Where do knife blades usually break?

[MT Borg]

Wondering where they usually break and how a maker can do better design elements to prevent it.

 

[23698012]

Most knives usually break at a "weak point", or the point where the design has the least amount of strength. Usually this is in places like the knife's point where the metal is very thin, or in areas where there the knife isn't as structurally sound due to large skeletonized holes in the material or deep choils, basically anything that "cuts" deeply into the metal. Basically, a knife is more likely to break where the metal is the thinnest and isn't as wide.

 

[Nathan the Machinist]

In my experience, at stress risers such as gut hooks etc.

However, I believe that for most people, the problems area tends to be the tip.

Knives don't generally break in "normal" use, but some people use knives in "non-knife" applications as pry bars and chisels. Knives used by certain people in uniform are generally used as regular cutting tools like any other knife, but may occasionally be used to pry open a glove box, pry open a crate or even pry open a door or used to open a hole in a wall etc. Some people are hard on knives, and when so it is frequently the tip that suffers the most. Hence the popularity of certain "tactical knives" with very stout tips.

 

[Rick Marchand]

In my experience, tips and plunges, mostly. Stress points like finger knurling(jimping?), gut hooks and sawbacks can be a place for fractures to start... especially if they were there during heat treat.

I use abuse my bush knives pretty hard. Thicker spines, no plunges or handle holes toward the front ricasso/tang junction are a few of the features(or lack of) that I like. There are always compromises among strength, toughness, slicing, weight, thickness, grinds, etc... you just have to find out what works for your style and technique.

 

[Mitchell Knives]

Points and plunges seem to be the most common failure points in my experience.

 

[james terrio]

About 1/8" - 1/2" from the tip, because some dipstick was too lazy to go get a screwdriver or prybar. To be fair, sometimes it's not a matter of being lazy or a dipstick, but an actual emergency.

As Nathan and Rick mentioned, many of us (including myself) build knives specifically with such use in mind, and there is a vast market for them. I have a special name for such knives, but it's not fit for publication

 

[Willie71]

They break when you overheat them, forge too cold, or in too fast of a quench.

 

[Stacy E. Apelt - Bladesmith]

My first thought was in the brine tank during yaki-ire. I have had a lot of blades break there PING!!!

The things to avoid, and how to make a knife less prone to breakage, are:
Have enough metal where you need it. This means not making the tip too thin on a heavy use knife. Also the ricasso should be the thickest area, as it gets the most force. A slicer can have a very thin edge, but a chopper needs some meat.

Avoid stress risers. Stress risers are where a stress forms in the steel from a starting point, and makes a weakness along the grain boundaries, thus providing a path for a fracture. Sharp inside corners, notches, jimping, and plunges are places where this can start. By making the transition in these places a slight curve instead of a sharp 90° angle, you will greatly strengthen the blade.

Tangs should be as wide as possible. The guard only needs 1/16" to rest on, so make the tang as wide as practical. Taper it back gently, not all at once.

Places that will put a torque on the blade. If you put a gut hook, or a bottle opener on your blade, it will torque the blade in use. This can lead to the blade snapping.

Abuse. Learn ( and teach others) proper blade use,..... and in using the right tool. A drop point hunter is not a pry-bar, and a skinner isn't made to baton firewood splints. More than likely, 99% of broken blades are due to being pushed beyond what the knife was made to do....usually by prying on something.

But, here is the cause of most breakage comes from, and how to prevent it:
Good HT. Getting the most out of the steel requires a HT that gets the best hardening and temper. I often read folks who say, "I am going to temper at 300F, because I want the knife hard." OK, a 300F temper will give you a higher Rockwell hardness..... but when it breaks, because it isn't tough, that hard edge isn't going to be of much use. Learn HT and what the steel type is made to do. 1084 is great at Rc58/59, CPM-154 and S35VN are best at Rc 60/61, CPM-M4 is good at RC64, and Hitachi white and some W-2 can go as high as Rc 64-65. Trying to make a 1084 blade at Rc 62 is just asking for it to break.

 

[james terrio]

But, here is the cause of most breakage comes from, and how to prevent it:
Good HT. Getting the most out of the steel requires a HT that gets the best hardening and temper.

Excellent point. For general-purpose/utility knives I err on the side of caution (as in CPM-154 @ 58Rc instead of 60), sacrificing a little edge-retention in favor of more toughness "just in case". I have not only hacked through, but dug holes through 2x4's with such knives, with no damage to the edge or point. On a dedicated hunter/skinner/filet knife etc, that compromise isn't necessary.

 

[Schleifstein]

In my experience, at stress risers such as gut hooks etc.

However, I believe that for most people, the problems area tends to be the tip.

Yeah tips are problematic. Especially when a passerby at your local heritage festival takes it upon themselves to commence an impromptu "stress test" on your display blade, say, by wacking it with a hammer while you have your back turned. That was my first knife. He was fortunate we were in a public place....

Rule of thumb: don't put anything that has sentimental value on display at you local heritage festival, or keep it strapped to your body at all times.

 

[HallHandmade]

I scanned through the thread and didn't see it mentioned, but even on a differentially hardened knife, the tip is still fully hardened, in addition to being the thinnest and least supported point. It's double trouble, and would be a fairly worrisome design feature if you stepped away from the fact that it's a knife and generally needs to be pointy.

Also, stress risers were mentioned a couple times above. If you do have any features on your blade that could potentially cause an area for stress risers, the insides should be finished with a grain pattern perpendicular to the direction of force, so in a knife blade, it would be lengthwise.

edit: Luckily most profile their blade edge with the proper grind direction, but sometimes features such as jimping are cross cut. I've seen blades snap cleanly when they had a very coarse raw ground finish, right along a deep scratch line from grinding.

 

[MT Borg]

Great comments everyone, thank you.

 

[butcher_block]

also making a tang too small or with sharp corners at the ricoso/tang junction is a good way to brake a knife (Kabar USMC knife have been known to snap off right in front of the handle ) not that a marine might abuse a knife