Will :
1.) What else, if anything, does the test demonstrate?
If a blade "passes", by definition, the elastic region of deformation has not been exceeded so you can infer that it is wide. Note all steels will return to the original state if stressed in this manner, there is only danger of fracture if you enter the plastic deformation region. I can bend my CPM-10V blade and even though it is a very high alloy steel at 62.5 RC, unless I bend it until it takes a set, there is no danger of the blade breaking. If you do go past this point, and the blade suffers deformation it can not recover from, it has entered the plastic deformation region and the knife will break at some point when pressed further, some steels will not see significant plastic deformation before they suffer brittle failure, some will.
2.) What predictions can reliably be made from this test, regarding strength, ductility, tendencies to roll rather than chip, etc.?
The strength would be correlated to how hard you have to press on the blade to induce a deflection. Of course the edge angle influences the result obviously, a thicker edge would need more force to be deflected so this has to be taken into account if you are comparing blades with different edge angles. Do the test on a scale if you want to get a number.
In regards to ductility (
assuming the level of deflection was constant), if the blade deflects and returns to true it is difficult to infer anything about the ductility as by definition that measures the extent of the plastic deformation region, how far a blade will take a set before it fractures (this is however very strongly correlated to how far it will bend before it takes a set). If the edge deflects and stays bent then you have shown that the elastic deformation region is not as wide as on the steel that returned to true, however ductility is good. You can put a number on the ductility by seeing just how distorted the edge will get before it fractures. Lastly, if the edge deflects and fractures you have shown that both the elastic and plastic deformation regions are shallow and that the ductility is low.
In regards to edge durability, assuming the test is always done under the same amount of force and on edges of the same geometry, if the edge deflects and returns to true this edge will be the most durable
under that particular level of slow lateral load because it will deflect and then return to true. The edge that deformed under stress will likewise deform in use, and similar for the edge that fractured.
There are however a couple of important considerations. First off all as you noted you can't ignore the amount of force used. Edges that chip out under the brass rod test are usually those of higher RC on high alloy steels and thus under the same amount of force that deflects a lower alloy softer steel, they won't deflect at all due to their higher strength. Thus, in use at that level of force, they will be more durable than the blade that "passed" the brass rod test because they won't even see edge deflection (or see it less severely) and thus will have a lower induced fatigue.
Now what about if the edge sees a lateral load that is above the force used in the brass rod test? Well in this case the edge that chipped out at a lower load will chip out again, no surprise there. The edge that deformed may now keep deforming or it may have its plastic region exceeded and it may fracture. The edge that "passed" may now see a load that is high enough to move it into the plastic deformation region and it may fracture or it may not depending on its ductility.
There is also another further complication, durability under slow loads is not well correlated to durability under sudden loads (i.e.. shock). In general the more ductile the steel the more shock resistant, but there are exceptions, and the relationship is not trivial (make it twice as ductile, it is twice as shock resistant).
You can put a number on the ductility by examining the performance under specific loads. For example, with edges that "fail" the brass rod test by deforming, they won't fail it under a very low load as they will have some region of elastic deformation, it is just very shallow. As well with edges that "fail" the brass rod test by fracture, they too have a shallow region of elastic and plastic deformation, so you should be able to both see them deform and return to true and deform and stay set. So basically do the work on a scale and proceed slowly. You should be able to map out the width of the plastic and elastic regions and the forces required to induced their onset. Once this is done you can make strong statements about the edges behavior under slow loads.
That sounds fairly interesting actually, I am glad you asked the question Will, I'll try to explore it in some detail this winter and see if I can't put some numbers on the above for a few steels.
Edge durability is a rather complicated aspect as it depends on the hardness, shock resistance and the width of the elastic and plastic deformation regions. I have some more results with Ray's blades on bone and concrete that I'll post up shortly with commentary on how the various materials apsects influenced the results.
1. Are results of this test equally significant on all blades, regardless of thickness, or does blade geometry play a part in interpreting the result?
No, and yes. Just as you describe the thicker the edge the harder the test. This is why Phil Wilson can take his 59 RC S90V fillet blades and bend the tip until it takes a set with no fear of cracking the blade. They are ground from 1/8" stock and have a full distal taper. With a 10" blade that means that most of the flex is through 1/32" of steel which has a full flat primary grind, and of course a fully rounded spine. The following thread shows the same aspect in some detail :
http://www.bladeforums.com/forums/showthread.php?s=&threadid=177876
In other words, doesn't a thinner edge have somewhat more elasticity than a thicker one out of the same material?
Yes, when you bend a piece of steel the amount of deformation that it sees internally is dependent both on the amount of curvature you induce and its thickness. As you move outwards from the center of the steel, the material has to proportionally distort to a further degree. Note as well that the size of the brass rod has a very dramatic effect as well. The smaller the bar the harder it is for the edge as the curvature induced is much greater. The edge finish is a factor as well, the higher the polish the better the edge will do. A convex profile will fare better as well. As will a blade that has a smooth primary -> edge transition.
Does the amount of pressure required to deflect the edge play a part in interpreting the result?
It would show the strength of the steel.
Why use a brass rod rather than some other metal, such as a smooth sharpening steel?
The edge of the knife has exposed carbides and they would readily cut up the steel if you used high pressure. By the same reason you don't want to use a really hard material as the opposite could be true as well. Plus brass is cheap, doesn't rust etc. .
Does the brass deflect as well, keeping pressure from being concentrated on too small an area?
The brass will indent on some level, however given the surface area of the contact and how both structures are supported, the edge of the knife will give way long before the brass sees significant enough deflection to effect the distortion induced along the edge.
Consider the following as an example of some of the above concepts :
Ed Fowler's 52100 blades have a very high ductility as once they go beyond the elastic deformation region (30 degree bend) and enter into the region where they stay bent, they can be pushed on very far before they suffer brittle fracture (180+ degrees). On the opposite end of the spectrum the INFI blades from Busse Combat have a much wider elastic region, they will go far beyond 30 degrees without taking a set, however they have a much more shallow plastic region, once they do take a bend, they can't be pressed on much further before they fracture, and it is no where near 180 degrees, < 90.
Anyway, in short; a blade that "passes" the test has a wide elastic region, one that "fails" by staying deformed has a shallow elastic region but a wide plastic one, a blade that "fails" by fracture has a shallow plastic region (and most likely a shallow elastic one as well). However as you noted, there are a lot of details about how the test is done that can make interpreting the results complicated. The best blade for a given situation may for example be one that "fails" the test but only does so under a very high load, one that would not be approached in use.
-Cliff
