Damage to Car glasses - do they crack and shatter ?

Dear (s]  -  Damage to  windscreen glasses – how they break  ??  -   {{May be most of this is known to you !!}}

Look at these photos.  Parked cars claimed to have been damaged by stone throwing.   Shattered glasses and apparent damages -  Looks simple.  Commonly, when we see road accidents involving truck, bus or auto - you see glass pieces strewn all around.  Do car glasses also break similarly ??

Interestingly, all the damages pertain to rear window glasses.  Enquiry with Motor Expert immediately revealed one facet - perhaps most of you know.  vehicle’s windshield is made of glass, and so are the door glasses, or windows, and the back glass. Not all of the auto glass around your car, however, is made of the same material or the same kind of glass.   If you had observed, generally, this is how the windsreeen glass of a car breaks.  The glass  breaks safely. May crack under impact, but typcially remains integral.

The windshields, in particular, are made from a different material or construct compared to the other glasses such as the back glass and the door glasses. The use of different materials for the auto glasses were made to ensure that each of the glasses would function just as they are expected.  Today, for greater safety, the automotive industry employs the laminated glass which consists of  two glasses plates and in-between, a plastic foil made  of polyvinil butyral.  In the unfortunate event of an accident, the glass  will not "explode", but the small pieces will remain attached over the plastic surface.

Before 1919, early windscreens were made from hand-cut glass and were extremely dangerous on impact. Then, in the interest of safer motoring, Henry Ford introduced laminated glass - two layers of glass held together by an inner layer of cellulose. This plastic layer absorbs much of the shock upon impact and keeps occupants from being ejected through the windscreen as well as preventing lethal glass shards entering the cockpit area.  Laminated glass does not shatter. In some instances, damage to laminated glass can be repaired. Although toughened glass is harder to break than laminated glass, there is a good reason laminated glass windscreens are mandatory in some countries. Today, Polyvinyl Butyral (PVB), a high-strength vinyl, is used in windscreens instead of inferior cellulose. The glass is held in place by urethane, the best adhesive for affixing windscreens.

Unlike the windshields that are made of two float glasses with a plastic laminate sandwiched between them, door glasses or windows are made of a single piece of glass. The glass used is a temperate plate and is basically similar to the glass used on the back or rear window of the car. The windshields proved tougher than the door glasses as the windshields are designed not to break easily when hit by debris, especially while driving. The temperate glass used on the windows, on the other hand, is designed to break easily when pounded. This is purposely made so that it would break easily in cases of vehicle rollover where the passengers are trapped inside the vehicle. While they would break easily, the door glasses would break into fine pieces, compared to the jagged formation of broken windshields, so that it would not cause much danger.

One of the safety glass manufacturer (in INDIA) claims that their products undergo the following tests before testified fit for your use :

1)  Impact Resistance test  2) Boil test  3)  Light stability test 4)  Optical deviation and distortion test  5)  Penetration Resistance test 6) Head Form test & 7) Visual Light transmission test.  Of these the penetration test is done with an iron ball to assess ball penetration resistance.

If you have some comments, please do share.

With  Regards,
S. Sampathkumar

PS :  circulated to my group on 5/7/2007