We have a reference pressure that we calculate or we are allowed to do a
rational analysis that limits working stresses in the glass as 25MPa for
parts away from edges and 20MPa for stresses along edges. von Mises stress
is a measure of "yield" in material and therefore if I use FEM like that in
RISA so long as the stresses are below those allowed in our standard, then I
can probably judge the glass as OK. It's a bit loosy-goosy but it does
result in simple calculations and gives agreement with thicknesses
acceptable to the local industry.
If I use principal stresses they generally seem to come out lower than the
von Mises so I'm conservative anyway.
Thor
-----Original Message-----
From: Christopher Wright [mailto:chrisw@skypoint.com]
Sent: Thursday, March 12, 2009 9:50 AM
To: seaint@seaint.org
Subject: Re: Point supported glass design
On Mar 12, 2009, at 11:07 AM, Thor Tandy wrote:
> I suggested rational analysis because we have a standard that has
> taken the statistical data and created an equation to determine a
> reference pressure allowed on the glass. We then apply a variety
> of factors for such things as duration, tempered, laminated etc.
> With that I can then do an FEA and check that I am below the von
> Mises values ... that's also why I suggested care with the boundary
> conditions because that's where the imperfections will become
> significant.
I'm real curious about a couple of things--
Why do you use the von Mises stress as a criterion, rather than
maximum (algebraic) principle stress? Glass is much stronger in
compression than in tension, and the von Mises stress carries no sign.
How do you get away without considering some sort of fracture
physics, like LEFM, in considering flaws?
Also you mentioned only pressure, but the real bugaboo with glass is
point loading, either at supports or from impact loads.
My experience with glass is dated, going back to the 60's when people
were looking to use it in submersibles. Glass and ceramic spheres
(more accurately paired and properly mated hemispheres) were
generally acknowledged to be great for resisting external pressure,
but everyone got really nervous about impact loads and local high
loads from supports and mis-mated interfaces. We had an instrument
housing made from glass hemispheres that simply vanished one day
during a fairly shallow dive. One second the gyro was working fine;
the next it was dead. Only an inconvenience at that point, but it was
always a consideration in discussions of monolithic glass viewports.
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania
1864)
http://www.skypoint.com/members/chrisw/
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