Windscreen-could it be stress fracture?
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I can't imagine many car windscreens fracturing other than by stress of one origin or another, so I'll agree, yes!
Number_Cruncher
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I assume the OP is alluding to the dealer poking about amongst the alarm wiring in the region of the windscreen, this is possible beacuse the volumetric sensor wiring usually runs down the inside edge of the screen.
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I assume the OP is alluding to the dealer poking about amongst the alarm wiring in the region of the windscreen, this is possible beacuse the volumetric sensor wiring usually runs down the inside edge of the screen.
Yes, this was what I was alluding to. But also I wondered if they'd had it up on a jack, whether it might have caused stress. Particularly if perhaps the original quality of the screen wasn't good or the fit was poor.
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The glass is under stress from the moment it is manufactured. It is the absence of a significant enough defect that prevents glass from failing catastrophically. So a nick or a chip or a heat source (solder iron) is all that is required to initiate such a defect.
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Red Baron,
All quite true - your use of technical language is interesting - do you work in the field of fatigue and fracture?
Number_Cruncher
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NC,
Yes, for my sin of venturing into the world of materials, I have spent 7 years working in the field of technical ceramics and glasses.
Red Baron
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NC, Yes, for my sin of venturing into the world of materials, I have spent 7 years working in the field of technical ceramics and glasses. Red Baron
A quick question, if I may? Apologies for it not being motoring!, but it is glass related.
Given a sample of low CTE, transparent, glass ceramic, what is the best way to inspect to find defects, and what would be the largest defect you might miss using this method?
The fracture mechanics calcs I have done suggest that I need to be concerned only with defects larger than 0.1mm.
Second, as the glass ceramic is low CTE, toughening is off the menu - are there any other methods and precautions which should be taken to avoid catastrophic failure under dynamic loading? So far, all of the surfaces which are not polished, I have specified to be acid etched.
Number_Cruncher
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NC,
A commonly used method for detecting flaws on larger areas is laser tomography. I have never used this and so can't really comment on its capability.
The ceramics and glasses that I have worked with in all cases, bar one, have been opaque. The low CTE g-c you mention, is it something like that used for glass-ceramic cooker hobs? 'Zerodur' by Schott springs to mind. This seems to withstand serious abuse.
Glass-ceramics by their nature are already toughened an so the toughening method used in the case of glass will definitely not work. With g-c's the opposite is true. Stress relief at or about the glass transition temperature is often used to prevent shattering, especially if you want to drill through it. If you find that you can't reheat the g-c in case you get crystal nucleation and growth, then the methods you suggest are correct. I would pay particular attention to the edges of the glass-ceramic and get these bevelled and also polished.
Your critical flaw size (0.1mm) is dependant on the type and nature of the glass-ceramic. Crystal/glass phases and grain size are the most important considerations along with the load to which the piece is subjected to. So your calculations may be right. 0.1mm is visible with the naked eye one so is quite a large flaw size in my experience.
Hope that helps...
Red Baron
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Hi Red Baron,
Thanks, your response is very helpful indeed!
You are right, it is Zerodur that we're using - as a stable base for an interferometer. It looks a little bit like this one;
www.physics.gla.ac.uk/~mplloyd/LISApf/S2-UGL-DRW-3...g
We are currently doing a polarisation based inspection, looking for isochromatic stress fringes - do you know a good reference on the subject of laser tomography inspection?
I will check to see if we can do a post machining heat treatment, and if so what the implications would be - the possibility hadn't occured to me.
I'm basing the fracture mechanics calcs on published fracture toughness data from Schott for the Zerodur grade we have specified (with a decent factor of safety!)
Thanks again!
Number_Cruncher
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Number_Cruncher
Now that I can see the application of the g-c, I can't imagine how a significant stress would suddenly arise that could cause the base to fracture. Glass cermaics can accommodate a certian level of flaw size, unlike glasses.
Certainly, before any machining or drilling is done, the g-c needs to be in a stress free condition. However, these operations will introduce stress raisers, mostly at the entrance (start) and exit (end) points of the operation. I have spent hours relieving stresses from quartz tubes so that I could drill holes into them. I have so far not had a machined or drilled glass ceramic fail on me, unlike quartz.
I would not advise heating the base to reduce any stresses introduced by machining as the base will deform. Even if you cannot see any sress fringes from the polarized light, there will still be sufficient stress in the base to cause it to warp - completely undoing all your polishing. I presume that the top and bottom surfaces of the base are as flat and parallel as you can get them.
I'm afraid that I don't have any references for Tomography.
Sorry I can't be more help.
Red Baron
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>>I can't imagine how a significant stress would suddenly arise..
The "vehicle" that we are going to bolt the base into is subject to some pretty violent accelerations. ;-)
>>Sorry I can't be more help.
Not at all!, I'm very grateful for your insights, I'm used to working with flaws in steel, but brittle materials, with their low fracture toughness are an entirely new and scary subject to me.
The range of the skills, and the willingness to help of the people who use this forum never cease to surprise me.
Cheers
Number_Cruncher
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Dirt in crack?
Actually in the crack and beneath the surface of the glass or is it flush with the surface.
If it is a speck of dirt within the glass (or maybe between the laminate layers) I find it odd that the crack has opened up in two directions. Are these two directions in a more or less straight line?
Is the speck of dirt flush with the surface of the glass. If yes, then maybe someone was a bit careless with an angle-grinder nearby. Red hot metal fragments can easily initiate a crack.
As to the guy spotting it straight away...if you know what a crack in the glass looks like, you'll spot it instantly.
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>> If it is a speck of dirt within the glass (or maybe between the laminate layers) I find it odd that the crack has opened up in two directions. Are these two directions in a more or less straight line? Is the speck of dirt flush with the surface of the glass. If yes, then maybe someone was a bit careless with an angle-grinder nearby. Red hot metal fragments can easily initiate a crack.
The dirt seems to be flush to the glass but it's so small it's really quite difficult to tell. It almost looks like a small oil smudge. You can't see it unless you look from one angle. The crack has gone in two directions and is straight. The crack seems widest where it meets the edge of the screen.
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I would think that is most unlikely that the garage has caused the crack in your windscreen. Weighing up all of the possibilities I think it is most likely that something has flicked up from the road and chipped the window and thus cracked the glass. I have had a similar experience in my truck. It had a new windscreen fitted, and within 7 days of having the new screen had developed a vertical crack around 10 inches long right in front of the drivers seat. I am fairly sure that it happened on a trip from the Midlands to Hampshire and back. I was the only person to drive the truck and do not recall anything at all hitting the screen. At first I thought it might be a faulty screen or had been fitted badly but upon inspection there was the tiniest of tiniest chips right near the bottom of the crack. The moral of the tale, windscreens are almost a wear and tear item these days. Of the fleet of trucks at the company I work for (around 35 vehicles) I would think that each year at MOT time between 33% and 50% of the trucks have to have a new windscreen due to cracks and chips.
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Sounds like weld spatter?
madf
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