Shinai stress points?

[Phlebas]

I was oiling and assembling a new shinai yesterday and I had a thought... has anyone ever studied the breaking "patterns" of shinais? I am pretty new to Kendo, and I haven't broken one yet, but *IF* there was a study done, where do you think most of the damage occurs? Kissaki? Sakigawa? How many breaks or cracks occur below the Nakayuki?

I suppose we should assume that the study should be done on shinais used in competition bouts by higher ranking kendoka, so we know that bad form doesn't enter into it as much, and there aren't sloppy beginners (like me) stressing the shinai improperly. Of course, I'm not talking about carbon shinais... do they even break at all?

I think I've just started the study I was musing about! Give me your thoughts on this, all you experienced kendoka...

[AlexM]

I can see it now:

"Stress and impact fractures in bamboo constructs: The shinai and how it breaks"

A doctoral dissertation presented to the physics department of X University.

Maybe you could get a grant?

[Marine_Boy]

I work in a materials laboratory and when I was maintaining my shinai, i had found that after oiling, stress fracture lines showed up to the naked eye across the bamboo staves.

On examination under a microscope, I found that the cracks did not go in very deep. Now if you look at the staves from the end, you should see that the bamboo has the circular cells (running the lenght of it) closer to the outer edge. But on the inner edge, there is a layer of "spongy" material.

I believe that if the stress lines go beyond this spongy layer and into the circular cells, then there maybe a stronger possibility that the bamboo stave may fracture.

Just my findings.

Stan

[Phlebas]

Hmmm... I never thought of that... perhaps the International Budo University would give me a grant for this. I could actually call myself a shinai scientist! Marine_Boy sure sounds like a better candidate then me, however. Interesting comments, and he even works in a lab! :)

[Paburo]

i'm new to kendo too, but let's say i have a long(?!) experience with broken shinai.

they almost all break above the nakayui. from hitting the mengane(begginers!) or hitting other shinai.

they also break often in the middle area, or the area you use to block the most in kirikaeshi and keiko.

most carbon shinai breaks i've heard of, are because excesive bending.

[S. Takamori]

Guys,

I've a quiet long experience in using bamboo shinai...(23 years). So, in my opinion, most of the shinai will breack when using them with bad motodashi. Lots of people cannot receive correctly a men strike. So, in this case it doesn't matter how expensive your shinai is... only good partners can make your kendo cheaper.
Of course, for those outside of Japan and its humid climat, don't put your bamboo near the central heating as I saw many times. Allow them to absorb some humidity by (for example) putting them in the bathroom or any othe humid space in your house...when you're not practising at the dojo.

[kendomushi]

Where and if a shinai will break depends on many factors. These include the amount of use, skill of the user, surface being hit, accuracy of strikes, power used in strikes, storage of the shinai, humidity, tension applied by the tsuru and other parts, and probably more. Most shinai I have seen break were near the nakayui on the sakigawa side of it.
I have even seen my sensei swing a bamboo shinai at the men of one of our members who tried to do an oji do (stop the shinai by a block to the right and come down to strike do), and the shinai split across the narrow width of the stave about 10cm above the tsukagawa. The amazing thing is that this now broken shinai was of carbon make, not bamboo......................

[Kuri]

Think of the shinai as a cantilever

Hands |---------------^Force

Where the force is applied, the internal resisting moments (turning forces) are zero. As you move along the shinai towards your hands, the internal moments (bending moments) increase linearly. Therefore, the maximum moment will coincide with the position of your hands (which stop the rotation). If the force is low, then the maximum moment will be low and it will not excede the capacity of the shinai in resisting that moment. Increase the force until that capacity is eceeded, and then things start to break.

As the shinai does not have a uniform cross section all the way from the tip (kensen) to the handle (tsuka), the bending moment capacity of each section will be different. Also, if there is a sharp change in the cross section, it acts as a stress raiser which can magnify the stress at that point.

Also, as the nakayui acts to restrict the movement of the bamboo staves, then they all act as close to "one piece" around that point. Towards the kesen, they are not so restricted and so (I'm hypothesising here) their stiifness properties are reduced (lower second moment of area due to their ability to move into a position of the lowest energy state). Combine this with an incresing bending moment going towards the tsuka and the nodes (incresed diameter at the end of a chamber) which increase stiffness at that point and reduce them before and after, it would appear logical to have a break around that point.

Due to the material properties of bamboo that change with the length of time that impact occurs (strain rate), a lower force with a long impact time will fracture the shinai as compared to the same force with a shorter impact time. (A good reason to use more tenouchi and save your shinai!)

Is this all a bit confusing? Sorry, but it's very difficult to explain it without diagrams and the ascii figures really aren't much help. The above is for the easiest condition which is hitting someone with the tip of the shinai. If you hit lower (towards the nakayui) the above explaination needs to be modified to account for bending at the point of impact. But even if this is the case, the bending moment of the kensen will still be zero.

[Kuri]

The previous post is to have a fracture running across the bamboo staves. Anyone that's swung a shinai long enough knows that the staves also split longitudinally.

The above is still valid, but we should also consider the complimentary shear stresses. This means that if you apply a force (and consequently a stress) in one direction, a complimentary force perpendicular to the first will be induced.

Think of a diving board. Now, if you placed another plank on that board pefectly matching its size (unattached) and then stood at the end (ready to dive in), the boards will no longer have their edges coinciding. This is due to bending, and the boards are free to move once they overcome the force holding them together (friction). Glue the boards together and they'll act as one, until the shear stesses are high enough to ovecome that bond, split them apart and then the boards will act as 2 seperate pieces once again.

Normally, the strength of the material is high enough to overcome this effect, but in materials that are anisotropic (different properties in different directions), such as bamboo, the shear strength in the longitudinal ditection will probably be lower as it relies on the strength present between the longitunical cells.

Hopefully I don't have to write a part 3 :eek:

[Raígma]

My god!
How long did you study physics?

[nodachi]

Makes me glad that I changed out of engineering when I did, not that I have anything against engineers, they are some of the coolest people I know.

:)

[Steve]

As a mathematician, I've heard that engineers are just physicists who can't do real physics, and physicists are just mathematicians who can't do real math.

just something i heard.... ;)

[Neil Gendzwill]

You got me there. I haven't done a lick of real physics or real math in decades. Fell into programming out of college (EE degree) and have been on the dark side ever since.

Most mathematicians I know are programmers anyways. Academia or computers, not a lot of jobs in the middle for math geeks.

[Karaken]

Guess I shouldn't mention I'm a EE fallen into computers.. zz