Optomechanics* – Jade Brittle Fracture Analysis Tool

*A note from Teale Hatheway: As I pored over Dad’s emails to organize and archive AEH’s Optomechanics Newsletter, I came across this gem which, dated 7-18-18, was never shared. I also discovered an abundance of conversations sparked by these communications. It’s no wonder Dad enjoyed his work well past “retirement age”: your camaraderie combined with his intellectual pursuits gave him great joy. He would’ve called it “sport.” With well over 100 Optomechanics Newsletters published since 2006, the experience of compiling these brought me renewed clarity on Al’s life work. From AEH product announcements, to carefully worded client stories, to occasionally revealing his trade secrets, I hope you will continue to enjoy his wit and his wisdom here… So here we go! The last Optomechanics. I sure hope he finished editing it. Enjoy!

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Colleagues:

Put fracture mechanics to work for you!

Fracture mechanics says that glass is sensitive to static stress corrosion fatigue effects.  Glass parts have finite fatigue lives when operated in a normal moist environment.  If the fatigue life, T, is defined as the time to fracture under the conditions of service, that life may be calculated from

noting that KIi is the initial stress intensity, KI is the instantaneous stress intensity, KIc is the fracture toughness, v is the instantaneous crack growth rate and the local stress, s, is time-variable and it is inside the integral.

AEH recruits our Jade Brittle Fracture Analysis Tool, to perform the numerical integration and determine T, the fatigue life.  For Jade it’s a piece of cake!

For instance, take this thermal-structural stress transient for an aircraft’s window requiring 10,000 flight hours of service life:

FULL STRESS CYCLES AT FRACTURE= 3563
TIME TO FRACTURE= 44896587.75
SECONDS= 12471.274375 HOURS
CRITICAL INITIAL TENSILE STRESS= 55002270.5466022
INITIAL CRACK DEPTH= .000149

If the engineer applies a reasonable factor of safety, say 1.5, to the critical initial tensile stress (the ultimate load) it will provide a proof test load (82.5 MPa in this case) that will assure the safety of the window throughout its service life.

There’s no way to get there with closed-form solutions and it’s way too big for an Excel spreadsheet.  Jade goes through the complete calculation in just seconds, allowing the engineer to try various combinations of design variables to optimize a safe concept.

Now available from AEH.

Al H.
7-18-18

Optomechanics – Fabricate Glass to Meet Proof Test Requirements

Colleagues:

Well, AEH has had another full year with no broken glass!

We do this by defining, for the glass fabricator, a proof test that his product must pass to meet the project’s service life requirements.

The strength of glass is knowable to the engineer if the glass suppliers will provide some simple fracture properties for their glasses:  the critical stress intensity factor and two or three points on the stress corrosion curve.  The glass suppliers tend to not publish these data.  So most of AEH’s successes are steering designs into using optical glasses for which the data have been published (or using that client’s proprietary data).  The balance of AEH’s successes have used larger factors of safety with conservative estimates of the glass’s fracture properties.

In either case, AEH feeds the service conditions (the thermal-structural dynamic stress profile and the initial surface crack size) into Jade with the appropriate fracture properties to analyze the service life of the glass product.  The independent variable is the initial surface crack size.  With the acceptable initial crack size determined AEH then designs a static proof test for the glass fabricator to demonstrate that the glass product will meet the required service conditions.  The glass fabricator may then design the fabrication process (grinding and polishing) to meet the proof test requirements.

There are some disappointed glass suppliers but it’s their choice whether (or not) to publish the fracture properties of their glasses.

Spring arrived right on time.

When the safety of glass is at risk AEH has these tools too!

Al H.
3-28-18

Optomechanics – 2017 In Review

Colleagues:

Ah… That Joyous Season is upon us once again.

In the Hatheway household it runs from about now ’till Twelfth Night, our daughter’s birthday.  We’re careful to let Guy Fawkes Day slip quietly by before we start to celebrate.  (We’re all for Parliament but burning effigies is a real downer.)

And, what a grand year it is shaping up to have been (yes, in the future perfect tense).  Speaking of which…

AEH fixed the wedge caused by thermal gradients….
… witnessed the solar eclipse in the Alleghenies…
… chaired a conference in San Diego….
… was snowed in Massachusetts…
… and got lost in Irvine.

That last one was a real embarrassment since your author was raised about 20 miles from there. But that was before Irvine “grew up.”

Ivory and Jade are having a terrific year also, assuring the performance of systems from zoom lenses to gimbal controls and assuring the safety of critical lenses and windows by helping designers select suitable materials and specify proof tests.  No broken glass so far.

So, here’s a wassail to you, one and all, for your companionship and cheerfulness throughout the year.

God Bless you, Tiny Tim!

And everyone else, Enjoy!  Here we go-o-o-o-o-o…..

Al H.
11-10-17

Optomechanics – Using CodeV Prescription in Ivory and Jade to Find Structural and Thermal Weaknesses

Colleagues:

I’ve been known to lecture my students and colleagues on the need to keep their tools sharp.  Some time ago AEH was invited to a design review as an observer and since I had no direct participation I sat at the back of the room, behind John, the systems engineer who was controlling the projector.  The technical sessions went well but about half-way through the schedule and budget sessions he suddenly blackened the screen and turned on the overhead lights.  He slowly turned and surveyed those of us sitting behind him.  His gaze settled on me!  “What, John?” I asked.  He stared at my hands which were holding my pen knife and its sharpening steel.  “Just keeping my tools sharp,” I declared sheepishly.

