Buttress thread fatigue failures
Buttress thread fatigue failures
(OP)
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 designs. One I inherited that had a very high axial stress in the wall- 26,000psi. I thickend up the wall and have reduced the axial stress to 9,000psi. This did nothing for the life of the tube. After about the same number of cycles I still end up with the same failure.
Any ideas on what else I should be doing to fix this problem?
Any ideas on what else I should be doing to fix this problem?





RE: Buttress thread fatigue failures
what thread form ?
net thickness (at the root of the thread) ?
basic thickness (how much material is removed by the thread, how much local bending is happening) ?
how did you determine your stresses (hand calc, FEM) ?
did you predict a fatigue life ? what Kt ?
can we change material ?
RE: Buttress thread fatigue failures
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 choice could be changed. I'd like to stick with aluminum because of weight. We try to stick with 6061 on pressure vessels since when they fail its not as likely to fragment as something like 7075.
RE: Buttress thread fatigue failures
i don't know your thread form but there isn't much info. out there on Kts ... is it a profile dsigned with fatigue in mind (generous root radius would be something to look for). thread Kts tend to be big, something like 8.
not surprised about the location of the failure. you could try unloading the last thread by thinning down (stepping) the end-cap wall (making it a little more flexible, encouraging the load to stay in the cyclinder); but i'd've thought this'd be a small effect (given your large change in nominal stress).
how did you accomplish this ... locally thickening the wall of the cyclinder ? did this extend somewhat beyond the thread (to make sure it was working as you expect) ?
RE: Buttress thread fatigue failures
Don
Kansas City
RE: Buttress thread fatigue failures
1)Threads do not equally share the load. The first thread at a bolt-nut interface can carry 1/3 to 1/2 the load.
2)The stress at the thread root usually exceeds all other stresses.
3)No matter how low the cyclic stress, aluminum will eventually fail from fatigue.
RE: Buttress thread fatigue failures
What diameter is the cylinder o.d & i.d and I assume that the full axial load generated by the pressure is taken on the thread?
I may be able to help just need some more info first.
regards
desertfox
RE: Buttress thread fatigue failures
RE: Buttress thread fatigue failures
The failure originates at the root of the first thread then works it's way out to the surface. We then end up with a tear/crack that goes about 1/2 way around the OD.
The thread is being cut on a CNC lathe with a carbide insert made to the thread spec. I looked breifly into thread forming. The quick reading I did said it didnt work well with buttress threads.
One other point of interest I did some FEA work on it today after my 1st post. I found if I changed the thread pitch from 10 to 12 there was ~20% reduction in the stress at the thread root. For reference that reduced the thread height by .010" and made the root radius smaller by .002"(.007 to .005).
RE: Buttress thread fatigue failures
Or taper the pitch surface of the threads; similar reasoning.
If at all possible, I'd eliminate the joint by machining the cylinder and cap from one piece of billet, or make it an impact extrusion.
Mike Halloran
Pembroke Pines, FL, USA
RE: Buttress thread fatigue failures
Or, switch to titanium.
Mike Halloran
Pembroke Pines, FL, USA
RE: Buttress thread fatigue failures
I was going to suggest you look at the stresses on the threads but you already have.
Is the tear your seeing going around the threaded portion like a helix?
If so you need to look at the combined stresses of the tension and shear as a helical crack can be formed by the
principle tensile stresses running at approx 45 degrees to
axis of the thread.
Using the Von Mises theory for failure of ductile materials might help.
regards
desertfox
RE: Buttress thread fatigue failures
at 3000psi, the hoop stress is 3000*(7.25/4)/(.75/2) = 14.5ksi and the endload stress is 7.25ksi ... maybe in focusing on the endload on the thread you've forgotten the hoop stress component (seems higher than the stresses you quoted above).
i guess part of your answer is even though you reduced the nominal stress significantly, but didn't see an improvement in fatigue life, is that you're possibly still in the plastic range of 6061.
could you shrink a collar onto the outside of the part ... this would cause compression stresses in the cyclinder and help the cyclic fatigue ?
i'd be hesitant to reduce the thread root radius as this has a direct relationship to the Kt.
RE: Buttress thread fatigue failures
RE: Buttress thread fatigue failures
Also, look into using a spiral retaining ring (e.g. www.smalley.com). We use these a lot, and like the way they perform. Calculating the Kt for a single groove is a lot simpler, and could allow you to design in more fatigue capability. Clamping the ring down to hold it flat under load (using a cover plate that screws down to the cap) is a way to improve the spiral ring capability beyond that shown on their website (which gives the failure stress in the body for a cantilevered ring). Talk to their rep about your design, and ask them to give you some numbers.
RE: Buttress thread fatigue failures
Rather, the triaxial stress should be used since radial and longitudinal stess add to elemental principle loads. Probably the Von Mises-Hencky model is better suited. (i.e. twice the gradient squared equals the cross product of the gradient with itself PLUS three times the shear stress. set shear to zero to get the principle stresses)
Using your numbers: P=3000 psig, D=4.0 in, d=3.25 in I would get a stress of 15,290 psi. This is 5.447% greater than that stated by earlier posts. The error is outside that of standard measurement, probably of interest in this case since we are dealing with failure analysis.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Buttress thread fatigue failures
The value of 15,290 psi calculated by Cockroach exceeds that.
It would seem a redesign is in order to reduce stress or use material with higher properties. Most aluminum alloys, whether cast or wrought, have similar fatigue strengths.
Ted
RE: Buttress thread fatigue failures
if you changed to 2024T3, and guess a Kt=5 ('cause that the highest in the book), fatigue life = 100,000 cycles
(for 6061T6, life with Kt=1 is about 1E8 cycles)
RE: Buttress thread fatigue failures
Ed Danzer
www.danzcoinc.com
www.dehyds.com
RE: Buttress thread fatigue failures
stresses. Certainly a different approach.
RE: Buttress thread fatigue failures
vM = sqrt((s1^2+s2^2+(s1-s2)^2)/2) = s1*sqrt((1+.25+.25)/2) = 0.87*s1
which is what i thought happened with vM when the stress state was bi-axial tension (the secondary tension stress "relieves" the primary tension)
RE: Buttress thread fatigue failures
? = (?*(ri^2)*pi)/(?(ro^2 - ri^2))
where ri=inner radius
ro=outer radius
pi=pressure
i used these values
ro=2inches
ri=1.625inches
pi=3000psi
we get the following axial stress = 5824psi.
this axial stress can be used for calculating fatigue failure for buttress threads
the above equation is for thick walled cylinders exposed subjected to internal stresses.
RE: Buttress thread fatigue failures
RE: Buttress thread fatigue failures
one more thing is 3000psi the mean stress or the maximum stress?
RE: Buttress thread fatigue failures
hoop stress = pr/t
axial stress = hoop stress /2
now r = mean radius = ro+ri/2
and t = ro-r1
but no pi ...
RE: Buttress thread fatigue failures