Sciencemadness Discussion Board

Glass too breakable, Lexan too flexible

Nixie - 11-3-2009 at 12:48

I need 3 by 8 foot windows that are unbreakable. They will be protecting expensive screens about a 1/4 inch behind them, so they can't flex more than that when hit with a hammer in the middle, and damage the screen. But a Lexan sheet of that size will flex unless it's something crazy like an inch thick. As for glass, even tempered and laminated glass will at least fragment, even if the laminate film holds it together. So is there some transparent composite or other material I can use? Whatever it is, should be under 1/2 inch thick.
I also considered mounting a Lexan sheet under tension, but to be effective, the tension would have to be enormous, probably a couple thousand pounds in each direction.
It's like I need something halfway between glass and Lexan. What do do?

[Edited on 11-3-2009 by Nixie]

DJF90 - 11-3-2009 at 12:58

Maybe try acrylic? Its pretty tough stuff and shouldnt flex too much, at least in sufficient thickness (5-10mm?)

Nixie - 11-3-2009 at 13:31

Lexan is polycarbonate; acrylic is actually much more flexible than it.

watson.fawkes - 11-3-2009 at 15:00

If the screens are LCD ones, mount them right on the window. Then it's not the total deflection that matters, but rather only the local deflection in a region the size of the screen. If that doesn't work for weight bearing reasons, you could add a floating mount system that supports the mass of the screen in the z-direction (parallel to gravity) but does not restrict it in the y-direction (perpendicular to the window).

Twospoons - 11-3-2009 at 15:04

I would have thought that if you were to use 1/2" lexan rigidly bonded to a stiff steel frame you'd get away with it. Is there a reason you can't move the screens back another 1/4 " or more?

If you had lexan on the impact side laminated to glass on the screen side it might work - you get the stiffness of the glass (it would be under tension on impact) with the impact and point load spreading of the lexan.

In my experience, (cast) acrylic is slightly stiffer than lexan, but more brittle. You can shatter acrylic with a hammer.

You will have to watch your mounting method - a plastic window that size will bow when it gets hot.

Nixie - 11-3-2009 at 15:16

Twospoons, there's an optical multi-touch touchscreen system in the frame, with beans scanning just over the window surface, about half a mm, and the interaction sucks if the distance between the surface the users are touching and the display screen is more than 1/2 inch. We're talking 65" inch portrait mode displays here.

watson.fawkes, the impact would be spread out better that way, but then all of the impact energy would be transferred to the screen electronics, whereas the Lexan's bending would otherwise absorb more of it if there was a gap. Soft mounts is an interesting idea, though. Too bad there isn't a material that's transparent yet soft, to use as a damper between the Lexan and LCD.

[Edited on 11-3-2009 by Nixie]

not_important - 11-3-2009 at 21:56

Quote:
Originally posted by Nixie Too bad there isn't a material that's transparent yet soft, to use as a damper between the Lexan and LCD.


Silicon rubbers would do, although expensive. I've also seen some urethanes that are colourless, transparent, and quite soft, but I've no idea if they were commercial products.

bfesser - 12-3-2009 at 10:28

http://en.wikipedia.org/wiki/Transparent_aluminum

Nixie - 12-3-2009 at 11:05

bfesser, that is incredibly expensive for the size I need.

[Edited on 12-3-2009 by Nixie]

watson.fawkes - 12-3-2009 at 11:42

Quote:
Originally posted by Nixie
watson.fawkes, the impact would be spread out better that way, but then all of the impact energy would be transferred to the screen electronics, whereas the Lexan's bending would otherwise absorb more of it if there was a gap.
Damage from impact is not dependent on total energy, but on instantaneous forces on the target object that push it past its yield strength. Time-spreading an impact from 0.1 ms to 10 ms, for example, reduces forces by two orders of magnitude. The polycarbonate itself will act as your time-spreading spring. The electronics should well be able to handle the resulting impulse.

