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Re: Point supported glass design

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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(--nospam--at)skypoint.com   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/members/chrisw/



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