shinai hairline cracks

[moetl]

hi folks,
i bought the "musim" shinai from kendoshop 2 or 3 weeks ago - now it shows hairline cracks on the inside as pictured in the attachment.
i sanded it and oiled it with a cloth wrapped around, which i dipped in raw linseed oil before. the cracks did not show before i oiled it - maybe they were made visible through the oiling process.

my questions are now: can i still use this shinai safely for practise? and where do these cracks come from?

thanks!

[Sentunim]

I had those when I got a shinai from my dojo, I also had an H shaped crack on the surface of the outside. It seems to be okay (mine atleast), even with beginners pounding on it(im a beginner too, and I've done my fair share of pounds). I could probably sand off the surface crack.

I don't know what to say about those things though.

[GBN]

I've seen such things before on my own shinai's. None of them have ever broken. I'd say you are probably ok.

Brian

[Zaphiel]

hi folks,

my questions are now: can i still use this shinai safely for practise? and where do these cracks come from?

thanks!
they might come from these little beetles that eat bamboo.......
this happened to me 2 weeks ago......
try if white powder comes out of the shinai......as for the use:
I don't see why you shouldn't still use it.....

[moetl]

they might come from these little beetles that eat bamboo.......LOL, i don't think there are little bamboo eating beetles living in my shinai :D

sorry, couldn't restist - it just sounded too funny. i know that these varmints exist ;)

[Sentunim]

Exactly, mine appeared in the winter, and I live in canada, and it was reaching -30 this winter. Couldn't have been beatles.

[Zaphiel]

just an idea........sorry

[streetcleaner]

i use only musim shinais last 2 year - this lines are from oiling bamboo and they are not cracks - its just some lines.

[moetl]

zaphiel, i meant no harm. no need to say sorry :)

streetcleaner, i think this are cracks, because the lines do not only appear on the surface, but also go deep - about half a millimeter.

[mystic_kendoka]

where are the cracks? under the tsukagawa or on the daitotsubu?

[moetl]

the cracks are erratically distributed in a few "bundles" on the inner part of the shinai, exept were the tsukagawa is. additionally there are some cracks at the joints.

[Marine_Boy]

I have viewed these cracks under a high powered microscope and found that they do not impare the structural integrity of the bamboo what so ever.

A good coat of oil would do.

[mystic_kendoka]

you actually DID that? :S:S:S

[Masahiro]

I have viewed these cracks under a high powered microscope and found that they do not impare the structural integrity of the bamboo what so ever.

A good coat of oil would do.

you have my applause. .. ...<clap, clap, clap, clap>

[samurai999]

hi folks,
i bought the "musim" shinai from kendoshop 2 or 3 weeks ago - now it shows hairline cracks on the inside as pictured in the attachment.
i sanded it and oiled it with a cloth wrapped around, which i dipped in raw linseed oil before. the cracks did not show before i oiled it - maybe they were made visible through the oiling process.

my questions are now: can i still use this shinai safely for practise? and where do these cracks come from?

thanks!

(rolling up sleeves and trying to utilize engineering know-how..)

You should still be able to use the shinai. The bamboo's strength is greatest if stressed in the "fiber direction". I don't want to go too much in to engineering principles, but the cracks in the shinai (in your case) are usually caused by high axial stresses in areas where there are no fibers (ie no "inner support"). Why do you think that every time we hit men or kote that the shinai never breaks? The forces on the shinai are pretty much perpendicular to the surface of the shinai everytime the shinai impacts something and the resulting stresses are always put on the fibers.

You will have a problem if the shinai is "split". I never use a shinai that is split in half. The split crack is along the direction of the fibers and therefore will propogate each time you give the shinai a good, hard whack. Why is that? There are fibers running the length of the shinai, and between, there is usually something filling the void. Some membrane that holds the fibers together. That membrane is very weak when stressed and will fail easier compared to loading the shinai in the previous example. If you load the shinai in that direction, CRACK!!!. Plus, the fibers flake off from thereand is pretty dangerous.

my 0.02$(US),
Tim

[samurai999]

Its great when I can actually attempt to apply the principles I learn in school to something I do in real life. :D LOL

Tim

[moetl]

...the cracks in the shinai (in your case) are usually caused by high axial stresses in areas where there are no fibers (ie no "inner support")...thanks for the info and for your long reply - this is quite plausible! just one question left: where do these axial stresses come from?

most likely from the manufacturing process...? where else could these axial stresses appear. i mean - i didn't wring the shinai :D. it would be very interesting to see, if an expensive shinai (=high quality bamboo+proper manufacturing process) shows the same cracks.

initially i posted this thread, because i saw in the book "kendo, lehrbuch des japanischen schwertkampfes" an illustration, where such a horizontal crack widens if you bend then shinai (inner side streched, outer side compressed). the crack does not continue to go deep, but horizontal - a tongue froms. this seemed rather dangerous to me..
btw zaphiel, they also mentioned those bugs ^^

but since streetcleaner has used for over 2 years and nothing has happened so far, it can't be such a problem..

