Optomechanics – Fracture of Cemented Lenses at Temperature

Colleagues:

I recently completed a job as an advisor to an airborne multi-spectral sensor system project.  My tasks included advice on lens mounting (metal rings vs. adhesive bonding), environmental sensitivities (temperature, vibration and shock), alignment procedures (tooling and adjustments) and manufacturing processes (bonding, glass finishing and failure analysis).

A major concern was the fracture of cemented lenses at high and low temperatures.  Understanding the physics of glass fracture was instrumental in both understanding the causes of the fractures of cemented assemblies and designing them to avoiding failures in the service environment.  The figures below show the analysis and test of a cemented glass coupon.  The contour plot shows the stress distribution in the test coupon and the photo shows the resulting de-lamination and fracture at low temperature.

In one of the finished cemented doublets, analyzed below, the fillet in Glass 2 needed special finishing to strengthen it against the stresses at low temperature.

This project was very challenging.  Great sport!

I hope your summertime was as interesting as mine. 

Look out, here come the Holidays.  Cheers!

Al Hatheway
10-12-07

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