Asking For Help : Understanding Knife Mechanics

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nullack

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As part of my new objective to produce the best survival knife I possibly can, I figured it was time to call a colleague of mine. He is well known as a metal guru and has established a reputation for saving companies large sums of money byby applying better suited materials to tooling.

So I call him and explain what I'm on about. His first question "Are you seeing abrasive wear, adhesive wear or a mixture of both?". Then I was embarrassed as I realised I was not properly prepared for the talk and would call him back.

The more I think about the mechanics of a knife in a survival knife type situation the more I get confused. Talks seems to be pretty general on it. Like people say, oh, it needs to resist wear. Right! I can think of occasions where a survival knife would have adhesive wear. Abrasive wear would be more common at a guess but I dont know how much of one vs the other might be typical for a survival knife.

Then people say it needs to be tough. In engineering terms do they mean:

Fracture toughness
Notch toughness
Impact toughness

And in what proportion might a survival knife have priorities over the different types?

Strength seems to be the easy property of the lot as I would say there that you want the highest possible yield strength so as to absorb all forces in the elastic range and avoid plastic deformation.

Ive tried to think of it in terms of tasks like:

Cutting animal skin
Cutting animal flesh
Cutting animal bone
Cutting different woods
Wittling different woods
Being used a pry tool
Being used as a digging tool

Then I have the same problem of relating those into quantifiable engineering terms.

Thanks for any help.
 
Whow. Here's the thing Survival Knife. I would recomend a good carbon blade steel.1080-1095. Differintialy tempered (edge at 57-59 RC). the hard edge and softer spine will make for a tough blade. Size-4" blade,flat or convex grind. Handle rounded easy on the hand. You will use this knife alot in a survival situation. Full tang construction would be best. If you need to dig sharpen a stick and use it. If you need to split wood, use a stick as a batton and strike the back of the knife. If you need a prybar find something else to use. Your knife will be alot more usefull as a survival tool in one piece.
I would study survival tecniques, then decide what knife would be best. The above is just my recomendation based on knowledge from the US Army survival schools in Panama and Alaska. Good luck in your search
 
+1

A knife is always a cutting tool. It should never be used for another purpose as doing so would result in damage. If you find the need for another tool, use the knife to fashion it.
 
I think the word "best" isn't helping you. Given the open ended nature of a knife's use, it's probably better to target "an interesting set of trade-offs" than "best."

Other factors that impact your steel decision in different ways are going to be corrosion resistance, edge geometry, field sharpening, heat treatment, blah blah blah.

For my own personal entertainment, I would opt for CPM-M4, with DLC, heat treated to HRc 64, with flat blade and edge geometry, and serrations that fit whatever round file I could carry in the handle. But that's a pretty idiosyncratic set of trade-offs.
 
hey gabe i like your thinking but M4 at 64 rc good luck sharpening it with a file that's just a point harder (65 maybe )
not then wrap that file in good sand paper and you ll be on yur way
 
When I think of "toughness" in a survival knife situation, the first thought that comes to mind is whether it would support my weight (significant ;)) with the blade shoved in a crack in a rock wall. If it could do that it'd probably do anything else you might call on it to do. For what it's worth, that is a situation I've been in - so no one should think it's a wholly unreasonable expectation. You'd have to decide which kind of toughness that would be, I don't know.

As to wear, there must be a tradeoff between ease of sharpening, which I think would be a high priority, and abrasive wear resistance. My personal inclination would be toward ease of sharpening, since that is something you cannot control in the field. Since your cryo experiment suggests the process improves "sharpenability," you might not have to make as much of a tradeoff as I think. This might also depend on whether you choose to include a sharpening method targeted toward your steel in its sheath or handle. Someone I read here years ago said he carried a piece of 400 grit paper in his billfold for that purpose. I'm not sure what adhesive wear is.

I won't offer a steel suggestion since your idea of 52100 or A8 seems pretty reasonable to me. As to blade size, anything over 5 or 6 inches (12-15 cm?) would probably be overkill. I doubt you'd have to deal with an Alien or a Predator. :D I think a flat ground blade would be more appropriate for chopping, which could reasonably be a task it'd be called upon to do. I might leave a heavier point, rather than a fully distally tapered blade. Edge geometry is something too. I don't know whether I could maintain an accurate sharpening angle if I was cold and using a rock to sharpen the blade, so maybe a convex edge would be a good choice. But hey! All my knives are flat ground with convex edges. :D

Please keep us posted as you work through your decisions, this is pretty interesting to me. Maybe when you get the ideal survival knife worked out we can make a trade of some kind. :thumbup:
 
As you are seeing, survival means different things to different folks. Not only that, but different survival situations require different knives. A survival knife kept in the cabin of a sail boat and one to be kept on the wall of a cabin in the mountains will be very different blades.

To me, the main things that make a knife a survival knife, are :

1)Ability to survive in rough use (care and concern for damage go out the window in survival situations)

2)Ability to be stored for long periods between use ,and be usable when needed (how often do you need to survive)

3)Multi use ability (Chopping wood, used as a spear,cutting meat, defense, etc.)

