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RE: Point supported glass design

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can/cgsb 12.20-m89

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-----Original Message-----

From: "Gary L. Hodgson and Assoc." <design(--nospam--at)hodgsoneng.ca>
Sent: Fri, 13 Mar 2009 07:59:28 -0400
To: seaint(--nospam--at)seaint.org
Received:  13 Mar 2009 07:59:27 -0400
Subject: Re: Point supported glass design


Thor,
Can you tell me what standard you are referring to? TIA.
Gary

Thor Tandy wrote:
> 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(--nospam--at)skypoint.com]
> Sent: Thursday, March 12, 2009 9:50 AM
> To: seaint(--nospam--at)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(--nospam--at)skypoint.com   | this distance" (last words of Gen.
> .......................................| John Sedgwick, Spotsylvania
> 1864)
> http://www.skypoint.com/members/chrisw/
>
>
>
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