Safe Drive Away Time (SDAT)
The windscreen and adhesive industries have invested significant dollars and man hours to establish the science of 'safe drive away time' (SDAT) says the Auto Glass Association (AGA).
Almost a third of a vehicle's structural strength is derived from its windscreen. That’s' why extensive research and development goes into determining the SDAT, which specifies the period of time a vehicle should be out of service after having a new windscreen fitted.
AGA president Rick Janssen comments, "Striking a balance between safety and the vehicle owner's desire to get back on the road is an auto Glass industry science, and like so many facets of the industry it's one that's constantly evolving ."
To a large extent a vehicle's total restraint system, which includes airbags, seat belts and belt tensioners, relies on the integrity of the windscreen and the adhesive bond securing the glass to the vehicle.
In a front end collision where occupants have not used their seat belts, the windscreen supports the inflated airbags and restrains the passengers.
"Quite literally, the lives of a vehicle's occupants can depend on the integrity of the windscreen and its correct installation in an accident. Not only do installers need to follow installation procedures, they must also use the right products and stick strictly to all the instructions from the manufacturer, Janssen says.
"With the dynamic and competitive nature of the market there is a constant quest for new direct glazing adhesives and processes that result in faster setting and more accurate SDAT recommendations."
The Australian Standards AS4739:2017 Direct glazed automotive glass replacement—Light vehicles lists that a direct glazing adhesive system shall meet or exceed the frontal impact test requirements of FMVSS 212 to establish safe drive away times.
The current FMVSS 212 standard refers to the windscreen mounting and therefore relates to the direct glazing adhesive.
Tests show airbags are completely inflated about 30 milliseconds (ms) after deployment and the front seat passengers make contact with the airbags about 50ms after that start of a crash. The entire crash typically takes about 100ms.
"Given the complex variables, several direct glazing adhesive manufacturers saw the need to develop a reliable method of determining SDAT. Advancements in Finite Element Modelling (FEM) and computer simulation of FMVSS 212 meant the forces involved in a crash could be understood, including the crucial interaction between the vehicle occupants, airbags, seatbelts and windscreen,".
It is understood the forces on the windscreen during a crash can be categorised as (listed in the sequence they occur):
1. Inertial forces of windscreen mass
2. Forces from the pressure increase in the passenger compartment during deployment of airbags
3. Forces transferred to the windscreen via airbags from the impact of the passengers
"To better understand crash dynamics, computer simulations can accurately determine the direction and level of forces transferred to the windscreen adhesive from the passenger via the airbag and windscreen. High impact testing and 'real-life' crash test give the final green light for newly developed products.
Direct Glazing Adhesive manufacturers achieving the test certificate from an accredited test authority provides all stakeholders the confidence a particular direct glazing adhesive meets the requirements of FMVSS 212 (crashes vehicles at 48km/h) or Euro NCAP (crashes vehicles at 64km/h) at the tested SDAT.
There are some assumptions and considerations that factor into a SDAT recommendation. Ambient temperatures during the application and curing of the one-part direct glazing adhesive can also affect the SDAT. Installers should always check with the adhesive supplier or the technical data sheet for the SDAT on the particular product they are using as not all products are the same. Usually, a direct glazing adhesive is not fully cured when the SDAT is achieved. However, it has developed a level of strength sufficient to meet the particular test standard requirement.
ABOUT THE BEAD
It is also important to note that achieving the required SDAT and weathering performance is reliant on the correct application of the adhesive to achieve an optimum bond.
The use of a properly shaped adhesive bead is critical when installing replacement auto glass.
Most direct glazing adhesive manufacturers recommend the use of a triangular (V) bead because it is the only shape that guarantees no air bubbles can get trapped between the fresh adhesive and the glass or pinchweld when the glass is placed into the vehicle.
As the glass is installed, the peak of the triangle is the first point of contact. As the glass is lowered into place, the top of the triangle is pushed down into the adhesive bead. This forces the top of the bead to bow out as the glass is lowered further. Since the adhesive is being pushed out from the centre, the contact moves from the centre to the final edge of the bead. Air is forced away from the bead and prevented from becoming trapped.
Also, many direct glazing adhesives are fast skinning and using a triangle (V) bead helps a lightly skinned product to break and optimise “wetting” of the DGA to the other substrate.
Square beads can present problems when the nozzle is cut poorly. If any dip is present in the centre of the bead, the outside edges will make contact with the glass or pinchweld first. This can create an air pocket that can result in leaks or adhesive failure in the event of a crash.
A similar problem exists with round beads. If the bead rolls as the glass is installed, or uneven pressure is placed on it, an outside edge can make contact with the bonding surface before the centre of the bead, creating air pockets.
The following questions should be asked and answered by the installer before starting a replacement:
- Do I understand the data that my supplier has provided?
- Am I clear on the airbags installed in the vehicle?
- Will the weather impact my process?
- Do I trust the published test certificate SDAT information that protects the safety of my customers and my business? Is the test certificate from the manufacturer and from an accredited test authority?
It’s important to use a respected direct adhesive suppliers’ product and follow their application procedures as they will have performed a tremendous amount of lab testing, engineering and crash testing to verify its SDAT recommendations.