BK 9 vs Battle Rat

[Cliff Stamp]

Frank :

When you double the thickness of a knife blade, you double the amount of material and the strength as well.

It depends on the vector of the applied load, it is dimension specific. It is far harder to pry with a knife for example in the edge to spine plane than though it, this is trivial to demonstrate with any non-square object. This would not be the case if the bend strength was linear in all dimentions as the cross section is equal in all orientations. The strength is linear only in the perpencidular planes because the slip plane deformation requirement doesn't change with thickness in those dimensions.

[thin blade]

... it will deflect further with out reaching it~Rs elastic limit

Yes, which does help you out some, and which is why as noted in the above, you can get those large bends with tapered blades. However you still have to be careful of the elastic limit. There is however a rather significant gain in knife durability from lower cross section which can counteract the strength loss significantly. If the user has enough skill, the applied loads will be lower given specific task [outside of direct prying of course] as the knife is more efficient. In that case the durability doesn't 't fall off as fast as the raw strength loss. This only holds for overbuilt knives though [based on skill level] and thus there is a lower limit.

Are you saying that prying with the edge or tip is not likely to cause damage?

In those noted vectors, again it has to do with the nonlinear strength in those dimensions.

-Cliff

 

[frank k]

Originally posted by Cliff Stamp
It depends on the vector of the applied load, it is dimension specific. It is far harder to pry with a knife for example in the edge to spine plane than though it, this is trivial to demonstrate with any non-square object. This would not be the case if the bend strength was linear in all dimentions as the cross section is equal in all orientations. The strength is linear only in the perpencidular planes because the slip plane deformation requirement doesn't change with thickness in those dimensions.

Cliff, I am not quite sure what you mean. The difference efficiency in prying form the edge to spine and prying form tip to handle, is of coarse, simply due to greater leverage.

Since increasing the thickness would not radically change the shape of the blade or orientation of the grain – the increase in strength is directly proportional to the increase in the amount of steel. Even if you were to change the shape, the leverage of any applied load would change, but not the “strength” of the steel itself.

There is however a rather significant gain in knife durability from lower cross section which can counteract the strength loss significantly.

I would tend to agree, but that seems to be in direct opposition with your original statement: “...the loss in strength very fast (*far* above linear)...".

In those noted vectors, again it has to do with the nonlinear strength in those dimensions.

I still don’t get what you where trying to say in your original statement about prying. Bending strength is measured in foot pounds (or other force*length units); the lever is in direct proportion to the applied force. Both the applied force and the lever are needed to define the bending strength – you can not define it be a force alone. The blade will “see” the same stress if you increase (or decrease) the lever and lower (or raise) the force proportionally.





- Frank

 

[Cliff Stamp]

Frank :

Strength is the ability of a material to resist external forces.

It is a lot more specific. Strength is the resistance to deformation. The defination you gave also covers properties such as corrosion and wear and fracture. These happen by very different internal activities than deformation which is a simply shifting of the internal planes, which is why they are governed by different materials properties and specific tests. Strength is usually characterized by the tensile strength, in psi, which is linear in cross section. However this linearity isn't directly relevant to knives because you can't fail a knife in this way as noted in the above, ie. you can't pull a knife apart in the tip to choil, or edge to spine. Knives fail by bending forces and these are not linear in the dimension parallel to the applied prying load which was the specific dimension being discussed in the above (edge thickness).

The difference efficiency in prying form the edge to spine and prying form tip to handle, is of coarse, simply due to greater leverage.

The direction of force is what is different in the above descriptions I gave. Take a blade and lock the tip in a vice or pound it into a piece of wood. Now grab onto the handle and pull up. This would be applying a load along the tip to the spine. Take the blade and drive the edge into a large block of wood or lock it in a vice. With the edge pointing towards the floor, haul up on the handle. This is applying a load in the edge to spine line [to be really specific there is a small bending torque induced here so this isn't strictly a linear case either]. Neither of these is likely to cause failure in the any blade, the first is near impossible . The massive difference in load required to break a knife with these methods as opposed to general prying, or bending is is precisely because the strength is nonlinear in prying with the dimension parallel to the applied load. As a very trivial example of this, take a butter knife and lay the flat on a table with the handle extending over the side. Press down on the handle with one hand and on the flat of the blade with the other. You should be able to break it without difficulty. Now repeat it, but this time hold the edge into the table. The knife is now stronger by the ratio of its width to thickness (about ten). If the bending strength was linear in all dimensions you could break the knife with equal ease in both cases.

.. that seems to be in direct opposition with your original statement

No, the strength goes down far beyond linear, however the requirement can drop as well depending on the skill level, this of course isn't a contradiction to the base properties of the blade, as well the drop isn't as fast, and as noted this is only a consideration for overbuilt knives and thus it would not hold from one user to the next. But it is something that each individual should take note of, when you do alter the geometry, take care not to just use the blade as before, take advantage of the greater cutting ability but altering your methods. Of course the basic point remains that decreasing the edge thickness induces very small changes in cutting ability, but very large changes in resistance to rippling. It is of course a loss that can be worthwhile depending on the knife. I have several knives that are 0.005 - 0.010" thick behind the edge. You just have to understand what you are inducing, as the overall scope of use change is tremendous with even a very small alteration because both effects are nonlinear in the worst ways. You don't gain a linear increase in cutting performance and you lose strength faster. The ratio then goes many times faster than linear.

-Cliff

 

[frank k]

Originally posted by Cliff Stamp
Strength is the resistance to deformation.

