"When a sufficient load is applied to a metal or other structural material, it will cause the material to change shape. This change in shape is called deformation. A temporary shape change that is self-reversing after the force is removed, so that the object returns to its original shape, is called elastic deformation. In other words, elastic deformation is a change in shape of a material at low stress that is recoverable after the stress is removed. This type of deformation involves stretching of the bonds, but the atoms do not slip past each other. When the stress is sufficient to permanently deform the metal, it is called plastic deformation."
"Yield strength can be explained, in engineering and materials science, as the stress at which a material begins to plastically deform. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed."
We thought that we would begin this post with a couple of definitions for reference later but first we would like to thank KG, Grepper and Bud for their most recent and insightful posts. We're sure that the rest of the followers of this thread appreciate their input as well as all must be tired of listening to only us droning on.
So what have we accomplished so far with our tests? We have pushed the apex of a sharpened knife edge to one side using a fixed amount of force and then measured how much the result of that "push" has affected the cutting ability of the edge. We have discovered then that our "push" exceeded the yield strength of the edge apex and resulted in a plastic deformation, or "permanent rolling", of the edge. We suspect three things now as a result of our tests and we cannot emphasize enough the term "suspect" strenuously enough because our test results are far from mature. The results are certainly definitive enough though to give us good reason to investigate further.
We suspect:
A. That very sharp edges are are more prone to rolling than less sharp edges and significantly so. This suspicion is not difficult to get one's arms around from a logical standpoint. Thinner things bend more easily than thicker things of the same material.
B. That edge apexes assume new mechanical attributes that are different from the metal they emanate from. We can suspect that the sharpening process is responsible for this phenomena but it might easily be something inherent to the steel itself. Ductility is a prime example. Once again, not difficult to see how this might be true in that edge apexes can be rolled (bent) nearly 180° without fracturing. The constituent metal they usually emanate from cannot.
C. That hardness ratings of the knife blade material do not translate to edge apex strength in a direct or semi-linear fashion. This is likely the least mature aspect of our testing. We suspect that hardness is definitely a contributor to apex stability but perhaps not as directly translatable as some may think. We think that metallurgical science lends support to this supposition in that burr formation is regarded as the result of plastic deformation. The mechanical characteristics of metals that have been plastically deformed are changed. Does that mean that all of the hardness qualities have been lost through the deformation process? It doesn't seem so in our tests. It remains to be seen what percentage of hardness, if any in the final analysis, is lost. Deformation may not turn out to be the culprit at all. Foil properties are a real consideration here. It could be foil and it could be deformation - or both or something else entirely.
We appreciate your taking this ride with us because "a ride" is exactly what this is. We are not performing a year or two worth of careful research and then publishing a white paper of our findings. We are asking you to ride along as we conduct experiments and talk about the results. We'll likely go down some rabbit holes during this process and then have to dig ourselves back out but that is how discovery works. Experiments need to be repeated and data confirmed. Right now we're just trying to figure out where the fences are. Once we and our outside testers establish the parameters then, perhaps, we can start driving stakes in the ground. Until then, we can use all the help and suggestions we can get.
Here's an experiment that all PT50 or ID75A owners can conduct. Power your instruments up and place your finger on the force plate and press down. You'll get a feel for exactly how little 150 grams, the force level we conduct these measurements at, actually is.
EDIT POST - Case in point. After posting this morning we decided to retest one element of our $5.00 knife test. While the results from the end of this knife that we had dulled to 292 seemed perfectly within bounds the factory ground edge did not even though we had confirmed the measurement. We were suspicious though of exactly what kind of edge the $5.00 factory had left us with so this morning we sharpened it using our techniques and equipment. Yesterdays test on this section of blade began at BESS 168 and this mornings BESS 164. Yesterday, after one set the factory produced edge rolled to 592 and our in-house sharpened edge produced this morning rolled to 435 and we confirmed the measurement at 442 . Those numbers make more sense. Just one more thing that we have to be careful about. We're going to have be careful about trusting the sharpening efforts of low end knife manufacturers because, obviously, it can have a significant bearing on our test results. We're going to edit yesterday's post and place an asterisk next to yesterday's numbers.
