Windshields are more than mere glass

By T Uncle

MOST car owners would be surprised to know the significance of the windshield in the overall safety structure of their vehicles.

The fact is that the contribution by the windshield to the strength of the upper body structure is, on average, something like one third -- meaning that without the windshield properly fixed in place, the vital "glasshouse" area that is subjected to significant forces during accidents has only two-thirds of its normal resistance to deformation.

Maximum strength is particularly vital in the event of a rollover, where the upper structure plays a vital role protecting passengers by resisting crush forces.

Using the windshield as a structural component in car design did not come into popular use until the mid-1980s when carmakers started bonding the glass into place with specially formulated adhesives.

This proved to be a good way of saving overall vehicle weight, while actually increasing overall structural rigidity.

At the same time, carmakers universally adopted laminated glass (although it had been around for years on more expensive vehicles) which was made up of two sheets of glass bonded together with a vinyl interlayer, for greater strength and much improved resistance to stone damage.

A laminated windshield not only resists damage caused by objects such as flying stones, but also usually restricts the damage to a small chip, or maybe a small crack in the outer layer which can often go unnoticed for years.

Previously, windshields used toughened glass that would shatter when impacted by a flying stone, leaving a small zone- toughened area through which the driver could see.

The shortcomings were obvious: The instantly shattering screen and the resultant loss of visibility was a hazard in itself, while the need for replacement was an expensive, often inconvenient by-product.

The advent of bonded, laminated windshields largely overcame the problems of windshield damage on open roads, but it did not eliminate them entirely.

Today, the problem is more likely to be the tendency of drivers to ignore any seemingly small chip or crack suffered by the windshield.

What most people do not realize is that small chips or cracks can often be legally repairable.

Standards determining the repairability of windshield damage are based on the size, type and location of the damage. If the damaged area is in the driver's line of vision, the standards determining what is an acceptable degree of repairable damage are particularly strict.

The repair is effected by injecting an optically matched resin into the damaged area. The results are usually that the damage is virtually undetectable.

In the event of a laminated, bonded screen needing replacement, the matter of installing new glass is critically important.

Because the glass is a structural component, it is vital that the windshield frame is correctly prepared with special primers to accept the new screen. So, the incidence of urethane adhesive UV degradation is lessened, while protection is also provided against the formation of rust.

Because the bonding agent holding the screen in place performs such a significant role in the structural strength of the vehicle, it is important too that the material used to attach the glass not only is applied correctly, but also that a specific "curing" time is allowed before the car may be driven.

Until this process is complete, the windshield is not an integral part of the vehicle and can even become dislodged by something as simple as slamming a door with all the windows closed.

In some instances, it is recommended that a curing time of six hours, or longer, is needed before the vehicle can be safely driven, although a time of at least one-and-a-half hours is more often stipulated.

This is especially important in the case of a car with a passenger's side air bag, where the windshield actually plays a role in its correct deployment.

The air bag is usually designed to direct the inflating air bag into the correct position for protecting the passenger.

Without the screen securely in place -- a risk in the case of incorrect installation, or if insufficient curing time has been allowed -- the air bag will simply not function correctly, therefore, putting the occupant's safety at further risk.

Bonding agent technology has also advanced in recent years. Many carmakers are now using a special "high modulus", nonconductive adhesive that is not only more rigid than other bonding materials but, because of its nonconductive nature, is also able to be used in aluminum-bodied cars such as Audi's A8, or the high-performance Honda NSX.

The new bonding material's strengths are also used to advantage by other carmakers, such as Volkswagen, BMW with its 5- and 7-Series, Jaguar and Mercedes-Benz with its current-model E- Class.

So, the windshield is an important part of the car's overall safety package and should be regarded as such.

It is a lot more than a just sheet of glass through which the driver views the world ahead and could, in extreme circumstances, mean the difference between life and death.