Stress riser question

[CUTS LIKE A KRIS]

I am a bit confused as to one aspect discussing stress risers. When discussing broken blades from HT factors like scratches from previous grits, grinding equal passes from one side to another etc are often given. But if a scratch on a blade can cause a stress riser, why isn't the same seen in regards to filework on the spine or handle. Often these decorations are sharp points, drastic directional changes, and cut across the entire thickness of the blade. I have yet to hear filework blamed for failures, yet less aggressive features get blame. Could it be that wrong factors are commonly blamed? Is there some rule in metals engineering that would negate the effects of large surface pieces of metal being removed. I send out for HT, but have often wondered if I was making things more difficult with the filework.

Why did this not cause blade failure but a scratch may:

What am I missing? Any clarification is appreciated.

 

[Stacy E. Apelt - Bladesmith]

In reality, stress risers are more of an issue in bending in use than in HT. Unless the blade is getting a high stress quench ( water) or has some large thickness variations, most blades will not crack along stress risers in HT.

In use, stress risers are places where stress can concentrate, and start a crack. Once started at the source location, it will rapidly and usually fatally proceed across that blade. Places at thinner sections are the most likely starting points. Thus, you avoid sharply angled indents along the edge or at thickness changes ( like the ricasso). File work on the spine is already at the strongest part of the blade. If it breaks starting at a file grove, it would break almost as likely without it. Obviously, lateral stress is the most worrisome issue, so those places should be looked at most when evaluating stress areas. The plunge line is the most likely spot for a blade to fail due to a stress riser. For this reason, a plunge line should be rounded in an ogee, not a perfect 90 degree cut. Avoiding the plunge going fully from edge to spine is also a good practice when the blade will get high lateral stress.

Stress risers are really overplayed by most folks. There are situations where they matter, buy with most knifes and steels you don't really have to worry much about it.

 

[mete]

The radius of the groove you cut should be smooth , as large a radius as you can get for the design. Fatigue applications are far more critical than just a single blow.

 

[stezann]

Quoting the above as has been said, i would add that in industry, when HTing complex shapes (blades, filework or not, are simple shapes) the keyword is eveness.
Eveness in the crystal structure, obtained by thorougly termal cycling with normalization and grain refining; eveness in heating and cooling.
The fileworks and plunges may pose a lesser problem than uneven and harsh, uncooled grinding... if not followed by proper preparation before hardening. Even rough grinding lines itself are not the real threat, but the condition they left the steel in when they where grinded.
hope it helps

Stefano

 

[CUTS LIKE A KRIS]

Thanks for the answers, it is a subject that I have thought much about, glad to have some clarification.

 

[gte805i]

If you'd like to read up on stress concentrations, try to find a copy of Peterson's either in print or electronically. It is generally accepted as the authority on the topic by mechanical engineers. While you'll find that the influences are real (3x the stress for instance), you'll also likely determine that the "usage" stresses in your blades aren't all that significant to begin with.