Quote:
Originally Posted by Hendog
Dumb question, but what's a "stress riser"?
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Not a dumb question. That is a very complicated and important question.
A crack has to start somewhere. If you can keep a crack from having a place to start it can multiply the toughness many times. The most important component in stress resistance is to try and make the stress act equally as possible across a part.
For a basic example take a coat hanger, straighten out the wire and grab each end of it and flex it. You can do that for hours and it won't break. Now take a 2" diameter bar of the exact same type of steel that is the same length and turn it on a lathe so just one spot is the same diameter as a clothes hanger. You could say that you have beefed up a cloths hanger every where except just one spot. If you flex that a one or two times it will break. All the stress is concentrated in one spot. That is your basic stress riser.
The part where this starts to get more counter intuitive is really important...
A crack has to have a place to start.
This is REALLY important. Cracks always start on the surface. If you can make the surface extra resistant to cracking you can make a part WAY tougher. A very common way to do this is to compress the metal at the surface. That way when the metal is starting to flex the surface isn't be stressed,
It is being relaxed! This is why shot peening (physical compression) and surface treatments (chemical compression) like nitriding improve toughness so much.
Corrosion can be a major contributor to initiating cracks which is why Porsche went to such lengths to make sure the torsion bars couldn't corrode.
This isn't the reason I would automatically reject a slightly marred bar like was shown here. The real question is what did the marring do to the surface treatment? The answer has to be "Nothing good" so just toss and replace.