Edge Structual Weakness - Plastic Crushes Steel

[grepper]

This test was performed on a a piece of the cold rolled steel used in the EOU SET tests before hardening.  It is unhardened cold rolled steel.  I sharpened it to a 20 dps bevel using a well used 600 grit 3M Microfinishing film belt and gently deburred using a leather belt on the Kally.  Obviously I did not do a good job of removing burr.
 
The shiny line at the top of edge is remaining burr.
 
The dent in the edge is from the test media, and clearly penetrates well below the edge.  The edge tested 840.

     
 
As the sharpness test was performed, first the burr was crushed against the edge increasing edge width and providing enough support for the test media to allow for more pressure.  As more pressure was applied, the steel edge started to collapse, and as it crushed under the test media load the edge became wider and duller further preventing cleaving of the test media. 
 
As still more pressure was applied, the steel collapsed further and the edge became duller allowing for even more pressure to be applied.  Because the edge was a 20 dps bevel, the edge width continued to increase until the test media finally severed @ 840g.
 
I’m pretty sure I could actually feel that process happening.  The harder I pressed the more pressure I could apply.  It was an interesting feeling.  I’m not really sure the final edge actually cut the test media or the media just ripped in half.  840g is a lot of pressure.
 
It is very interesting that the tiny little plastic test media only of only .009” diameter can actually crush steel.  Yes, the steel was not hardened, but it is steel nonetheless and test media is just plastic.  How can that be?
 
I think it demonstrates how structurally weak knife edges are because they are so incredibly thin.  Even hardened steel rolls when stressed under only 150gf.  That’s not much force.  It takes far more force than that to slice a carrot or potato.
 
While HRC 62 steel may be harder than HRC 50 steel, that hardness difference is more or less irrelevant because our sharp edges are so thin and therefore structurally weak.  It’s the supporting a car on one egg or two type of thing.   While two eggs would provide twice the support the difference is grossly insufficient under that amount of load.

Anyway, I found actually observing a tiny little piece of plastic crushing steel to be a good example of how structurally weak very thin steel edges are.

[KnifeGrinders]

This test was performed on a a piece of the cold rolled steel used in the EOU SET tests before hardening.  It is unhardened cold rolled steel.  I sharpened it to a 20 dps bevel using a well used 600 grit 3M Microfinishing film belt and gently deburred using a leather belt on the Kally.  Obviously I did not do a good job of removing burr.
 
The shiny line at the top of edge is remaining burr.
 
The dent in the edge is from the test media, and clearly penetrates well below the edge.  The edge tested 840.

 
 
As the sharpness test was performed, first the burr was crushed against the edge increasing edge width and providing enough support for the test media to allow for more pressure.  As more pressure was applied, the steel edge started to collapse, and as it crushed under the test media load the edge became wider and duller further preventing cleaving of the test media. 
 
As still more pressure was applied, the steel collapsed further and the edge became duller allowing for even more pressure to be applied.  Because the edge was a 20 dps bevel, the edge width continued to increase until the test media finally severed @ 840g.
 
I’m pretty sure I could actually feel that process happening.  The harder I pressed the more pressure I could apply.  It was an interesting feeling.  I’m not really sure the final edge actually cut the test media or the media just ripped in half.  840g is a lot of pressure.
 
It is very interesting that the tiny little plastic test media only of only .009” diameter can actually crush steel.  Yes, the steel was not hardened, but it is steel nonetheless and test media is just plastic.  How can that be?
 
I think it demonstrates how structurally weak knife edges are because they are so incredibly thin.  Even hardened steel rolls when stressed under only 150gf.  That’s not much force.  It takes far more force than that to slice a carrot or potato.
 
While HRC 62 steel may be harder than HRC 50 steel, that hardness difference is more or less irrelevant because our sharp edges are so thin and therefore structurally weak.  It’s the supporting a car on one egg or two type of thing.   While two eggs would provide twice the support the difference is grossly insufficient under that amount of load.

Anyway, I found actually observing a tiny little piece of plastic crushing steel to be a good example of how structurally weak very thin steel edges are.

