# 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 – So Many Tiger Teams

Colleagues:

Hold that Tiger…

I told you a while ago about attending a Massachusetts “TigerTeam” review of an external store for the F-16.  It took three days of talking about everything else to finally get to the crux:  High thermal gradients in the slab made the cavity unstable!  Surprise, surprise!  After the final session I got a tour of the brassboard’s laboratory.  The mechanical engineer confided to me that the brassboard would not work when it was up-side-down (talk about a tender cavity structure for a tactical aircraft application).

I’ve also told you about the two day “Tiger Team” in Texas for a laser, the brassboard of which worked just fine but the flight system lost power when a laboratory door was opened or closed.  In the final session the mechanical engineers described how the system’s flexure mounts for the cavity had been replaced (since they were not space-rated) with hex-head bolts!

Then there was the four-day “Tiger Team” in Illinois that reviewed the flight tests for a two-color recce system.  When I asked to see the imagery they declined and when I pressed for some quantitative data on the image quality they said they were unable to measure it.  And that was their “improved” design.

And the Culver City “Tiger Team” that couldn’t get their encrypted light back into their fibers.  The optical designer and I both stumbled onto the fix for that one at the same time.

Well, the Massachusetts folks managed to pull it out and Culver City folks went into production in Texas.  As “Tiger Teaming” goes two out of four ain’t bad.

…and, here comes 2018.  Hold onto your hats too!!!

Al H.
1-5-18

# 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 – The Industry Needs to Know the Strength of Optical Glasses

Colleagues:

Fall is coming in two weeks and the Great Pumpkin is right down the road.  It’s time to kick-back and just look around a bit at this Glorious Summertime…

I’ve had about four weeks on the road lately, touching base with all of you (or most of you, I hope).  Then I cruised around SoCal in my spare time chinning and jawboning with the local industries’ participants.  So, what are the “hot items” for optomechanical engineers today?  Well, how ’bout…

precision dimensioning and tolerancing of optical structures
repeatability and stability of instrument performance
broken optical glass

I’d have trouble identifying the most important of these, it all depends upon what’s happening NOW.  All three of these have kept me pretty busy since Valentine’s Day.  The greatest uncertainty for the engineer, however, is in the strength of optical glasses.  Its not just mirrors and windows.

I’ve participated in two projects in which the optical design had to be revised (compromising performance) to replace glasses that could not pass the environmental tests.  Perhaps, someday, the glass houses will start to provide strength information on their data sheets near the Young’s modulus and Poison’s ratio.  Today the engineer can’t be sure until the tests are passed.

And we, all of us, managed to squeeze-in a terrific conference with SPIE in San Diego last month (32 papers published).  How we were able to pull that together, I just don’t know.  But, it would never have happened without you and the terrific staff at SPIE who keep us on the right track.

Here comes Guy Fawkes Day. Get your bonfires, and marshmallows, ready! All our prayers this day are with those who may be in harm’s way.

Al H.
9-7-17

# Optomechanics – Poor Optical Performance is a Mechanical Problem

Colleagues:

Hey!  It’s been one Heck-of-a-Year…

…horrible images, tricky windows, cracked laser rods, soft adhesive, savvy clients, a challenge from the nanoradian and writing a book-and-a-half.  What’ll it be next?

Our industry is terrific fun.  Optics is an art that was just meant to work, until it doesn’t.  Then the solution is almost always mechanical:  a longer stroke on the focus mechanism; find a way to install a “corrector” lens; fold the system here or there; find a new adhesive; brace the objective doublet; stiffen the cold finger, control-grind and polish the edges, etc.

And that’s all as it should be.  Once the optical designer finishes his design it’s virtually impossible to improve it.  If that were not true the optical designer would probably not stay employed at the firm.  All deviations from the prescription are the mechanical engineer’s contribution.  Poor performance is therefore a mechanical problem; something is improperly located, dimensioned or deformed and the solution is also mechanical, correcting the mechanical engineer’s positional or dimensional specifications or stiffen the structure.

The errors are almost always detected after the system is assembled and that’s what makes it so much fun.  The mechanical engineer has to think ahead and analyze ahead to avoid the problems in that first assembly.

