Why do some materails dull a knife faster than others?

[Adamantium]

Hi. Okay, I know that stuff like hemp robe and cardboard and other fiberous materail dull a knife quickly. While other stuff won't do the job nearly as quick. I mean, I know that they do this, but I'm unsure on why and how they do.

So, for everyone who does, could you please shed some light on the subject.

Sincerely,
Adam

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Self improvement is a hobby of mine .

 

[Rick]

The material that dulls a knife quickly is simply more abrasive. I don't think it's any more esoteric then that.

 

[Sean Perkins]

Rick's right, and the level of abrasiveness has a great deal to do with the material's hardness as well; nothing dulls a blade quite as quickly as steel wool or aluminum oxide sandpaper.

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Sean Perkins
perkinsknives.com
seanperkins@yahoo.com

 

[Adamantium]

Sean, Rick: Yeah, anything that is super hard will make turn the edge nothing. Because when ever the edge strikes it it makes a little ding in it. But what about things that are much softer than steel?
So is it safe to assume that what makes a materail more abrasive is all the little fibers that are part of it?

Sincerely,
Adam

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Self improvement is a hobby of mine .

 

[Mike Turber]

It also involves the density of the material. The closer the density of the material is to the density of the material doing the cutting the faster it will dull.

Also the temperature of the item doing the cutting can alter this effect.

Hence the saying "cuts like a hot knife through butter".

A hot kife will slice through plastic, which is not notably dense but strong.

Many factors must be weighed in when determining the cutting ability of a knife. To many to make an effective formula for comparison reasons.

I am trying to come up with a formula that can be duplicated for comparison tests. My first use of this formula will be with my upcoming Trailmaster -vs- Basic #9 test.

Here is part of my idea.

I will do the cutting on top of a digital scale and in front of a video camera. This way you can tell I am not apllying more pressure on one knife as compared to another. This should stop the BS of saying that a test was fixed. Each cut which goes over a specified pressure will not be counted. I will use 1" hemp rope for the test. At the end of the test I will devide the number of cuts by the cost of the knife and this will give a overall score for this test. My other tests will use similar methods.

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Best Regards,
Mike Turber
BladeForums Site Owner and Administrator
Do it! Do it right! Do it right NOW!
www.wowinc.com

 

[Cliff Stamp]

Adamantium, water is much softer than rock but if it can still wear it away same thing as with knives. However the biggest problem with knife dulling is rolling and not the edge abrading as unless your knife is made from copper the latter takes a long time.

Mike that sounds pretty cool. However here is something for you to think about. Recently I was comparing the cutting ability of a custom fixed blade to a CS Recon Scout. I was surprised to find that the CS was actually easier to using cutting up boxes. This made no sense to me as the custom was much thinner and should have sliced though much easier.

The reason that the CS had the advantage was that due to its vast increase in weight and size (altering my grip) much more force could be exerted during a cut with me experiencing the same comfort level (feedback strain to my hand).

With this in mind I would be more interested in how the knives compare if you used both of them so that they causes you equal levels of strain / discomfort. This is how you guage performace in the field. The disadvantage to this method is that strain/discomfort are fairly difficult to measure. You could do timed runs or even hook a biofeedback monitor up to guage stress levels if you wanted to be really precise.


-Cliff

 

[Jeff Clark]

I don't think water channel erosion is a good analogy for cutting edge wear. Water weathers rocks in many ways. Most stream bed erosion comes from rocks and sand transported by the water scouring the banks. Another big effect is the solvent action of the water breaking apart soluable minerals that bind some rocks together. Then there is that ice expansion effect that cracks rocks apart. All these would be unusual mechanisms for edge wear.

A lot of materials will have dirt particles embedded in the structure. Quartz is pretty hard and a common soil contaminant. Abrasion that removes particles from the edge would depend on the hardness of the material that you are cutting and it's contaminants; however edge rolling is still the biggest dulling mechanism and primary to explain.

I think that you need to think in terms of forces against the edge. If you have a stiff material and on a microscopic level high spots push against the side of the edge unevenly there will be a tendency to push spots of the edge over sideways. The steel may have a higher elastic constant and yield strength, but sheer bulk can win out and lean over bits of the edge. The softer material will compress more than the steel, but you don't really notice. For example I can bend over a thin metal fence post with my much softer body. I lean my body more than the post leans and my palms compress more than the surface of the steel, but over it goes.

If everything were perfectly symmetrical and homogenous the edge would only roll due to pressure on the edge causing "column buckling". This would only happen if the edge is very thin. I would expect this to occur on a "wire edge" if it was standing straight up. Think of the edge like a sheet of paper. A solid slab of paper is tougher than the equivalent amount of wood, but when paper thin you can deflect it with something as soft as mashed potatoes. Pressing on the edge of a sheet of paper will cause it to bow. Continued pressure causes it to fold and not return to original shape.

This is why I try and finish my sharpening with some light strokes with a ceramic rod at a higher angle to try and cut off any wire edge. It's more involved than that, but you do want to minimise the edge burr if you want an edge that will last. Once an edge starts to roll the on-edge forces have an easier time to roll the edge further.

Back to the original question: Why do some materials dull a knife faster than others? I think that materials which are harder to compress (in local spots) will put more local bending pressures on the edge. Corregated cardboard may be weak as a structure, but the underlying paper fights being compressed or parted and can push back against an edge harder than bread or meat. Hemp fibers individually are hard to compress and have a high elastic modulus. Things like these dull knives more than you might think.

[This message has been edited by Jeff Clark (edited 21 June 1999).]

[This message has been edited by Jeff Clark (edited 22 June 1999).]