4)Ease in field sharpening (If used it may get dulled and damaged)

Now, as to the mechanics and metallurgy used to attain these needs:

1) Make the spine thick and the blade fairly heavy. In a survival situating you don't need to worry about the blade breaking in heavy use. The blade should be differentially tempered to give as soft a spine as practical. The edge should be hard enough to cut, but soft enough to avoid chipping. blade/edge geometry should be one of the prime design considerations.Longevity of the edge is of little concern, durability of the edge is of the utmost concern.

2) The knife may be stored in a car/truck, boat, back pack, or garage for long periods, unattended, and in a poor environment. The steel should be able to maintain usability without continued maintenance.Use of a stainless steel is a good choice. Super steels are fine, but simple stainless steels, such as 440C ,are adequate. If the knife is made in carbon steel, 5160, 9260, 1080/84, S7,3V,M4 and others will work, but will require more care in oiling and scheduled maintenance.

3) As well as overall design, the steel selection and heat treatment should be designed to allow misuse of the knife in survival situations. The knife may be used as a hammer, axe, spear, piton, shovel, food prep tool,weapon, screwdriver, tool, pry-bar. No one steel or blade geometry does all these. A convex or flat grind are probably the best edges in abusive situations.The spine should be as thick as practical. The edge should be somewhat thicker than normal. The spine temper should be as soft as possible,Rc 50-54 is good, and the edge slightly lower too, Rc 57-58. Full tang construction ( or as wide and heavy as the handle design allows) with rugged durable handle material (Micarta, G-10, tough wood, paracord, aluminum, leather)

4) The use as described in 3) above will be detrimental to any edge.The lowered edge hardness will make sharpening easier. Mounting a small pocket on the sheath ,with a carborundum stone in it, is a good idea. However, a rock may be the only available sharpening medium in a survival situation, so the use of high hardness steels and heat treatments (M4 at Rc64) may be less than desirable. With abusive use, the blade may need severe edge repair, and spending hours sharpening a blade in a survival situation will be impractical, if not detrimental.


Final note:
Folks often ask. " What is the purpose for the ABS test requirements.- I don't want a blade that can chop wood, shave hair, bend 90 degrees, and still hold an edge for a while." ......Well someday you might just need a blade like that - a SURVIVAL KNIFE.The ability to understand the forging (or grinding) and heat treatment requirements to produce such a blade show that you understand the physics, mechanics, and metallurgy necessary to make one.

Stacy
 
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Thanks allot everyone.

Stacy, I think you make an excellent point about storage and maintenance. I was hoping to deal with corrosion through a surface coating and and getting away with super tough materials that arent stainless. I realise the uncoated edge portion would still be subject to corrosion but at worst I was hoping the edge could be fixed up via honing. In emergency use I thought the coating maybe worn, but could always be renewed following. Is this is a viable option? Allot of appealing benefits are lost in using stainless.

Gabe I think also your dead right - it is open ended. Knives will be subjected to adhesive and abrasive wear in different situations and its hard to quantify proportions. Same with the different types of toughness.
 
The choice of steels often more personal that practical. For a survival knife,the biggest appealing benefit of a carbon blade is the ability to HT yourself,with much more control. A properly coated blade will sit for a long time with little care. It is, indeed, a good option.

The semantic points to keep in mind here are:
If it is a real survival knife, then it may never be used....and if it is, probably only once. Care, resharpening, and maintenance are not of prime concern. Availability for immediate use is.

If it is a survival style knife, and will be used in camping/hiking/bushcraft/etc. - then make a strong and tough camp knife to your liking. It will be constantly maintained and there should be no problem with any choice of steel compatible with the use intended. This is what most folks call a "survival" knife.

If it is a survival looking knife, such as a Rambo style. Then all you need to do is make it look like whatever you want it to look like. It will be more for show than work. Most knives of this class would fail miserably in actual survival situations.
Stacy
 
OK ,
ya'll got me to thinking.
Never made a knife with survival its main purpose.
I think it is a tool , so being a carpender all my life ,I go to my tools.Now I know that
my toughest ,by far, is a good flat pry bar. So I go from there.IT'S A TOOL SO,

AUTOMATICALLY, I WOULD THINK TOOL STEEL.Now to me 0-1 is tops
Now because my typing sucks and the tip of my typing finger is sore I'll make it short

I would say---3/16 o-1 7" blade 5" handle full tang diff ht flat grind macarta or g10
cutting edge not over 57

Just my way of thinking
Jerry Bond

Think I'll go make one right now
 
Am I wildly mistaken or has not Kevin Cashen debunked the idea that differential heat treating provides any real toughness benefit in a knife? I was under the impression that a fully hardened and tempered blade will resist bending (and will, instead, flex) far past the point that a soft-backed blade will (in terms of the force applied) and will break no sooner than the softer blade. Seriously, did I completely misunderstand his lengthy post on the matter? (a very real possibility :D)
 
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Impact toughness does decrease as Hrc values rise. However some materials do exhibit relatively high elastic deformation ranges before yielding into plastic deformation at pretty hard Hrc values.
 
