INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Impact Testing on Windows

Impact Testing on Windows

(OP)
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

RE: Impact Testing on Windows

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

RE: Impact Testing on Windows

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

RE: Impact Testing on Windows

(OP)
zappedagain,

I am designing my enclosure to the ATEX 94/9/EC ExP 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

RE: Impact Testing on Windows

Quote (drawoh)

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

RE: Impact Testing on Windows

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

Z

RE: Impact Testing on Windows

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/20130726-14...
http://www.fema.gov/media-library-data/20130726-16...
http://www.fema.gov/media-library-data/14188374717...
http://www.fema.gov/media-library-data/14526226199...

TTFN
I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers

RE: Impact Testing on Windows

(OP)
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 EXd. We are working to the EXp 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

RE: Impact Testing on Windows

> 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.

TTFN
I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers

RE: Impact Testing on Windows

(OP)
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

RE: Impact Testing on Windows

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.

TTFN
I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close