Bicycle Mechanics - Spoke failure question

I have a wheel set built up with Open Pro rims,DT Swiss swagged spokes, DA-7700 hubs.

The wheel went out of true. So I check the spoke tension, I find a spoke that's loose. So I try to tighten it up, no luck. I run out of thread before proper tension is built up, it's like the spoke is too long. I take out a good spoke and the problematic one, the problematic one is about 7-8mm longer. I know I used the same spokes for this wheel set and didn't have any odd length ones.

This is the second time I've had a spoke elongate itself like this... Last time this happened, I thought I just used a wrong spoke by mistake when building the wheel. I tensioned the wheel fine and ~250 miles later same thing happens... The first time this happened, I've had about 3k miles on the wheels.

I've never built up a wheel with swagged spokes before, only straight gauge. Is this how swagged spokes fail?

Spokes don't stretch like that. If they really did, they are terribly defective.

Are you sure you're not comparing drive side and non-drive side rear spokes? They will be different lengths.

Stretching is the overtension failure mode of butted spokes (see Brandt's book). I'm a little surprised that you were able to get that much tension on the spokes!

I can almost guarantee that the butt sections are the same length as on all the other spokes, just the narrow piece in the middle stretched.

Plain guage spokes will not fail in the same way, they'll just break.

Later

Mel

Are you sure you're not comparing drive side and non-drive side rear spokes? They will be different lengths.

Yeah, but not 7mm or 8mm different. There's something goofy about this post.

I'm def. comparing same side spokes. They are both off the drive side of the wheel. I might be exaggerating
with 7-8mm difference. I eye balled it, but it's way off. The difference is enough where that one drive spoke is tensioned less than the non drive spokes.

The butted parts are about the same length, so I would assume they stretch towards the center.

I'm not superman, the tension is set around ~120 Kgf on the drive side spokes. I don't think I could stretch the spoke with just my spoke wrench. I would think that the nipple will round off before I reach that point. I set the tension and rode them about ~250 miles. I didn't check the wheel until wheel went out of true. This is when I noticed that this one spoke can't be brought up to tension. Previously it was tensioned evenly like the rest of the drive spokes.

I've seen broken spokes before, but straight gauge. I've never seen this happen before. This is my first set of wheels that I built with butted spokes. So I guess this is how they fail.

With 7 to 8mm of stretch in the butted section, you would see flaws in the surface of the material. Something else is wrong here.

Gremlins