You can get fracture without significant deformation. You can also have deformation with out failure, if it is within the elastic limit.

The direction of force is what is different in the above descriptions I gave. Take a blade and lock the tip in a vice or pound it into a piece of wood. Now grab onto the handle and pull up. This would be applying a load along the tip to the spine. Take the blade and drive the edge into a large block of wood or lock it in a vice. With the edge pointing towards the floor, haul up on the handle. This is applying a load in the edge to spine line [to be really specific there is a small bending torque induced here so this isn't strictly a linear case either].

What you are describing is a form of tensile strength, and it is (unlike bending) very difficult if not impossible to damage a blade with tensile stress in normal use. When I asked what type of strength you were referring to you replied:

Parallel to the edge thickness, rippling in other words.

Which is not tensile strength but bending.

Edge to spine prying isn't a worry because now you have an even greater strength because the width and thickness have reversed and the strength has been increased by their ratio (which is usually very large, on the order of 10 or so), tip to choil is nearly impossible to do any damage as there isn't a person alive with the necessary grip strength to hold onto the knife.

This statement might make some sense if you meant pulling apart instead of prying. I still don't get what you are saying about reversing the width and thickness, since tensile stregth is dependant on cross sectional area.

The massive difference in load required to break a knife with these methods as opposed to general prying, or bending is is precisely because the strength is nonlinear in prying with the dimension parallel to the applied load. As a very trivial example of this, take a butter knife and lay the flat on a table with the handle extending over the side. Press down on the handle with one hand and on the flat of the blade with the other. You should be able to break it without difficulty. Now repeat it, but this time hold the edge into the table. The knife is now stronger by the ratio of its width to thickness (about ten). If the bending strength was linear in all dimensions you could break the knife with equal ease in both cases.

The reason that it is more difficult to break hardened steel by tension than by bending, is that they are very different types of strengths.
The reason that a knife will bend with less force tip to handle then from spine to edge is a due mostly to the difference in leverage (force times distance).

No, the strength goes down far beyond linear, however the requirement can drop as well depending on the skill level, this of course isn't a contradiction to the base properties of the blade, as well the drop isn't as fast, and as noted this is only a consideration for overbuilt knives and thus it would not hold from one user to the next. But it is something that each individual should take note of, when you do alter the geometry, take care not to just use the blade as before, take advantage of the greater cutting ability but altering your methods. Of course the basic point remains that decreasing the edge thickness induces very small changes in cutting ability, but very large changes in resistance to rippling. It is of course a loss that can be worthwhile depending on the knife. I have several knives that are 0.005 - 0.010" thick behind the edge. You just have to understand what you are inducing, as the overall scope of use change is tremendous with even a very small alteration because both effects are nonlinear in the worst ways. You don't gain a linear increase in cutting performance and you lose strength faster. The ratio then goes many times faster than linear.

There is no exponential increase in strength, either in tensile or bending, by increasing the thickness of a blade – these strengths would be proportional (at best, due the greater flex of the thinner blades in bending), to the increase in the cross sectional area.



-Frank

 

[Cliff Stamp]

Frank :

You can get fracture without significant deformation.

Yes, that would be the property described by toughness. When the load is applied too quickly for the slip planes to dissipate the energy through internal movement and they instead break away from each other. The rate of applied load, and the difference induced in internal mechanics is exactly what separates the two (toughness / strength), of course the external failure characterists are quite different as well.

You can also have deformation with out failure, if it is within the elastic limit.

Yes, that is when the internal strain still has not exceeded the resilience of the material, the "grains" are then still in a strained state, once they pass a certain limit this energy is released and can thus no longer be recovered, so they stay in the new position and the material is permanently deformed.

... very difficult if not impossible to damage a blade with tensile stress in normal use.

Yes, that was the point. Blade strength is linear in dimension when it is loaded so as to be characterized by tensile strength. However I was ignoring this when I said strength falls away faster than linear because the break point of tensile loads are too high to be any concern even for the weakest knives. Only bending loads are significant for blade failure, and are thus the only aspect needing to be considered when discussing strength failures.

Now to be clear, bending and tensile strengths are essentially the same characteristics materials wise as the same internal processes take place in each. The difference is how the strain is produced from the stress. Tensile loads are applied uniformly over a cross sectional area, inducing a uniform strain over said area. Bending loads induce a nonuniform strain which both compresses (inside radius) and stretches (outside radius) a much more focused area.

[referring to the butter knife demonstration]

The reason that a knife will bend with less force tip to handle then from spine to edge is a due mostly to the difference in leverage (force times distance).

There was no difference in leverage or cross sectional area in the butter knife example, yet the bending strengths are very different (~ by the ratio of the width to thickness). It doesn't have to be a butter knife you can try this out with anything that is rectangular in cross section and notice the difference depending on which dimension you try to bend through. If the strength was linear this orientation would not matter but it is immediately obvious that it does.

There is no exponential increase in strength, either in tensile or bending...

No it isn't exponential, non-linear doesn't imply exponential. To be specific it is ~quadratic in the dimension through which the load is applied, it is linear in the perpendicular dimensions. This is easily deomonstrated as noted in the above, and can be understood from some very basic principles as described. Using knives for prying tasks it also becomes immediately obvious as rather small changes in thickness induce very large changes in strength.

-Cliff

 

[Lukers]

Cliff
Is there a significant difference in edge holding ability between the BK9 and BR9.
Thanks
Luke

 

[Cliff Stamp]

I have not used the CU9 yet, Andrews experience is in favor of the Swamp Rat which I would tend to expect.

-Cliff