[Marine_Boy]

you actually DID that? :S:S:S

Yes I worked in a materials lab for a few months and they had both high powered and electron microscopes.

Tim, great analysis! :smoker:

[samurai999]

Ya.. We just finished a composite bridge project at my university and the main principles are similar to bamboo or any type of wood for that matter so the words sorta "flowed".

As for the high axial stresses, the main equation for stress is
Stress=Force/x-sect Area (so it is like pressure). The main idea is, as the cross sectional area gets smaller, you get bigger stresses and BOOM... Failure. The forces applied in kendo commonly impose bending stresses on the bamboo with the greatest forces on the slat that impacts the men. This is assuming of course that you hit perfectly straight on. These types of stresses vary along the thickness of the bamboo and is usually greatest at the inner and outer surfaces. There are other types of stresses such as shear and torsion. I doubt you'd twist the shinai on purpose unless you tsuki and twist so the shinai doesn't see too much torsion. You should see shear as well, but from your picture, it seems to be cracking due to axial stresses caused by bending. Other things may be factors such as anisotropic properties (non-uniformity) througout the x-sect of the slat. Also note the high points of the inside of the slat. These high points would have the greatest bending stresses imposed on it.

Where is the slat pictured located on your shinai? I'm guessing it is the slat that impacts the men. That type of cracking (if I see it correctly) is caused by tensive stresses. This means that the slat (in that local area) is being pulled apart. If there are no fibers or reinforcements in this area, you only have the weak "membrane" to hold it's own so it starts to crack since due to high bending stresses. This is what I propose happened. As for the crack propagating, you don't have to worry about it too much since there are fibers there to reinforce the rest of the slat. Again, like I said before, I would worry about it if the bamboo is cracked along the length or split. I have shinai that cracked like yours and it was still ok for shiai. It is weakened obviously, but not to the point where it is an immediate cause for concern.

Any engineers out there want to validate or disprove my theory? Sorry for the long response, but this stuff is very detailed. Plus, I sorta wrote kinda quick so some things might be a bit hard to understand... Anyways, there are books and books and books on this stuff and this is what I have learned.

If you were in my dojo, I'd bring a shinai slat and demonstrate for you.

Tim

[ben]

... in the book "kendo, lehrbuch des japanischen schwertkampfes" an illustration, where such a horizontal crack widens if you bend then shinai (inner side streched, outer side compressed). the crack does not continue to go deep, but horizontal - a tongue froms...

My experience with shinai would agree with this statement. IMHO you should be wary of such cracks Moetl. Whenever I've come across them, one of them has always been the place where the whole slat breaks, forming this "tongue". So the shinai might be alright "for now", but how much longer? I'd recommend completely dismantling that shinai every week to see if any have worsened.

I've never come across these cracks in new shinai, only ones that I've been using for some time. In my own mind I call them "percussion cracks" because they seem to be a result of repeated blows. However I am no engineer and this is just educated guesswork.

The fact that Kendoshop are shipping new shinai with these cracks is enough for me to think twice (at least) before ordering their shinai.

b

[Masahiro]

it would be very interesting to see, if an expensive shinai (=high quality bamboo+proper manufacturing process) shows the same cracks
Indeed, if you consider "Akatsuki" shinai from e-bogu to be the "expensive, high quality bamboo+proper manufacturing processed" shinai then I can tell you from experience (as I do practice with Akatsuki shinais) that these same "hair line" cracks also occur in the expensive shinais.(ranging from Akatsuki to the top madake eguchi shinais) However, it has been my experience. Shinais that are naturaly dried (not dried by a kiln) exhibit few "hair-line cracks". Only one or two exist on the tenouchi part of the bottom slat (the one which makes contact with the mengane upon men-uchi) on my Akatsuki. If you want my hypothesis, I would say these cracks result from a combination of lack of sufficient oil absortion, constant bashing against an opponent's mengane, and old age. (yes, bamboos do have an age limit after which you shouldn't use)

