Bayonet Blood Grooves

Th232 said:
Indeed you're right Enkidu! I stand corrected. If I may ask though, we've been taught the parallel axis theorem through first and second years (the I(around axis) = I(around centroid) + A*dist^2), where is that meant to be used then?
Click to expand...
AFAIK, the parallel axis theorem still applies to calculations of the 2nd moment of area. I think you were close, you just miscalculated the 2nd moment of area through the centroid. I'll try calculating the 2nd moment of area using the parallel axis theorem:

Parallel axis theorem applied to the moment of area

(Inertia around axis x) = (Inertia through centroid parallel to axis x) + (Area * (distance from axis x to centroid)^2)

Simplified = (First Term) + (Area * distance^2)

Code: 
AAAAAAAAAA
AAAAAAAAAA -- (axis through centroid of A parallel to x)
AAAAAAAAAA
  BBBBBB
  BBBBBB
-------------- (axis x)
Calculating the first term:
(moment around axis parallel to x through centroid) = Integral [ -1.5, 1.5] r^2 10mm = 1/3 r^3 10mm over (-1.5, 1.5) = (10mm * (1.5mm)^3 / 3) - (10mm * (1.5mm)^3 / 3) = 22.5mm^4

Area of shape = 10mm * 3mm = 30mm^2

distance = 3.5mm

so Ix = 22.5mm^4 + (30mm^2 * (3.5mm)^2) = (22.5 + 367.5)mm^4 = 390.0mm^4
 
A. G. Russell said:
Antonio,
I just erased 3 paragraphs that would have harmed my image as a person of enormous patience. If you will not take the word of those speaking the language of science and you will not take the word of those speaking the language of experience, what exactly are you asking for?
Click to expand...

Thank you for your courtesy and patience - my comments were not meant to be derogatory - merely stating my skepticism. I trained and worked as a scientist (not engineering) but have worked with designers and engineers long enough to know the difference between scientific theory, scientific fact and "the voice of experience" - I was also trained to be skeptical until the body of empirical and reproducible evidence provided sufficient proof.

OK - I just read the engineering theory on I-beams and I presume that the formulae resulted from mechanical tests. But were these formulae based on static or dynamic loads? What an engineering formula predicts and what actually results do not always correlate - bridges and buildings do collapse. Can you reliably translate the theoretical strength of an idealised I-beam into the very different shape of a knife with fuller?

There also seems to be some inconsistencies in the replies from the engineers
mjolnirman said:
hawkings is right. if you simply grind out steel, it won't be stiffer
Click to expand...
- so who's right?

What I was asking for was a properly conducted and comprehensive stength/stiffness/torsion test between 2 blades (or other bayonet thickness flat steel bars) that are identical in every way except for one of them having a fuller - not I-beams, tubing or square rod. Until I see that empirical data I will remain skeptical. Show me the data and I will be convinced
 
I think part of the complication comes the unclear definition of the "stiffness" of a blade. In the calculation of the stiffness using the 2nd moment of area, yes, creating a fuller decreases the mathematical stiffness of a blade to a constant force of deflection (let's call this absolute stiffness). But, when you consider a wielded blade, much of the force of deflection comes from the weight of the blade itself. So if you can lighten the blade by 16%, but only reduce the stiffness by 3% (as in our example), the perceived stiffness of the blade (resistance to flexion along axes through the hilt) should increase considerably. If you consider design variations where you take away more material in the fuller, but thicken the blade around the fuller (even a 5% increase in thickness has large implications on side to side stiffness), you can readily calculate that absolute stiffness through the two axes through the hilt (perpendicular to the blade and through the spine into the edge) can change only slightly (or even increase) while weight can drop by large amounts. As shown above, this should result in a large increase the "perceived stiffness" while having little or no effect on "absolute stiffness". Also, the longer the blade, the greater the effect since the effect of changes in weight increases with the square of the distance from the axis/hilt (see the parallel axis theorem above). I think the entry from AG Russell's knife encyclopedia supports this analysis.