And that’s why we have a conference in San Diego this August with SPIE, to share with each other and our colleagues the discoveries and methods for making optical systems work in spite of it all!

Attached is the Conference Announcement.  Just click on “submit an abstract” on the bottom of the second page to reserve your position in the Program.

Look out 2017, here we come!

Al H.
12-28-16

# Optomechanics – Align the Image Plane over the Entire Detector Array

Colleagues:

I mentioned earlier that there is a great deal to optomechanics.  Even full time practitioners can lose track of the current scope of the optomechanical arts.  And with a well corrected optical design the onus falls to the optomechanical engineer “to make it all play.”

I was working with another dear friend shortly ago.  He was contemplating a compound hyper-spectral imaging system and wondered if I knew anyone who could help stabilize the instrument.  Of course, I immediately raised my own hand.  He said that I didn’t understand:  The entire image plane had to stay in strict alignment over the entire detector array all the time.  He behaved incredulously but, generously, heard me out.

I explained:  AEH’s Ivory Optomechanical Modeling Tools operate over the entire exit window (field-of-view on the image plane, if you prefer).  They tell the optomechanical engineer what’s going on at the edges, the corners and the center or anywhere else in the image.  The Tools can do this because their Optomechanical Constraint Equations determine the alignment of the entire image plane (position, orientation and size) over the entire detector array.  He said, “Show me.”

So, I used the Ivory OMT to model the image’s corners, edges and center in shock, vibration and thermal environments.  Initially the image’s stability over the entire detector array was barely marginal.  But with a little tweaking of the structural design, informed by the influence coefficients from Ivory, we got it within specification with a comfortable margin.  This system flies today with confidence.

Just Bridging the Chasm one more time.

I hope you all enjoyed a Great Thanksgiving. It’s down-hill from here to 12th-night!

Cheers.

Al H.
11-29-16

# Optomechanics – Implementing Intentions

Colleagues:

Ok, ok.  I left you hanging.  But, in spite of that, a number of you offered your opinions ranging from the Hawthorne Method to Witchcraft!  It turns out that some of you studied Industrial Engineering while others studied Theology!  What a group.

Well, the engineering lesson I thought I learned (or actually re-learned) was,

“Just because it’s in the drawings doesn’t mean its in the hardware.”

I cut my teeth as a Liaison Engineer in a factory fixing the butches that factory workers create when assembling airplanes at Boeing’s plant in Seattle.  That’s a whole different industry but whether it’s optomechanical or aeromechanical, the principle is the same:  Someone has to assure that the engineer’s intentions are implemented.  In our culture that task usually falls to the Quality Assurance organization.  Gene took up that challenge.  And, it felt just like Boeing’s factory, except for the photons.

Thank you for hanging-in-there with me.

Enjoy early Autumn.

Al H.
9-20-16

# Optomechanics – How to Overcome Resistance from Management

Colleagues:

There is so much more to optomechanics than meets the eye (or ear or touch, even).  I learned a great optomechanical engineering lesson from a Quality Assurance engineer!

Gene was recruited (and offered a “bounty”) by a major manufacturer of lasers for printers to improve the quality of their products, ~1/3rd of which were failing in the 1,000 hour burn-in test.  Gene, in turn, called me in to supplement his electronics industry experience.  (Full disclosure:  We had worked together at one of our previous employers.)

We ran into resistance from both the Mechanical Engineering management and the Factory management.  The mechanical engineers would not let me “review or analyze” any of their design work and the factory would not let me “observe” their operations during working hours.  Hmmm….  Both Gene and I were dumbfounded.  But, I must say that Gene was a resourceful Devil.

He invited me to coffee at a nearby bistro and swore me to secrecy.  Then he laid out his plan:  He would dress me in a white QA smock, give me a clipboard, a jeweler’s loupe, a marking pen and spools of red, yellow and green adhesive dots.  I’d spend most of the day in his office but I’d go out to the factory floor on coffee breaks and lunch times.  I’d cruise the floor, stopping at empty work stations and pretend to inspect the stations, one at a time, applying the loupe and the colored dots (annotating a few, especially the red ones) while making notes on the clipboard.  At the end of each break I’d return to Gene’s office and hide-out.  It took about three days to cover each of the work stations twice.  Then he released me telling me to “disappear.”  I went back to AEH and made myself busy elsewhere.

