Shock waves reflecting from grind "lens".

nozh2002

nozh2002

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This idea came to me from other topic and I doubt it will be possible to discuss it there.

When blade is hit - acustic shock waves start traveling inside blade body. Blade has grind concave or convex. I am wondering if this grind may reflect this shockwaves and accumulates them in certain point inside blade body to cause breakage?

Convex and concave grinds looks like optic lenses or mirrors and may reflect those waves same way as optic lenses or mirrors reflecting light. In result we may have strong accumulated impact which may be way stronger then initial impact and so strong blade may collapse without visible reason from weak hit.

Any ideas?

I hope Noss4 will see this - he has most experience in this matter.

Thanks, Vassili.
 
It sounds unlikely that acoustics could be even a minor effect in shearing metal.
Sound waves move freely in a good conductor like metal.
Acoustic shock depends on a build up of pressure
I could not see there being enough sound pressure to break even an existing fracture

The physical shock from a blow on the blade would be far far greater, which would shear the metal
 
neeman said:
It sounds unlikely that acoustics could be even a minor effect in shearing metal.
Sound waves move freely in a good conductor like metal.
Acoustic shock depends on a build up of pressure
I could not see there being enough sound pressure to break even an existing fracture

The physical shock from a blow on the blade would be far far greater, which would shear the metal
Click to expand...

May be I use wrong term - pressure waves from impact inside steel I call acoustic - may be I am wrong, but not like it break from sound of hit. From the hit pressure will travel through the blade and may reflects from grind surface back inside blade body combines with wave from opposite grind, making in that point more pressure then at the hit point itself which break blade - something like this.

Thanks, Vassili.
 
Sort'a like the critical resonance frequency, I take it.

I could see it, but I think it would be at a very high frequency and energy level such that one would not normally observe it in the use of a knife.
 
Dude, you should make a sugestion to Time Warp, and get them to film it.
I think the transition from choil/guard to blade is more important than what kind of grind.
Think wave propagation, going from deep water to shallow, all of a sudden
the wave rises to 50 feet, and SNAP :)
 
Regarding wave reflection, focussing of the waves would only occur with convex grinds, looking at a convex surface from the "inside" would result in a concave surface (I'm not sure I'm explaining this bit right) allowing for focussing, but the point of focus would be so far away from the blade itself that the increase in strength would be negligible. Flat or hollow grinds would cause no focussing or weakening of the wave, respectively.

In either case, there's a massive dampener in the form of the person holding it, so I can't really see the shockwaves getting to this point without the person in question being either severely injured by the source of the shockwaves, or dropping the knife because of said shockwaves. At least not in any practical use.
 
Th232 said:
Regarding wave reflection, focussing of the waves would only occur with convex grinds, looking at a convex surface from the "inside" would result in a concave surface (I'm not sure I'm explaining this bit right) allowing for focussing, but the point of focus would be so far away from the blade itself that the increase in strength would be negligible. Flat or hollow grinds would cause no focussing or weakening of the wave, respectively.

In either case, there's a massive dampener in the form of the person holding it, so I can't really see the shockwaves getting to this point without the person in question being either severely injured by the source of the shockwaves, or dropping the knife because of said shockwaves. At least not in any practical use.
Click to expand...

Batoning is what I am talking about. Edge is flat on the wood and hit by let say stone right on top of the wood. without visible shaking near handle blade just broke 4" away from hit point.

Thanks, Vassili.
 
I don't think so. You have so many points of contact that act as dampeners that it would be unlikely.

I think you have to look at more tradtion reasons like micro cracking that caused stress raisers etc. Especially with 30V at high hardness this would seem a more obvious path. Did CRK ever analyze the knife for why it failed?
 
how about the big honking 'cracks' that are the serrations. Both blades had them, both broke in similar spots.
 
orthogonal1 said:
Sort'a like the critical resonance frequency, I take it.

I could see it, but I think it would be at a very high frequency and energy level such that one would not normally observe it in the use of a knife.
Click to expand...

nozh2002 said:
Batoning is what I am talking about. Edge is flat on the wood and hit by let say stone right on top of the wood. without visible shaking near handle blade just broke 4" away from hit point.

Thanks, Vassili.
Click to expand...

