Sudden glass shatter

Ergo606

I was recenlty on the phone with my friend who moved into a new house. The house has been around for a little while...nothing too long. He was on the phone when his mom told him to hang up and look at this. As it turned out, a pane of glass in the bathroom shattered. He looked at it and watched the cracks get longer right in front of him. It began from the middle and worked its way towards the edges. There was no one in that room. The facts that I know are that it was like about room temperature in there. Most of the windows in the house were open. The glass was about ten years old. The water wasn't on and no apparent forces were acting on it. We're really curious to find out what happened considering that we're both really into physics. His guess was that a sudden pressure change from opening a door might have bent the glass slighlty enough to cause it to crack and send small pieces on the floor. More facts....his mom used CLR on the glass to clean it three weeks before the incident. Any help or suggestions would be appreciated, thanks!

James R

The glass may have been pre-stressed, though this is more common in car windscreens than in bathroom windows.

PerennialII

"Cool!" Initial defects, residual stresses, sudden loading (??? really windy out there ....) combined with the first two .... considering a simply supported / uniformly streched etc. plate the cracks starting from the middle would be a logical place stress - wise -- crack path curvature / crack branching (if there is any) might imply residual stress effects and crack path orientation about whether the residual stress or an initial defect & applied loading were the dominant causes (curved against straight crack paths relative to panel width). Wonder how glass does with respect to environmentally assisted cracking (always considered it inert if there is one?), stress corrosion cracking arising from the cleaning ?

... probably really difficult to find any initial cracks (especially afterwards ), but sounds like something having a fracture mechanical explanation.

FredGarvin

I highly doubt the pressure difference theory. The only way that would hold water is if the rest of the room were sealed, including the door.

If it's a new-ish house, my first thought would be an improperly framed window opening. If the header for that window was not large enough or not installed properly, the roof loading could have been transfered through the window. I'd bet if that window was removed, the window frame itself is twisted slightly.

StirlingA

Which way was it cracking?? Vertically? Horizontally?

My guess would be a settling house.

Danger

That's kinda strange. At least I've never seen a stress fracture start anywhere but the edge of a pane. Are you sure that nothing impacted it, like maybe some kid with a pellet gun or something? I'm not saying that it doesn't happen, but I've never heard of it. I have friends who own a glass shop; I'll see if they can shed any light. It might take a while; I don't get around so well any more.

Ergo606

Thanks guys for all the responses. Yeah, only two ppl live in that house. My friend who was on the phone playing his guitar in his room...and his mom who was in her bedroom on the bed. No one else was in the house. The crack was vertically straight down the center.

PerennialII

In common stress fracture theories one rule of thumb is that the crack propagates from the edge with about 89% of the load that it does from the center.

Danger

Could you clarify, please? Do you mean that it takes 89% as much stress to start one from the edge as it does from the middle?

PerennialII

Yeah, if the initial defect is identical in size and the applied stress can be considered far - field tension. It comes from one 'principal' solution of linear-elastic fracture mechanics.

mapper

houses are always shifting, specialy new ones. All that happened was the house shifted ever so slightly to put more stress on the frame of the window and poof.

Ergo606

Well if it helps, I thought of the settling house theory. Then I found out that the glass nor the frame touch the cieling. Would it be possible that one wall moved inwards causing more stress on the glass? Btw, you guys are fast at responding, lol.

FredGarvin

Nor should it. However, the framing that holds the window definitely does. The whole purpose of the header above a window or door is to transfer the roof/ceiling loads to the two supporting side members that hold the window. If that header is not right the load or part of it will go through the window as well. See the diagram below.

http://www.hammerzone.com/archives/w...raming_2xq.gif

Danger

Ya learn somethin' every day... Thanks.

Ergo606

Oh, oops. I failed to mention that this wasn't a 'window' window....Its the glass in a shower....I can't explain better. It was like the picture below only not as fancy. It was also sliding instead of on hinges.
http://www.glassnet.co.za/images/pic17s.jpg

mapper

lol its a shower door window. That stuff should be tempered and shouldnt break like that. Its most likely defective. How old is it, about the same as the house i would imagine. Over time the steam and heat from the showers may have weakened it. That or ghosts.

Ergo606

Haha, yeah. A 'shower door window', there we go. I don't know what else to say. What I told him is what all of the suggestions I read combined together were. Basically, defect+deterioration+some other things.....Yeah.

Steam would deteriorate tempered glass over a while? Whoa.