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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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