"Yield strength can be explained, in engineering and materials science, as the stress at which a material begins to plastically deform. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed."
We thought that we would begin this post with a couple of definitions for reference later but first we would like to thank KG, Grepper and Bud for their most recent and insightful posts. We're sure that the rest of the followers of this thread appreciate their input as well as all must be tired of listening to only us droning on.
So what have we accomplished so far with our tests? We have pushed the apex of a sharpened knife edge to one side using a fixed amount of force and then measured how much the result of that "push" has affected the cutting ability of the edge. We have discovered then that our "push" exceeded the yield strength of the edge apex and resulted in a plastic deformation, or "permanent rolling", of the edge. We suspect three things now as a result of our tests and we cannot emphasize enough the term "suspect" strenuously enough because our test results are far from mature. The results are certainly definitive enough though to give us good reason to investigate further.
We suspect:
A. That very sharp edges are are more prone to rolling than less sharp edges and significantly so. This suspicion is not difficult to get one's arms around from a logical standpoint. Thinner things bend more easily than thicker things of the same material.
B. That edge apexes assume new mechanical attributes that are different from the metal they emanate from. We can suspect that the sharpening process is responsible for this phenomena but it might easily be something inherent to the steel itself. Ductility is a prime example. Once again, not difficult to see how this might be true in that edge apexes can be rolled (bent) nearly 180° without fracturing. The constituent metal they usually emanate from cannot.
C. That hardness ratings of the knife blade material do not translate to edge apex strength in a direct or semi-linear fashion. This is likely the least mature aspect of our testing. We suspect that hardness is definitely a contributor to apex stability but perhaps not as directly translatable as some may think. We think that metallurgical science lends support to this supposition in that burr formation is regarded as the result of plastic deformation. The mechanical characteristics of metals that have been plastically deformed are changed. Does that mean that all of the hardness qualities have been lost through the deformation process? It doesn't seem so in our tests. It remains to be seen what percentage of hardness, if any in the final analysis, is lost. Deformation may not turn out to be the culprit at all. Foil properties are a real consideration here. It could be foil and it could be deformation - or both or something else entirely.
We appreciate your taking this ride with us because "a ride" is exactly what this is. We are not performing a year or two worth of careful research and then publishing a white paper of our findings. We are asking you to ride along as we conduct experiments and talk about the results. We'll likely go down some rabbit holes during this process and then have to dig ourselves back out but that is how discovery works. Experiments need to be repeated and data confirmed. Right now we're just trying to figure out where the fences are. Once we and our outside testers establish the parameters then, perhaps, we can start driving stakes in the ground. Until then, we can use all the help and suggestions we can get.
Here's an experiment that all PT50 or ID75A owners can conduct. Power your instruments up and place your finger on the force plate and press down. You'll get a feel for exactly how little 150 grams, the force level we conduct these measurements at, actually is.
EDIT POST - Case in point. After posting this morning we decided to retest one element of our $5.00 knife test. While the results from the end of this knife that we had dulled to 292 seemed perfectly within bounds the factory ground edge did not even though we had confirmed the measurement. We were suspicious though of exactly what kind of edge the $5.00 factory had left us with so this morning we sharpened it using our techniques and equipment. Yesterdays test on this section of blade began at BESS 168 and this mornings BESS 164. Yesterday, after one set the factory produced edge rolled to 592 and our in-house sharpened edge produced this morning rolled to 435 and we confirmed the measurement at 442 . Those numbers make more sense. Just one more thing that we have to be careful about. We're going to have be careful about trusting the sharpening efforts of low end knife manufacturers because, obviously, it can have a significant bearing on our test results. We're going to edit yesterday's post and place an asterisk next to yesterday's numbers.