Safety Glass - Identification and Maintenance
BULLETIN NUMBER:IB01-11-S001
ISSUE DATE:
FEBRUARY 2001
GROUP:
BODY
AUTOMOTIVE SAFETY GLASS:
IDENTIFICATION AND MAINTENANCE
AFFECTED VEHICLES
All Isuzu Vehicles
SERVICE INFORMATION
Safety glass as the word says it provides some form of protection for occupants in automobiles. From the introduction of the windshield in the early 1900's, there have been so much importance placed on the windshield from hue/road debris deflectors to becoming an important part of the vehicle's safety structure. The early windshields may have provided a more comfortable drive, but once flying debris strikes the windshield, the windshield is fractured causing dangerous glass fragments to fly into the cabin and possibly injuring the occupants. Today's windshields has been engineered with more advanced technology giving it enough strength to prevent occupant(s) in the vehicle from being thrown out of the windshield and resiliency to reduce injury in case of occupant impact on to the windshield.
There are two types of commonly used automotive safety glass. They are laminated glass and tempered glass, which will be covered in this bulletin. Also, covered in this bulletin are valuable information for safe use and handling of automotive safety glass.
Identifying Vehicle Windows
Glass Properties
1. The basic ingredients of glass are silica sand, soda ash, and limestone. More ingredients are added depending on the glass application. As basic a glass may seem, glass also requires some maintenance to provide the longevity in its usefulness. Dirt, sand and airborne dust particles may contain metals and substances that are harder than glass. Glass (i.e.. automobile windshield. etc.) can be damaged/scratched from dirt, sand and airborne dust particles that may come in contact with it if not removed. Dirty wiper blades imbedded with dirt; sand and airborne dust particles can damage the front windshield if used without first rinsing the windshield clear of them.
2. Glass subject to external forces may be damaged from scratches. or break as a result of high impact on the glass.
Types and Characteristics of Automotive Safety Glass
There are two types of commonly used automotive safety glass. They are laminated glass and tempered glass.
(1) Laminated Glass*
Laminated glass is a type of glass with a tough plastic film (Poly Vinyl Butyral a.k.a. PVB) is sandwiched between two sheets of glass bonded together in a vacuum under controlled temperature and pressure forming a single sheet, to provide a high degree of safety. This is followed by extensive tests prior to receiving clearance for usage. Laminated glass (used for automotive windshields) found in practically all automobiles produced, are part of current safety regulations for production vehicles.
*Properties of Laminated Glass:
^ The interlayer cuts ultraviolet rays.
^ The plastic interlayer adhesion prevents the two glass layers from splitting and shattering into the cabin (resulting in occupant injuries). This ensures undistracted driver field of vision, thus lessens possible injuries.
^ The automobile windshield (made of laminated glass) is designed to break but not shatter, will absorb the impact (lessening bodily injuries) of the human body in the event of an accident (head-on collision) than tempered glass.
(2) Tempered Glass**
Tempered glass consists of a tensile stress layer sandwiched between two compressive stress lavers. This three-layer glass is produced by heating a glass plate to its softening point (approx. 700°C) and quenching it with air. Tempered glass is used in automobiles for side door windows, backlight windows, rear quarter windows. sun-roof/moon-roof glass and other windows except the front windshield.
**Properties of Tempered Glass
^ The outer layer (compressive stress layer) of the tempered glass resists given external forces (tensile forces). This layer gives the tempered glass three to five times impact strength than a normal glass plate of the same thickness.
^ The tempered glass will shatter into small particles (with no sharp edges) if breakage occurs to reduce possible injuries.
^ Tempered glass can resist sudden temperature changes (thermal shock).
Scenarios of Damaged Front Windshields (Laminated Glass)
Breakage in Laminated Glass
Scenario:
A crack is found on the windshield after returning to a parked vehicle.
Cause:
A small scratch on the windshield was unnoticed, which developed into a crack. This may have developed as a stress crack emanating from vehicle load stress concentrating on the vehicle's weakest point (the windshield).
NOTE:
A flying pebble may have caused the scratch (which mainly occurs from the external surfaces of the glass.
Why cracks develop?
Cracks develop when stress is applied to the impact site. The impact site is the actual location of the outer glass layer where an object (i.e., flying pebble) struck the windshield, chipping away a small piece of glass.
Stress is defined as:
^ Vibrations result from wind pressure while the vehicle is moving.
^ Vehicle body stress from turning and moving (i.e., rolling, pitching and yawing).
^ Thermal stress (temperature difference between interior and exterior temperatures of the vehicle).
Here is an example of a scratch caused by a flying pebble on the windshield.
PREVENTATIVE TIPS FOR WINDSHIELD CARE:
[ ] Select a parking location where the vehicle is not exposed to potential hazards from flying pebbles or gravel.
[ ] Avoid driving behind trucks at a close distance, especially the ones that haul gravel. (Truck tires may sometimes-pickup loose gravel/pebble in its tire grooves and may sling shot to your windshield.)
Scratches on Laminated Glass
Instructions for Proper Handling of Laminated Glass
Laminated glass should be kept away from high heat or heat generating equipment/appliance during transport and/or storage. The interlayer may bubble or become deformed it left exposed to high heat.
Scenarios of Damaged Backlight Glass and Door Glass (Tempered Glass)
[ ] The glass broke while the vehicle was being driven.
[ ] The glass broke while parked for a short time.
[ ] The glass broke when the door was closed.
[ ] The glass has scratches on the inner-side that is visible from the outside.
Causes:
Several causes may play a factor to tempered glass (i.e., backlight glass or door glass) damage during vehicle operation or on a parked vehicle.
^ Impact of flying pebble causing a scratch. (See Scenario 1 and 2).
^ Various stress factors from wind pressure and vibrations while operating the vehicle are continually applied to the "unnoticed" scratch (from the flying pebble) until the scratch reaches the inner tensile stress layer, will lead to glass breakage. (See Scenario 1 and 2)
Glass Breakage Flow Chart
Scenario 1: Damage reaches the inner tensile stress layer of the tempered glass to cause breakage.
Scenario 2: At the initial impact, damage remains in the outer compressive stress layer. While operating the vehicle, various stress factors from wind pressure and vibrations are continually applied to the scratch (from the flying pebble) until the scratch reaches the inner tensile stress layer, that leads to glass breakage.
Backlight Glass with Damaged Defroster Lines
This section describes failures that have recently been observed in station wagons and SUVs with tailgates. These are defective heat generation by the defroster-printed lines.
Cosmetic Defects in Side Door Glass
The vehicle's door glass is one of the most exposed glasses to heavy usage in the vehicle, as well as susceptibility to damage by the user. Here are some examples to common scenarios.
Other Related Defects
Damage due to mishandling
Maintenance of Soiled and Stained glass
Glass Molding Surface Damage