What kinds of damage are you anticipating, anyway? I assume HE grenades are not part of the threat model, to pick an absurd upper bound. What <i>is</i> your upper bound, really?

unionised - 12-3-2009 at 12:11

Just a thought, is there a glasss reinforced version of polycarbonate? You would need a glass fibre with the same refractive index as the polycarbonate to get the composite transparent, which might be more difficult.

Twospoons - 12-3-2009 at 13:58

Quote:
Originally posted by Nixie
there's an optical multi-touch touchscreen system in the frame, with beans scanning just over the window surface, about half a mm, ]


You wont be able to use plastic then. The heat from the screen will cause the plastic to bow, and this is likely to affect the touchscreen. Speaking from experience here.

If you can let me know what brand of touchscreen, I may be able to be more helpful - I work in the industry.

Have you considered chemically toughened glass? We use 2mm chemically toughened glass in one of our products, and it survives the impact from a 100g steel ball dropped from 1m. Crazy.

[Edited on 13-3-2009 by Twospoons]

[Edited on 13-3-2009 by Twospoons]

Nixie - 12-3-2009 at 17:43

The touchscreen is custom from Israel. LCD screens, unlike plasma, get most of the heat out of their back, and there is an airflow system that gets it out, so I don't think it will be a problem. With a gap between the screens, some of the air will flow between those as well. We're leaving the backlight at default value anyway, so it's only around 200 W per 65" screen.

How much does glass like this cost? BTW, you have backing to your glass I assume, it's not suspended at the edges, in your test ;)

[Edited on 12-3-2009 by Nixie]

densest - 12-3-2009 at 18:33

"Bulletproof glass" or "bulletproof plastic" is usually a laminate of polycarbonate, glass, and acrylic. The glass and polycarbonate work together - the polycarbonate prevents shattering and the glass provides rigidity and compressive strength. The acrylic is (I believe, this may be way wrong) a spacer - it is there to allow fragments to embed and lose energy. Also, the thickness makes the whole thing more rigid. I -think- the order is polycarbonate-acrylic-glass-acrylic-polycarbonate. The acrylic is optional.

1.25" polycarbonate alone is listed by some plastics suppliers as sufficient to withstand a .45 caliber bullet. 1/2 inch polycarbonate is considerably less expensive and can be laminated to glass "reasonably" easily - a sandwich of 1/2 inch polycarbonate outer sides and 1/2 - 3/4 inch glass would be pretty resistant. Add 1/2 - 3/4 inch acrylic to absorb energy and I think your hammer wielder would get very frustrated.

Something like $30/sq foot for the polycarbonate, maybe $10 for glass, $15 for acrylic, so $55/sq ft? It would be pretty much impervious to impact. There are coatings for the polycarbonate which make it much more scratch resistant.

Maybe consider finding out what transit vehicles use? A train window has to be pretty sturdy and not flex much.

If you don't anticipate violent impacts with huge energy then acrylic & glass laminate is much cheaper. The acrylic is soft and will scratch unless coated.

Nixie - 12-3-2009 at 19:19

Who makes this? I only see glass/acrylic or glass/polycarbonate. Also, I'm a bit concerned with having the outer surface being plastic due to the lack of scratch resistance.

Twospoons - 12-3-2009 at 20:15

Sorry, I don't have cost details on the chem toughened glass, but it can't be very much or we wouldn't be using it. I'm not sure what the test conditions were, but remember this glass is just 2mm thick. Even with backing during the test it has to be pretty damn tough to handle that sort of abuse. Ours is sourced in China somewhere - sorry, I'm just an engineer - I don't get involved in our supply chain.

I'd agree on the scratch resistance - you really want glass on the outside. All it takes is some idiot cleaner with a gritty rag ...

Nixie - 13-3-2009 at 09:04

So when I read about scratch-resistant coatings, are they actually scratch-resistant? ...