On 27 May, 11:48, Osprey wrote:
Time for another project ... need to put a 90 degree bend in some
1.25" square section tube.

Not a hope, even if you fill it beforehand. It will crimp (it has to -
can't do it with pure stretch). So either indent it, notch it, or join
it.

Indenting is pretty strong and simple on a production line (look at a
lot of office furniture) but hard to do neatly as a one-off. You
crumple the inner surface deliberately, thus providing somewhere for
the excess steel to fold up fairly neatly. The outer circumference
remains pretty constant, i.e. no stretching required, so you don't
need tooling that has enough grip to exert a longitudinal tension.

Notching (Wikipedia describes) is how I'd generally do it in my
workshop, but then I'd MIG weld it back together afterwards.

Joining it is simple cut & shut, perhaps even with a plastic plug
joiner in there afterwards. Dead simple, only tool is a hacksaw (or
for Doctor Drivel, a pipe cutter) but the result's not so strong.