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 crosssectional area of the circular bar before being hollowed out and A2 is the crosssectional 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^2d^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. 


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. 

"Never Knows Best"
GO BUCKS! 
Yeah, but blood grooves are F'n tactical!


I have no useful skills, therefore I supervise.

Now do "Is a fluted barrel stiffer/stronger than a nonfluted barrel?"


8/26 +2810 8/31 +3386 9/10+4300 9/20 +4950 10/1 +5966 (450)

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. 


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. 

"Life is a comedy for those who think... and a tragedy for those who feel" Horace Walpole
"I put on my robe and wizard hat" bloodninja "I'll bet you five bucks that when you die you crap your pants" South Park 
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. 

8/26 +2810 8/31 +3386 9/10+4300 9/20 +4950 10/1 +5966 (450)

The "blood groves" on a sword actually make the blade weaker. But they do look "cool", FWIW.
D 


You are correct. A fluted barrel with a larger diameter that a nonfluted barrel, but made with the same amount of material as the nonfluted 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. 


AR15.COM is the worldâ€™s largest firearm community and is a gathering place for firearm enthusiasts of all types.
From hunters and military members, to competition shooters and general firearm enthusiasts, we welcome anyone who values and respects the way of the firearm.
Subscribe to our monthly Newsletter to receive firearm news, product discounts from your favorite Industry Partners, and more.
Copyright © 19962017 AR15.COM LLC. All Rights Reserved.
Any use of this content without express written consent is prohibited.
AR15.Com reserves the right to overwrite or replace any affiliate, commercial, or monetizable links, posted by users, with our own.