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[Sean T. Stevenson via Personal_Submersibles]

Minimum usage factor specified by ABS is 1.25, specific to the inter-stiffener strength failure mode, which is far more predictable than the instability modes. Consequently, the usage factors on those modes are greater so that the design failure pressure is always deterministic. When you build from tough steel though, as is the case with ASTM A516 grade 70 N which many of us are using, the steel can actually yield and undergo plastic deformation to a significant extent (slow crush) and may not even reach its ultimate strength limit before hitting the safety factor on a buckling mode (implosion).

Thus, the maximum allowable working pressure specified by ABS is the pressure at the yield point at an interstiffener strength failure divided by 1.25, but this is still somewhat removed from the actual survivability limit, which has a probability distribution that effectively begins at the inter-stiffener strength failure with usage factor 1.0, and extends past the buckling modes (also at 1.0).

Sean

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-------- Original Message --------
On Jun. 18, 2020, 06:49, Jon Wallace via Personal_Submersibles wrote:

> I ran the numbers again in the calculator and got the same numbers. If I change the usage factor to 1.0 then I get 884 feet. I suppose when trying to ascertain a theoretical crush depth a usage factor of 1.0 would be acceptable in the calculator. It's been my understanding that ABS, Lloyds, etc, look for a safety factor of about 1.5 which would put the 350 at 525 feet. That may explain the 600 foot test depth you mentioned, but even so, my opinion is that's overkill.
>
> Jon
>
> On 6/17/2020 1:25 PM, Rick Patton via Personal_Submersibles wrote:
>
>> Jon
>>
>> OK sounds good. I was asking for the crush depth of a K-350 and the unmanned test depth for one hour is 600' so that doesn't sound correct. Someone told me a while ago that Ketteredge had put a 350 when first developed in a hyperbaric chamber that was only rated for 1,200 and pushed it down to that depth to see if it could take that pressure without imploding and nothing happened so he knew that that design would survive at least to that depth without failure. Can't remember who told me that but does anyone know if that story is correct?
>>
>> Rick
>>
>> On Wed, Jun 17, 2020 at 4:13 AM Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>
>>> Just point them to the website Rick, if they ask. There's a link to facebook from there.
>>>
>>> I get 665 feet for the pressure cylinder and 576 feet for the hull caps, but those are theoretical best case limits. So 500-600 feet would be a fair statement. Given all the fabrication variables there is no way to predict a specific depth which is why we use safety margins.
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