School me about stress risers

[riclaw]

Mighty fine information. Thank you for posting!

[Marc_645]

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.

That makes sense.

Once a crack forms, the crack tip becomes the primary stress riser in that region

As already mentioned by others, changes in geometry (thread roots, holes, sharp edges/corners, etc.) amplify the stress level at the location of the geometric discontinuity. The sharper the geometric discontinuity is, the larger the stress multiplier, and crack tips are about the most egregious geometric discontinuity a component could have.

So by drilling out the crack tip, one is basically removing the most sever stress riser in that region.

[Marc_645]

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?

Speaking in general about steel components , I can think of two methods by which to identify a steel component that at one point was potentially close to fracturing or is close to fracturing due to mechanical stresses.

1. Permanent Deformation: Permanent deformation of a steel component indicates stresses exceeded the yield strength of the components material. It should be noted though that knives are often heat treated in such a manner to prioritize steel strength and hardness over ductility and therefore these knives often don’t exhibit any significant deformation before fracturing.

2. Presence of a crack: If one is lucky, especially when dealing with high hardness knives, one could potentially notice a crack before a complete fracture of the blade occurs. Various industries use NDE (non-destructive examination) methods to detect and identify cracks which may be below the surface or which may be too small to identify with the naked eye. Common NDE methods include: magnetic particle inspections, liquid/dye penetrant inspections, ultrasonic testing and X-rays.

[shunsui]

https://knifesteelnerds.com/2019/04/15/ ... en-blades/

[Bolster]

https://knifesteelnerds.com/2019/04/15/ ... en-blades/

How did I miss that? Thanks!

[Wowbagger]

Pardon me for not catching up on this thread before posting . (maybe someone has already posted this very thing)
I've spent my life dealing with stress risers / warranteeing the results in retail / mechanics for customers as the result of stress risers etc.
So not so intent on following this thread ; too much like my work .
BUT
when the thread first apeard I had a clear image in my head from seeing "questionable" sharpening choil designs in the forums over the years . I didn't remember the brands of the knives , I just glanced and , lets say , changed the course of my shopping path.
I then spent way too much time searching past "what's in your pocket today" threads (and knife vendor sites) and never found images of the examples I wanted to post in this thread .
Finally I gave up .

Until TODAY !
Here you go :
Look in that other forum under 'Which knife or knives are you carrying today" for today's posts . See the Allen folder with light wood handle . ( I would post the model and maybe try to steal an image to post here but I don't know the model and I get a bad taste from negatively critiquing another individual's work . Catch 22 / rock and a hard place )
also see
same thread and the Northfield slip joint knives just bellow the Allen . Now that is a knife model made by a "company / committee" of people who should know better so I'm not so averes to critiquing those (((has , in the past , been kind of a side hobby for some of us but I try not to indulge as much as I used to)))

From my fifty years of chasing stress risers , lets just say , those both look like tools I would not want to stress day in and day out if I had to depend on the tool with no back up .

PS: other stress risers that I have seen cause regular and predictable part failures have been something as innocuous as the embossed / forged in brand moniker in a forged and heat treated aluminum part and weld beads that are convex in cross section on thinnish structural , heat treated , 7000 series aluminum .

The preferred weld bead cross section would be at least flat and the much preferred bead would be a little concave . I am by the way an experienced TIG welder by trade .
Among many other things involving machining and machinery .

. . . just saying . . . curves are preferred rather than abrupt notches at the base of a moment arm.