School me about stress risers

[Bolster]

I was reading another forum's discussion of "stress risers," the locations in steel where stress is concentrated -- holes, notches, grooves, sharp corners. Stress risers are the areas along which failure or breakage is relatively more likely to start and propagate. Obviously knife blades have some of these features, such as the hole in Spyderco blades. I read one commentator who said a sharp plunge line running through a hole is asking for trouble, and recommended at minimum a gently curved plunge line that gradually rather than abruptly transitions from bevel grind to ricasso.

Now, I've never had a Spyderco blade break or crack, so you could call this merely a theoretical interest on my part. Those of you who know more about this topic (which is probably everyone on this board!) help me learn a bit more about stress risers and blade design, I'm interested. Would a gradual plunge line be a significant improvement, or is a sharp plunge line a non-issue, from an engineering and blade-strength standpoint? What other features on a blade constitute stress risers, besides the plunge line? Sharpening choils? Does the heel of the edge meeting the kick at 90 degrees constitute a stress riser?

[zhyla]

I don’t have much depth in this area but the Wikipedia article has a good description with some pictures.

https://en.wikipedia.org/wiki/Stress_concentration

[Doc Dan]

I never gave that much thought as I don't do things with my knives that are likely to break them. However, that has led me to some more thinking on the subject. I'll have to look into this.

[Fireman]

[Evil D]

This isn't really adding to the topic but I would be all for softer/blended plunge lines. I just think they look nicer, and if there's any science behind them being stronger then great.

I like this little detail on the Southard.

[Naperville]

It is well known within the Corvette community that if a crack appears in a fiberglass body panel that you drill a hole at the crack tip and it distributes the stress to stop the crack.

[Aladinsane]

Hmm… well, when my supervisor wants me to stop a machine so that he can “check something real quick” and then it doesn’t want to start up again is a definite stress riser for me!

[Pokey]

It is well known within the Corvette community that if a crack appears in a fiberglass body panel that you drill a hole at the crack tip and it distributes the stress to stop the crack.

Years ago the Beretta 92 started having issues on a part called a locking block. Each side had an inside angle that was machined to a sharp 90 degrees. During normal operation of the pistol the stresses were concentrated there and the "ears" of the blocks could crack. Beretta's answer was to radius the inside corner on each side rather than leaving it at a 90 degree angle.

That radius is barely visible on the bottom edge of the larger part in this picture. (It's in the corner of the "L" shaped angle.)

[riclaw]

I remember something about the Respect having a stress riser where the blade meets the tang of the knife. I think someone broke one?

Hey Pokey, are you a fellow Beretta lover?

[RustyIron]

I read one commentator who said a sharp plunge line running through a hole is asking for trouble, and recommended at minimum a gently curved plunge line that gradually rather than abruptly transitions from bevel grind to ricasso.

Therein lies the challenge of listening to "commentators" on the interwebs. Every youngster who took a science class in high school and gets a micrometer and a slide rule for his birthday thinks he's now a rocket scientist.

By now you've figured out that we're talking about a chunk of material where the geometry is such that stress does not distribute evenly, where a feature of the shape causes a localized concentration of stress. Folks who aren't trying to impress others with their vast knowledge will just say "weak spot."

Sitting here at my desk, I can find a hundred of these "stress risers" of which we speak. If I pull on the metal clip on my Ridge wallet, I can tell you exactly where it will permanently deform. On my triplet loupe, I can tell you where it will break if I purposely twist it. The cap on the tube of hemp lip balm will crack in the same place every time. An ability to recognize where something will break doesn't make one an engineering savant. Even chimpanzees know where a tree branch will break if they climb out far enough. If you can't write an equation for the weak spot, then you shouldn't use the term "stress riser." Just say "weak spot" like the rest of us chimps.

Does anyone think for one second that the designers at Spyderco don't know that their knives will break? Of course they do! This is a typical discussion at the weekly meeting at every knife company since the invention of knife companies:
Engineer 1: We need to make the blade on the new knife 0.100" thick. It will cut like a laser!
Engineer 2: No way, Brohoss! That's a girlie knife! We need to make it 0.150" thick!
The Boss: Boys! 0.125" will satisfy the needs of 95% of our customers. The %5 can buy a different model.

The Tik Tok Jockies on these other forums are just strutting around trying to impress each other with their insight. If they were sensible and honest, discussions might sound more like "My Spyderco Tuff can't even cut through sashimi! " or "My Spyderco NAND won't even split logs!" Spending time quibbling about "stress riders" is like arguing how many angels can dance on the edge of a Manix 2.

[cycleguy]

Losing a knife is a known “stress riser”…

CG

[RustyIron]

Losing a knife is a known “stress riser”…

There you go “rising my stress…” or should I say, “pointing my weak?” Yesterday I got home from a trip. Before leaving, I hurriedly gathered up all my good knives and locked them up. Now I can’t find my Manix 2 LW CPM-15V.

[Fireman]

Losing a knife is a known “stress riser”…

CG

This is why air tags on lanyards is a great idea when your knife is worth multi Franklins. Just clip it on whatever knife is in rotation.

