The Science of SDAT
The Ins and Outs of Safe Drive-Away
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.”
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
• 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.
© Copyright 2012 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.