Of course, going too far will weaken the blade to torsional and buckling failure. Torsional and buckling failure modes are considerably more complex to calculate and anticipate.

On second thought, it may be clearer to distinguish between absolute stiffness and wielded stiffness, since calling it "perceived" stiffness may give the incorrect connotation that the measurement is only in the mind of the user. This is definitely not the case. Wielded stiffness (perhaps some calculation of absolute stiffness to the moment of inertia of the blade around the hilt) is measurable and quantifiable.
 
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enkidu said:
I think part of the complication comes the unclear definition of the "stiffness" of a blade. In the calculation of the stiffness using the 2nd moment of area, yes, creating a fuller decreases the mathematical stiffness of a blade to a constant force of deflection (let's call this absolute stiffness). But, when you consider a wielded blade, much of the force of deflection comes from the weight of the blade itself. So if you can lighten the blade by 16%, but only reduce the stiffness by 3% (as in our example), the perceived stiffness of the blade (resistance to flexion along axes through the hilt) should increase considerably. If you consider design variations where you take away more material in the fuller, but thicken the blade around the fuller (even a 5% increase in thickness has large implications on side to side stiffness), you can readily calculate that absolute stiffness through the two axes through the hilt (perpendicular to the blade and through the spine into the edge) can change only slightly (or even increase) while weight can drop by large amounts. As shown above, this should result in a large increase the "perceived stiffness" while having little or no effect on "absolute stiffness". Also, the longer the blade, the greater the effect since the effect of changes in weight increases with the square of the distance from the axis/hilt (see the parallel axis theorem above). I think the entry from AG Russell's knife encyclopedia supports this analysis.

Of course, going too far will weaken the blade to torsional and buckling failure. Torsional and buckling failure modes are considerably more complex to calculate and anticipate.
Click to expand...

Thank you. If I read the above correctly you're saying that there will be a slight but insignificant loss of strength from a fuller. This allows for a thicker (=stiffer blade) without increasing weight - hence the belief that "blade with fuller is stiffer" would be true only if it were to be "slightly thicker blade with weight reducing fuller is stiffer" - which is what I've maintained all along. Otherwise a star picket would be stiffer than a solid bar of the same diameter - and I know which one of those I'm capable of bending
 
antonio_luiz said:
Thank you for your courtesy and patience - my comments were not meant to be derogatory - merely stating my skepticism. I trained and worked as a scientist (not engineering) but have worked with designers and engineers long enough to know the difference between scientific theory, scientific fact and "the voice of experience" - I was also trained to be skeptical until the body of empirical and reproducible evidence provided sufficient proof.

OK - I just read the engineering theory on I-beams and I presume that the formulae resulted from mechanical tests. But were these formulae based on static or dynamic loads? What an engineering formula predicts and what actually results do not always correlate - bridges and buildings do collapse. Can you reliably translate the theoretical strength of an idealised I-beam into the very different shape of a knife with fuller?

There also seems to be some inconsistencies in the replies from the engineers
- so who's right?

What I was asking for was a properly conducted and comprehensive stength/stiffness/torsion test between 2 blades (or other bayonet thickness flat steel bars) that are identical in every way except for one of them having a fuller - not I-beams, tubing or square rod. Until I see that empirical data I will remain skeptical. Show me the data and I will be convinced
Click to expand...

Take a piece of thin sheet metal and bend it. Then take another piece of the same metal and corrugate it. Then bend it in the direction that crosses the corrugations. Same principal.
 
mjolnirman said:
never said forging made the steel stronger. forging the fuller in would make it stiffer because it introduces the shape without removing steel
Click to expand...

assuming the two blades have the exact same cross section, it should be the same.


adding fullers makes the blade lighter, and almost as stiff as if you left solid slabs. If you removed the same amount of steel evenly from the sides (making the blade thinner) it would be weaker than if you added a fuller.

