Re: Viewports gobbed on with silicon

[protek@shreve.net]

Paul,
    I am NOT an expert on the mechanical properties of plastics and I
certainly do not claim to be one.  But just consider this; if you read
Stachiw's book it will overwhelm you as to the degree of testing his group
put into acrylics for viewports.  It would have been easy to model a flat
plane window for example on computer or just put one in a test situation
and burst it once and derate it for normal operation, but they took many
windows of numerous sizes and thicknesses and not only did one-time failure
testing, but cycle tested them.  Many times these samples went through
cycling, under different conditions.  I know of no such data for
polycarbonates.  Their may be some but I do not know of it.  Now think
about this; I once called Stachiw and explained that I was constructing a
sub for 200 feet and that I wanted to have the front window a 5" by 7"
(Rectangular) 2" thick window of acrylic.  He did not discourage this, but
told me that I should definitely test it by cycling it following the
criteria of his tests on round windows.
    My point is that the window that I specified, and it had large radius
curving on all four corners, in his opinion was not a good idea unless it
was tested.  You could drive a tank over that window!  But plastics can
have unusual failure modes and any time you change the conditions from
standard ones you are out there on your own with no data behind you.
    Your Lexan window may perform better than acrylic, but before I use it
I want proven data.  Harold LeTourneau, my friend from Texas who has a sub,
used it and made 100+ dives some to 100 feet, but he said you could see the
Lexan (0.5 inch x 6") bow under pressure.  Not me!
    Two problems that you may have can be difficult to engineer around
without the proper equipment.  First, you need the spherical section for
the dome as round as possible.  Deviations from roundness are not
desireable.  When heat forming a spherical window, the thickness may be
less at the top than the sides.  This can be a problem.  Unless the dome is
an exact 1/2 sphere you might have problems with the seat design.  Once
again these problems have been examined in the industry and allowable
deviations defined.
    I have thought of spherical window construction but if I ever get that
bold to tackle a project like that I would build a pressurized oven and
cast the basic shape out of acrylic, then shrink cycle it, machine it,
polish it, and anneal it.  That is why they get the big bucks for those
windows.  I think the problem for me would be the machining.  You would
have to construct a technique to machine something like that inside and out.

Gary Boucher



At 07:31 PM 8/12/99 PDT, you wrote:
>Gary, Thanks for the references, I'll see if I can't get a copy. I agree 
>that you shouldn't re-invent the wheel, but my design is a little radical 
>and is going to require some re-engineering. The only thing I don't like 
>about acrylic is that it has an Izod impact resistance of  0.5 ft-lb/in 
>compared to polycarbonate which has an Izod impact number of ~ 15.0 
>Otherwise, the basic plastics are very similar in strength and mechanical 
>properties. One of the problems with poly besides it's UV stability 
>(yellowing) is that it requires a higher temperature to process than 
>acrylic. Because of this, you have to "dry" it and minimize the absorbed 
>moisture before forming. Since my canopy is going to be 1/2 inch to 3/4 inch 
>thick, I'm going to have to form it myself. It would be easier if I used 
>acrylic, but I'd feel a lot safer with poly. It's overall strength is about 
>20 % higher than acrylic in most categories (Tensile, elongation,yield, 
>compressive, flexural). One of my major concerns is adequate sealing of the 
>canopy to the inner hull. I essentially have a biplanar (two planes) sealing 
>problem. Unlike a hatch cover and gasket, which seal on one plane, I will 
>have to insure that the pressure and loading of the canopy seals a "V" type 
>of configuration. Ahhhh.. I know, lots of work, but hey, that's half the fun 
>of building this thing. Thanks again for the help.
>Suds
>
>
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