Edge Fatigue -- Real or B.S.?

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Sharperthansticks

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Sep 19, 2017
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I've heard this "edge fatigue" term occasionally bandied about, and I'm not sure if it corresponds to an actual physical, measurable phenomenon, or if someone just made it up. Does anyone have some detailed information on this? I've never heard Cedric & Ada even mention it.
 
Never seen the term used.
However, the only "edge fatigue" I can envision would be most commonly found in kitchen knives.
There are two ways that a knife will lose its edge.
1) wear
2) the edge bends.
It's why you steel a kitchen knife...to straighten the edge. The edge bends over from hitting a hard surface and must be straightened. After a number of straightenings, the steel fatigues and breaks. Then the knife must actually be sharpened.
 
knarfeng said:
Never seen the term used.
However, the only "edge fatigue" I can envision would be most commonly found in kitchen knives.
There are two ways that a knife will lose its edge.
1) wear
2) the edge bends.
It's why you steel a kitchen knife...to straighten the edge. The edge bends over from hitting a hard surface and must be straightened. After a number of straightenings, the steel fatigues and breaks. Then the knife must actually be sharpened.
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What you are describing sounds like what I think used to be called "beer-canning." In any event, I just heard the term "edge fatigue" today again -- this time in a video by Big Brown Bear, where he claimed that the "wrong" abrasives could cause this phenomenon.
 
Sharperthansticks said:
What you are describing sounds like what I think used to be called "beer-canning." In any event, I just heard the term "edge fatigue" today again -- this time in a video by Big Brown Bear, where he claimed that the "wrong" abrasives could cause this phenomenon.
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I believe that to be "suitable for lawn enrichment".
 
From wikipedia:

Fatigue (material)

In materials science, fatigue is the weakening of a material caused by cyclic loading that results in progressive and localized structural damage and the growth of cracks. Once a crack has initiated, each loading cycle will grow the crack a small amount, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.

If the loads are above a certain threshold, microscopic cracks will begin to initiate at stress concentrations such as holes, persistent slip bands (PSBs), composite interfaces or grain boundaries in metals.[3] The nominal maximum stress values that cause such damage may be much less than the strength of the material, typically quoted as the ultimate tensile strength, or the yield strength.
 
Currawong said:
From wikipedia:

Fatigue (material)

In materials science, fatigue is the weakening of a material caused by cyclic loading that results in progressive and localized structural damage and the growth of cracks. Once a crack has initiated, each loading cycle will grow the crack a small amount, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.

If the loads are above a certain threshold, microscopic cracks will begin to initiate at stress concentrations such as holes, persistent slip bands (PSBs), composite interfaces or grain boundaries in metals.[3] The nominal maximum stress values that cause such damage may be much less than the strength of the material, typically quoted as the ultimate tensile strength, or the yield strength.
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Yes.
Which is why the the choice of abrasive media itself is separate from anything which would cause fatigue, unless you are putting so much pressure on the blade that the pressure is enough to fatigue it. But that is "amount of pressure", not "choice of abrasive media".
 
knarfeng said:
Yes.
Which is why the the choice of abrasive media itself is separate from anything which would cause fatigue, unless you are putting so much pressure on the blade that the pressure is enough to fatigue it. But that is "amount of pressure", not "choice of abrasive media".
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I don't know much about it, but I wonder if this is true at the macro scale but not at the micro scale? I'm thinking about molecules hitting molecules during sharpening.
 
Currawong said:
I don't know much about it, but I wonder if this is true at the macro scale but not at the micro scale? I'm thinking about molecules hitting molecules during sharpening.
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It doesn't work that way.
 
Well then how the heck does it work lol. So far this is a debate between two people who haven't said anything.
 
Currawong said:
Well then how the heck does it work lol. So far this is a debate between two people who haven't said anything.
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I've said how it does NOT work. (choice of abrasive in and of itself)
And I've provided a possible scenario for what might actually be happening. (too much pressure)
 
Currawong said:
I don't know much about it, but I wonder if this is true at the macro scale but not at the micro scale? I'm thinking about molecules hitting molecules during sharpening.
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Steel is not comprised of molecules.

I haven't watched Bear's video yet, but Knarfeng's thinking aligns with mine.

If there is a lot of pressure on the apex, I could see metal fatigue setting in on a very small part of the edge, especially if the edge angle is especially acute. I suppose large abrasives could push the apex out of line, and if done repeatedly, it could cause very small, very localized fatigue. But I don't get the sense when sharpening with coarse stones that I'm doing anything to move the apex -- just the burr.

I'd guess that steeling would cause the most edge fatigue. I once rolled the entire edge of an old Sebenza. It steeled back into shape easily, but that re-rolled edge was not very durable. I had to resharpen it.

It will be interesting to see Bear's video explanation.
 
Are you talking about a kind of work hardening, like when you bend a wire paper clip back and forth until it becomes brittle and breaks? I suppose if you're too heavy handed with a honing steel...:confused:
I don't see that being an issue with normal sharpening.
 
Twindog said:
Steel is not comprised of molecules.

....
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I was going to 'correct' you on that first part (weren't we all taught molecules were basically everything?) ... but then decided to actually see what's up, and wiki says:

The definition of the molecule has evolved as knowledge of the structure of molecules has increased. Earlier definitions were less precise, defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties.[13] This definition often breaks down since many substances in ordinary experience, such as rocks, salts, and metals, are composed of large crystalline networks of chemically bonded atoms or ions, but are not made of discrete molecules.
 
Thanks Twindog. Yes good point, molecules was the wrong word, I just meant small particles like carbides or atoms. Something must hit something at the micro scale otherwise how do you grind away metal? Bear talks in the linked video about this happening in high hardness / large carbide steels. Which makes me think about particles / carbides in the stone and the steel hitting each other repeatedly at the micro scale and causing fracturing rather than bending. I think what he's saying is that something of lower hardness (particles in a non-diamond stone) hitting something of higher hardness (e.g. rex121 at 70rc) over and over could cause fractures at very small scales, and in a very thin edge this might weaken the apex.
 
Currawong said:
Thanks Twindog. Yes good point, molecules was the wrong word, I just meant small particles like carbides or atoms. Something must hit something at the micro scale otherwise how do you grind away metal? Bear talks in the linked video about this happening in high hardness / large carbide steels. Which makes me think about particles / carbides in the stone and the steel hitting each other repeatedly at the micro scale and causing fracturing rather than bending. I think what he's saying is that something of lower hardness (particles in a non-diamond stone) hitting something of higher hardness (e.g. rex121 at 70rc) over and over could cause fractures at very small scales, and in a very thin edge this might weaken the apex.
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We really need Bear to explain what he means. I did watch his video. He says the edge fatigue occurs with stones softer than diamonds and CBN. I think the idea is that softer stones are unable to cut the large, hard carbides in the high-carbide steels; so they plow up the softer steel matrix, leaving the carbides standing proud on the edge. In that condition, they can be knocked out of the apex, leaving a fracture point that damages the edge and reduces cutting performance.

With diamonds, the carbides are sharpened along with the matrix, so you don't get that carbide pullout.

I think this effect would be most likely in steels like 440C, D2 and ATS 34, where the carbides are large and often clumped together.
 
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