Standards Development

Scope

This document specifies/describes test methods and procedures to evaluate the cover lens component
used to protect the display surface by characterizing the strength of the cover lens, taking into account
two of the major sources of breakage:
• surface strength
• edge strength
The test results are limited to evaluating the strength as a component, of the cover lens alone. The
strength evaluation of a fully assembled display within the vehicle cabin is out of scope of this
document. This is because the overall strength an assembled display cover lens may depend on
multiple variables and components of the display system. The structural features supporting the cover
lens and installation into the vehicle cabin are very influential and dependent on each vehicle design.
This document does not cover the evaluation of strength of finished display product and is therefore not suitable to be uniquely used as proxy to quantify the display susceptibility to impact test.

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. 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.
Cover lens components are exposed to manufacturer-specific production processing damage, which
may create surface flaws impacting strength. Thus, informative material is provided in a supplementary
annex for convenience.

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) (U.S. Department of Transportation, 1998), UN/ECE-R21 (EU) (Official Journal of the European Union, 2008), and others. These regulatory tests are for occupant safety and simulate head impact events, where cover glass performance and systemlevel 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 (Pisipati, 2014). By some accounts the surface stress on the glass can be upwards of 1GPa (Layouni, 2019). 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 (ASTM C1499, 2023) and four-point bend for edge strength (ASTM C158, 2023). It should also be noted that the strength of glass can be influenced by many factors including a length, area, or volume effect (Wereszczak, 2010) (Quinn, 2003), an environmental effect (Wiederhorn, 1967) (Wiederhorn, 1974), and a loading rate effect (Ritter, 1971) (Aaldenberg, 2016). 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 (Karlsson, 2010) (Haldimann, 2006) (Layouni, 2019). 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 (Kist, 2016), chemical etching to blunt atomistical sharp flaw tips (Sglavo, 1993), or introducing a compressive layer, by thermal tempering (Gardon, 1980) or chemical tempering (Gy, 2008), 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 (Chen, 2015) (Sakata, 2014) 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.