Questions about Lateral Strength

[James Terrio]

I'd like to focus this thread on lateral strength only. I try to design my blades and choose steels and HT protocols based almost entirely on cutting ability for the intended mediums.

However, many of my knives are "tactical" or "survival" or whatever you want to call them, and it very often comes up that someone wants to know if they can stab it into a tree trunk and stand on it to escape a bear, or jam it between some rocks to arrest a nasty slide down a cliff, or dig their way through a concrete wall to get out of a fire. (I know, I know... but for everyone who says "Seriously, how often is that going to happen? Knives are only for cutting things," someone will say "It happened to me! I'd be bear poop today if my knife had broken.")

I'm not trying to design a skinner that will chop down trees or pry open car doors. For the sake of discussion let's consider blades 5" or longer, made from stock 3/16" or thicker, intended for rough use, not fine cutting.

Sooo... is there any way a guy can meaningfully measure lateral breaking points with a torque wrench? (I happen to own a couple). Or should I just jam them into a wall and stand on them? Hang progressively more weight from them like in the Cold Steel and noss4 vids?

I understand that geometry (mainly width vs thickness, and cross-section profile) is very important for lots of reasons. I presume the longer a blade is, the more important geometry is simply because you can put a lot more leverage on it.

How important is overall mass compared to grind style? Say a thinner spine with a saber grind compared to a thicker spine with a full-flat grind, both having the same amount of material in their cross-section?

For a given width, length, steel, HT etc, how much "convexness" (more mass along the blade) is needed to noticably improve a blade's resistance to breaking, compared to a similar size/shape blade with a full flat grind?

How does a distal taper affect lateral strength?

FWIW I don't much care if my knives can flex and return to true, I'd rather have them not flex but also not break. I'm working mostly with O1, CPM-154 and CPM-3V, so alloy-specific recommendations are welcome.

 

[EdCaffreyMS]

I suspect your not getting much input on this because it can be such a controversial subject, and because if you design a knife with the intend functions in mind you mentioned, it pretty much ceases to be a knife, and becomes a "knife shaped object". Seems lately I have been using the quote "trade-off" a lot, and it certainly applies here. In this case if building/owning/using a "knife" that you wish to be able to do those thing you mentioned, then there is little doubt in my mind that your going to sacrifice it's usability to cut things...and if you want it to cut things, you will sacrifice it's ability to do those thing you mentioned. You could certainly create something that could do all of those things, with the specific steels you indicated, but in order to do so, would require building in attributes that would only detract from using the item as a knife.

I can address one area you mentioned, that being distal tapers. Distal tapers will not produce a blade that will not bend...what it does do is allow a blade to flex without breaking. Let's start by asking the question....What is the strongest structural shape? An Arch. Now let's do some comparison... a blade that maintains a constant thickness over it's entire length may logically seem strong. But, if you introduce a lateral force to this type of blade, the "force" will seek to concentrate itself in a specific area of the blade, that area always being the weakest point. Particularly if the blade has sharp, square edges at the spine....the blade will tend to "kink" rather than create an arch such as a distal tapered blade would do.... and as a result, the blade will generally kink or break. Now, a blade with properly created distal tapers....when a lateral force is applied, will create an arch. The natural shape of an arch seeks to distribute the force (in this case stress) EQUALLY along the length of the arch. The effect is further enhanced when the spine of a knife is rounded...removing the sharp/sqaure corners. That does not mean that if you stab a blade with distal tapers into a wall and stand it...it won't bend....it will. The idea is that with distal tapers, you create a lighter, thinner blade, that will withstand more lateral force over it's length due to the distal taper educing the arch shape, and in most case will not kink, or take a "set", and will endure more lateral force, with less damage than a blade of full thickness throughout it's length. (relative to a full thickness blade of equal proportions)

As knifemakers we get all kinds of crazy requests....I've gotten my fair share, and turned most of the down. But I also know that each of us must decide whether we want to make knives, or make things that look like knives, but are intended for other purposes. I know it's an old argument, but if your going to make a knife, make a knife, understanding it's limitations within it's intended design. If someone whats a knife to fillet fish, and also be able to drive it into a tree and climb on it, first try to educate them, and failing that, send them somewhere else, because they have very unrealistic expectations. IF you agree to build it for him/her, you're only going to cause yourself grief, simply because the client/customer will be expecting it to do those things, and while you might achieve one or more of the attributes sought, it's a good bet you simply won't be able to achieve them all.

 

[James Terrio]

If someone whats a knife to fillet fish, and also be able to drive it into a tree and climb on it, first try to educate them, and failing that, send them somewhere else, because they have very unrealistic expectations.

