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Author Topic: Thoughts on Bend Testing  (Read 3280 times)
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davidm
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« Reply #15 on: September 10, 2014, 03:49:29 AM »

I am not sure i would consider the blade that bends under moderate forces the stronger blade, but more a tougher blade.. Like a rawhide chew to a dog vs a bone. 
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ChrisAnders
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« Reply #16 on: September 10, 2014, 10:05:36 AM »

The thingthat needs tobe done is to define specifically what one cosiders failure.  Ed appears to define failure as the breaking of the bladewhen laterally flexed or bent.  The effort (torque) required is not limited.  In that circumstance, only a knife with the ability to flex or bend will pass.  A knife that is thin enough and with a high enough hardness will flex essentially infinitely in terms of number of flexes, though strength may suffer.  If a torque minimum is used, the thickness must be very carefully chosen to meet the minimum and still flex to 90 degrees w/o cracking. 

If a knife is allowed to bend, then a soft back can be used and the edge hardness and width can be controlled. 

The other option is to set failure as breakage below a certain torque value.  Again, cross section and hardness can be controlled to meet the goal.  However, a fully hardened blade will take more effort to reach a given degree of flex past a certain point.  That degree of flex can also be set as a limit.  For most threads here, it is 90 degrees.  There is nothing saying a maker cant set the torque at 100 ft-lbs and the degree of flex/bend at 45.  It just needs to be stated and then perhaps a reason given. 

Example from Ed's testing (approximate):

Full convex grind on 3/16" x 1" x 5" drop point blade
70 ft-lbs to flex 90? at least once
The knife blade cannot break fully into 2 pieces. 

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Ed Fowler
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« Reply #17 on: September 10, 2014, 11:29:15 AM »

You have the idea. What we are talking about is what I call functional balance:
a blade is functionally balanced when it is more than adequate for the tasks it was made to achieve.

Each maker is free to develop the functional balance of the blades he makes. The essential aspect of functional balance is that the maker has tested his blades and knows their limitations from his own knowledge and honestly communicates these relationships to his clients.

For example, I make knives to serve me in my environment.  I have no interest in making kitchen knives and would be a lousy kitchen knife maker. 
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John Silveira
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« Reply #18 on: September 10, 2014, 10:13:59 PM »

Ok - i like what i'm hearing here.

Last year i went for the first time to the American Bladesmith Society hammer in at Tulare Ca.... I think it was there that i heard for the first time that most newbie knife makers over build their blades - ie: too thick at the spine .

below is the first knife i ever made about a year ago now. It's over built ( 3/16ths at the spine ) for such a little blade - but one thing for sure you'll be hard pressed to destroy this blade - it's even thick at the tip. For some reason i did a thing or two right on this blade in my opinion. Point being - who sets the parameters for what's a correctly built blade anyway?


for me - i guess when discussing blade performance ( specifically the 90 degree bend ) i guess i need to be more clear , my point is i see a knife as a tool that i want to depend my life on if need be. I keep thinking if i ever needed to stick a knife between two rocks to save my life from a fall while climbing that the blade won't snap under my weight. So not only would i like a blade to support a 90 degree bend but i see it as needing to support my weight as well to some full degree. But there again - i weigh about 190lbs - what if i weighed 290 !! And have i actually hung my weight from one of my blades to see if i could hang from it ?  NO - i guess i should to see where my blade actually stacks up to my personal ideal. 

So like Ed - i may make a kitchen knife or many ( or maybe NOT ) but they would not be what i consider a HEPK type blade ...

What this talk has helped me realize is : different steels will perform differently regarding toughness or flexing ability for HEPK type blades. And for a while i was asking myself " What can i do to alter the parameters of a certain steel ( 52100 or 5160 for example ) to achieve what my best performing blade in those steels would be.  So there seems to really only be one way to correctly Heat treat one of these blades. From there it's about temper to adjust what i'm looking for in my best blade - and i suppose the actual blade dimensions ( thickness of spine ) needed to build a blade that fits my needs.

