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

Very interesting, I'd also be interested to see the modeling on this one, although I'm afraid my education may not be up to the task. The explanation makes some sense; I guess I'd also wonder how shape, size, etc varies it. I'll be interested to see more about this topic for sure!
 
I got an e-mail. His analysis is contained in a Word .docx file.

Hmm. Going to be a bit of a challenge to figure out how to post this.
 
Would a .pdf work better? I thought it would be more understandable with a little description. I hope I'm not making this too difficult for you Jamie
 
interesting, i gather you are treating the blade as a cantilever with the impact forces applied at or near the free end. the stress is also dependent on the target material, as noted hitting a wooden 'soft' target distributes the stress over a longer time period, with less acceleration forces than if hitting a harder rock. the inverse is true if hitting a softer target, like a person. the impact forces set up standing waves in the beam which vary with the time profile and the beam's slenderness ratio and length, the 'sweet spot' is normally at a node near the free end, but another node is found nearer the tang/grip junction. like in a baseball bat, hitting away from the sweet spot can produce nasty vibrations that affect the stresses and can increase stresses to the breaking point... hitting slightly off the vertical can also induce side-to-side stresses to further complicate the equation. an example is a bad hit on bamboo with a katana. or how to destroy a folded steel piece of artwork in a few microseconds; or the infamous tacoma narrows bridge where the wind induced vibrations caused by the wind's force on the side had spectacular results.

[video=youtube;pl0vzOMYsUU]http://www.youtube.com/watch?v=pl0vzOMYsUU[/video]
 
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bric said:
.. "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 ..
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In the Malaysian tradition, our Golok Rembau and Golok Perak has a notch at the ricasso to place index finger when performing small delicate tasks!

CRGP056GolokPerakhandle.jpg


And the Dayak tribes of Borneo made Parang Ilang (i.e. mandau) and Parang Nyabur with a krowit and an brass inlay at the ricasso as well.

ParangDayakIbanJohnnyBarangan20Ilang.jpg


And I never heard anybody in Malaysia mentioning about such a notch and a krowit meant to provide stress relief!
Now it sounds real interesting to me :)

mohd
 
Hi Everyone,

Sorry I got a bit busy this week. I just made a google site and posted some stuff.

http://sites.google.com/site/hepkatfea/cho-fea

I think considering the natural frequency of vibration is looking at strength under an entirely different loading condition than I considered, although harmonic vibration in a knife is an interesting question, and I think we've all felt vibration transferred to our hands after chopping with a blade.
I haven't done much work with modeling stress waves. I think to do this you'd have to treat the metal as a viscoelastic solid which would require viscoelastic material properties to be known. I don't know if this is readily available or not, although I think the software can handle it. This is getting a little out of my area. Dr. V probably knows more about this. Physics guys are good at knowing a bit of everything!
 
Nice Hepkat, thanks for posting. Can you import shapes into your program? If so here's a picture optimized to define the shape of the Khukuri. I wonder if the angle of the blade affects the stress pattern. Also, maybe I don't understand exactly you did to simulate impact, but 0.1in deflection seems more than the blade would deflect - but maybe you just added the deflection to simulate the force of impact...also isn't 340 lbf fairly low. I could be way off but I think the force of the blade hitting wood would be in the tens of thousands lbf.

WWII Black and White
WWII Silhouette

Measurements of this particular khuk:
Spine thickness = 7/16in
OAL = 18in
Handle (Tang) = 6in
Blade width ranges from 1 5/16 to 1 15/16

Here's a good post on the differential hardness of the blade (section 2 of first post) if you want to adjust the mesh.
http://www.bladeforums.com/forums/showthread.php/720287-(WiP)-A-kukri-s-quot-working-edges-quot

May also have to consider that the tang edge is not exactly "fixed" - that the hand holding it will "bounce" and shift with the impact.
 
most such finite analysis systems allow for a variety of different end supports, including shock absorbing ones as well as hard fixed ones. a kinda upside down comparison would be to a diving board, impact stress to a flexible cantilever beam where the fixed end is a rubber bushed pivot and there is an unconnected adjustable support (ie a rubber 'bumper' under the diving board) to absorb some energy after the diver is propelled off the end. the diver can adjust the support to move the vibrational node to suit. kinda like choking up on a khuk or bat depending on what you are intending to use it on.

View attachment 230953
 
Interesting conncept, I'm watching to see where this goes.

If indeed this is the case the devil is in the details. I have had kuks form cracks AT the cho due to sharp notches.
 