One of your HRC56 Victorinox knives, if sharpened at 8 dps, will show similar denting when pressed against the test line and score 400-800 BESS in the dent.
- pretty similar to the unhardened steel edge at 20 dps.

Once for fun I was testing an unhardened blank honed so successfully that its edge was catching a hair, but it would crush on the BESS media line and score near 1000 BESS.

We've amassed plenty of facts that a well sharpened edge is a matter entirely different to the blade body.

[EOU]

it is an interesting situation. DE razor blade edges don't dent, microtome edges don't dent and knife edges that measure 25 don't dent. If these edges did dent then these edges would measure much higher than 25 or 50. Edges formed from soft metal can dent and sometimes, according to KG's experiences, edges formed from some hardened steels can dent. We have witnessed test media denting on medical edges composed of non-hardenable stainless steel.

There is no component used in the makeup of BESS test media that even approaches the hardness of any steel, soft or hard. It would seem that even the sharpest edges that are formed in a structurally sound manner don't dent. We can say that the presence of burr (or a  wire edge) often plays a key role here but only with edges that are structurally weak to begin with. When the burr folds over the structurally weak edge, several hundred grams of force may now be applied to the edge. If the underlying material is weak, it may dent. It's not difficult to detect a dented edge, they measure from 600 to maxing out the BESS scale.

[KnifeGrinders]

it is an interesting situation. DE razor blade edges don't dent, microtome edges don't dent and knife edges that measure 25 don't dent. If these edges did dent then these edges would measure much higher than 25 or 50...
We can say that the presence of burr (or a  wire edge) often plays a key role here but only with edges that are structurally weak to begin with. When the burr folds over the structurally weak edge, several hundred grams of force may now be applied to the edge. If the underlying material is weak, it may dent. It's not difficult to detect a dented edge, they measure from 600 to maxing out the BESS scale.

This structural weakness in the edge must be caused by Burnishing.
The way I finish our edges, edge-trailing on paper wheels, goes with burnishing, dragging metal from bevels over the edge apex.
When the knife is of a quality steel I get sharpness testing <= 50, but when it is a lower-end budget/junk knife it tests as Mike describes.

For example, a cheap knife sharpness scores. The edge was set on #400 at 14 dps, and then deburred on 2 paper wheels - one with a 5 micron diamond paste at a higher angle of 15.6 dps, followed by a paper wheel with 0.5 micron diamonds at the exact edge angle of 14 dps.

Off the #400 (visible burr) = 555 BESS
Paper wheel 5 mcrn @ +1.6 degree higher twice, alternating sides = 165 BESS
Paper wheel 0.5 mcrn @ exact edge angle once, alternating sides = 495 BESS (visible dent)
Paper wheel 5 mcrn @ +1.6 degree higher twice, alternating sides = 125 BESS
Paper wheel 0.5 mcrn @ exact edge angle, alternating sides: once = 330 BESS; twice = 700 BESS (visible dent)

Apparently, in honing at the exact edge angle burnishing creates the structurally weak edge apex that then behaves as Mike has described, i.e. folds allowing to apply more force against the test line to crush and dent. While the high-angle finish allows to get a stable shaving edge.

Todd Simpson on his scienceofsharp.wordpress.com has shown by SEM that straight razors finished edge-trailing on a strop with 0.5 micron diamonds invariably get a wire edge. Todd recommends to finish straight razors by high-angle stropping on a hanging strop with a metal polish (they commonly have abrasive particles of 3-6 microns in size).

This reminds me of the Mutant Burr thread - that  was observed by Grepper when manually stropping on a leather strop; nowadays Grepper strops on a leather or cotton belt on a belt grinder, and the burnishing effect should be more pronounced.

Paper wheel or belt grinder stropping drags enough metal from bevels over the apex to overlay a new extra-thin edge.
This edge will split a hair, but in cheap stainless steel blades will not hold a load.
This edge is also prone to the post-sharpening loss of sharpness discussed in the neighboring thread

We've been hashing these phenomena long enough to see the two contributing factors that play role in each: the stressed edge apex we create when sharpening, and the weak ultra-edge we create by burnishing when deburring.

The wire or foil edge created by burnishing has properties different from the underlying edge, it resembles the unhardened steel in behaviour and in how it tests on the sharpness tester.