One thing I wanted to add is that to do a FEA analysis of a pressure hull with hydrostatic pressure on it, of which then has a collision with something, is another thing to check for. Placing the collision in different areas, multiple areas at once, and at different collision speeds at different temperatures, as well as different depths of dive can be done with the right software and tech person. Also the shape, size and type of material(s) of the collision object can be checked.
The other point I felt was very important in a collision on a standard ribbed pressure hull between the ribs at depth was that the dent between the stiffener rings does not only have the psi for the few square feet or less that is dented, but also has a larger part of the combined psi the is on the entire pressure hull leveraging it's self against that dented area. Much like if you dent an empty aluminum pop can between its stiffened ends and then add pressure to the ends.
One of the most common ways for an I beam or T beam to buckle is for the unsupported flanges to fail in a wavy ribbon type fashion. This is why a lot of new pickup trucks are being made today with fully boxed frames, compared to the open C channel type frames they made before. This is yet another reason I want to add horizontal weldments to my pressure hulls, whether they be internal or external. For external horizontal weldments you need to leave holes in the areas that would normally allow water to just sit in the part, so it can drain out.
Regards,
Brent Hartwig
Date: Mon, 28 Jan 2008 09:31:59 -0800
From: cliffordredus@sbcglobal.net
Subject: Re: [PSUBS-MAILIST] Hydrodynamic Hatch Viewport Arrangement
To: personal_submersibles@psubs.org
FYI, I do not consider myself to be an FEA expert but have done enough in regards to hull design to add just a bit of background on hull failure analysis.
This video clip of a FEA analysis of a stiffened cylindrical hull sections is quite good to get an understanding of one of the three primary failure modes, i.e., yielding of the shell between stiffener rings in an accordion fashion. This is easiest solution for an FE analysis and the most predictable. There are however two other primary failure modes, buckling of the shell between stiffener rings in which the stiffeners hold but the shell buckles between the rings in the form of dimples and general instability which is characterized by failure of multiple rings where the entire shell/stiffener assembly caves in. These two additional primary failures modes are tricky for FEA programs to do and normally require additional computational modules. It is good to run the ABS stress analysis spreadsheet that is on the pubs.org site in conjunction with any FEA work to nail down which
type of failure mode is expected. Designers try and make the stiffeners strong enough and spaced close enough so that the predicted failure mode is the one shown on the video, i.e., yielding of the shell between stiffener rings in an accordion fashion. In other words, if you run the ABS spreadsheet, and it predicts failure due to general instability (overall buckling) then, beef up the stiffeners and space them closer until the spreadsheet indicates the failure mode is the preferred accordion style.
One other point on the video, the displacements (movements) of the shell are dramatically magnified to make it easy to see how the parts will move under load, in the real world, these displacements are almost imperceptible for this failure mode.
Thanks Brent for posting the clip.
Great works are performed not by strength but by perseverance. Samuel Johnson
Cliff Redus Redus Engineering USA Office: 830-663-6445 USA mobile: 830-931-1280 cliffordredus@sbcglobal.com
----- Original Message ---- From: Brent Hartwig <brenthartwig@hotmail.com> To: personal_submersibles@psubs.org Sent: Monday, January 28, 2008 6:03:20 AM Subject: RE: [PSUBS-MAILIST] Hydrodynamic Hatch Viewport Arrangement
James,
I wish you luck with your drilling attempt number two. Yes Solidworks can do a lot of different types of finite element analysis in there Office Premium package of which I have. I've only used there basic Cosmos Express program that only will test one part at a time, for a limited number of things. But it's a fast way to make sure your on the right track and see were the weakest part of the part is. So now that I have the full Cosmos Works, I've got my homework layed out for me. I was planning on doing some finite element analysis of the K-250 CAD assembly model I'm planning to finish this winter. Having a K-350 CAD assembly model made up as well to test, would be interesting.
Close to the time when I first joined the group, I was playing around with the Cosmos Express program to show the general accordion failure mode of most submarine pressure hulls, and why I believed horizontal
weldments to be useful for not only hydrostatic pressure, but collision protection as well. Here is a link to the short video clip I put together back then.
http://www.youtube.com/watch?v=rhdzRc_tOVY
I have not done any further work FEA type testing since then, so I guess I better get with it.
Regards,
Brent Hartwig
From: james@guernseysubmarine.com
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Hydrodynamic Hatch Viewport Arrangement
Date: Mon, 28 Jan 2008 11:06:57 +0000
Hi All,
I think i fixed my viewport. Well, ive got it back to how it was at the start.
I heated it, welded it. Let it cool slowly and filed the weld flat.
But im still not sure what will happen when i drill it again. Will the heat have made it easier to drill? Not sure.
Have a look here.
Brent. I agree that hatch picture looks great. Does solidworks do finite element analysis as well?
Cheers
James
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