In article ,
says...
In article , Ned Simmons
says...

.62 in^2 x 20000 lb/in^2 = 12400 lb.

Which is pretty close to the yield of the full (unthreaded)
wall of the pipe.

Again, the trouble arises with the sharp V form
at the root of the thread, and also the die-cut
threads. Those are the two *worst* situations
to have when one is trying to optimize the
strength of the joint.

Each of those will substantially reduce the
load carrying ability of a connection like that.

A proper fastener will have a) rolled threads,
with b) a bit of a radius or flat at the root.


The power was out here for a few hours, so I did a bit of
reading and playing after cleaning up in the dark.

First, like you, I recalled the NPT thread as sharp, which
is not the case. It's truncated, though obviously not
nearly as much as the UN thread. Neither requires, though
both permit, a radius in the root.

Socket caps over 2", and nonstandard lengths, have cut
threads. Even socket cap screws are allowed to have certain
"discontinuities" (laps and seams) in the thread, though
not below the pitch diameter.

For a ductile material like low carbon steel, stress
raisers have little effect on static strength. They do have
a marked effect on fatigue, though the stress
multiplication factor used for figuring fatigue (Kf) is
often less than for static stress concentration.

The endurance limit for carbon steels varies from 25 to 75
ksi.

Using the 3/4 pipe from before, and assuming a Kf of 3, the
joint should not fatigue below about .138 x 25000 / 3 =
1150 lbs. The 25 ksi is pretty conservative. Kf=2~3 seems
typical for cut threads in low strength fasteners. The
allowable load could be higher if you could predict the
number of cycles and/or avoid strain reversals.

Failure under static load would be closer to .138 x 60000 =
8280 lb.

While the lights were still out, I did a quick experiment.
I threaded a length of 3/8 pipe from the scrap bin thru a
hollow 12 ton porta-power cylinder and put some heavy
washers over the pipe, then screwed a couple random CI
fittings onto the ends. Upon pumping up the cylinder,
initial yield occurred at about 5400 lb tension; tensile
failure was at approx 6800 lb. The failure was exactly
where we both predicted, where the threads exit the
fitting. This corresponds to 57 ksi and 72 ksi
respectively. The material spec for black pipe is 35 ksi
yield and 60 ksi tensile, minimum.

Ned Simmons