[reaperman]

> That thin glass cracks under thermal stress should be no great surprise.

Actually, the thinner the glass is, the less likely it is to crack due to thermal gradients. It is thicker glass which experiences greater stress. The equation for thermal stress is:

thermal_stress = (coefficient of thermal expansion) ∙ Young’s modulus ∙ ∆T

which for glass, works out to about 0.63 MPa per 1 degC temperature difference between the centre and edge of the glass. Thinner glass has less temperature difference, so lower stress. Furthermore, surface roughness and type of imperfections lower the amount of stress needed to cause a failure, so Apple's obsessive polishing makes the glass withstand higher stress than usual.

[brookst]

Thanks for the info and model! But if the thin glass is bonded to the aluminum frame by glue, would that not change the calculus? If we imagine hot air on one side of the thin glass and cold aluminum on the other?

I’m very skeptical of the “charging thermals cause cracks” theory, seems more likely that minor manufacturing defects might do so when the headset cools from a session. But I suppose we’ll hear more than we want to in the next few weeks.