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Re: [PSUBS-MAILIST] FEA Analysis Options
One caveat about using FEA - it is (relatively) simple to learn to use the
tools. Having the experience and judgement to set up relevant load cases is
another matter entirely. For things like hydrostatic pressure, lifting
loads, etc., one can reasonably assume a quasi-static equilibrium and
apply "worst-case" loads and reactions to the FE model to solve. In the case
of a collision, the analysis is not quite so simple, since the forces are
applied over a very short period of time and the nodal reactions are very
dependent on their instantaneous displacement, and the rate of change of
nodal forces. Many of the traditional FEA tools fall apart with this sort of
analysis, and you need to move to a tool that does fully dynamic iterative
FEA, such as LS-Dyna or similar.
In the quasi-static case, you can approximate the force developed during a
collision with reasonable accuracy, but cannot subsequently model how the
structure will collapse (beyond predicting the mode of collapse) without a
fully dynamic simulation. As an example of where this sort of thing would be
useful, in some submarines a watertight bulkhead is used to separate spaces
within the pressure hull to control compartment flooding, and these bulkheads
are designed for flooding emergency only - i.e. to buckle and deform
plastically (exceeding yield) but maintain sufficient integrity to keep the
water out. Such a structural yielding can only be accurately modeled with a
fully dynamic FE simulation.
-Sean
On January 29, 2008 07:52:25 Brent Hartwig wrote:
> 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.
>
> http://www.guernseysubmarine.com/index_files/Page9229.htm
>
> Brent. I agree that hatch picture looks great. Does solidworks do finite
> element analysis as well?
>
> Cheers
> James
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