Optomechanics – Zero Chance for Failure Over 15 Years

Dear Colleagues:

I had one terrific Summer.  I hope you enjoyed it as much as I did.

One highlight of the Season was the analysis of a meter-class optical window to assure safe operation over a 15 year service life in transport-type aircraft.  Safe means zero chance for failure over 15 years (no Weibul statistics for these folks!).  Fortunately, they had selected a glass for which there is good engineering data, i.e. the fracture toughness, the crack propagation rates and the stress-corrosion effects of moisture had been well defined and quantified.  My job was to define the proof test for the new windows that would assure the product safety for 15 years and another proof test to be used at the periodic reinspections.

The procedure is a deterministic one.  The engineer integrates the crack propagation velocity repeatedly over the time of the stress transient until the crack grows to critical size at which time complete fracture occurs instantaneously.  I use MSC/Nastran finite element code to define the stress transient and I use AEH/Jade brittle fracture software to perform the numerical integration.  AEH/Jade allows me to easily vary the initial crack size and the integration time step.  The latter allows me to demonstrate convergence in the time domain which may be very important in complicated stress transients.

Once an acceptable initial crack size has been determined the defining proof test becomes a piece of cake.

Well, that was one of this Summer’s adrenalin highs.

I’m sorry for having been away so long, but I was having way too much fun!  More later, I promise.

Look out!  All Hallow’s Eve awaits you.

Al H.

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