[PSUBS-MAILIST] pressure test

[Sean T. Stevenson via Personal_Submersibles]

Most of what I do is destructive testing, but an intended nondestructive test wherein the specimen fails unintentionally acts identically. When I collapse a pipe, for example, typically both the specimen and the volume between the specimen and the test chamber are filled with deaerated water to remove any gas spaces. Then, both volumes are interconnected and the pressure is increased to the maximum test pressure. At that point, the two volumes are isolated from each other, and the internal pressure of the specimen is gradually reduced. The pressures are automatically controlled / maintained, and the volume of water leaving the internal vessel, as well as the volume of make-up water entering the external vessel, is precisely measured. This generally exhibits a linear relationship to the pressure differential within the elastic region. Once the internal vessel (specimen) begins to fail by yielding, that relationship diverges, and that is recorded as the failure point. We then have the
option of continuing to propagate the failure to any extent we choose by metering the water flow in and out. We have crushed a number of 42" OD, 2" wall pipes this way.

I'm not sure if you are planning a water filled vessel or gas filled? If the latter, you don't have any check against a complete violent collapse if you do happen to exceed yield, other than the ability of the external water volume to sustain the applied pressure as the internal vessel deforms - depending on how that pressure is controlled, it could react in a variety of ways. They are probably just being diligent to ensure that your test will be conducted entirely within the elastic region.

Sean


On November 17, 2016 5:32:02 PM MST, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Sean,Again, thank you, you are very generous with your time to answer
>these questions.  I hope to have a test date tomorrow, but they are
>asking about materials I used to build the sphere.  They are likely
>worried about damage to their equipment.  Hank 
>
>On Thursday, November 17, 2016 5:09 PM, Sean T. Stevenson via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
>
>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 wou!
>ldprovide 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
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