[streetcleaner]

Where is the slat pictured located on your shinai? I'm guessing it is the slat that impacts the men. That type of cracking (if I see it correctly) is caused by tensive stresses.
Tim

wrong guess ;) as i said - this cracks appears on a completely new shinais _after_ been oiled for a long period of time. i oil me new shinais about 2-3 weeks before practice with them. if not oiled, bamboo on this shinai does not have this cracks or line

p.s. i dont know about any other sinais - i bougt only this musim shinais for about 3 years. they are bad, but they a very chep

[moetl]

..Where is the slat pictured located on your shinai? I'm guessing it is the slat that impacts the men...not quite. as i wrote before, these cracks appear in "bundles" - a number of cracks close together - (as can behope fully seen in the attachment) and those bundles again appear in no special order on the whole slat, except where the tuskagawa is - on all 4 slats..! what would be interesting to know too is how these bundles are distributed... but my shinai is assembled at the moment and right now i'm too lazy to disassemble it :)

so the theory that these cracks appear because of hitting a target can be abandoned IMO. streetlceaner too said that the cracks are also on shinais that have NOT been used before. i'd rather stick to the theory that the cracks on MY kendoshop shinai are a result of (bad?) manufacturing. however, i'd agree with the theory that such cracks can also develop through hitting - as tim, ben and masahiro pointed out.

i believe, that cracks caused by manufacturing and hitting are different. i'm not sure if you can see on the picture that "my" cracks are not straight, but rather angular (sorry for the bad voc.). i could imagine that cracks which have their origin in hitting are straight AND do not appear in bundles? just a guess.

i'm not quite sure what to think of streetcleaner's oiling theory though.

[samurai999]

Oops! my bad. I thought it was present only after you did some shiai. My theory (as usual) is null and void. LOL :D

There are effects to be considered during shipment however. If the shinai is being shipped over long distances, high heat to extreme cold conditions which could thermally expand and contract the shinai which could result in warping and some hairline cracks. Also, moisture due to condensation can set in as well, though dessicant packs can keep the moisture from setting.

my 0.02$(US) yet again...

Tim

[mystic_kendoka]

if their in random bundles, maybe it could be the way they were packed? wires to tie them together? does anyone know the commercial way of packing shinai's for shipping?

[samurai999]

if their in random bundles, maybe it could be the way they were packed? wires to tie them together? does anyone know the commercial way of packing shinai's for shipping?

They normally ship them with the red string around the outside of the bamboo slats (ie the shiny surface) with the 4 slats "assembled". THey never ship them (at least the ones i know) wrapped with wires or tie wraps.

By the way, the slat has those cracks on the inside of the slat. If the wires were wrapped too tightly, you'd see the imprint of the wires on the inside instead of cracks.

The only other way i can see those cracks happening with the wire is if they wrapped it so tightly with the wire around the outside that the slats were packed "bent". That plus the environmental conditions during shipping may have had some effect. Again.. This is one of my new crackpot theories. :D lol

Tim

[mystic_kendoka]

its MY theory, u just developed it! :P

[Sentunim]

Well, normally, I think they boil them in oil and leave them to dry(for like 6 months or something) when they make them. Maybe when they are oiled by the user the cracks just appear more prominently than before. I dont know though. That would mean they appear on everyone's shinai, which they don't, I gather.

[Zaphiel]

hi folks,
i bought the "musim" shinai from kendoshop 2 or 3 weeks ago - now it shows hairline cracks on the inside as pictured in the attachment.
i sanded it and oiled it with a cloth wrapped around, which i dipped in raw linseed oil before. the cracks did not show before i oiled it - maybe they were made visible through the oiling process.

my questions are now: can i still use this shinai safely for practise? and where do these cracks come from?

thanks!Sorry:
....hey moetl bin etwas verwirrt!
serenity hat mal einen namen erwähnt....glaub martina(bin nicht sicher aber was mit m am anfang) bist du das???

[mystic_kendoka]

what did he know? please translate... this is excruciating as i only understand half of it..

[moetl]

Sorry:
....hey moetl bin etwas verwirrt!
serenity hat mal einen namen erwähnt....glaub martina(bin nicht sicher aber was mit m am anfang) bist du das???ja das bin ich, aber was hat mein thread damit zu tun (ich meine, warum hast du das gequotet)? und warum bist du verwirrt?

mystic: it's nothing contributing to the thread ^^ just official PM, you know...