I completely understand. A hunter or skinner is one thing, and a tactical or survival knife is something else entirely! I like to "trade off" as little as possible, so when I head out camping I bring a big honkin' knife and a small, handy cutter. Different tools for different purposes.

Now, a blade with properly created distal tapers....when a lateral force is applied, will create an arch. The natural shape of an arch seeks to distribute the force (in this case stress) EQUALLY along the length of the arch.

That's exactly the kind of information I'm asking for, thank you for explaining it so clearly, Ed.

I'm not trying to stir up controversy, just have lots of questions. I thought it would be better to start a new thread to discuss these things specifically, rather than clutter up other people's threads

And if anyone says "a survival knife is whatever knife you have on you" I'm going to SCREAM. This thread is about designing a knife to take a beating, not building a shelter with an SAK. Fair enough?

 

[EdCaffreyMS]

I like to "trade off" as little as possible

There in lies the difficulty....as knifemakers, most of us know and understand that often we have to do the "trade-off" thing. But, a customer who requests/wants a "knife" that can be used as a pry bar, tree step, or whatever, do not, and often they simply cannot comprehend why an object that you could do those thing with, simply will not function well as a knife.

this thread is about designing a knife to take a beating

What kind of a beating? Are we talking about something that will be used as a knife, and take a "beating" as it relates to knife use? If so, that's one thing. If it must take a "beating" as a hoe, tree step, can opener, etc., then that is an entirely different matter....see what I mean?
If the emphasis is on "knife" characteristics, the heat treatment and geometry are going to play the pivotal roles (within the chosen steel type). Once you get into any of those other areas (sticking it into something and standing on it, digging with it, hammering it through something, prying with it.....) then the emphasis must be placed on it's ability to do those things, and in doing so, the "knife" characteristics are going to suffer. What YOUR job is, is to decide which of those characteristics is more important, and which of the other characteristics your willing to sacrifice to achieve them. To survive the latter tasks, the blade is gong to require mass, and be heat treated in such a manner that it will no longer perform cutting tasks as well as it could without the mass and "tougher" heat treating.
Actually, a person could write an entire book on this subject and never cover it all......hopefully others will chime in with their input!

 

[R.Keith]

Actually, a person could write an entire book on this subject and never cover it all

Ed,
If you do I'll buy a copy, and I think I remember the distal taper being explained well on your
Basic Bladesmithing video also. I really enjoyed it.
Thanks for sharing your thoughts.

 

[Frank Niro]

I've had a friend tell me several times that he has I believe a"Rat" (?) made by Busse I believe that is unbelievable in cutting ability and also will do all those things for strength. It is made very thick . Any information on these knives from Busse. Yes, it is a straight knife. I tried to get into the site but they want me to join a club and pay money on top of that to do it. Frank

 

[Doug Lester]

One of the problems with Busse is that I went to "Blade" to check out their adds because I thought I remembered the "rat" being advertised there. Well, I found adds in two editions for the "swamp rat" only in one add it was something like a narrow bolo type knife and in another it was identified as straight spined "regular" length blade with a swage on the spine.

I remember a while back that a maker was approached by some Ranger type military member to make a knife that could do rough chopping and dig trenches in an emergency. He came up with a tool, to use Ed's deffinition-and a very good one to my estimation, with a wide blade that as I remember was probably a bit over 12" long that had a chissel grind and one of those angular "tanto" points. The maker said that his customers realised that the tool's ability to funtion as a knife was somewhat compromised but that's what they wanted. I can't recall if it had a "saw" back or not but I recall reading in knife books that during WWI that the German engineers were issued bayonettes with a saw back spine. If the allied troops captured any of them with than weapon/tool they were summarily exicuted as war criminals. I guess that it doesn't always pay to be too inventive.

Doug Lester

 

[James Terrio]

Perhaps I was wrong to start this thread, or maybe I just worded my introduction poorly.

I don't want to chase around in circles defining what is a knife and what is a tool and what is a sharpened pry bar. I was just hoping to learn more about what physical attributes make any given blade stronger laterally.

 

[Fellhoelter]

One of your original questions was about testing.
It seems to me that the best test for your lateral breaking strength would be to do just what you proposed.
Hang weight on them until they fail !
I tested my ECS locking mechanism that way, though I never did make it to failure, simply because it held my body weight.
I took the rig apart to inspect, and just never put it back to make it to total failure.

I would say mass, no sharp corners for your stress to concentrate at, and good heat treat are keys to what you are after.

I'm going to spend a while contemplating Eds assertion that a tapered piece forms an arch, while a parallel piece does not, as I don't instinctively see why this would be the case.
Especially since the forces we are talking about here are creating the arch, not being applied to an arch.

 

[James Terrio]

Actually, a person could write an entire book on this subject and never cover it all......hopefully others will chime in with their input!

