I already presented some material on this subject in the post describing bandsaw blade cracks and the post of the broken round stock shear blade.
The increase in peak stress near holes, grooves, notches, sharp corners, cracks, transitions in mass or grain structure due to heat effects and other situations is called stress concentration. The agent of the concentration is called the stress riser.
Blacksmiths frequently encounter this in both design and production phases of work. A common example is seen in forging an upset square corner. It takes practice to end up with a strong radiused inside corner and not a weak folded in crack in the corner.
I encountered another example in a 10’ pocket steel tape I always carry. I’ve had two of these break over the years in the same place - at the second hole where the catch element fastens to the ruled tape element. I’m sure there are many considerations which go into the decision of why the tape is constructed in this manner and all things considered there may not be a better choice. Still, I find it interesting to ponder.
Some stress riser references:
http://en.wikipedia.org/wiki/Stress_concentration
http://www.ewp.rpi.edu/hartford/~ernesto/Su2012/EP/MaterialsforStudents/Aiello/Roark-Ch06.pdf
http://www.kokch.kts.ru/me/t1/SIA_1_Stress_Concentration.pdf
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