Catastrophic spoke failure. While stored..

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Re: Catastrophic spoke failure. While stored..

Postby NATURAL ANKLING » 11 Mar 2019, 12:03pm

IIRC I read some where that stainless spokes are chemically / heat processed on their surface to give them the strength?
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Re: Catastrophic spoke failure. While stored..

Postby Brucey » 11 Mar 2019, 1:12pm

the only surface treatments (apart from polishing) that I know of on stainless spokes are chemical/oxidising treatments that are carried out to render some stainless spokes black. It isn't clear to me that such treatments don't make the spokes a little more likely to break.


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Re: Catastrophic spoke failure. While stored..

Postby CJ » 3 Aug 2019, 1:49am

531colin wrote:See if I've got this......
The butted bit, being thinner, will be closer to yield than the elbow, being thicker. Reverse way round to fatigue, because fatigue happens where it flexes. Ok, yeah, that makes sense.

Strictly speaking, all parts of the spoke flex, they simply flex by differing amounts due to differing factors. Fatigue cracks initiate and grow where the flexing and the variations in tensile stress that cause it, is greatest. Usually it's greatest on the inside of the bend on a traditionally bent spoke. That's because the bend does not fit with perfect snugness into the hole and against the flange, so the head tends to straighten out as the spoke is tensioned - and regain its orginal bend whenever tension is reduced by the section of rim to which it's connected being pushed upwards by the road. As you'll know very well, making the spoke fit more snugly in the hub does a lot to reduce the chance of it failing.

The butt is another place where stress and changes in stress may locally be higher, but here due to the manner in which a change in shape affects stress at the surface. If the butt is abrupt or if the tooling that formed the butt is worn or faulty such that it makes a sharp inward corner, rather than a smoothly radiused transition, a small amount of metal in that corner will see a much higher stress and flex more than the metal in the core of the spoke, albeit on a very small scale. Once a crack forms however, that itself acts as a stress concentrating factor upon the metal in the crack-tip, which is what causes fatigue cracks to grow ever faster as they deepen.

That said, it's unusual for the butt to be the weakest point in a spoke and generally indicates a faulty spoke. Though whether the fault lies in the material or the process of shaping it will be hard to say without recourse to specialist knowledge and equipment.
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