One of AEH’s sharpest tools, other than a pen knife, is Ivory’s Optomechanical Modeling Tools.  It’s been under continuous development incorporating many of my personal insights working as a mechanical engineer in the optics industry.  I recently put together an updated version and released it to all users of Version 3.  That’s another way I keep AEH’s tools sharp (and protect AEH’s Ivory subscribers, too).  Ivory is AEH’s prime tool for engineering thermally and structurally reliable optical systems.  It’s designed to work in both Excel and Nastran and its application early in the design process prevents much embarrassment and saves many labor-hours from preventable failures that may occur later in qualification tests and service.

Somewhat more recently AEH was invited to participate in a “Tiger-Team” review of a sub-contractor.  The initial issue was broken glass.  The first thing I did was get a copy of the physical optical prescription (CodeV) and read it into Ivory (for the structure) and Jade (for the broken glass).  I could then quantitatively infer where the principal structural and thermal weaknesses might be.  With that insight I was able to form an independent assessment of the completeness of the design team’s engineering effort, which undergirded my report to the prime contractor.

I hope to see all of you at SPIE’s Optics+Photonics in San Diego come August.  I’ll be teaching (Optomechanical Analysis), chairing (The Optomechanical Engineering Technical Group and Optomechanics 2017), presenting and publishing (on a new diffraction grating capability in Ivory) and begin planning our next SPIE Conference (Optomechanics 2019).

That also keeps AEH’s tools sharp.

Hasta luego, caiman.

Al H.
6-5-17

Colleagues:

At AEH we study closely the ways things fail.

It is often said that an engineer’s job is to make things work.  Well, that’s nice.  Tinkerers can do that too.  What’s really required from engineers is to make things work every time.  That’s a little different discipline.

So, as engineers AEH studies how things fail in order to better know how to prevent bad things from happening:

In optical systems, virtually all “optical” failures result from some defect in the mechanical implementation.  These failures are never corrected by changing the optical prescription.  Well, almost never:  There was the Hubbell primary mirror fix.

Optomechanical problems are best spotted early, while the design resources (size, shape, mass, arrangement, interfaces, etc.) are malleable.  Those resources can quickly become depleted, even unavailable, as they are claimed by other interests:  bearings, servos, electronics, cryogenics, subcontractors, etc.

Early detection requires special tools for the optomechanical engineer.  To assess optomechanical problems the optics and mechanics must be coupled by the engineer, hopefully from the first publication of the prescription, perhaps during the proposal effort even.

The results of this early optomechanical coupling may only be estimates, but they’re essential.  They give the engineer who uses them a sense of how to guide the design to his desired…, no, to his required destination:

Spot-on performance with a trouble-free service life.

Early assessment of optomechanical problems is one way we help our clients.  AEH has the tools:  longhand, ten-key, spreadsheet and Nastran. We’ve got all that plus Ivory, Ebony and Jade to interpret the optomechanics for you.

Of course, we can often help after problems materialize and the corrective options have become more restricted.

Joy to all for the Autumn season.  It has arrived!

Al H.
10-6-14

Optomechanics – Keep Your Tools Sharp

Colleagues:

If spherical surfaces didn’t make pretty good images our optical industry would be entirely different.  As befits a technology that basically works as intended, cliches and rules-of-thumb perform a yeoman’s service.  And they work!  I’m glad that many of you enjoyed my parable about “kinematic” mounts.  Well, that is, they work until they don’t, as in that misunderstanding between the laser physicists and the mechanical engineers.  Thanks for all of your comments.

More recently I’ve been inspired by some of my students to publish the optomechanical influence coefficients of diffraction gratings (i.e., the ratios of a spectrum’s motions to the grating’s motions).  Gratings are often simulated as mirrors.  But the grating’s influence coefficients differ slightly from the mirror’s and there are more of them.  I’ll present my results in Mark Kahn’s conference, “Optical Modeling and Performance Predictions VI,” at SPIE’s meeting in San Diego this August.

Imaging spectrometers (using gratings of course) are particularly challenging to the optomechanical engineer because the images of both the far-field object and the near-field slit (the spectrum) need to be stabilized simultaneously on the detector plane.  The slit operates as a field stop and the two images behave somewhat (and sometimes importantly) differently.  “Mining” the resulting “data cube” requires close registration between the spectrum and the far-field object’s image.  The grating will work in my Ivory Optomechanical Modeling Tools software.

In San Diego I’ll also present a paper in my own conference, “Optomechanical Engineering 2013.”  This presentation will describe the use of my Jade Optomechanical Modeling Tool.  Jade models the subsurface cracks induced by grinding and polishing.  I use it to engineer, for structural safety, components made of glasses, ceramics and other brittle materials.  As an example I’ll show how I applied Jade to meter-class optical windows for a civilian transport-class aircraft.  The windows have been in service for years.

Engineers develop tools to keep themselves out of trouble.  In the public works domain these have developed into codes and standards that engineers are obligated (by their insurance companies) to follow.  Elsewhere, engineers develop tools for themselves.  In optomechanical engineering there are few rules of thumb to help.  There are, however, a few cliches.

Summer is coming!  Get out the sunscreen and water skis again!

Hasta luego, caimán.

Al H.
6-18-13

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.
9-24-12