Somehow I can't help but feel misunderstood here on this thread. I really don't give a flip about how those blades bent or flexed. I did what I had to do to pass the performance test.
My main question here is if there is a correlation between where my test blades became un-flexible by hand, and then returning to that same point after releasing pressure with the cheater bar.
Is that simply coincidental, or a mechanical anomaly?
Karl,
It has been a few years since I took mechanics of materials, but I'll try to rememer the basic concepts and you can figure it for yourself.
In steel, any movement in the elastic range goes back to zero. Any movement beyond that range, the difference is permanent. On a theoretical tensile specimen of some unknown hardness and long length, lets say you can pull it longer by 3/4" and it will go back.
But if you pull it out by 1 1/2" it will go back but not to 0, but will be 1/2" longer. That means it yeilded at 1" This is an over simplification, and it isn't really perfectly 1:1 like that because the hardness of the steel changes too, but we'll ignore that.
Lets apply the numbers you saw in degrees in a bend test, to inches in a pull test on what would have to be a very long thin specimen (to make the numbers work).
Let say you have a long thin wire and you can pull it to 20 inches before you just can't pull it any farther, you let go and it goes back to 0. Then you put a wench on it and pull it 90 inches. It yielded and went back to 20. That means its yield point was somewhere around 70 inches. If you would have been stronger you could have pulled it to 40 inches and it would have gone back to 0.
Now, these number won't really apply to a knife because, unlike a wire in tension, the loads are different throughout the cross section because it is bending. Somewhere around the center of the cross section is a neutral plane which is not stretched or compressed, but everything outside of that is in tension on one side, and compression on the other. And it increases geometrically the farther you get from the neutral axis. Thus in a bend, it is possible for only the outside "skin" of the blade to yield. In fact, at some point around the neutral axis of your test blades there is steel that did not yield. My point here is that applying tensile concepts to a bend question is an over simplification, but I think it illustrates the concept.
I'll finish with this juicy tidbit. When you double the width of your bend specimen, it becomes 2 times stiffer (duh), but if instead you double the thickness it becomes 8 times stiffer. That is because there is more material farther away from that neutral plane.
Being 8 times stiffer is great if you want to resist deformation, but if you're going to subject it to a specific amount of deflection it is more likely to break than a thinner specimen because the material on the outside of the bend is being asked to stretch much farther, and thus may exceed it's ultimate strength.
I have always assumed that flex was just a side effect of having a blade thin enough to do the job and folks somehow started to focus on the flex instead of the proper blade proportions. What am I missing here?
(in a class), I have a lot to learn. Enjoy coming over here and taking class occasionally.
Now I am back to square one, wondering how those things stay aloft. Makes one question one's education.
. If you think cryoed bainite would cause me to go spastic you should see what would happen if you tell a metalurgist about molecules in steel, and spell it metalurgist in the process