Why blades lose their sharpness

[Ken-Hawaii]

After many years of swinging swords, teaching tameshigiri, I always question why blades don't stay sharp. I ran into a fascinating study that might shine some light on that, https://www.zmescience.com/science/shaving-razors-blade-chips-46373453/

 

I've always had the impression that more extensive or complete cutting occurred with tangential cuts, but this kind of cutting may play a role of more rapid loss of sharpness.  Thinking about it, perhaps a tangential cut places more stress vectors on the side of the edge that makes an obtuse axis to the object. Fractures probably occur in a laminar fashion, parallel to the edge not supported by the material being cut.

 

This is what keeps tosho, & razor blade companies in business. Just speculating.

[Fuuten]

That gif gives me the creeps.. and I'm unsure what it is.

 

[Bruce Pennington]

I've always been puzzled that something like tempered steel can dull by cutting softer stuff.  That article discussed the study's discovery that cutting the hair created micro chips and cracks, but I didn't see any explanation as to HOW hair (or anything else softer than steel) can bust steel, even at the microscopic level.

 

Where's our metals guys (@IJASWORDS)

[PNSSHOGUN]

From what I could ascertain: when the blade edge is deep in the hair it creates torsion on the microscopic edge, resulting in cracks or chips?

[Ken-Hawaii]

That's right, John. The blade isn't actually unsharpened, jsut micro-chips forming, from what I gathered.

[Shugyosha]

2 hours ago, Bruce Pennington said:

I've always been puzzled that something like tempered steel can dull by cutting softer stuff.  That article discussed the study's discovery that cutting the hair created micro chips and cracks, but I didn't see any explanation as to HOW hair (or anything else softer than steel) can bust steel, even at the microscopic level.

 

Where's our metals guys (@IJASWORDS)

 

I shave with a straight razor pretty much daily at the moment. The received wisdom is that cutting a human hair is the equivalent of cutting a copper wire of similar diameter so it distorts the edge over the course of a shave and creates the need to strop the razor ahead of the next shave. Stropping doesn't make the blade sharper but realines the edge (and presumably smoothes out any micro chips) and greatly adds to the comfort of the shave.

 

Presumably cutting a tatami, particularly with the force created when the blade impacts the target is going to have a similar effect and I suppose this is where "niku" or the profile of the edge can make a difference to its durability and ability to retain sharpness.

 

Ken, how much cutting does your weapon of choice take before it needs "fettling"?

[Ken-Hawaii]

About 10 years ago, I was teaching tameshigiri at least once a week, & I had it polished twice that year. It's a Cold Steel katana, & one of the toughest modern blades that I own. Excellent for cutting.

[16k]

I second that! Cold 

steel ,ale beasts, their sword may not look like the real deal but they cut like hot wire in butter. And they are sturdy!

[celt72]

I studied Ishi Yama Ryu under Russell McCartney years ago and was involved in his Senbongiri event back in 2000. He performed 1181 cuts from kata without a miss over 1 hour 25 minutes. The sword he used was a Nakata Kanehide blade that was remounted and freshly polished. What was interesting was that over the course of the 1181 cuts through damp igusa goza, the friction of the cuts heated the monouchi up and caused the edge to fold over slightly! The fact that it performed so well was amazing. I was deeply impressed with the strength and cutting ability of the blade and after that Kanehide became one of my favorite smiths.

[b.hennick]

I am impressed with the man weilding the sword. 

[Jacques]

It's just a matter of physics, there is friction and when there is friction, there is always wear whatever the material cut. If you look at a cutting edge under a microscope, you will see that it is not smooth but micro-serrated... these are these teeth that tear off during friction and when the cutting edge is smooth, it is thicker and does not cut anymore.

[Bazza]

8 hours ago, celt72 said:

I studied Ishi Yama Ryu under Russell McCartney years ago and was involved in his Senbongiri event back in 2000. He performed 1181 cuts from kata without a miss over 1 hour 25 minutes. The sword he used was a Nakata Kanehide blade that was remounted and freshly polished. What was interesting was that over the course of the 1181 cuts through damp igusa goza, the friction of the cuts heated the monouchi up and caused the edge to fold over slightly! The fact that it performed so well was amazing. I was deeply impressed with the strength and cutting ability of the blade and after that Kanehide became one of my favorite smiths.

Well, ffolkes, I went a-looking and found this:

 

https://www.youtube.com/channel/UCTkrKjXeHog5CaktlRn8gvw

 

WoW... 

 

And the sword smith himself:

https://new.uniquejapan.com/wp-content/uploads/2015/09/22-ujka163-Kanehide-Katana.pdf

 

BaZZa.

Edited May 30, 2021 by Bazza
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[Ken-Hawaii]

It would be interesting to ask a high-level togishi what he "feels" when he starts repolishing a heavily-used blade.