Impact Testing on Windows

[drawoh]

Has anyone here analyzed optical windows for impact loading? I am analyzing some windows for an impact of 4J, and I am not in the ballpark. These things will shatter. I have analyzed at least one window I believe has passed physical impact tests, and it is not in the ballpark either.

I am using Roark's equations for various simply supported flat plates, with a load concentrated in the middle. I am working out the spring rate of the window, the force required to absorb the energy, and the resulting stress. Does this sound right?

The window will be mounted in gaskets which will absorb energy. I am ignoring this, for simplicity and safety.

--
JHG

 

[jedward]

John Pepi has done much work on the impact/ safety of/on to aircraft windows-- check him out.

 

[zappedagain]

For reference IEC/UL 61010-1 for lab equipment calls for 5 J testing for metallic and nonmetallic enclosures; you can test as low as 1 J, but need to include risk analysis why 5 J isn't required (Clause 8.1). This test is for any surface required upon for safety (i.e. mechanical, thermal, or electrical hazards within the surface).

The standard mass for impact is a steel sphere with a mass of 500g+-25g and a diameter of approx. 50 mm that drops onto a horizontal surface or swings into contact with a vertical surface.

Z

 

[drawoh]

zappedagain,

I am designing my enclosure to the ATEX 94/9/EC Ex_P standard. We will slightly pressurize it with filtered, non-explosive gas. The customer has specified 4J[ ]impact. I have tried alternate materials. Tempered soda lime glass worked for one of my windows. I can make Acrylic or Lexan pass the impact test for some other windows. These windows are for an optical system requiring high light transmission in all sorts of interesting wavelengths. None of these materials are suitable for our optics.

I have estimated what would happen if our soft rubber gaskets compress under the impact. This appears to keep the impact forces within the limits of my materials, but I have little confidence in my model. Does the gasket compress in the response to the impact? Does a shock wave pass through the glass window before the rubber does anything?

--
JHG

 

[MacGyverS2000]

Does a shock wave pass through the glass window before the rubber does anything?

I don't have to run a simulation to almost guarantee this will happen... the question is, will that wave be of high enough amplitude to stress the glass to the point of fracture...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[zappedagain]

I haven't designed for explosion proof; only intrinsically safe. Good luck.

Z

 

[IRstuff]

What exactly are the requirements, i.e., what is the canonical projectile? I thought that this was a challenge, but a 2-lb bird hit by a 60-kt windshield has over 400 J of KE to dissipate. 4J is something like a 5-oz object traveling at 10 kt, or 1-oz object traveling at 18 m/s.

Impact resistant windows are typically not just glass, but laminates of glass and some sort of polyvinyl. FEMA design guidance has a bit about blast resistant windows, but your KE seems to be well below that.

http://www.fema.gov/media-library-data/20130726-1455-20490-4910/fema426_ch3.pdf

http://www.fema.gov/media-library-data/20130726-1627-20490-4755/fema459_ch3.pdf

http://www.fema.gov/media-library-d...7ee15436712a3e82ce709/FEMA_P-320_2014_508.pdf

http://www.fema.gov/media-library-d...65d0e2483f6ed46aca0/fema_p361_Jan2016_508.pdf

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[drawoh]

IRstuff,

I am actually not sure. The atmosphere inside an explosion-proof enclosure is the same as the atmosphere outside the enclosure. Some electronic device causes a spark and KABOOM!. But just a little kaboom, because the enclosure wall, access hatches and windows contain it. The window impact would come from inside. Ours is not explosion proof to EX_d. We are working to the EX_p standard, which is for an IP66 or NEMA[ ]4X enclosure slightly pressurized with non-explosive gas so that sparks do not set anything off. There is a gas detector inside that switches the system off if anything volatile is present.

If something smashes the window, there would be a time delay from when the outside atmosphere gets inside, and the gas detector shuts everything down. There is no good reason to not shut our system down immediately if the window gets smashed.

I have looked very briefly at the laminated glass. I think this solves a different problem. All of the explosion proof enclosures I have seen use a single piece of glass. 80% transmission in the visible range is okay if you are trying to read a gauge. Our optics people won't tolerate this, and, as noted above, our wavelengths are interesting.

--
JHG

 

[IRstuff]

> you don't specify what size or shape the object has. That alone makes a gigantic difference. Your 4J requirement is very low energy; it's a golf ball going at 30 mph, which does not seem particularly harsh. I'm fairly certain that my car windshield won't shatter if it was going 30 mph and it hit a stationary golf ball. How are you doing the impact? What shape are you using? What speed?

> you say "interesting wavelengths," but if they're not between 400 nm and about 2500 nm, glass can't even be used. Moreover, if the wavelength spread is large, you'd need an AR coating.

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[drawoh]

IRstuff,

4J is a requirement from our customer. So far, my calculations have been for spring rate, force and stress, and it does not matter how I apply 4J. If I work out shock waves, this could change.

--
JHG

 

[IRstuff]

What are your window dimensions? What are your object dimensions and weight? I can't help but think you've over-complicated the problem somehow. A large Corningware bowl dropped dropped from 3.5 ft will have just about 4 J of KE at impact, and will generally survive.



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