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Volume 7    Issue 3            May/June 2005

Putting Repair to the Test
by Chris Davies

As part of Belron’s effort to understand the impact of repair on vehicle occupant safety, in 1999 it asked Transport Research Laboratory (TRL), an ex-government agency active in transport research in the UK, to investigate a number of vehicle, occupant and glazing interactions. The goal was to understand how a repaired piece of glass performed with regard to the criteria specified by the standards for automotive safety glass. 

The areas looked at were:
• The effect of the repair on the glass strength;
• The impact of a repair on the driver’s visibility;
• The effect of a repaired windshield on the occupant’s safety in the event of a collision; and
• The impact of a repaired windshield on the vehicle structure.

In all cases, tests were undertaken comparing undamaged and damaged and repaired windshields and, in all cases, TRL’s conclusions were that repair has little impact on safety. Such conclusions lend weight to a continued push of repair as a service to our customers.

Let’s look at each of these areas:

Glass Strength
The required functionality and performance of safety glazing for automotive applications is contained in standards such as ANSI Z-26.1: 1996 and BS AU 178a:1992. These standards specify a number of tests any piece of automotive glazing should pass in order to be deemed safe for use. They include ball drop tests to measure laminate adhesion (and set limits to how much glass can become detached in an impact) and windshield penetration. Other tests include dart drop tests and head form tests. A number of tests were selected that would determine the essential properties of the windshield required to ensure the safety of the vehicle occupant in an accident. These tests were carried out on a range of samples including parts of or whole windshields and undamaged and damaged and repaired glass. After undertaking all the tests on the relevant samples TRL concluded that a repaired windshield did not affect the required properties of the glazing as set down in the standards.

Driver’s Visibility
One essential requirement of any windshield is the ability for the driver to see through it clearly. It was vital, therefore, to investigate the impact of repair on visibility. The TRL test track was used to investigate driver visibility in a number of static and dynamic tests aimed at creating a rigorous but controlled environment for measurement. In order to test the worst-case scenario, all experiments were carried out at night with oncoming vehicles in order to increase potential glare. 

After a large number of static and dynamic runs, TRL statistically analyzed the data and found that there was no significant difference between both groups of results. 

It was decided to test to see if a repair had any adverse affect on injuries to a vehicle occupant if he struck the windshield during an accident. This was tested using a sled impact and an Occupant Protection Assessment Test (OPAT) dummy. 

When a person strikes a windshield the fracturing and detachment of the glass can cause lacerations. The degree of laceration potential is measured when a dummy with a chamois facial cover makes contact with the windshield. The number, size and shape of the cuts caused are then examined. For the TRL tests, the dummy was also fitted with accelerometers to measure the potential for head and brain injuries. The dummy was loaded into a 1997 Honda Accord, which was accelerated on a sled to strike a fixed target at 23 mph.

The Triplex laceration index was used to compare the facial damage caused by each test and the head accelerations monitored. TRL’s conclusions were that there was no increase in lacerative damage from a repaired windshield over an undamaged piece of glazing and that the accelerations measured in the dummies’ head were also very similar.

Vehicle Structure
In the event of a collision, it is essential that the windshield not only resist ejection of the occupants but also provide some resistance against the vehicle being crushed.

Also, the rigidity and strength of the glass laminate provides a torsional sturdiness, which ensures that the car is both stable and safe to drive. TRL set up a series of experiments to look at these factors. 

A vehicle’s safety normally is assessed by undertaking crash tests representing either front- or side-impact collisions. TRL used a FEA-based computer modeling method to simulate the effect of a crash test on the vehicle. Physical testing was used to measure the torsional rigidity.

A commercially-available digital model of a Ford Taurus was used for the computer simulations. This was augmented by accelerometers and 17 intrusion beams at critical points in the car’s interior to measure the amount of crush the vehicle suffered. The frontal crash test simulation used the Euro-NCAP (European New Car Assessment Program) crash test methodology, not the usual FMVSS 212. The Euro-NCAP crash test, while not a mandatory federal standard (it is actually a consumer based test that is used to rate new cars on a number of safety categories and can award up to 5 stars per category), was used as it has a more realistic crash barrier and a higher impact speed of 43 mph. The results of the multiple simulations showed there was no impact to the safety of the vehicle occupant during a frontal impact when the windshield had been repaired.

The side impact simulation also used the appropriate Euro-NCAP methodology. For the side impact test a movable, deformable barrier was brought into contact with the vehicle. For this particular test simulation a model of a European Side Impact Dummy (EuroSID) was used and fitted with accelerometers. Again the results showed that a repaired windshield did not compromise the safety of the vehicle occupants.

In addition to impact tests, the resistance to roof crush was also tested by computer model. Again an undamaged windshield and a damaged and repaired windshield were tested and in each case it was found that the vehicle would pass FMVSS 216. 

The torsional stiffness of the vehicle was measured by physical tests on a Honda Accord, and it was found that a repaired windshield had no effect. 

Chris Davies has worked with Belron since 1994 as an academic consultant on a variety of projects including many relating to windshield repair. Since 2002 he has been employed by the company as its innovation manager, responsible for developing new technologies.

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