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The time-dependent phenomenon of delayed elasticity was studied at room temperature and is presented here for Zerodur and ULE. The effect is believed to be related to the alkali oxide content of the glass and to the rearrangement of the ion groups within the structure during stress.

A failsafe design of a BK-7 glass window for commercial aircraft use is presented which meets the stringent requirements of the Federal Air Regulations. This may be the first ever commercially approved all glass design for aircraft use in the USA.An all glass design is often essential for both visible and near infrared photography in order to meet the stringent specifications and resolution demanded by high acuity optical systems. However, glass is subject to slow crack growth and static fatigue in the presence of moisture and tensile surface flaws. Exposed surfaces on aircraft are subject to scratches from handung and cleaning, as well as impact due to hail, windblown sand, and high altitude, high velocity airborne dust. Additionally, defects are present due to the manufacturing process itself. Such flaws may grown quickly in the presence of high stress, as induced by cabin internal pressure, thermal gradients and soaks, aircraft racking loads, and aerodynamic pressure.In order to yield an acceptable design, a dual pane glass concept is presented which exhibits a safe life in excess of 10,000 hours and is also failsafe in the unlikely event of a catastrophic failure of one of the panes. Reliability of the design is based on extensive coupon testing by the National Institute of Standards and Technology, validating crack growth parameters and survival probability. These coupons were subject to induced scratches, hail, sand, and dust impact (performed by General Research Corporation). A full-scale failsafe test is also made to show the ability of the dual pane design to withstand maximum loading in the event of a single pane failure.