Blade choils. Please make me get it.. (and please read my whole post.)

[Ankerson]

Gary you use of knives and mine are entirely different. I don't do much kitchen work, still I can see the knives you pictured are perfect for your needs. I have a family "French Cheff's Knife" that is a cross between the two you have photos of in your post the bolster is integral and protects the wood scales, been in the family since the early 40,s, cut up many turkey, hams and roasts and is a wonderful knife for what it was made for. Most of this knife's history looks to be in the center of the blade.

Dorito: Two similar blades, one with a chiol or nick, the other without were tested, the one with the choil did not make it past 12 degrees of flex before it broke into two pieces, the force was so light the torque wrench did not register and force, the fracture line started at the deepest part of the nick and ran straight up to the spine.

The one I had ground the choil out, reducing the volume of steel in the blade made it to 90 degrees with 10 lbs of torque and did not break on the first bend..

These were cheap factory knives - same model - and due to finances were not a representative sample of their knives, but I feel significant indicators of a stress raiser by design.

Yes any knife will break. Some are much more fragile than others, you can chose the knife that pleases you, just do so with the knowledge of what you are buying. knives with choils have served man for a long time and continue to do so well.

I find it is easier to teach an employee how to sharpen a knife with a choil and will be fewer cosmetic errors than to teach how to sharpen one without a choil.

What it really comes down to is the intended use of the knives in question.

As you know better than most Heat treatment and tempering has more to do with the strength of the blades than anything else as you can make a blade that can be bent into a circle without breaking with or without a chiol if you wanted to.

The same as Custom Fillet knives in High Alloy steels that can flex and go back to true.

 

[Ed Fowler]

Chris - I believe you understand the variables.
To put it simply, any round hole or notch in a blade can be a stress raiser. Some claim success through heat treat methods, I have not tested all blades made by others.

I have found that any scratch, even a makers name on the side of a blade can be a stress raiser. The discussion about the inspector refusing to accept any propeller blades with a scratch on them is a very good illustration. I would bet dollars to donuts he did not put his stamp of approval on the blades themselves.

I remember reading about a propeller blade failure where an employee stamped a serial number on the blade of a propeller, this small seemingly insignificant stamp resulted in a stress raiser that caused the propeller blade to fail during a flight sending a large portion of the propeller through the fuselage and a fatal crash of an airplane.

This does not mean the blade will fail catastrophically in normal use, some may break due to significant variations in temperature. What it indicates is that the blade without any stress raisers by design will probably not be as prone to failure as one without a stress raiser.

Stress raisers included in design can be a "cool" factor that helps to sell knives, we can't blame the manufacturers for selling knives keeps them in business, these knives will probably not be as strong or tough as clean blades without them and can aid as an indicator to predict of where the blade will break when use demands enough force to exceed the strength of the blade.

We tested one blade that had a small round hold drilled and tapped down into the spine of the blade for a thumb stud - the blade was fairly strong, but broke with 20 ftlbs of lateral force. The fracture ran straight through the center of the hole in the spine of the blade. It was one of the strongest folder blades we tested.

Many old factory folders have the name and other information stamped into the ricasso, they did this to reduce the potential of failure by putting the stamp in the strongest portion of the blade, I have never seen a ricasso break, they are usually the strongest portion of the blade.

If you get a chance to look at a folder made by Bill Scagel, you will note that the hole he put in his folder blades was 3 times longer than the maximum depth of the hole, he did this for a reason. A hole that is 3 times as long as it is high will not be as significant a stress raiser when lateral force is applied than a round hole.

Next time you see a semi hauling tubing to a oil field, note how carefully they pad the tubing while hauling it. My friend examined a length of tubing that failed, the oil company blamed the manufacturer, the source of the failure was a stress raiser probably created when they unloaded the tubing and dropped a length of the tubing creating a dent in side of the tubing.

There is a good book about stress raisers, "The Prevention of Fatigue of Metals Under Repeated Stress" by the Battelle Memorial Institute for the US Army Air Command at the beginning of WWII. A careful reading of this book and a little research into the references at the end of the book will provide the reader with a lot of knowledge about stress raisers.

The quality of steel is also a significant variable.

Ankerson: you were writing at the same time I was writing. Thank you for the kind thoughts!
I would never put a choil or round hole in one of my blades, I feel it would create more problems to solve than it would be worth.

 

[Chris "Anagarika"]

Thanks for educational feedback. I'm not a metallurgist, so the understanding I have is more from geometry perspective.

In the example of above knives, to reduce stress raiser effect, I have to make the transition from spine to the bottom of notch having large curvature or even infinite curvature (straight), in the manner of factory waved Endura.

That is if lateral stress during use is expected.

 

[Ed Fowler]

Geometry is what it is all about, the theory and practice is simple and easily forgotten by specialists in other fields.

 

[Twindog]

This M4 blade was improperly rehardened to HRc 64. From Benchmade, it came stock at HRc 59.

It's obviously brittle, but you can see the break came right where you'd expect, from Ed's perspective. I don't know if I'd call it a stress riser, but rather a weakness in the blade caused by such a large hole. It broke with just hand pressure. My guess is that the thin top part of the circle broke first, creating some kind of torque or twisting action that caused the bottom of the blade to shatter in two places. But that's my guess.

 

[Ankerson]

Ankerson: you were writing at the same time I was writing. Thank you for the kind thoughts!
I would never put a choil or round hole in one of my blades, I feel it would create more problems to solve than it would be worth.

I was just pointing out your skill level and that you could do whatever you wanted.

 

[Ed Fowler]

Twindog: when you study photomicrographs of steel surrounding holes drilled in steel there are very real stress raisers radiating from the hole into the surrounding steel.
Seeing that blade broken into three pieces is something I have not seen before, very interesting, thanks for posting it up.

Ankerson: again thanks for the compliment.