So, if I understand correctly, the traditional cho (with "tail") does not reduce stress, but a choil or elongated cho would? It seems most, but not all, cho shapes fit into the category of the half circle with tail. Interesting how the "tail" changes things and brings back the spike in stress. Again, not having seen much in the way of blade failures, it's more acedemic than a real world concern IMHO, especially concerning HI's overbuilt field models. But it is very interesting none the less. Thanks again Hepkat and please let me know if my understanding is incorrect. Take care.
 
Can you get photos of this?
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4.jpg


Hi Howard,
Notice the tiny cracks developed along both sides, before merging into the cho?

I was thinking of rounding it off but will leave as it is for now.
 
the 'cho', 'kaura' or 'kaudi' is usually thought of as a cow's hoof-print partially across the blade, cows being sacred in hindu. the 'vee is thus actually the space between the cows toes. some have this shape as more of a pagoda, or other shapes like a fleur-delee. they are either cut or punched, but if done improperly can have a very acute (sharp) inside angle which is indeed a crack inducing stress raiser. if you have one like that a small jeweler's round file can be used to radius & round off that sharp corner. the closed cho of course doesn't have that problem, so should be the best stress reducer. sharp OUTSIDE corners like the point of the Vee are NOT a stress raiser and are not a problem.

if the punching or cutting of the cho causes stress cracks, as above, they also can be filed back to good metal. in the above photo, the rounded corners of that 'pagoda' shaped on would have been perfect, except for those tiny cracks.

View attachment 231345

more chos:
View attachment 231348

more buzz words and concepts:

the locii of points along a stressed beam which are not under stress is called the neutral axis. the area above or below that axis is either in compression or tension, which is higher the further you get from that neutral axis. in an i-beam or simple flat beam, that axis runs down the middle of the web of the beam. drilling a reasonable sized hole in the web centred on the neutral axis will reduce the beam's strength negligibly. if you ever need to drill an access hole in a beam, drill it on the neutral axis on the web, not on the flange. conversely drilling away from the neutral axis or removing material away from the neutral axis will reduce the bending strength of a beam, the further you drill, the worse it is. note again that sharp inside corners in a hole are stress concentrators, oval or round holes are best. square holes are poor , jagged holes left by a acetylene torch are worst.
 
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Salyan.JPG


This Salyan pic was found at Howard's old webpage; Notice the distinct hole in between the cho.
It serves to "cancel" or shift the building stress developed along the corners of cho towards the hole which,eventually disappeared.
 
I just got to the google page.

Interesting idea, but I've never heard of a khukuri breaking at the cho area. There's a lot of steel there. Breaks sometimes occur at the blade/tang junction. This appears to be due to improper heat treat in that area. I had it happen to me once, and it was with the first, quite light, chop with the knife.
 
I think there is an anecdotal evidence of stress concentration from the show Myth Busters. The crew built a katana swinging machine for some sword breaking myth and the first version of the machine basically slammed the katana into a block of gelatin.

Without fail, every katana snapped right at where the blade met the guard/throat.

It was not until the machine was revised to simulate the "push and pull" technique that it was able to slice through the block.
So the moral of the lesson is that swords do break if it is improperly used such as attaching it to a giant sword swinging machine.


In any case, thank you for this very interesting investigation. Cho is a rather unique design element that I have yet to encounter in other historic weapons!
 
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Was in the midst of brainstorming session about geometrical design inputs with a simulation specialist (currently running into blanks on a medical product that i designed ) and just curious i popped the questions about cho.

He explained it with Young's Modulus that the cut-out (Kaudi) is crucial for "little plastic deformation" with resultant force acting along the curvature of khuk itself, simulate this into several thousand cycles of chopping that eventually shared or damped the stress built up around neck/bolster of the blade. (If the shock is transferable you will experience vibration on your wrist, forearm or even joints.). If the stress hits yield then the edge's simply too hard, hence brittle and it chipped.

I often wonder what's the function of the cut-out holes on the blade of Eli in Book of Eli?
Weight reduction or stress reliever?
Key in the factor of differentially hardening as well as bevel height, fullered/hollow forged, such variables can alter the lifespan of khukuri itself.


Once again, i might just be totally wrong.
 
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JayGoliath said:
... (If the shock is transferable you will experience vibration on your wrist, forearm or even joints.)...
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Another interesting question is if the cho has any effect on the vibration of the handle, or the energy transfered back into the hand.
 
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