Standards Development

Scope

This document specifies a mechanical performance testing procedure for cover glass used in automotive display modules. The document focuses on the quantification of surface fracture stress after flaw introduction via Oscillating Taber abrasion similar to ASTM F735. After abrasion, the retained surface fracture load is measured with the method documented in ASTM C1499 and converted appropriately to failure stress. The failure stresses are plotted using a Weibull distribution for communication of strength statistics. NOTE: The actual display product strength required to survive an impact test is largely dependent on the construction, supporting structure, its deformation while in use as installed into a vehicle. It is recommended to quantify stress in a glass cover lens component using commercially available FEA software to understand what stress a glass component may need to survive. The strength evaluation results obtained by the test procedure described in this document may not directly correlate to the durability of a final display product as installed in the cabin compartment.

Purpose

As the automobile market evolves, OEMs are looking to further differentiate themselves by providing a
unique interior experience, which now includes large glass displays. Because these large displays tend to extend from behind the steering wheel, into the region of the center console, and sometimes
extending fully across the dashboard, the display module and cover glass are subject to global motor
vehicle safety standards, including FMVSS201 (US), UN/ECE-R21 (EU), and others. These regulatory
tests are for occupant safety and simulate head impact events, where cover glass performance and
system-level architecture play important roles for successfully meeting the requirements. This includes
acceptable acceleration of the head-form, but also that the glass survives without breakage. The stress on the glass imparted by the head-form impact test (HIT) can be significant.
Quantifying the stress on the cover glass component in HIT can be done by using commercially
available Finite Element Analysis (FEA) software, where the model would also incorporate a full
display, dashboard, and all mounting components. By some accounts the surface stress on the glass
can be upwards of 1GPa. This may not be representative of all cases but shows the importance of
knowing the strength of a glass component that is being supplied for a display subject to HIT. Common
strength tests for glass include concentric ring-on-ring (ROR) for surface strength and four-point bend
for edge strength. It should also be noted that the strength of glass can be influenced by many factors
including a length, area, or volume effect, an environmental effect, and a loading rate effect. It is
recommended to be aware of all test conditions and test implications when comparing strength
distribution results.
Glass strength is controlled by flaws that are generated during glass processing and subsequent
handling, which for the surface flaws, may be on the order of 5-10 µm in depth. Breakage of the cover
glass during HIT would force an OEM and/or supplier to redesign their product, mounting structure, and or components. Therefore, it is important to have high confidence that a cover glass will pass HIT
without breakage. Achieving high strength in glass can be realized by multiple means including
mechanical polishing to reduce flaw depths, chemical etching to blunt atomistically sharp flaw tips, or
introducing a compressive layer, by thermal tempering or chemical tempering, to be overcome first
before the flaw sees tension. In all cases the strengthening approach can be rendered ineffective if
downstream processes introduce similar and/or more severe flaws prior to processing.
Knowing the flaw distribution on a surface of glass is difficult if not impossible, especially when the
sizes of these process flaws are not visible through optical inspection methods with the human eye.

Other optical, non-destructive methods have been devised to try and identify small flaws but they are
also size limited and require high precision components. Therefore, it may be suitable to evaluate a
cover glass’ strength across a range of flaw depths that may be representative of going through a
supply chain. A glass showing high strength in this type of “retained strength” test may yield a higher
probability of survival during HIT. Controlled abrasion can be used to distribute representative flaws
across a glass surface then tested in ring-on-ring geometry to evaluate the remaining glass strength.
This retained strength distribution can then be used, in combination with FEA-modelled stress
magnitudes, to quantify likelihood of glass survival in high energy impact events with high confidence.