The next I heard from Gene was a phone call from Norwood, Massachusetts.  The plan worked spectacularly, he had collected his “bounty,” left that company and moved with his family to more stable employment in Norwood.

The laser manufacturer is gone and I think Gene would approve of me sharing his secret with you now.

The lesson?  I think you got it.

Summer’s over and it’s downhill to the Great Pumpkin .  . Shhhhhhhhh!

Al H.
9-13-16

# Optomechanics – Forensic Observation on Shock Test

Colleagues:

I was peering into the AEH archives the other day and got a shock.  I needed the source code that I use to analyze the effects of a variety of shock pulse shapes.  What I found was a complete set of mechanical engineering drawings for the medium-weight hammer blow shock test machine at the Hughes-Fullerton facility!  John Martin, who ran the test lab, gave them to me after AEH succeeded in getting a client’s Optical-Nav system qualified for Navy shipboard minesweeper service.

AEH’s job was to specify the shock isolators.  To understand the problem I had prepared a massive Nastran model of the system (6 feet high, two feet square and 550 pounds) mounted on an array of Aeroflex wire-rope isolators.  The numbers had said it should pass the shock test, but it failed.  AEH was called to the lab.  The bolts holding the isolators had stripped the threads in the isolator flanges!  I spent lunch-time with the Nastran output file reviewing the loads and forces and couldn’t make sense of the results.  The client thought that larger/stronger isolators were required.

I went back to the test lab and re-inspected the failed isolator mounts.  I found that the threaded ends of all the failed bolts were flush with the inside surface of their isolator flanges and that all of the un-failed bolt-ends protruded into that sway-space by about 3/16ths of an inch.

I returned to the Nastran file, looking at displacements in stead of forces.  It predicted that the isolators would use virtually all of their sway-space.  The failed bolts had actually been pushed out when the isolators prematurely bottomed on the bolt-ends, not pulled out when the isolators stretched to their limits.  The bolts were too long.  Larger isolators were not necessary.

One fix was to put extra flat washers under the heads of the bolts so their ends would not protrude into the sway-spaces.  With that accomplished we returned to test the next day and PASSED!

That was AEH’s second successful adventure in John’s shock-test lab.  He graciously offered AEH the drawings for his test machine and invited me to present a paper at the annual conference of the Institute of Environmental Sciences, of which he was President.  I accepted both.

Engineers need to keep their forensic skills sharp, too!

Thank you John Martin!

Al H.
6-1-16

# Optomechanics – An Engineer’s Tactile Sense

Colleagues:

It’s 2016 so, here we go….

For starters, I moved the AEH offices again.  See the new contact info below.

Now, I’ve told you the story about the number of rivets in the Queen Mary.  Well here’s one about my calibrated thumb.

Early in my career my boss, Wilford, invited me to the environmental test laboratory.  We found a space payload mounted on the shake table.  He turned to me and asked, “What’s the fundamental resonant frequency of this beast?”  I turned to go up to the Structures Department but Wilford called me back.  He said something like, “We’re the mechanical engineers and should be able to get a pretty good handle on this right here.”

Wilford asked me, “How much do you think it weighs?”  I estimated the dimensions, studied the construction and made a guess.  He raised an eyebrow, but nodded.

Then Wilford walked around the payload and pressed on it several places then beckoned me over suggesting I press on it, which I did.  He asked how much it had moved.  I hadn’t noticed the motion so I pressed again and told him how much motion I had observed.  He asked how hard I had pressed but I had no idea.  He took me to the Inspection Department and had me push on the scales with the same force I’d pushed on the payload.

We went back to Wilford’s office and on his whiteboard he wrote “2 x pi x f = (k/m)^.5,” “k = force/motion” and “m = weight/gravity.”

The next day the test engineer reported that Wilford’s resonant frequency estimate, f, was 3% high.  Not bad.

My estimate wasn’t as good as his but I learned a lesson:  It’s useful for an engineer to maintain a tactile sense of the magnitudes of forces.  I cater to my right thumb.  What about you?

One more tool to keep sharp in 2016.

As I said up-top… “Here we gooooooooo!”

Al H.
1-5-16