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9/22/2017 12:11:25 AM
Posted: 10/2/2005 10:47:32 AM EDT
I had posed this in the "It happened at the gun show" thread, but I decided to move it into its own thread since so many inquiring minds seemed to want to know.

Does making a "blood groove" on a knife make it stronger?

It's all about moment of intertia.

For example, consider the shear stress on a linear elastic circular bar (as a simplified example)
where D is the diameter, pi = 3.14159... and Iy is the moment of inertia:

Iy_solid = (pi * D^4)/64

Now, take that same bar and hollow it out, such that it has an inner diameter d:

Iy_hollow = (pi * D^4)/64 - (pi * d^4)/64 = [pi * (D^4 - d^4)]/64

It has been shown that maximum shear stress, tau_max, on a circular bar subject
to a maximum torque T_max is:

tau_max = (T_max*D)/Iy

So, let's say we want to reduct the weight of the circular bar by removing half of the material,
where A1 is the cross-sectional area of the circular bar before being hollowed out and A2 is
the cross-sectional area of the circular bar after being hollowed out:

A1 = (pi * D^2)/4

A2 = A1/2 = (pi * D^2)/4 - (pi * d^2)/4

((pi * D^2)/4)/2 = (pi * (D^2-d^2))/4

Thus, d = .7071 * D

Going back to moment of inertia,

Iy_solid = (pi * D^4)/64

and

Iy_hollow = [pi * (D^4 - d^4)]/64 = [pi * (D^4 - (.7071 * D)^4)]/64 = (pi * D^4)*3/256

Therefore, the ratio of moments is

Iy_solid / Iy_hollow = 1.333

And, since

tau_max = (T_max*D)/Iy

tau_max_solid/tau_max_hollow = .75

Therefore, a 50% reduction in weight only results in an 33.3% higher maximum shear stress.

The knife geometry would be more complicated, of course, but I kept it simple here for the purposes of illustration.

Yes, the solid shaft is stronger, given the same outer diameter.

However, a larger hollowed out shaft can be made that will bear just as much load as the solid shaft with a considerable weight savings.

That is why your car's drive shaft is hollow, bicycle frames are hollow, etc...

So, making a "blood groove" on a knife cannot possibly make it stronger, unless a larger knife was made with the groove material removed to make it comparable in weight to the smaller knife, in which case we would no longer be talking about the same knife.

If anything, the "blood groove" will introduce a stress concentration and make the knife weaker than just shaving off material from the blade to achieve the same weight.
Link Posted: 10/2/2005 10:58:12 AM EDT
[Last Edit: 10/2/2005 10:59:20 AM EDT by TheCynic]
Reading this math makes me want to test out your
calculations by using one of my SKS bayos with a
"blood groove" to free my intestines from their gooey
prison.
Link Posted: 10/2/2005 11:03:01 AM EDT
Yeah, but blood grooves are F'n tactical!
Link Posted: 10/2/2005 11:09:16 AM EDT
Now do "Is a fluted barrel stiffer/stronger than a non-fluted barrel?"

Link Posted: 10/2/2005 11:14:07 AM EDT

Originally Posted By Phil_in_Seattle:
Now do "Is a fluted barrel stiffer/stronger than a non-fluted barrel?"




Given the same barrel diameter and given that the material is not rolled, heat treated, or otherwise processed after the fluting, I would have to say no.

Fluting only saves you weight. it is done under the assumtion that the fluting will not affect barrel performance enough to be a concern.
Link Posted: 10/2/2005 11:15:54 AM EDT

Originally Posted By TheCynic:
Reading this math makes me want to test out your
calculations by using one of my SKS bayos with a
"blood groove" to free my intestines from their gooey
prison.



Arfcom should support mathematical equation formatting.

Then the calculations would be prettier.
Link Posted: 10/2/2005 11:17:39 AM EDT

Originally Posted By CAR_16:

Originally Posted By TheCynic:
Reading this math makes me want to test out your
calculations by using one of my SKS bayos with a
"blood groove" to free my intestines from their gooey
prison.



Arfcom should support mathematical equation formatting.

Then the calculations would be prettier.



I was having flashbacks from strength of materials class!!! 20 years ago!!! Hard to read without good notation, I agree.
Link Posted: 10/2/2005 11:18:02 AM EDT

Originally Posted By CAR_16:

Originally Posted By Phil_in_Seattle:
Now do "Is a fluted barrel stiffer/stronger than a non-fluted barrel?"




Given the same barrel diameter and given that the material is not rolled, heat treated, or otherwise processed after the fluting, I would have to say no.

Fluting only saves you weight. it is done under the assumtion that the fluting will not affect barrel performance enough to be a concern.



Come on that's the easy answer..... but essentially correct

A fluted barrel has much less rigidity than a plain barrel of the same diameter, and more rigidity than a plain barrel of the same weight. But those comparison barrels are not, cannot, be the same. The plain barrel of "same weight" will have a necessarily smaller diameter than the corresponding fluted barrel. The material in the fluted barrel is arranged, due to its greater major diameter, more advantageously to resist bending than the plain barrel's material.

Link Posted: 10/2/2005 11:19:17 AM EDT
The "blood groves" on a sword actually make the blade weaker. But they do look "cool", FWIW.
D
Link Posted: 10/2/2005 11:23:03 AM EDT

Originally Posted By Phil_in_Seattle:

Originally Posted By CAR_16:

Originally Posted By Phil_in_Seattle:
Now do "Is a fluted barrel stiffer/stronger than a non-fluted barrel?"




Given the same barrel diameter and given that the material is not rolled, heat treated, or otherwise processed after the fluting, I would have to say no.

Fluting only saves you weight. it is done under the assumtion that the fluting will not affect barrel performance enough to be a concern.



Come on that's the easy answer..... but essentially correct

A fluted barrel has much less rigidity than a plain barrel of the same diameter, and more rigidity than a plain barrel of the same weight. But those comparison barrels are not, cannot, be the same. The plain barrel of "same weight" will have a necessarily smaller diameter than the corresponding fluted barrel. The material in the fluted barrel is arranged, due to its greater major diameter, more advantageously to resist bending than the plain barrel's material.




You are correct. A fluted barrel with a larger diameter that a non-fluted barrel, but made with the same amount of material as the non-fluted barrel, will be stronger.

But then we are not talking about the same barrel any more.

What I was saying is that adding fluting to an existing barrel will not make it stronger, only lighter.
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