Hello Again Dan,
That's a good idea to run the same test on basically the same viewport but with a conical configuration to see how much more of a FOS is obtained. I was trying to decide if I should keep the viewport the same OD or the same viewing area diameter to get a fair comparison. I'm going to go with the viewing area staying the same. I'll see if I can run that tonight after I remove some of the wall of snow off my road by continuing to sharpen my skills at the business end of a shovel. I figure that at the rate of 55 feet of road cleared a day, that I'll be able to drive my rig out in six weeks. I just had a friend in Florida tell me it was 88 degrees F at there place today. They really know how to cheer a snow wookie up. ;)'
One slight problem I have with my FEA test of this basic flat viewport we are discussing, is that the way I'm restraining the seating area from moving at all is not totally accurate to a real world window on it's seat. In my test the seating edge is not allowed to lift off or slide at all. For a real acrylic window like this I would expect that at some point the outer edge of the window that is next to the seat would lift off the metal slightly at great pressure and perhaps the inner area that is also seated would slide in ever so slightly towards the center. I think that even with this issue, I'm getting some valuable data from the test. I'll see if I can do a test that more reflects what I'm talking about.
I think your right Dan about a layer of urethane reducing the stress concentration on the inner seating edge of the viewport window tested. One reason I want to add a soft rubber type gasket just under my retaining ring is to keep the retaining ring from scratching the window like it did on the two conical windows on the older K-250 here.
In regards to your question about warping imperfections in the seat I'll make a warped seat and test it with the window. Of course there are a massive number of variables in that basic configuration but I'll keep it simple and run both the basic and conical viewports in the sizes we are talking about, and only warp the seats a small amount. I would think that a high percentage of viewports will only have a small amount of warpage. In those cases I would suspect that if the viewports were sealed with a O-ring on the outer edge like your conical hatch viewport assembly is, that it wouldn't take much hydrostatic pressure to fully seat the acrylic window to it's warped seat. I would expect a very small amount of warpage in this configuration to change the failure depth rating on the acrylic window very little if any, and since we don't plan to get any were near the windows failure depth in the first place, we should be well with in are over kill rating.
If you fill the slightly warped space with a good harder type urethane sealant and let it fully cure, I would think you should be in very good shape to closely match the FOS of perfectly flat window that is in a perfectly flat seat.
"The biggest troublemaker you'll probably ever have to deal with, watches you from the mirror every mornin'." ~Unknown
Cheers,
Brent Hartwig
From: jumachine@comcast.net
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] FEA of a Basic Flat Acrylic Viewport
Date: Thu, 28 Feb 2008 08:02:51 -0500
Very interesting Brent!
Is it possible for you to rerun your analysis with a transitioning taper on the inner edge of the viewport housing and reduce the concentrated loading of the lens? I realize there wouldn't be much actual deformation of the lens when in operation but the ring of stress concentration should be able to be spread out more by playing around with the housing geometry.
In PVHO it recommended that you install a urethane gasket before installing the lens. I set my lens in a bed of uncured urethane. I suppose either of these will reduce that stress concentration.
Another thing to consider is, by the time you get your viewport housing welded into place it probably isn't going to be flat anymore. And also, I doubt the surface to the acrylic is perfectly flat either. How much do these imperfections reduce the actual failure depth?
Nice work, Dan H.
----- Original Message -----
Sent: Thursday, February 28, 2008 5:23 AM
Subject: [PSUBS-MAILIST] FEA of a Basic Flat Acrylic Viewport
To begin the process of verifying that I'm getting the correct results, and that I have the
right material properties and constraints dialed in. I decided to run a basic flat acrylic
viewport, and compare the resulting data to what I acquired on the PSUBS.org Plane Disc
Viewport Calculator.
The software I use subdivides the model into a mesh of small shapes called elements. I ran a even finer mesh then I have before on this viewport, which is the finest setting allowed with my computers resources.
Element size 0.17 Element tolerance 0.0085 This will give me a much more accurate result.
For a flat acrylic disk measuring 15" OD by 12" viewing area, which gives me a Do/Di Ratio of 1.25 and a t/Di Ratio of 0.18 and being 2.16" thick, I'm getting a
FOS of 7.7996 for a operating depth of 350 fsw/106 meters, of which gives me a failure
depth of 2729.86 fsw. When I ran the same specs on the PSUBS.org Plane Disc Viewport
Calculator, I got a Failure Depth of 2130 fsw. So I have some more checking to do.
I need a FOS of 6.08571 to match the PSUBS. Calculator.
Perhaps the PSUBS viewport calculator puts in an average strength of the acrylic over it's life span of the number, or the total duration, of pressure cycles of 10,000 dive cycles or 40,000 hr, respectively. As well as for UV damage and different states of moisture content through out it's life span.
Also different temperatures need to be factored in. ABS rules require that the operating tempurate is to be within a -18 degrees C to 66 degress C (0 degress F to 150 degress F) temperature range.
I'm now using the full version of CosmosWorks Designer that allows me to add in a lot of different factors into a finite element analysis of a part and/or assembly model with different part materials and stresses. Temperature, collision, gravity, force, pressure, restraints, centrifugal forces, bearing loads, stress cycles to test fatigue over time, and a lot more. I have not discovered as of yet, if I can test hydrodynamic load stresses to simulate stress loads on a subs hull as if moves through the water in different ways.
I've added a 8 screen captures of this viewports FEA test. The deformation is exaggerated primarily a visual aid.
http://www.flickr.com/photos/12242379@N05/2295406063/in/photostream/
I know this is dry stuff, but I figure that if I don't get it right, I'll be even more wet behind
the ears. ;)'
Regards,
Brent Hartwig
|