I've had a similar discussion about fluting rifle barrels. A fluted barrel will be less stiff than a bull barrel of equal diameter, but lighter. The fluted barrel will be stiffer than a solid barrel that is the same weight, and a smaller diameter.
 
fudo said:
Take a piece of thin sheet metal and bend it. Then take another piece of the same metal and corrugate it. Then bend it in the direction that crosses the corrugations. Same principal.
Click to expand...
yes, but the corrugated metal will still be less stiff than sheet of metal as thick as the corrugations. It will just be a whole lot lighter.
 
mjolnirman said:
never said forging made the steel stronger. forging the fuller in would make it stiffer because it introduces the shape without removing steel
Click to expand...

pointless observation, since you would be comparing a lighter blade with less mass to a heavier blade with more mass. The method of creating the fuller is inconsequential.
 
SaMX said:
yes, but the corrugated metal will still be less stiff than sheet of metal as thick as the corrugations. It will just be a whole lot lighter.
Click to expand...
Correct. But if you cantilever both by fixing one edge and holding the sheet out horizontally, the thick metal will flex more than the corrugated metal, because it is much heavier and only marginally stiffer.
 
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true, but a bayonet isn't really long enough for there to be a long enough moment arm to cause any real bending.

For the most part. I've seen a Swiss bayonet from WWI that seemed about a yard and a half long.
 
While you eggheads have been jabbering I have stabbed you 3 times. Stab first, calculate later.
 
enkidu said:
AFAIK, the parallel axis theorem still applies to calculations of the 2nd moment of area. I think you were close, you just miscalculated the 2nd moment of area through the centroid. I'll try calculating the 2nd moment of area using the parallel axis theorem:

Parallel axis theorem applied to the moment of area

(Inertia around axis x) = (Inertia through centroid parallel to axis x) + (Area * (distance from axis x to centroid)^2)

Simplified = (First Term) + (Area * distance^2)

Code: 
AAAAAAAAAA
AAAAAAAAAA -- (axis through centroid of A parallel to x)
AAAAAAAAAA
  BBBBBB
  BBBBBB
-------------- (axis x)
Calculating the first term:
(moment around axis parallel to x through centroid) = Integral [ -1.5, 1.5] r^2 10mm = 1/3 r^3 10mm over (-1.5, 1.5) = (10mm * (1.5mm)^3 / 3) - (10mm * (1.5mm)^3 / 3) = 22.5mm^4

Area of shape = 10mm * 3mm = 30mm^2

distance = 3.5mm

so Ix = 22.5mm^4 + (30mm^2 * (3.5mm)^2) = (22.5 + 367.5)mm^4 = 390.0mm^4
Click to expand...

Argh! Thankyou very much for that.:thumbup:
 
fudo said:
Take a piece of thin sheet metal and bend it. Then take another piece of the same metal and corrugate it. Then bend it in the direction that crosses the corrugations. Same principal.
Click to expand...

Nope - not the same principle. Your explanation is incorrect as once you bend a thin flat sheet into a C- or S- shape you significantly alter the X, Y, Z ratios - increasing the dimension in the Z axis significantly and effectively making the metal much thicker through the curve - as in the roll put into steel drums - and it is this extra 'thickness' in the Z axis that imparts stiffness, (ignoring the effect of metal hardening from being worked). A fuller starts with a thicker flat (or shallow angled) flat bar and incorporates a thinner grooved section that reduces the Z axis so it's not the same thing at all.

As I said - engineers calculations (based on I-beams that have entirely different X, Y, Z ratios to a bayonet) may theorise but will not prove how something will behave - and with all due respect to other BF members, the very subjective "voice of experience" is not proof either. Only comparative empirical data can confirm that a fuller does actually stiffen a blade. Fullers have been in use for many years so I would assume that test data exists - so where is it?

While I remain skeptical, I am prepared to be convinced, but please don't try to give me more explanations - just show me the actual test data.
 
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