The Ins and Outs of Safe Drive-Away Time

Original article from:

by Bob Stenzel

When your customer drives away after a windshield replacement, he is relying on your skills and attention to detail to provide a safe installation. As a shop owner or technician, you already understand the high level of responsibility required to perform a quality installation that will keep your customers safe in the unfortunate event of an accident. What you might not realize is the tremendous amount of testing and engineering that goes into providing a urethane adhesive system could also play an enormous role in the safety of your customer.

Safety and quality have always been the top priority for most companies and individuals in the auto glass replacement (AGR) industry. This dynamic and competitive market is on a constant quest for new adhesives and processes that result in faster and safer safe drive-away time (SDAT) recommendations. SDAT is defined as the minimum amount of time that a vehicle is required to remain out of service until the installed auto glass part can operate properly as a safety device.

History of SDAT
In the late 1990s, the Automotive Glass Replacement Safety Standard (AGRSS)™ was established by the AGRR industry through the fair and open ANSI process. The AGRSS Standard continues to be refined by input from the industry, all of whom support and follow these standards.

The AGRSS Standard (ANSI/AGRSS 002-2002) defines the minimum drive-away time to be “the time necessary for a given adhesive system to attain minimum drive-away strength after an adhesive bonded glass part is set in place,” where the minimum drive-away strength is “the minimum properties as defined and specified by the retention systems manufacturer or private labeler to meet the requirements of FMVSS 208 and 212.”

So how does a windshield adhesive manufacturer generate accurate SDAT charts for each adhesive formulation they offer?

In the late 1980s, several urethane manufacturers were on the cutting edge of innovation and technology and began to crash test vehicles according to U.S. Federal Motor Vehicle Safety Standards (FMVSS 212/208) to prove the reliability of their adhesive systems. These crash tests were often performed under severe climatic conditions to simulate mobile installations performed in outdoor elements.

In the mid-1990s, most adhesive manufacturers typically relied on simple adhesive strength data to promote the performance of their products. The strength information used initially was generated using a pseudo-static (slow speed) lap shear test method. While this was important information to acquire and analyze, it did not really portray an accurate account as far as determining precise SDAT. As air bags were introduced and mobile installations increased, the need for more precise recommendations and shorter SDAT also increased.

The windshield forms an integral part of the restraint system and is designed to prevent occupants from being ejected from the vehicle during a crash. To a large extent, the total restraint system, which includes airbags, seat belts, belt tensioners and knee bolsters, relies on the integrity of the windshield and on the adhesive bond that secures the glass to the pinchweld. In the case of a front end collision, where the occupants have not utilized the safety belts, it is the windshield that supports the inflated airbag and restrains the passengers.

As most windshield replacements are performed using urethane adhesives, ambient temperature and humidity play a large role in the ability of the adhesive to retain a recently replaced windshield during a crash. Cold and dry conditions will slow the cure of polyurethane, while hot and humid conditions will increase the cure speed. However, as uncured polyurethane is exposed to cold temperatures, it becomes more viscous, and therefore, much stiffer, potentially resulting in shorter SDAT due to added green strength of the chilled adhesive. Green strength is the uncured strength of a urethane.

“The total restraint system, which includes airbags, seat belts, belt tensioners and knee bolsters, relies on the integrity of the windshield and on the adhesive bond that secures the glass to the pinchweld.”

Determining SDAT
Airbags deploy at speeds of up to 200 miles per hour (mph) and exert tremendous force on the windshield. The adhesive then becomes a critical component of the retention safety system. Test measurements show that the airbag is completely inflated about 30 milliseconds (ms) after initial deployment (see diagram 1). The front-seat occupants of the vehicle begin to make contact with the airbag about 50 ms after the start of the crash. Typically the entire crash sequence takes about 100 ms and is literally over in a blink of an eye.

The size of the windshield and mass of the vehicle are of little consequence when compared to the force exerted by the occupants who impact the airbag.

Given all of the complex variables involved, several urethane manufacturers recognized the need to develop an alternative method to determine SDAT. Through advancements in Finite Element Modeling (FEM) and computer simulation of FMVSS 212/208, the forces involved in a crash could finally be understood; including the crucial interaction between the vehicle occupants, airbags and the windshield.

Through the use of these tools, it is now understood that the forces on the windshield during a crash event can be categorized as follows (listed below in the sequence in which these loads occur):

  1. Inertial forces of windshield mass;
  2. Forces due to the pressure increase in occupant compartment during deployment of the airbags;
  3. Load of passenger transferred via the airbag onto the windshield.

Both the inertial load of the windshield and the increased compartment pressure due to airbag inflation occur at the time of maximum deceleration, about 30 ms after the start of impact. These forces are applied at a separate time from that instant at which maximum load is realized (approximately 80 ms) due to an occupant coming in contact with the airbag. As a result, the inertia loading and pressure spike associated with deployment play a much smaller role in the total maximum force applied to the windshield.

In order to better understand these crash dynamics, computer simulations have been employed to accurately determine the direction and level of forces that are transferred to the windshield adhesive from the passenger via the air bag and windshield. After completing these theoretical (mathematical) evaluations, partial vehicle sled tests have been used to validate and compare to the computed values. These sled tests were able to validate that the results of the computer simulations were representative of a crash situation. Additional validation data was achieved from full vehicles used in actual FMVSS frontal crash tests.

The data compiled from these various inputs have provided very specific information regarding the strength and energy absorption requirements of the adhesive to retain the windshield during the crash test conditions set forth in the FMVSS specification. As it is not practical to repeat the FMVSS testing at the full ranges of environmental conditions, special customized equipment and test methods have been developed to perform these tests at the high strain rates identified in the computer simulations and confirmed using data obtained, while conducting both partial (sled) and full vehicle crash tests. It is this need for additional data, along with the realization that the test speed (strain rate) must match that of real crash scenarios, which spurred some adhesive manufacturers to design and develop high-speed tensile strength testing equipment capable of acquiring more than 30,000 data points per second.

The correlation of the lab-generated high-speed strength data for windshield adhesives with the results of full vehicle FMVSS 212 offers the final piece of the SDAT model puzzle. Some windshield adhesive manufacturers have invested significant resources in order to fully develop and validate this type of SDAT model. This allows the adhesive manufacturer to determine at various climatic conditions the amount of time that is required for each different adhesive formula to build up to a level of strength that is sufficient to meet the FMVSS 212 requirements for windshield retention. Only after generating and analyzing this type of high speed physical strength buildup test data against a validated SDAT model for a variety of climatic conditions, is it possible to create accurate and reliable SDAT charts that are ready to be published for a given adhesive formulation. The published SDAT matrix chart used to determine when a vehicle is safe to be driven can consist of many pieces of test data and some assumptions regarding seat belt use.

So the next time one of your customers drives his vehicle away from your shop or a mobile installation after waiting the appropriate amount of time as determined by the adhesive SDAT chart, realize that your adhesive supplier has performed a tremendous amount of lab testing, engineering and actual crash testing to verify their SDAT claims. If you are staking the safety of the customer and their family, your hard-earned reputation and possibly your business on the accuracy of your adhesive supplier’s SDAT recommendations, then you owe it to yourself to ask the following questions;

  • Do I understand the data that my supplier has provided to me?
  • Am I clear on the assumptions regarding seat belt use that has been made in the representation of the SDAT chart?
  • Do I trust that the published SDAT information protects the safety of my customers and my business?

Bob Stenzel is a senior auto glass replacement application engineer for Sika Corp. in Madison Heights, Mich.