That's true. The Tactical Knife by Ayres is a pretty good overview, but from a buyers perspective. It doesn't have a lot of solid info for makers. I wish I'd taken some engineering courses somewhere along the way!

I would say mass, no sharp corners for your stress to concentrate at, and good heat treat are keys to what you are after.

I can't argue with that :thumbup:

 

[RandyR]

Ed,
I believe there is a bit of a misconception here with respect to the strength of a tapered blade versus a straight one. I say this with all due respect to your incredible talents and skill as a knifemaker, far exceeding mine without question. I was a practicing mechanical engineer for 30 years and, during that time, I had the opportunity to design complex structures for satellites that required every bit of skill and knowledege of materials you could obtain to meet the sometimes conflicting goals of light weight and strength. Applying some of those concepts to a knife stuck point first into something and then loaded laterally, I reach a different conclusion than what you present.

One thing I hear quite often is people confusing strength and stiffness. If we assume we have two blades of identical materials and heat treat, then we can also assume the material of each will have approximately the same strength. However, the stiffness of the tapered blade will be less than the straight one so that under identical loads it will indeed deflect (arch if you will) more than the straight one. The highest stres in a situation such as this will occur at the position where the point enters the material it is stuck into. In engineering speak, there will be both a shear load and a bending moment. The shear load is the same as the load applied and the bending moment is the product of the force applied and the distance to the point we wish to calculate the stress. If we assume the distance and angle of the load does not change with deflection (which may or may not be a great assumption here) the stress created by these loads will be higher in the thinner point of the tapered blade than the thicker straight blade (again assuming the same load). The stress is strictly a function of the cross sectional properties of the blade called the area moment, with the thinner blade having a lower moment. The actual stress calculation is bit complicated because we would call this situation statically inderminate which requires some mathematical tricks to resolve. However, the result will be that, given equal material properties and loads, the tapered blade will indeed bend or deflect more but it will break (or yield) at a lower load due to it having higher stress where they are stuck into the wood (or concrete, god forbid).

Submitted with respect,
Randy

 

[EdCaffreyMS]

I agree to a point. If you're talking about a blade being "stuck" into something, then you create a fulcrum...and if that fulcrum is in a a thinner cross section, I agree that it would take less force to deflect that specific point.

However, if you place bending force on a distal tapered blade (without creating a fulcrum point) and do the same thing to a blade of equal thickness throughout, you will find that the tapered blades does indeed form an arch, and will "arch" and return to a truer form. If you apply the same force to a blade with full thickness for it's entire length, that blade MAY form a very slight arch, but as the bending force increases, the stresses will seek to concentrate themselves, finding the weakest point long the length, and the blade will either "kink" or break. I also agree that the intent of distal tapers is not an over whelming resistance to bending, but rather aids in the blade's ability to flex and return with minimal damage. Distal tapers will also make the blade lighter, "faster", and it will feel much better in the hand, and will be much less fatiguing to use.

I'm not trying to be a butthead, but I was anticipating someone coming along with the kind of response you wrote. I know I won't change anyone's mind for or against the benefits of distal tapering knife blades...all I can say is that if a person takes the time and effort to experiment with each configuration, as it relates to a knife blade, the results will verify what I have said. Each individual maker will have to determine what they wish to do with the knives they produce, and decide how to best achieve the characteristics they desire. My whole point of responding to this thread has been that there are limitations to what we do, within the material(s) we use. And to underscore the fact that much of what we do as knifemakers is "give and take"....meaning that in order to achieve a specific characteristic/attribute, we very often must sacrifice something else. In the end, the primary purpose of a knife is to cut...if we wish a "knife" to do other things, it often results in the degradation of that function. The individual knifemaker is the one who must make a decision and choose.

 

[James Terrio]

Thank you again Randy and Ed. Those two posts alone made starting this thread worthwile. Interesting stuff :thumbup:

 

[Kevin R. Cashen]

It is a pretty heavy, and dry, subject, I have often depicted heat treating as a sliding point on the straight line of the proportional limit of the stress/strain curve. To the left of that limit it is all about the amount of material in the cross section (or its distance from the center i.e. area moment) heat treatment deals with what happens to the right side of that proportional limit line. While working in the proportional range a distal taper should provide a curve that resembles a fishing rod with a healthy walleye on the line. If you want the pole to resist this you make it thicker, if you want to be sure the pole to fold up early on without worry of breaking you heat treat it softer, if you want even the thick pole to flex like a thin pole but eventually break when you have the mother of all marlins on the line, you leave it harder. With no distal taper the curve will be equal throughout with any flaw, deviation or weakness at any point being the area to beware of, the taper sort of allows the smith to at least direct the forces, but mainly it distributes mass for the function of the blade in motion, a tip heavy smallsword stinks, and a handle heavy machete is tiresome.

