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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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