Kevin did indeed say that differential hardening is not a good way to do it and I agree. For those who pry a full hardened spine defeats the purpose .A thicker blade is better for prying also.
 
jackrabbitslim said:
Am I wildly mistaken or has not Kevin Cashen debunked the idea that differential heat treating provides any real toughness benefit in a knife? I was under the impression that a fully hardened and tempered blade will resist bending (and will, instead, flex) far past the point that a soft-backed blade will (in terms of the force applied) and will break no sooner than the softer blade. Seriously, did I completely misunderstand his lengthy post on the matter? (a very real possibility :D)
Click to expand...

This is one I would rather just watch play out on its own without any interference from me, but since my name was evoked I thought I would at least clear up any confusion I may have created in the past with a less than adequate explanation.

The concept that “I” personally debunked anything, that is really just common knowledge in the rest of the metalworking world, does not bode well for our craft. The confusion can come about when concepts of flexibility, ductility and toughness all get piled together or used interchangeably. Toughness is most often associated with impact in industry, and impact is very different from flexing versus bending in a prying type of operation. My vinyl siding can be bent stretched and twisted all day but if you hit it with a weed whacker it shatters like glass, this is a prime example of how ductility and flexibility are not at all the same as impact toughness. (and please don't ask how I know the weed whacker thing:().

For practical considerations softening the spine does nothing for adding an advantage in prying operations since it cannot affect the stiffness in any way at all (that is a solid fact of physical law, those who doubt this, please research Young's modulus) however since it will substantially lower the yield point it can in fact detract from that operation. e.g. how many pry bars have you ever seen that were designed to easily bend? The best way to make a knife behave more like a pry bar than a knife is to change it cross section, i.e. make it thicker or increase the moment of inertia with ribs, ridges or flanges.

Toughness in impact can however be increased by lowering the overall hardness but it will be at the expense of overall strength, i.e. the blade will bend when you hit a rock instead of chipping or breaking. However many ideas about hardness vs. softness being the only way to deal with this issue, and having an absolute situation of compromise, are based upon pre-alloying type of thinking. With steels with alloying options beyond just iron and carbon it is possible to keep higher hardness and greatly increase toughness, this is why L6 or S7 can easily double or triple the impact strength of 1080 at the exact same hardness.
 
Here is a link to an explanation of adhesive wear and abrasive wear. I would think adhesive wear is of less concern for knives than abrasive wear. At the same time the idea that cryo treatments efect carbide size could be best demanstrated via adhesive wear testing. This is because adhesive wear is increased by large carbides. From what I understand adhesive wear takes place when cutting soft (sticky) metals.




http://www.thefabricator.com/ToolandDie/ToolandDie_Article.cfm?ID=719

I wil post more in a bit but now I need to go with my wife into the woods to pick out a xmas tree.
 
All of this aside,a real survival knife will be the one at hand when you find yourself in an emergency situation in which your survival is an immediate concern. It is a wonderful thing to be able to say "this is a survival knife" I would have to go along with Stacy and first ask that the term "SURVIVAL KNIFE" be defined as you see it. To me a survival knife is a general purpose knife that will perform well in a variety of survival type tasks that is still civilized enough looking that I can carry it daily without causing alarm and unwelcome attention from the gendarmarie. If I do not have the knife easily at hand at the point where survival is an immediate concern, I can call it anything I want, it's still NFG

just my opinion

-page
 
I have never been in a survival situation nor owned a survival knife.
How important is the tip shape in a survival knife. Would a "RAZEL" style knife
be better as a survival tool than a spear-point? It certainly could be made sharp and beefy at the same time
 
Steelshaper, good article but he doesn't use the term 'galling' [cold welding ] for some reason. Adhesive wear really is not a problem for our knives as we don't usually cut soft metals repeatedly.
 
Guys adhesive wear is not limited to metal on metal wear. Its defined as an action of one material sliding over another with surface interaction. The asperities interact with each other and cause wear under load. Im not a Tribologist, but I have professionally worked with them and it is far more complex that what is intuitively apparent to a casual person.

Due to the opened ended uses of a survival knife the more I think about it, the more I realise a horrid way to torment a Tribologist is to ask them to publish a paper studying wear on survival knives :)
 
Kevin R. Cashen said:
However many ideas about hardness vs. softness being the only way to deal with this issue, and having an absolute situation of compromise, are based upon pre-alloying type of thinking. With steels with alloying options beyond just iron and carbon it is possible to keep higher hardness and greatly increase toughness, this is why L6 or S7 can easily double or triple the impact strength of 1080 at the exact same hardness.
Click to expand...

Good stuff Kevin. Yes, the modulos of elasticity was a good point you made.

Thats why I was interested in A8 modified, as it shows considerable impact toughness and yield strength even when treated to a pretty hard Hrc value (charpy 162j @ 59). I want to stay away from stainless as I loose this, for the advantage to corrosion, which I believe can be managed through surface treatments. Not to nitpic, but plain old steel is still an alloy :)

To take it further, some materials exhibit over double the abrasive wear resistance at the same Hrc hardness value.

Its likely my a8 modified test samples with be hardened using nitorgen gas quenchant in a high pressure industrial vacuum furnance.
 
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