On Apr 23, 7:57 am, Peter Cole wrote:
Ben C wrote:
The evidence we would expect to see for residual stress being a factor
just isn't there.

The point I made by posting the source was that overloading was a
recognized technique to manipulate residual stress -- either to reduce
or increase it depending on the desired outcome.

If a spoke is laced with an elbow angle that is too large, there will be
a bending stress in operation (load stress) that will put the outside
skin in tension. If the angle is too small, the load stress will be
tension on the inside skin. If the load path for a spoke is straight
from the hub to the rim, there will be no moment (bending stress), only
uniform tension across the cross section and shear stress.

By overloading the spoke, any existing notch conditions (small cracks,
threads) yield in tension and after unloading have residual compressive
stress which retards crack growth (see reference). The important factor
is that the static load plus overload plus residual totals to greater
than yield, if only in very local spots where stresses become naturally
concentrated.

As for the claim that spokes always crack from the outside of the elbow
(which doesn't agree with my limited experience), it's a certainty that
cold forming a ~90 degree bend will leave micro cracks on the outside
skin. Stress relief will yield these and generate beneficial
(compressive) residual stress in the immediate vicinity (see reference).
It does not matter if the residual skin stress from forming was
compressive, the stress relief will mitigate the fatigue effect of
surface flaws and provide additional benefit. As Jobst has frequently
pointed out, these effects are at the microscopic level, the source I
cited explains the mechanism.

Stress relief by brief overload before a part is put into service is a
well established method for improving fatigue life. The only requirement
is that the overload be applied in the same direction as the service
load. The literature abounds with examples, I just cited one source.
This can only be controversial via willful ignorance.

There are other requirements than direction of applied proof load,
namely ductility and defect size. For a large defect in a ductile
material, you will in fact get plasticity and residual compression
when you release the load. If the defect is very small, or the
material is brittle, proof loading may form a crack. There will be
some residual compression at the crack tip, but not enough to make the
part stronger than it was before it was cracked.