[PSUBS-MAILIST] pressure test

[Sean T. Stevenson via Personal_Submersibles]

According to the ABS rules, the usage factor eta as implemented in the ABS calculations runs from a minimum of 0.5 (2x safety factor) for buckling modes, to a maximum of 0.8 (1.25 safety factor) for inter-stiffener strength. This implies that ABS is ensuring that for equivalent critical pressures, strength failure occurs before buckling by varying the value of eta, such that the limit pressure for any particular failure mode is the theoretical onset of failure (i.e. crush depth), and the limit pressure multiplied by the corresponding eta value for that mode of failure gives the maximum allowable working pressure. The lowest of the maximum allowable working pressures for each mode gives the maximum allowable working pressure for the vessel, which corresponds to the maximum allowable rated depth. Without any additional safety factor imposed by the operator, this maximum rated depth is also the maximum allowable operational depth, which would provide a minimum safety factor of 1.25
from crush, which would occur by inter-stiffener strength failure.

Thus, for a nondestructive chamber verification test, you could test to any depth within the maximum allowable, which would be at 0.8 times the design failure pressure.  For destructive testing (i.e. a verification of a novel design), you would use the 0.2% limit strain criterion to determine the failure point, and accordingly define the maximum allowable working pressure to be 0.8 times that value, or the design value (whichever is less).

Sean


On November 17, 2016 5:55:28 AM MST, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Hi all,I am just getting a pressure test organized in a chamber in
>Burnaby, but I have some questions.   Does the safety margin change
>with deeper diving subs?  What is the rate of pressurization?Hank
>
>------------------------------------------------------------------------
>
>_______________________________________________
>Personal_Submersibles mailing list
>Personal_Submersibles at psubs.org
>http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.whoweb.com/pipermail/personal_submersibles/attachments/20161117/30e88c6d/attachment.html>