Cumulative Damage to Window Panes

Three types of tests were employed: static tests, fatigue tests, and tests inside the Sonic Boom Test Facility (SBTF). Uniform loads across the glass surface were applied to glass panes in all tests. In the static tests, the panes were subjected to increasing pressure until they failed (typically between 30 and 90 sec loading). Fatigue tests sinusoidally loaded the panes at 3 Hz frequency until they failed or 24 hrs had elapsed (259,200 cycles). SBTF tests involved exposing the panes to up to 5000 sonic booms or until failure, Static tests were used to establish the strength of panes under a variety of conditions and to assess the variation of the glass strength for any particular condition. This included assessing the strength of new glass, after aging, with different edge conditions, flawed glass, and after other tests. Fatigue tests allowed testing at a significant fraction of the anticipated breaking strength and loading the panes more slowly than in the SBTF. These two factors increased the chance of breaking and, thus, of detecting glass fatigue behavior. The SBTF provided the most realistic simulation of repetitive sonic booms and, thus, the most credible demonstration of the importance of cumulative damage to the assessment of sonic boom damage to glass.
Prepared in cooperation with BBN Acoustic Technologies, Canoga Park, CA.