Guard Notch Effects on Strength

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me2

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Oct 11, 2003
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For any out there that use a notch and slide and/or solder a guard into it, does the notch reduce strength, or does the fact that it gets filled back and then bonded eliminate the stress concentration usually associated with notches with sharp corners? Also, are the corners sharp in the first place. They certainly look like it in the pictures I've seen, but the radius could be very small and difficult to see in catalog size pictures.
 
It is my understanding that any sharp corners or 90 degree angles are potential stress risers and that area of the knife is a very crucial area not to have any in. The slot for the handguard should be smoothly rounded with the blade area tightly matching it, or so i've read...
 
Certainly they have too weaken the blade, but what are you using the knife for ? If it is a chopper build it with a guard that is part of the blade bar. If it's a cutter then how would that be a concern? Built with construction for its intended use , and it will be fine. Frank
 
In my career as a metallurgist I have seen a huge number of failures due to sharp corners !! Both in impact and fatigue situations. Sharp corners are not a good practice regardless of use. Frank is right a slicer wouldn't be a problem.
 
Frank Niro said:
Certainly they have too weaken the blade, but what are you using the knife for ? If it is a chopper build it with a guard that is part of the blade bar. If it's a cutter then how would that be a concern? Built with construction for its intended use , and it will be fine. Frank
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I don't know, I actually like this aswer a lot. I now eliminate all sharp lines at the guard juncture, but do it for my customers peace of mind not mine. I have seen often first hand how much much the slightest notch can halve the impact toughness of steel and I am still not worried about it on a small knife blade.
 
I believe whether in the guard area or anywhere on the blade, you have then created a weak point and a stress location on the blade if there was any sudden impact to it, it would crack/break right there. Always made sure to round off your corners even it it's a itty litty bit as the corner will be a hell of a lot stronger. Just think of it as a piece of wood, if you cut a perfect 90 degree angle in it vs. a rounded 90' degree corner, which is easier to snap?
 
Now that I have some time to post I can elaborate, I saw mete posted the same time I did, right before I got grabbed by the ear to go to church.

mete is correct that any sharp corners or notches will create a weakness in the steel object. I am surprised how much just a small score on the opposite surface will drastically reduce impact toughness on charpy samples. And I would NEVER have a sharp inside corner before heat treatment, it is an excellent way to make two small peices out of one blade.

This is why I have taken to doing all round corners that are cleaned and smoothed before I heat treat and then not mess with it any more. I used to draw the shoulder area back softer after the heat treament and then cut my corners and do the guard. I had even done this on larger blades and never saw an issue if one was using a knife as a knife, however the methods I now use make more sense for efficiency, strength (due to not having to soften the steel to make shoulders), and in appealing to the consumer's overblown ideas of what sort of supernatural things knives should be.

So while it is correct that any such notch or corner will create a much weaker point, I just feel that the whole broken blade paranoia that has bladesmiths doing all kinds of things to make a blade that will not break if you drive your truck over it is a bit much when all you have to do is not drive your truck over your knives.

I think this reaches its ridiculous extremes when I see debates about whether it is acceptable to stamp your initials into your blades as it could results in a fatal stress riser:rolleyes: Give me a break! (no pun intended;))
 
For stock removal blades for the guard juncture use a half round notch milled-filed-cut into the blade and will be astrong junction. The straight lines will get you into problems. Mike
 
Kevin and Mete and others are perfectly correct, in both directions. That is, I've broken a bunch of blades (ABS-itis) and many have had perfect right angles at the bottom portion of the ricasso where the guard would be fitted. None, read zero, have failed there. All have bent/broken at or somewhere near where they are clamped in a vise. Now, of course, the whole thing is unnatural and in no way mimics what a knife should be doing, but I digress. Basically, build some blades and TRY to make them fail at the ricasso. It is a very hard thing to do, right angles or no.

John
 
John this has been my experience as well, but I would be a hypocrite if I fell back on the old "I have never had a problem with it so thus the problem does not exist" argument that I hate so much. It think this paranoia does have a reasonable basis in fact, and that origin would be the dreaded rat tail tang. I am sure there have been many tnags snap off from knives that had 90 degree cuts dropping down to less thatn 3/8" for a tang, This is just foolish whether you radius down to that little rod you call a tang or not.

I would confirm again that such a notch will cut your toughness at least in half, but it is the equivelent of having an area of wiring that drops from 10amps to 5 amps on a curcuit that is fused at 1 amp. Factors in the blade and edge will be far more suseptable to failure before that area will. I have impacted various damascus patterns and have found welds running perpendicular to the edge will also have these effects yet I am confident to use such patterns in almost any knife blade because those type of forces are not commonly encountered with knives, even ones that see impact. However get a notch in the edge and...:eek: Even the slightest nick in a hardened edge could result in failure should any forces be applied from the opposing side.

The broken blade is such a fear among makers that we have come to believe in its certainty; edge quenching, spine drawing, bainite, 3/8" thick spines, endless variations on lamination all to over come the invevitable broken blade, yet I have asked time and again for first hand accounts where blades broke in normal knife use, that could not be attributed to bad heat treament or serious flaws, it is apparently an exceedingly rare thing that weighs so heavily on our minds. More folks should try to break blades and see how tough a material steel really is for a job as simple as cutting things.

It is a tough call for me since I am always harping on and splitting hairs about heat treating factors that will never be noticed in the majority of knife uses, yet they are still a factor, so as I previously mentioned I don't do the 90 degree corners either:o.
 
You make good points and i (while not having any experience with making a knife like this) would try to round it as much as possible to not create any more stress. no matter how little failure you should have in normal knife use how many people have never used a knife for something it wasnt made to, i bet no one here can honestly day that they never had, i bet a lot of them wish they could say they hadn't because they broke a good knife by doing it (like i did) but it think that kind of has added to why people get so caught up in this.

making sure everything is perfectly rounded might be like using a 40 cal to shoot squirrels but theres always that one time you get the giant rabid squirrel that wants to eat your soul so you need the extra strength.

-matt
 
So how big should the radius be? Minimum 1/32"? Or use a 3/16 file to cut the whole notch?
 
me2,
In my opinion I dont believe that filling in a notch with solder or glue would add any substantial strength to that stress raising notch. This bond would likely break long before you reach the stresses required to cause failure at the notch. Agreeing with the sentiment of the thread, failure is unlikely unless you use your knife as a crowbar or to chisel concrete.

Lang
 
Surprisingly 1/32 radius might be a big help, but I would try for at least 1/16 (the equivalent of having drilled a 1/8 hole by the junction).
 
I must confess to pondering this subject a bit since yesterday, and I have come to a conclusion that my initial attitude was a knee jerk response to the uber-unbreakable blade sales hype idiocy that becomes so tiresome. Avoiding the stress risers from such corners is one of the few ways that makers can use to get a stronger blade that does not compromise its primary function of cuttingin any way. Looking at it like this it is a win/win situation.:thumbup:

Hey I knew there were darned good reasons that I did it ;)
 
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