[moetl]

i disassembled my shinai today and the hairline cracks stayed exactly the same as they were when i started this thread. they did not widen, not split, not increase, nothing - just stayed hairline cracks.


thought it might be interesting to now :)

[mystic_kendoka]

i got those cracks as well!

i didnt have them when i bought them, but after 5 months usage, the cracks have formed... i guess they are from stress applied on the wood as samurai999(tim) said..


the cracks are erratically distributed in a few "bundles" on the inner part of the shinai, exept were the tsukagawa is

mine has cracks on the tsuka as well.. :S maybe its because of my desperate tsuka-doh-blocks...

[Zaphiel]

ja das bin ich, aber was hat mein thread damit zu tun (ich meine, warum hast du das gequotet)? und warum bist du verwirrt?

mystic: it's nothing contributing to the thread ^^ just official PM, you know...
warum???......nun ja die PMs funktionieren nicht mehr.......

[Zaphiel]

what did he know? please translate... this is excruciating as i only understand half of it..
you can speak german???......wow I'm really impressed!
what's your name?
I know it's not really possible but I once knew a korean guy who moved back to south-korea.........well I just hoped it could be you...
his name was Su-hjuk(this isn't right...but the name sounds like this) Chi.

[mystic_kendoka]

umm.. i never moved back to south-korea.. im still in belgium... and have been for all my life...

i dont speak german but i understand half of it, since im fluent in dutch/nederalands/flemish whateva u wna call it.. its quite similar so i understand some but not all..

and my name is Seoung Houn or Seung Hun or Sungi for short
(the reason my official name is SeoungHoun with 2 extra useless letters is because the guy who made my birth certificate was either stupid or drunk..)

[Zaphiel]

umm.. i never moved back to south-korea.. im still in belgium... and have been for all my life...

i dont speak german but i understand half of it, since im fluent in dutch/nederalands/flemish whateva u wna call it.. its quite similar so i understand some but not all..

and my name is Seoung Houn or Seung Hun or Sungi for short
(the reason my official name is SeoungHoun with 2 extra useless letters is because the guy who made my birth certificate was either stupid or drunk..)
hahahahahahahahaha
I'm so embarresed......:-)
sorry for bothering you!
still...hahahahaha!
just thought you might live in korea because of the flag next to your name
.......why don't I have a german flag than?.......really I've got a lot to think about now......

[Armageddon]

Just wondering how can i look at the photo? I'm new on this forum and i can't seem to find the attachment of the hairline cracks.

[moetl]

hi armageddon,
you can't view the attachment, because i removed it. but the hairline cracks on my shinai look about the same as yours.

btw: the cracks on my shinai did not widen or spread

cheers,
martina

[Himura. Kenshin]

Hi,

Regards to hairline cracks, I got them too in my new shinai.

I got my shinai from nine-circle, and after oiling them, cracks started to form. Will see if they spread in the next month or so.

as long as it doesn't propogate, I think its not a problem !

[heri0n]

the cracks are most likely from the bamboo being dried out
when the oil seeps into them it makes it more visible..
basically u got a cheap quality shinai :P

[Himura. Kenshin]

I'm going to see how long it'll last hehe....it costs me 20 pounds ! in the UK ! It better last at least 4 months !

[Kendoka]

hi folks,
i bought the "musim" shinai from kendoshop 2 or 3 weeks ago - now it shows hairline cracks on the inside as pictured in the attachment.
i sanded it and oiled it with a cloth wrapped around, which i dipped in raw linseed oil before. the cracks did not show before i oiled it - maybe they were made visible through the oiling process.

my questions are now: can i still use this shinai safely for practise? and where do these cracks come from?

thanks!

Please show it to your sensei.

[EzzzE]

just got 2 shinai from kendoshop and want to inspect them, now my question is: how do i dissemble them the easiest way? is untying the sakigawa the easiest way? the tsuru seems a little bit more diffucult to untie...and how on earth do they get the cord so tight? does it have to be that tight that it makes a sound like pulling a guitar string?

greets

EzzzE

[Kendoka]

just got 2 shinai from kendoshop and want to inspect them, now my question is: how do i dissemble them the easiest way? is untying the sakigawa the easiest way? EzzzE
Untie the tsuru (string) where it holds the tsuka-gashira (handle cover), then slip the sakigawa off. You should be able to inspect the staves properly without removing the tsuka-gashira.

untie...and how on earth do they get the cord so tight? does it have to be that tight that it makes a sound like pulling a guitar string?
greets
EzzzE
Yes. More like a koto actually!