From my earlier message:
> I would very much
> appreciate if someone could run
an independently calculated design through
> it to see if the results
match!
Well, someone came through!
"ACJacques" was
kind enough to put two sets of hull data through his "under pressure"
software, and sent me the results. It was very interesting to compare them with
what I get using the my primitive little spreadsheet:
- The spreadsheet
does not calculate collapse pressures for the endcaps. Instead, it the cylinder
will fail first. Jacques' program does calculate endcap failure, and as expected
the cylinder was the weak point. (Disclaimer: this is only true if the material
and thickness of cylinder and endcaps is the same!)
- Both tools
predicted the same failure mode in both tests (shell buckling)
- The
failure depths were not exactly the same, but they were less than 10% apart. In
one test, the spreadsheet came out 6% more conservative. In the other, 9% more
aggressive.
- Even though the spreadsheet
does not calculate the endcaps, it does assume stiffener rings (they have a
dramatic effect, so any design I come up with is likely to use them). To use the
spreadsheet for a non-stiffened design, I had to hoodwink the formulas by
specifying a minimum of 2 "frames", which I made massive and set a
full cylinder length apart. I also had to say the "distance between
bulkheads" was equal to the entire cylinder length. In other words, the
spreadsheet formulas only worked if I treated the endcaps as bulkheads.
-
Because of my "fake bulkheads" the spreadsheet weight calculation
could not be checked.
MY CONCLUSION:
When I was going through the
literature to put this spreadsheet together, I found a number of alternative
formulas for calculating the same things, even in the same book. They are all
approximations, and anyway the actual failure depth will be heavily influenced
by "unpredictable" factors such as out-of-roundness, hull
pass-throught, material imperfections, etc. I
think it should be OK so
long as (a) the safety factor is MUCH greater than 6-9%, and (b) I do a physical
test whatever the theoretically calculated
result.
-Alec
PS: Thank you ACJacques!!!