The cylinder is a 4"ODx3.25" ID. The full axial load is taken by the threads. We added a radial seal before the cap that eliminates the hoop stress on the threads- this didnt help any with the fatigue life.
The failure originates at the root of the first thread then works it's way out to the...
The thread form is- 3.50-10 PUSH-BUTT-2B, on the current design.
The wall thickness at it's thinnest is .220", The depth of cut for the thread is .058".
I just calculated the stress for the wall outside the thread quickly by hand.
Fatigue life was determined by testing actual parts.
Material...
Well the tile sums it up we are having a fatigue failure that involves a buttress thread. I have a cylinder of 6061-T6 Aluminum we have cut an internal buttress thread in. After 7-10,000 cycles to pressure, 3,000psi, the tube fails at the last thread of the sealing cap. I've been through two...
