can i ask why you're using such a relatively non-structural Al in a primary structure ? ... part of the reason you can't find data is that because people generally don't use it, so not much investigation has been done.
i've had a quick browse through some of the data at my desk, and couldn't see anything on 6061.
Actually, my customer selected 6061-T6, not me. However, the application is the liner of a filament wound pressure vessel and this is a common material for this application. I need the crack growth rate to predict the cycle life of the liner before leakage occurs for a minimum acceptable flaw size. The liner itself is thin relative to the carbon composite and is basically nonstructural and simply functions as a bladder and a "fly away" tool. The liner is autofrettaged by yielding during an initial sizing pressure cycle, so a ductile, high elongation metal is required. For this purpose there are even better alloys available. However, the polar boss is integral with the liner and some decent strength is required to prevent spitting out the boss (the ultimate pressure requirement is over 15000 psi). Alloy 6061-T6 seems to be a good compromise between these two requirements. Anyway, I did find some data for R=0 in the Air Force Damage Tolerant Design Manual. This will probably work, but I'd really like to have data at other R values.
Thanks for your attention to my question. Any additional information would be greatly appreciated.
i think it's reasonable conservative to use R=0 data for stress cycles with R>0. i would be more concerned about the impact of your manufacturing process, it sounds like you're doing a lot of plastic deformation which might upset the crack growth.
actually that leads to a couple of questions about the stress cycle ... if its plastically deformed in manufacture, how does this interact with the applied loading (a bit like a cold worked hole) ... but then that may be why you want high R data ('cause of the mean stress from manufacture);
and, if the liner isn't particularly structural how (or why) is it taking load (maybe in proportion to stiffness ?)
if i understand, you're applied 15ksi as a service load (linear elastic) ... over the top of your residual manufacturing stresses, or does the boss have no appreciatable stresses ? if the boss cracks does everything get really bad really fast ?
