Hello Jon,
I was hoping you would fill me in as you have. I'm grateful for you putting the PSUBS.org plane disc viewport calculator together for use. I wonder how hard it would be to put a conical window calculator together some time in the future.
Right now I'm trying to put together all the proper material data for Plexiglas G, Plexiglas GP, and Acrylite GP, to input each type into there own custom material in Solidworks. Then I can see what kind of FEA results can be produced from those material properties.
"PSUBS highly recommends that viewports be fabricated out of Acrylic material that has been manufactured to meet or exceed ASME PVHO standards. Some brand names that meet these standards include Plexiglas G, Plexiglas GP, and Acrylite GP.
Always ask your supplier for the specification sheet that applies to the material you want to obtain, and verify that the material meets ASME PVHO standards."
http://www.psubs.org/design/viewports/1ATMFD/
I'm not sure I need a conservative number for the predicted failure depth on a window. Since I'm putting in a very conservative factor of 6 or 8 to get an general operating depth. Of course the material properties I'm using, is for brand new material with no damage from any thing, like UV, cycle life, moisture content, etc... So that of course needs to be considered.
Regards,
Brent Hartwig
From: jon@psubs.org
To: personal_submersibles@psubs.org
Subject: RE: [PSUBS-MAILIST] FEA of a Basic Flat Acrylic Viewport
Date: Thu, 28 Feb 2008 10:56:48 -0500
Dan,
Those are both good points (welding thru-hull, acrylic flatness). Stachiw makes a point in his book of recognizing that acrylic sheets are inherently not flat and that when calculating the strength of the viewport you have to make sure you use the dimensions of the thinnest area of the viewport. I suspect that most acrylic material is "relatively" flat and that we aren't talking about huge differences in material thickness.
Brent,
The viewport calculator on PSUBS.ORG is conservative and is based upon my interpretation of table 7.27 in Stachiw's book. Those graphs do not provide hard numbers for a given t/ratio, but rather are predicted failure curves that have to be interpreted since you have to account for the the thickness of the graph lines, the thickness of the predicted failure curve, and the fact that the graphs in the book may have been reduced or expanded from the original to fit the format of the book. If there was any doubt with interpreting where the failure curve intersected a particular psi/tratio intersection on the graph, I took a conservative approach and used the next safest data point which would always result in the calculator showing a somewhat less depth rating than what was shown in the graph. I rationalized that these were minor differences in the grand scope of things and it was better to be safe, than sorry.
In the viewport program, I have 1065psi defined as the predicted failure point for a .18 t/ratio. Here again, the calculator takes a conservative approach when converting PSI to depth by doubling the PSI rather than dividing by .44 for sea water. If I divided 1065 by .44 (the actual pressure per foot for seawater), the result would be a failure depth of 2,420 feet.
I'm not suggesting your program doesn't need more work as well, but if it were perfect, the descrepency would be explained by a conservative approach to interpreting the failure pressure for various t/ratios as shown in table 7.27 of Stachiw's book. Regardless, we need to be clear that we are talking about failure depth here, not operational depth. Given the descrepencies in material as pointed out by Dan H, I'm comfortable with the conservative approach of the calculator. Especially when you take into consideration that Stachiw's tables were formulated for viewports cut from ASME approved acrylic material, and viewports that have been properly annealed after all machining.
Jon
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
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