I agree with Orthogonal. The pressure wave from a baton strike does not travel that way through metal. You would need a much higher frequency and you would need a constantly repeating set of such waves such that a standing wave would be set up that might then be concentrated at a certain area due to shape. Repeated striking with a hammer won't do it. Think of something on the order of a tuning fork. Hundreds (Thousands?) of vibrations per second maintained for hours (weeks?) in order to set up that kind of failure.

Geometry does often cause a failure, but it would be merely from stress concentrations, not focusing of of waves. Sharp angles in a design are a no-no for stress resistance. But this has nothing to do with wave phenomena. A static stress load would produce the same failure.
 
You remember that thing that the Sharper Image (RIP) used to sell that had the five steel bearings suspended from two parallel bars? When you lifted one of the end spheres and let it drop, the energy of the impact would travel through the three inner spheres and cause the other end sphere to bounce away. There was very little energy loss and the bounce back and forth would last quite a bit. There was zero recoil unless you let both ends strike at the same time. The spherical shape is the ultimate convex grind. I think that you are conceptualizing the inside of the metal as if there would be a sudden increase in density as the wave gets to the threshold which would then cause a wave to exhibit some reverberation. In fact as the wave gets to the end of the medium, the sudden drop in density causes an immediate dissipation of the energy outward. I'm no expert and I may stand to be corrected on this but I did stay at a Holiday Inn Express last night.
 
Marcelo Cantu said:
In fact as the wave gets to the end of the medium, the sudden drop in density causes an immediate dissipation of the energy outward. I'm no expert and I may stand to be corrected on this but I did stay at a Holiday Inn Express last night.
Click to expand...

E in equals E out - conservation of energy.

If no energy is transfered out, where'd it go?
 
Actually, I have some issues with the model.

In the hanging ball bearing thingy (tick-tack machine, I guess), looking at what occurs to a single bearing would be more appropriate, I'd think.

Afterall, the knife would simply be a middle bearing in such a model. Inside each bearing, some internal reflection of energy must be occuring (just of negligible import).

As hardheart alluded to above, any structural irregularities, such as serrations (stress concentration factors), would be of more import in determining failures. Books have been written about and for determining the effects of surface irregularities for a reason.
 
That is one wild theory. Wow.

As Mr. Knarfeng says, you could break a knife (or a bridge) by vibrating it at it's resonant frequency. You won't get there with a hammer, though you might break the knife. And it won't be because of shockwaves reflecting inside the blade.

It would probably surprise many to see a FEA stress model showing a serrated blade versus a non-serrated blade with the stresses resulting from the same displacement.

Since FEA of stress could show us the locations cracks would likely start, namely the areas of greatest stress (if we only knew the displacement from the impact) I think some finite element analysis pictures are in order (Thank God we do use real science to design and investigate failures for some very important blades - turbine blades).


turbineblade.png
 
Broos said:
It would probably surprise many to see a FEA stress model showing a serrated blade versus a non-serrated blade with the stresses resulting from the same displacement.
Click to expand...

Why would this suprise people?

Everytime I used a FEA program to determine stress areas, the highest stress was rarely somewhere I didn't expect it to be.

Even in the above shown example, one would expect the blade root-to-center hub junction to be a critical area, with the area of thinnest material receiving the greatest stress.
 
Broos said:
That is one wild theory. Wow.

As Mr. Knarfeng says, you could break a knife (or a bridge) by vibrating it at it's resonant frequency. You won't get there with a hammer, though you might break the knife. And it won't be because of shockwaves reflecting inside the blade.

It would probably surprise many to see a FEA stress model showing a serrated blade versus a non-serrated blade with the stresses resulting from the same displacement.

Since FEA of stress could show us the locations cracks would likely start, namely the areas of greatest stress (if we only knew the displacement from the impact) I think some finite element analysis pictures are in order (Thank God we do use real science to design and investigate failures for some very important blades - turbine blades).


turbineblade.png
Click to expand...
What are you showing in the picture, interpret for us please.
Blue means no stress, red means max stress, yes?
 
Red is max stress.
Broos is posting the results of a Finite Element Analysis of a blade segment.

In the left picture, he shows that the blade segment is clamped and appears to be being torqued (judging by the shadow figures above and below the clamp).

I will use words,
To form a pressure wave, the impact has to be moving faster than the medium can deform. Steel can deform faster than any impact from a manually driven hammer. So there is no "wave" formed in the material. The steel just deforms as if in static load.
 
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