531colin wrote: ↑5 May 2021, 3:30pm
andrew_s wrote: ↑4 May 2021, 1:38am
.......................Cold setting doesn't mean there's no resulting stress.
When a spoke is bent to form the J-bend, the metal on the inside and the outside of the bend sees enough stress to permanently deform, but in the middle of the spoke, between the compressive stress on the inside and the tensioning stress on the outside, these are regions where the metal is both unstressed, and stressed not quite enough to deform.
If you cold set something, you'll notice it springs back some when you let go. That's the un-deformed metal in the middle attempting to return to its original position.
The closer a bit of metal is to the deforming stress, the fewer stress cycles it will take before it starts to crack.
Stress relief is adding enough extra stress, temporarily, that the metal that was previously just about to deform does so. This means that the remaining undeformed metal isn't so close the the deforming stress under normal use.
Andrew; thanks for this. Its something I don't find easy to understand in depth. (I have a background in biology, not engineering.)
Can I just check with you that I have understood properly your sentence which I have turned red?
"When metal is stressed into shape (ie. not permanently deformed) cyclic stresses imposed in addition to the static stress will initiate a crack. Crack initiation is quicker if the static stress is close to that stress which would cause permanent deformation, and with greater cyclic stresses."
I'm sorry, but I'm not sure that I understand what you are saying. Metal cannot be stressed into shape without permanent deformation. But what happens when it is bent is
The bending has changed the structure of the material, and these stresses can only be eliminated by changing the structure of the material again, through something like heat treating. This is what engineers & metallurgists think of as stress relieving. One of the things about this though, is that the process of bending also causes something called work hardening. Because the microscopic structure has been changed, it resists further changes. This is advantageous for spokes because we want them to be stronger at the bends, and heat treating would also take out the work hardening. Straightening the wire from how it was coiled, threading the ends, and cutting the spokes also introduces stresses, though other residual stresses are relatively small by comparison.
As Andrew noted, the stresses vary, and because in order to work the material, some bits have been brought beyond the yield (plastic deformation) point, this leaves some parts of the material at or near the yield point. If these are subsequently cycled repeatedly in that condition, they will break. It's like taking a wire & bending it repeatedly.
The point of stress relieving is to get these points in the material to relax. A spoke normally operates at some fraction (1/4th? 1/2?) of it's capability, so the 'firm grasp' method increases the forces enough to take these highly stressed bits in the metal structure beyond the yield point. It doesn't affect other parts of the spoke because they are not at or near yield.
The diagram was borrowed from https://www.thefabricator.com/thefabric ... d%20radius.