Of course once you get it all worked out for steady loading, as we are mostly discussing here, then you can throw it all out the window for sudden loading (like chopping) and many of your strengths will counter the toughness you may want. Either way a big old prybar with an edge on it will handle it all, it just won't cut as nicely.

 

[James Terrio]

It is a pretty heavy, and dry, subject...

Yeah I realize it's not a real barn-burner of excitement for most people. But I find it fascinating and this sort of thing keeps me up at night so I figured I'd ask. I do appreciate you all taking the time to share your thoughts. Next time someone asks you can just send 'em here instead of typing it all out again :biggrin:

 

[Fellhoelter]

If you're talking about a blade being "stuck" into something, then you create a fulcrum...and if that fulcrum is in a a thinner cross section, I agree that it would take less force to deflect that specific point.

My bad I guess???

I was under the impression that was exactly what we were talking about.

Stuck in a tree, and stood on, to escape a bear.
Although I find that analogy strange, since a bear can climb a tree better than most of us.

 

[Doug Lester]

Ok, James, I think that I understand your point better. Part of the problem is how do you want the blade to respond to lateral stress. Do you want it to resist bending, bend and return straight, or can it bend and the stay bent until straightened? If you want ti to resist bending you could add thickness or heat the steel to increase strength, though increasing the strength by heat treating could reduce the toughness of the steel. The temper and the the thickness of the steel would influence the springiness of the steel. Then, in the last case, you could choose steel and heat treat to increase toughness at the expence of strength. Whether you want to make a knife or a knife shaped tool-I would agree that some could see this a hair splitting-what we have is a series of trade-offs. As far as the ability to bust through cender blocks, that's going to require mass. Mass to trasfer energy (mass X velosity = energy) to a point on the block and mass to take the sudden loading of stress. You would also need hardness that will exceed the hardness of the object being cut and strength to resist deformation of the point of the tool.

I really didn't mean to sound critical of the "knife shaped tool". The comment about the WWI German Engineer's bayonette probably didn't add much to the conversation and I probably should have not brought it up. Like the Ranger types that I mentioned from the thread dealing with a like situation to what you have it was realized that what they were asking for was a tool that was a Jack of all trades and master of none, but that's what the customers wanted. Whether or not you decide to make these tools, and we must keep in mind that a knife IS a tool, is up to you. If you do you will deffinantly have choices to make between abilities and limitations.

Doug Lester

 

[RandyR]

It seems I almost started up the bend test discussion again and that was not my intent. I do have strong beliefs based upon my technical background but I honor everyones right to have their own and disagree with me.

I agree with Ed about carefully considering making something for someone who may have very unrealistic expectations about capabilities. I turned down a sale not too long ago to a friend of a friend because of similar concerns. At a hunting camp I watched him abuse a good quality factory knife until it was almost useless. He hammered it with rocks to drive it through bone, threw it into trees and pried open cans and wooden boxes, all with associated damage. To the knife's credit, it was still in one piece but not very pretty. He came to me after the hunt to buy one of my small hunters. I asked about his abuse of the other knife (carefully) and he told me he expected any knife he had to be be to handle that. I politely declined to sell him a knife and when he argued I told him the knife he wanted was designed and made to cut hide and meat, not to be driven through bones, hammered with rocks, used as throwing knife or to pry open cans and boxes. He decided I made crap and left so we were both better off.
Randy
Randy

 

[James Terrio]

Part of the problem is how do you want the blade to respond to lateral stress. Do you want it to resist bending, bend and return straight, or can it bend and the stay bent until straightened?

Actually any of those three would be fine, if it can be bent back close to straight in the field.

...the knife he wanted was designed and made to cut hide and meat, not to be driven through bones, hammered with rocks, used as throwing knife or to pry open cans and boxes...

I completely agree. I don't expect a "combat/survival/tactical" knife to be a fine field-dressing tool and I certainly don't expect a skinner or hunter to pry things open. Again, I'm not trying to come up with a magical Excalibur that can do everything, I think we all agree there's no such thing. Just trying to understand the factors that contribute to, or work against strength.

 

[Delbert Ealy]

Randy and Ed,
One of the factors nither of you have taken into consideration(at least that I have seen) in talking about stresses on a tapered or not tapered tang is the handle material. Granted some of the handle materials on the type of knife we are talking about here will not be a factor, but some like carbon fiber composite and G-10 the strength of the handle material and more importantly the adhesive or pinning materials strength. I really don't mean to throw gas on the fire here, it was something I noticed and I thought to add it for a complete description of materials and stresses. I think that more important for strength issues than the tang is the plunge grind. That can be a serious weak point especially when discussing lateral stress.
Thanks,