I can see that if you want a sunroof it's better to have a factory fit rather than an after-market jobbie. Not sure if that's what the OP means.
But surely a sunroof is different to a panoramic screen.
In the first you cut a hole in the roof and fit the horizontal equivalent of an electric window in it. In the second you bond the glass to the body - there's no gap to encourage flexing - and a bonded screen apparently contributes about 30% of a roof's strength.
So I don't see a problem though I do see that being hit by a projectile would be quite a shock if the glass started to opaque out from the damage. But a projectile into a steel roof would be pretty alarming too.
Edited by Optimist on 31/08/2008 at 20:12
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>> I remember than a decade or so ago several manufacturers actually recomended that you did >> not even fit sunroofs to their cars as it would weaken the roof structure... Which manufacturers were they?
manufacturers of cabriolets, rag tops, and drop heads i would assume? :)
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BL had its share of problems with the rigidity of their Hydragassed cars - Have you ever seen an Allegro with a sun-roof ?
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I had a small glass "sunhatch" fitted to a Peugeot 104ZS in the mid eighties. Made an otherwise samll car seem much more light and spacious without affecting the general bombproof feel of a very solid warmhatch.
If I remember rightly the national chain that fitted it offered a service on everything but the VW beetle and the Hillman Avenger. Possible though that both those cars had cross members in the roof making it impossible to cut the shheet sttel roof panel.
Friend of ours wrote off a 307SW in a collision with an HGV and walked away - I've never asked her for a detailed account but don't recall her telling of the tale invovling being showered with glass.
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In terms of stiffness, the glass itself is probably stiffer than the steel it replaces. The Young's modulus for glass is about 80 - 100 GPa, as opposed to about 210 GPa for steel. As the glass is typically more than 2 or 3 times the thickness of steel, overall, the glass itself can be stiffer.
If the glass is held in with an old fashioned rubber seal, then the connection to the car body is so weak that the stiffness of the glass itself is irrelevant - the rubber seal just deforms. If the screen is bonded in, then the bond thickness and material stiffness can be tuned to effectively make the glass behave as part of the structure - the bond will add some compliance, and this will compromise the stiffness ratio of the two materials as discussed above.
The screens are typically heavier than the steel they replace, and this weight is in the worst possible place, high up in the car - the roof.
In terms of stress, it is difficult to define a failure stress for glass. Even a basic steel has a yield stress of over 250 MPa, while glass has been known to break with stresses of the order of 10 MPa. This is because glass breaks via the growth of small defects or cracks, rather than by plastic flow which is the case for ductile metals. So, before you can say anything about glass failure strength, you need to know about the presence of existing cracks or defects, and the residual stress state in the glass as well as the magnitude and duration of the applied loading.
If, however, you have a toughened glass with a built in compressive stress in the surface layer, you are usually quite safe to apply tensile loads until the residual compressive stress is nullified.
The other detail, of course, is the post failure behaviour of the materials - after glass breaks, it offers little or no further strength, while steel can continue to absorb energy beyond the yield point - in fact, this is the regime where steel can absorb most energy, there's relatively little deformation before the yield point.
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Thanks, N-C, I knew you'd come good! I thought it might not be as strong and that rather coinfirms it - the key thing for me is that if its sunny I tend to drive with the visors down to reduce the glare, so the glass roof would be rather useless! Must be made for those who like open-top cars but don't want the wind ruffling their feathers!
P, I think you might be right... I think it was a BL car I saw the warning for (in a manual if I remember rightly!
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I didn't find what you had to say that easy to follow, NC.
Is the specially prepared glass as used in screens and panoramic roofs as strong as the steel it replaces?
Thanks.
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I didn't find what you had to say that easy to follow, NC.
You aren't alone optimist!
The technical problem is that unlike most metals, you can't define a strength for glass. So, in the way the question has been posed, it's difficult to answer.
For glass to break, typically, you need 2 things to come together;
1) a small defect or crack
2) tensile stress at the site of the defect which is big enough to make the defect grow to failure
If you have a sample of glass that has no defects (a typical example being a drawn fibre), you'll obtain a very high strength, but, it only takes 1 defect in the wrong place for the glass to have a much diminished strength.