[ChrisinHove]

One of the most famous issues regarding stress raisers was the catastrophic failure of hastily built merchant ships in WW2.

https://metallurgyandmaterials.wordpres ... -failures/

I would assume Spyderco know exactly where their knife blades will fail under excessive loads.

I also suspect this is why the Spydie hole is round and not square. Just kidding.

[Fireman]

One of the most famous issues regarding stress raisers was the catastrophic failure of hastily built merchant ships in WW2.

https://metallurgyandmaterials.wordpres ... -failures/

I would assume Spyderco know exactly where their knife blades will fail under excessive loads.

I also suspect this is why the Spydie hole is round and not square. Just kidding.

This is why I support Pykrete ships.

[Scandi Grind]

Sharp angles and holes are the most common stress risers in blades. With most pocket knives it isn't a super big deal because they aren't designed to chop and batton, but with longer blades that are designed specifically to be tough, paying attention to the locations that you introduce stress risers can be important.

One of the best examples of how stress risers can be a problem is on hidden tang knives. If the transition from the ricasso to the tang is a 90 degree angle, when you put the blade under much stress it can often snap right at the intersection. It is much stonger to have a concave radiused shape because it distributes forces well, instead of focusing it into a small area. It also gives you a little more mass at the transition which will also make it stronger. Here is a good illustration from "Wayne Goddard's $50 Dollar Knife Shop".

This shape is good for similar reasons that arches on a bridge are good for support.

[zhyla]

Every youngster ... slide rule

You can always spot the time travelers. They never can keep the centuries straight.

[bdblue]

I don't know where the term stress risers came from, stress concentration is probably a more correct term. To school someone about stress concentrations could get into a whole college degree in fracture mechanics. I took a single course in fracture mechanics in engineering school and I worked in the research lab on some projects related to fracture and fatigue.

Basically if you have an irregularity in a material it can cause higher stresses due to loads applied to the material. The importance of this will depend on the material. Everybody knows you can scratch a piece of glass and it will break right there. You may know but may not realize that you can scratch a piece of soft metal and it doesn't make any difference. It depends on the material and how ductile it is or how notch sensitive it is. If you take a steel wire coathanger out of your closet you can easily bend the wire. If you scratched the wire it would still bend at that point and not easily fail because that material is ductile.

Now apply this to knives- some knife materials are hard, high strength steels and are somewhat notch sensitive so they can break at discontinuities such as the handle tang example that someone mentioned. Blade holes, plunge lines, things like that can thus affect strength. Needless to say you don't use your PM2 as a pry bar and don't hammer on it like a chisel. Most of us have proven that the PM2 is sufficiently strong when used as a knife.

The other situation where stress concentrations is important is fatigue, as in the example of the Beretta locking block. If you subject some materials to repeated applications of tension stress they can fail through fatigue cracking. Depending on the material there is a level of stress that will allow cracks to form with enough fatigue cycles. Below that level of stress cracks won't form and the item would have infinite fatigue life. If you make an item with that material and it has stress concentrations, the maximum stress at the stress concentration can be high enough to allow fatigue cracks to form even though the overall stress in the piece is not that high. In my field the construction of things like steel highway bridges has to be done in a way that minimizes stress concentrations so the bridges don't fail through fatigue, and lots have failed in that way. None of this applied to knives because it may take 20,000 cycles of stress application to fail an item made of steel. Another example that I see sometimes on the internet concerns the British Comet aircraft right after WWII. There was a theory that the corners of windows caused cracking due to the cycles of pressurizing as the aircraft goes to high altitude. People like to point out that it wasn't the windows it was some access hatches, and it wasn't actually the access hatches it was the rivet holes. Well they are all stress concentrations. The access hatch opening is a stress concentration and increases tension stress around its corners, then you punch a rivet hole in that area, and the hole being a stress concentration it increases the stress further. Punching the holes causes little cracks around the edge of the hole and the tip of a crack is effectively a very high stress concentration so now it's the perfect storm of stress concentrations to cause the cracks to propagate until the aluminum skin can fracture.

Before I went to college I worked in my father's garage and I read about building high performance engines. I noticed that engine builders would grind the sides of connecting rods smooth and even polish them. Back then I didn't understand how removing material would make a connecting rod stronger but it relates to fatigue.

The last aspect of stress concentrations relates to stress corrosion cracking. I read about problems from this in the Ruger factory relating to the lubricant they were using in machining. Supposedly they would screw barrels onto revolver frames, the stress concentration where the shank meets the larger diameter of the barrel was under tension and also created a stress concentration, the lubricant in there caused stress corrosion which lead to fractures and the barrels would break off. I can't substantiate any of this.

[SpyderEdgeForever]

Are there signs of increased stress on a knife blade that one can look for such as minor discoloration or it is mostly invisible until it breaks?

[Bolster]

Wow, that's fascinating, @bdblue. Thank you!! I need to read your post a couple of times over. Where do I send my tuition money?