On 7/11/2018 10:16 PM, Jeff Liebermann wrote:
On Wed, 11 Jul 2018 13:48:47 -0500, AMuzi wrote:

I think the question isn't so much 'has the tube's ultimate
strength been diminished?' but rather 'is it yet strong
enough for expected application?'.

Nope. I'm not trying to measure if the tubing is strong enough.
Assuming identical lengths of tubing, I wanted to see if the presence
of a Rivnut significantly changed the tension required to bend or
break the tubing when compared to the identical tubing that did not
have a Rivnut inserted. If I'm able to pull hard enough, I should be
able to eventually break both tubes. If they break at the same
tension, then I'll declare the Rivnuts are safe to use. If there's a
substantial difference in tension, then I'll declare the Rivnuts
weaken the frame.

In theory and in absolute yes the tube is less strong. In
practice, from Santana ExoGrid tandems to Bianchi thinwall
tempered aluminum models, to their carbon bikes, rivnuts are
not a failure point.

Where are the Rivnuts installed on these machines? Water bottle
holders? The pictures don't show much detail:
https://www.google.com/search?q=Santana+ExoGrid+tandem&tbm=isch&
https://www.google.com/search?tbm=isch&q=bianchi+aluminum



Four pairs of bottle rivnuts on a Santana, two pairs on a
modern road bike.

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971