Now, because of item 2) demanding a tensile stress, if you can toughen the glass, by making the outer layers (where the defects are likely to be) have a residual compressive stress, you can ensure that there isn't a tensile stress field until the residual compressive stress has been overcome - i.e., by tweaking the residual stress in the glass, you can increase the service stress at which the glass becomes susceptible to failure.
In a crash scenario, however, the answer is much easier - once the glass is broken, it offers very little strength, and so, you would be better off with steel there, which does continue to absorb energy post-yield.
If there are any particular words, phrases, or concepts which I haven't cleared up, please point me to them, and I'll try and lift the fog!
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Pre VW Skoda was a manufacturer who advised against the fitting of sun roofs.
I've seen a few aftermarket one's fitted though, and in spite of the Skoda warnings, they still let the sun in.
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I owned three Skoda Estelles and two Rapids. One of the Rapids had no glass sun roof, but all the others had them, dealer-fitted here I think. They all had alloy wheels too.
The sun roofs had a not very good radio aerial incorporated into them, and were the easiest way to break into the car without doing any damage if you locked yourself out.
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I've read this with interest. Our Grand Scenic has the panoramic roof option, which consists of two big panels covering about 75% of the roof area. Of the two panels, the rear panel is bonded in, and the front panel opens (up and back) over the rear panel. It's a big area of glass, as this pic shows for those who aren't familiar with the design:
i-m.com.ua/pic/r/renault.grandscenic.jpg
So as I understand N_C's explanation, the front panel contributes nothing to the rigidity of the shell because it's not fixed (and butts against rubber when closed), but the rear does as it's bonded.
That has to affect the crashworthiness of the car, surely. Renault proudly trumpet their 5 star NCAP rating for the car, but I wonder if any comparison has ever been made between the version with the panoramic roof, and one without?
Cheers
DP
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I want to ask much the same question of NC.
Assuming the glass roof/windscreen is previously undamaged and the car rolls, are you safer in a car with a solid steel roof or with a panoramic roof? In either event I'm assuming a write-off so the later performance of steel over glass doesn't matter.
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It's more complex still!
In DP's example, where a piece of glass can move or slide, the roof around the glass will be effectively re-inforced by the mechanism and slideways, making the steel of the roof stronger than if no glass or sliding roof were there.
However, given the option of being forced into rolling in a car with glass for its roof or steel, I would get into the car with the steel roof, mainly because after the glass breaks, it offers nothing in terms of strength, whereas steel, even deformed steel, is capable of taking load.
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So if the car rolls the glass inevitably breaks?
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The Vauxhall/Opel panoramic roof seems to be an extended windscreen, i..e. nothing in front of you or above you but glass. Sure you must have seen the advert. Citroen do them too I think. So must therefore Peugeot.
I tried to reply earlier and the post failed and got lost. So will cut and paste as the original was lengthy.
At A level time we were told glass is actually stronger than steel. But it's more brittle due to the imperfections in the crystals as it cools. Hey Pilkington did a glass hammer after all.
So the strenth test was a fibre of glass and steel of the same diamter. The glass one was stronger. Touch both with a feather and the glass one now has flaws and fails much more easily under weight.
Now the problem with glass strength is that due to how it will fail you have to make sure it fails safely (who'd want plate glass windows in the car) so it is cooled quickly to make sure the imperfections are there and it shatters instead of having large lethal pieces. So it will now shatter. For windscreens it's laminated but still it is still not really strong.
So for these big panoramic windows I would think the glass is not a structural part of the vehicle and the strength is in the A pillars. Stands to reason. And if a convertible is "safe" without a roof up then these must be too. But I do no thing these can offer much strength and the car compensates in it's steel. Maybe lightter/higher strength steel is used - the Mazda2 uses this to improve strength with a lower mass.
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>>At A level time we were told glass is actually stronger than steel. But it's more brittle due to the imperfections in the crystals as it cools.