Doesn't seem like there's much to change with Heat Treating - but blade thickness/dimensions and temper are the only other variables we have to change performance of blades -  Am i about right ? And i know Ed's going to say " it's where you get your steel and the quality of that steel " - and - " it's about forging temps and how to keep control of grain " so already it's not as simple as i was making it out to be. Anyway - topic has helped answer some of the questions i've had rolling around in my head.     

thnx

cheers
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ChrisAnders
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« Reply #19 on: September 11, 2014, 05:02:10 AM »

The maker decides the overall design based upon a clear idea of what the knife should and should not be able to do.  That is the starting point.  Then the maker must prioritize the goals, as some may be in opposition to each other.  To use your goal as an example, being able to bend 90 degrees and being able to support my weight for purposes of a hand hold are somewhat at odds.  I don't want my hand hold bending under my weight.  I wouldn't want it to flex very far either.  Stiffness is what we're after and this particular trait is dictated almost solely by geometry: thickness, width, blade length, overall length, and the taper from spine to edge.  Under the constraints of holding me up in a climbing situation, heat treatment doesn't really even come into play.
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Ed Fowler
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« Reply #20 on: September 11, 2014, 09:26:04 AM »

I feel heat treat is always a very significant variable, for instance if you harden the tang you are in trouble if you plan on having to rely on the strength of the knife. Hardened tangs are very significant contributors to knife failure.
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ChrisAnders
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« Reply #21 on: September 11, 2014, 10:24:11 AM »

For purposes of this example, and based on the soft steel flex/bend test that started this thread, the amount of flex I'd be comfortable with in this situation is extremely low.  Low enough that whether the tang is hardened or left fully annealed would make no difference.  There is also the assumption that the tang is more than a cord wrapped stick tang and has a substantial material on it, such as horn, micarta, wood, etc.
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John Silveira
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« Reply #22 on: September 11, 2014, 12:38:11 PM »

variables variables and more variables.

One of the stick tang blades i'm finishing up is at the point of guard and handle fitting. The tang was left straight initially ( just slight curve downward ) and i finally settled on a curved Antler handle which meant i needed to bend the tang down a bit more. This blade is 52100 from a roller bearing, it's edge quenched and i was careful not to harden the tang. Low and behold when i laid the tang over the horn on my anvil and rapped on it a few times to put a slight more bend in it the tang actually cracked in one spot severely , enough so i'm going to grind it and wire weld up some extra strength there.  My mental note : "Be careful with the tang". It must have air hardened when i was doing some heavy grinding on it - sometimes during grinding i also dip in water . Anyway ... 

A little more on flexing a blade. Just spinning my wheels here. It would be one thing for me to make a blade that supports my weight - without flexing to 90 degrees. But i think i'd like a blade to support my weight for example but not snap under a QUICK prying motion - hence a 90 degree bend test. If the blade supports my weight but snaps during a QUICK hard crank on it i'd be screwed again.  If i make a blade that survives less flex i'm feeling i'd risk it snapping even though i build it to support my weight.   

Anyway - this could go round and round i guess. 

There's something else ( among the many others i don't understand ) and that's how or if what is called "The Nose" when heat treating. Haven't been able to study up on that yet but i wonder if there's a way to adjust performance levels working with "The Nose" during heat treat.   
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ChrisAnders
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« Reply #23 on: September 11, 2014, 03:45:01 PM »

There are ways to design for the impact loading like you are talking about.  The first is material selection.  For impact/sudden loads, you want to minimize the carbon content of your steel.  Just barely enough to get the tempered hardness you want, when tempering at the maximum I've seen Ed mention here when discussing Charpy testing, about 360 to 400 degrees for most steels.  5160, or 9260 if you can get it, or a few others.  Welded and other structural connections can also be designed for impact if desired.  Ultimately you'd just have to test it though.  Were I to design a knife with the high likelyhood of that scenario actually happening, it would be the following:

3/8" flat grind on a 1.5" wide blade, 6" long blade, full tang, 4340 steel, 55-57 hardness, tempered 390-400

On your other question, I think you're talking about the pearlite nose.  Steel makers publish cooling curves for their steels.  These show what speed to cool the steel and the microstructure that forms at the different speeds.  If you look at the diagram, pearlite (soft) forms at high temperatures in short times, and sticks out like a nose.  If you want to get 100% martensite/fully harden your steel, your cooling rate has to avoid this "nose" on the diagram.
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ChrisAnders
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« Reply #24 on: September 18, 2014, 06:33:04 PM »

Out of curiosity, has anyone done finite element analysis (FEA) on a blade while bend testing?  Has anyone added strain gauges to one?  A LOT of information could be gathered from FEA.  Slightly less would be available with a strain gauge.
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