin is offset by the softness of the edge being thin, and the stress on the edge is unaffected.

The problem is where the thin edge meets the thicker tang. The thicker tang is stiffer, and carries more force. Forces have to be equal and opposite, and this is transferred to the edge.

So right at the point where the edge meets the tang, is what we call a stress concentration. By cutting out the cho, they are not allowing the force to transfer directly from the tang to the edge, because they are no longer in direct contact with each other.

Therefore by cutting out the Cho, the maximum stress point on the blade is alleviated and the blade is actually stronger.

In my research as an engineering graduate student, I use a technique called Finite Element Analysis (to be fair I use a computer which uses that technique...LOL). FEA takes an object and chops it up into a mesh of thousands of little rectangles. This allows me to take a complicated problem and reduce it to thousands of not so complex problems.

I did this to a VERY VERY simplified model of a knife and saw that adding a cho reduced the maximum stress by one third.
Because I didn't spend a lot of time verifying my model, it's not very trustworthy. It does show that the cho has the potential to reduce the stress at the point where stress is maximum, and I think my model is trustworthy enough to demonstrate this trend (although the actual numbers should not be trusted)

I think this is especially interesting because it is so counter-intuitive. I wouldn't have thought that cutting a hole in a hunk of metal would make it stronger.

--Justin
 
@oldphysics, you're right. The mesh is an absolute mess. If you've been away from FEA for a while, with the newer software and computers we often approximate quality with quantity. I gave it 20,000 nodes and it took less than a minute to run! Still, no excuse for sloppy work.

That's a good idea with varying the parameters. If I can find some more time (stupid dissertation! lol). I'd like to play with this some more and see what an optimized cho would be like. I suspect it's going to be a lot like what a thousand years of trial and error by the Kami have produced.

@Everybody, thanks for all the kind words!

Also if someone could help me out with this, I'll post some cool pictures that show the stress concentrations.
 
Hepkat said:
@oldphysics, you're right. The mesh is an absolute mess. If you've been away from FEA for a while, with the newer software and computers we often approximate quality with quantity. I gave it 20,000 nodes and it took less than a minute to run! Still, no excuse for sloppy work.

That's a good idea with varying the parameters. If I can find some more time (stupid dissertation! lol). I'd like to play with this some more and see what an optimized cho would be like. I suspect it's going to be a lot like what a thousand years of trial and error by the Kami have produced.

@Everybody, thanks for all the kind words!

Also if someone could help me out with this, I'll post some cool pictures that show the stress concentrations.
Click to expand...

Well do I remember my doctoral work. That, of course, comes first.

Still, it will be interesting to see the stress concentrations.

You probably need an adaptive mesh, one that has a higher concentration of properly formed shapes near the highest gradients and a lower concentration where conditions are more stable.

If you e-mail those pictures, I'll see if they will post up. I can be reached through the e-mail function on this forum.
 
Justin, thanks for the explaination. It actually makes sense, even though it seems to oppose common logic. I have heard of khuks breaking at the tang juncture, which seems to be a proven weak point in any knife as you say. I should add that, to my knowledge, any catastrophic blade failure with HI seems extremely rare. I don't want anyone to get the wrong idea. The stress relieving cho idea is pretty darn cool and does make sense, in a way. But, knowing the tangs are also thick, I wonder if this would just make the cho a tang juncture of sorts. But, then again, I've never heard of a blade failure at the cho. It could be that moving that point out further away from the handle alleviates/mitigates some of the forces created during a chop. I also figure cut angles at the tang or cho play a part. I've already confused myself and probably anyone else trying to make sense of my post:) Anyway, thanks again to you and OldPhysics for some interesting insight. I am no engineer. In fact, numbers are part of my axis of evil;), so I'll let y'all do the heavy lifting on this one. Take care.
 
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Interesting concept. I wonder if there was any need for that type of stress relief in modern or historical khurkis or if they are more prone to isolated chipping/breaking at the point of impact or due to general hardening errors.

Here's the button to add an attachment when making a post.
attach.jpg


Take a look at this thread where the cho didn't help when using an irregular batoning technique - it also has an example of attachments on a thread (although you may need a paid membership to attach files)
http://www.bladeforums.com/forums/showthread.php/609283-Broken-Khukri

At the end of the thread you can see some serious stress relief by Auntie's customer service ;)
 
Registered users can't use the attachment features. His best bet is a service like photobucket or simply take OldPhysics up on his offer.

bric said:
Interesting concept. I wonder if there was any need for that type of stress relief in modern or historical khurkis or if they are more prone to isolated chipping/breaking at the point of impact or due to general hardening errors.

Here's the button to add an attachment when making a post.
attach.jpg


Take a look at this thread where the cho didn't help when using an irregular batoning technique - it also has an example of attachments on a thread (although you may need a paid membership to attach files)
http://www.bladeforums.com/forums/showthread.php/609283-Broken-Khukri

At the end of the thread you can see some serious stress relief by Auntie's customer service ;)
Click to expand...
 
Looking forward to the pictures. I often put mine in facebook with the privacy setting set to "everyone" and then link to it.
 
Bric, you just had to make me a fibber! Just kidding:) I never saw that thread, even though I've spent a fair amount of time in the archives. Broken CAK is something for sure! This whole thread just gets more interesting. I'd love to get a run down on every khuk failure to look for a pattern and see if you and OldPhysics could come up with a theory. Though as has been stated many times, over hardening seems the likely culprit. But it would still be interesting to see what info could be gleaned. Luckily, these sorts of failures seem so rare as to be a real oddity. IIRC Ferguson also had one, but I recall it being from the no longer made, aluminum handled museum model. I'll be checking in on this thread. Take care everyone.
 
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jdk - that was not meant as a rebuff to you. If I read your posts correctly, I am inclined to agree with you. "Even if" the cho provides stress relief under certain circumstances, there is still the question as to whether that is the "purpose" of the cho. To propose this theory I think would be suggesting that historically cho-less blades were more prone to failure and someone discovered that blades with cho were less subject to these failures. In modern-day HI Khukris I do not expect that this phenonemon (cho-less blade failure) is the case, take the Tarwar or Mongolian short sword for example. Also the blades stand up to batoning (hitting back of blade with heavy piece of wood) and in this use I would think that very little of the force gets distributed to the cho area.

As to whether historical cho-less Khurki were prone to failure...I would have no way to know. It does seem that chopping wood has it's own stress relief built in to some degree with the blade undergoing a less rapid decellaration than hitting a rock.

It seems like some of the greatest arguments come out of "purpose" discussions, which is also not my intent. Cheers Hepcat for a new application of Khukri fascination and curiosity!
 
No problem Bric. I was just giving you a hard time. I promise:). I'm glad you posted it since I'd never seen it before. The whole idea here is pretty neat. Whether it relieves stress by design or is incidental, it's a good thing and a very interesting topic. Like someone said on another subforum, it's more interesting than another "what knife are you EDC'ing this week" thread:D. Take care.
 
I'm sure he'll send them to me soon ... I provided him with my direct e-mail yesterday. Remember, he's a young fellow in grad school and has classes and his dissertation to worry about.

It will come soon!
 
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