Yes, that's true - in theory. The problem is that real glass, outside of drawn fibres is riddled with flaws and defects.
>>So the strenth test was a fibre of glass and steel of the same diamter. The glass one was stronger. Touch both with a feather and the glass one now has flaws and fails much more easily under weight.
Yes, it's amazing just how easy it is to damage a glass surface, and introduce a flaw. When you look at a surface scratch through a microscope, you can see the path of the embedding particle, and how the material almost tears and fissures in its wake.
>>so it is cooled quickly
With toughened glass, which is what we are both actually describing, the rapid cooling gives a locked in compressive stress zone near the surface (which tends to close cracks), but, across the section, this must be balanced by a built in tensile stress for equilibrium. It is the release of this locked in stress field as well as the presence of initial flaws which makes toughened glass fail in its characteristic, safe, way.
Will a glass roof always break during rollover? No, I'm sure rollovers on sandy beaches will be fine, but, as soon as there's a stone or anything applying local pressure, the screen will break.
>>And if a convertible is "safe"
I don't think there is such a thing - certainly not in an absolute sense. Rolling over in a convertible is inherently more risky than rolling over in a car with a solid roof. These panoramic roof equipped cars probably sit somewhere in-between on a scale of safety.
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NC
When I said ">>And if a convertible is "safe""
I know it isn't so the panoramic roof is no less safe. But not as safe was a proper roof.
I agree from my limited knowledge of glass from A Level Physics (nearly 20 years ago) that glass is strong but will have inherent flaws and therefore brittle. So the undamaged fibre is strong under load but under stress .... ;-) But we all know glass re-inforced fibre is strong.
So I personally think glass panoramic roof is not as safe as a normal one. And at times will let in too much light as well.
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>>When I said...
Sorry, I misunderstood.
>>I agree from my limited knowledge of glass from A Level Physics...
Not at all, you have written about the very nub of the problem.
Most engineers who have a background in ductile materials have real trouble when they begin to think about the failure of brittle materials. They naturally want to simply define a failure stress, and carry on with their structural calcs as before.
Even the type of stress that you are interested in changes. For a ductile material, you are typically interested in the yield point, and when calculating stress in 2D and 3D structures will typically use something like a Von Mises yield criterion.
For brittle materials, yield is absolutely irrelevant, and what you are interested in is the maximum tensile principal stress. This maximum tensile principal stress is then used with knowledge of typical defect sizes (typically, with good processing and quality control of the order of a few hundred microns) to produce fracture mechanics calculations to determine if the glass is OK.
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Pre VW Skoda was a manufacturer who advised against the fitting of sun roofs.
The post VW Felicia was available with a manufacturer fitted optional glass tilt sunroof - and it's a real problem with leaks because of the roof flexing.
Glass doesn't break, but the frame to roof sealing is a nuisance, as is the windscreen rubber - see HJ CBC.
Ours was done first time under third year warrantee, and twice since.
Aparently there's a bit more steel in the Volkswagenised version of the Felicia, but it could have stood even more.
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Lud,
The Skoda dealer I worked for at the time fitted them at customers request, the work being done by a sunroof specialist.
If I remember correctly, The alloy wheels and Dunlop tyres were fitted to all Estelles, by the importers, in place of the original steel and Barums.
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The cost of a new screen must be pretty high. I can see insurers either charging higher premiums or having bigger windscreen excesses with these cars.
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Has anyone seen one of the Vauxhaulls on the road?
Is it a real option, or just a copywriter's idea for the commercial?
It looks less strong than the rag top 2CV; doesn't even have a top rail to the windscreen area.
Anyone seen any of the crash test results?
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I work in this field, the majority of these panoramic roofs are made of toughened laminated glass, so they behave in exactly the same way as a windscreen. they will not shatter. they give strength to the bodyshell as they are bonded in place.
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"Which manufacturers were they?"
I know that Jaguar were so worried about their first venture into monocoque construction with the Jaguar Mk1 that they recommended that the a sunroof (sliding fabric probably) wasn't fitted..... but that was the late 1950's!!
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I have a feeling the one which mentioned it in the handbook was an early Metro I had... but I may be wrong.
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Interesting thread.
Talking of glass strength, I have just bought a bullet proof windscreen for a helicopter for one of my middle eastern customers.
I had to laugh when it came into the warehouse today in a box plastered with Glass / Fragile stickers.......
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I saw a huge battleship grey senior-Nazi Mercedes tourer on display in Plymouth in the fifties. Its side windows, some of which were still in position, were two or three inches thick and made of many thin sheets of glass laminated together with tough plastic.
The windows had been severely tested by playful squaddies with sten guns. They were quite deeply chipped and gouged in places, but hadn't been penetrated. Must have weighed plenty though, and the car itself was no shrimp.
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>>thin sheets of glass laminated together with tough plastic.
That's an extremely effective technique, the plastic acts as a crack arrestor, and prevents the crack propagating through the structure.
Crack arresting is one of the great advantages of a bolted joint over a welded one in metallic structures. Cracks originating on non-structural parts of an assembly have been known to propagate into structural members with catastrophic effect via such welded joints, and as such, all welded joints on a safety critical structure should be viewed as potential crack initiation sites during a structural assessment.
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There is no such thing as bullet proof glass though. There's bullet resistant glass but a big enough bullet will always get through ;-)
How thick is it? If only about 3" thick it might only stop small hand guns. But if the rest of the helicopter has not been reinforced is there much point?
I like the idea of one-way bullet resistant glass - let you fire back.
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one-way bullet resistant glass - let you fire back.
Ah, that would be the Mk XVIII Reversible Forcefield... Not yet available to civilians unfortunately, although sales are expected to be brisk in the US when the embargo is lifted...
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Lud, sorry to disappoint but there is such a thing as one way bullet resistant glass. But you might not do much harm to the person on the other side. It slows bullets in only one direction.
But a big enough calibre bullet will get through. An Apache AH-64's cannon firing depleted uranium 30mm rounds takes out tanks after all :-)
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Lud sorry to disappoint but there is such a thing
I'm not disappointed at all, just a bit surprised. I can see such a thing would be possible but that (as you say) even bullets fired from the easy side might be slowed, distorted and deflected. Have to say though that you wouldn't want to weaken the bullet-resisting capacity of the glass by making holes in it from the other side, not if people were outside shooting at you... :o}
And of course we all know that a dense enough projectile with the right mechanical qualities and travelling at sufficient speed will go through anything. It's not for nothing that the current standard US sniper rifle is a single-shot .50-calibre device. The big bullet carries much further with accuracy too.
Edited by Lud on 03/09/2008 at 18:34
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I sadly googled for bullet resistant glass and found some you tube videos. All to many rounds went through the glass.
The cockpit area of an Apache AH-64 is meant to withstand 23mm rounds. The fact you're firing 30mm DP shells might make them run away.
For car safety if needed, I think I prefer the South African flame thrower approach. Squirt petrol on the shooter and tell em to run.
This is getting off topic though.
I still think the GM/Vauxhall panorama roof is safe enough but I would not want one.
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I have a feeling the one which mentioned it in the handbook was an early Metro I had...
ISTR seeing the original "crash tested" Metro at the Design Centre, or some such place in about 1980. It had a sunroof.
But my memory isn't what it was... and I may be wrong.
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Think it might have been after-market versions they were warning about - there were some rather dubious ones around in the late 80s!
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i remember years ago ( yore) a documentary on the telly when a bloke fired a wax candle out of the barrel of a shotgun straight through a four panel door...must be something to do with the speed and weight of the projectile i suppose, is it the same has a H.E.S.H. round a chieftain tank used to use ?
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Are you perhaps thinking zookeeper of the Home Guard technique during WW2, opening the end of your no.4 12-bore cartridges and pouring molten candle wax in, then sealing the thing up and having a very deadly musket with all that shot stuck together in a horrible waxy lump?
I wouldn't have wanted to be a German invader shot with one of those, although my Schmeisser and extensive combat experience would probably have prevented that from happening.
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