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 – Tensile Strength of Glass

Colleagues:

OK, AEH hit you in May you with analyzing the tensile stresses in cemented glass doublets.  Then AEH hit you in June with analyzing the tensile stresses in ring-mounted glass lenses.  And you responded with numerous queries about AEH’s analytical methods… but they were surrounded by a massive silence.

No one asked, “What’s the tensile strength of the glass?”

It’s a characteristic attitude in the optical industry, that the strength of glass is not its problem, “Leave that to the structural engineers.”

Well, surprise!   The structural engineers don’t care either.   Glass, being brittle, is in no-way a structural material.  Structural engineers design with ductile materials like steel and aluminum.  The only brittle material they routinely design with is concrete and they insist that it never see tensile stresses.  That’s why they invented pre-stressed concrete: to eliminate the tensile stresses!

The tensile strength of a glass is determined by its “fracture toughness,” a material property of the glass that can be measured in a test lab and is repeatable.  However, among the silica glasses the values for fracture toughness appears to vary, based on the very limited data available, by factors between 3 and 5, depending upon the specific glass composition.  It makes a big difference which glass is used in high-stress situations.

Ah, just one of the challenges
(and one of the joys!)
of optomechanical engineering.

Al H.
7-2-18

Optomechanics – SOLVED: Elastic Theory’s Differential Equations for the Tensile Stresses in Glass Lenses Mounted in Threaded Metal Rings

Colleagues:

AEH has solved elastic theory’s differential equations for the tensile stresses in glass lenses mounted in threaded metal rings, and it’s good news.

Paul Yoder had originally proposed Delgado and Hallinan’s 1975 solution (Opt. Eng. 14) but their solution gave very high tensile stresses in the lenses, high enough that virtually all such lenses should have fractured.  None of my ring mounted glass lenses had ever suffered that fate.  I surveyed a number of my colleagues and none of them recalled a ring mounted glass lens fracture.

Delgado and Hallinan’s work was flawed.  To correct their flaw would require a new solution to the equations of elasticity that honored the appropriate contact geometry.  AEH finally made it happen and the result is surprisingly simple,

s = p(1-2u)/b,

where s is the peak tensile stress, p is the linear ring load, b is the radius of the contact ring and u is the Poisson’s ratio of the glass.  This stress is three-to-four orders of magnitude lower than that predicted by Delgado and Hallinan.

Using Nastran AEH was also able to verify the general shape of the stress distribution in spite of Nastran’s notorious difficulty at the point of load application.

Closed-form solution       >>>         Nastran solution

To learn more you have choices:  Either download AEH’s peer reviewed paper from SPIE [Optical Engineering 57(5), 055105] or go through the gory details with me in my tutorial,

“Optomechanical Analysis,”
SPIE’s Optics and Photonics Symposium in San Diego
8:30 AM to 5:00 PM on the 21st of August.Cheers!

Al H.
6-6-18

Optomechanics – Quick Checks for Stress in Glass

Colleagues:

Well, Ok.  It’s not that AEH hasn’t seen broken glass this past year, it’s just that it hasn’t been AEH’s glass that broke.  Cemented doublets were the principal excitement.  AEH’s research indicates that, for a quick check,

tensile stress in the glass =~ (E1 + E2)/2 x (alpha1 – alpha2) x deltaT

and

shear stress in the adhesive =~ 2/3 x tensile stress in the glass.

If stresses are marginal the engineer may then want to adjust for the edge thicknesses of the lenses and the Poisson’s ratios of their glasses.  The peak shear and tensile stresses occur at or near the edges of the lenses.  The only dimensions that influence the stresses are the edge thicknesses.  Center thickness and diameter have little influence on the stresses at the edges.

Does it all seem spooky?  Well, I’ll take you through the gory details in my tutorial,

“Optomechanical Analysis,”
August 21st in San Diego at SPIE’s Optics and Photonics Symposium.

And, I’ll toss in, just for you, the latest details on the stresses in ring-mounted glass lenses including a close-form solution and a finite element simulation!

I’ll see you all in San Diego.  Bring your sun-screen. Al H.
5-4-18

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 – Zoom Lenses

Colleagues:

Zoom lenses are getting hot again.  They seem to come in cycles for AEH.

There was the 20:1 MWIR zoom with a 300 mm entrance pupil.  It was a lot of fun!  Good for a mechanical engineer to cut his teeth on too.  As tough as it was, it actually inspired much of AEH’s optomechanical engineering practice.

The 8:1 visible microscope zoom for medical diagnostics was an application that brought its own challenges.  Using commercial optical assemblies where only the focal lengths were known required some lab work to find their principal points.  Those of you who’ve taken my short course in optomechanical analysis or used my Ivory Optomechanical Modeling Tools will appreciate that need.

Then there was the 10:1 NIR zoom with 36 individual images, five moving lens groups and two working distances.  And it had to be packaged into a convenient instrument case.  AEH used a customized version of the Ivory Optomechanical Modeling Tools to make that one happen too.  One of the advantages of having the source code is that I can make it do whatever is needed at the time.  And the current version’s users receive the upgrades as I develop them.

It’s a New Year with new opportunities and new challenges.  As I said last time,

Here we go-o-o-o-o-o…….

Al H.
1-9-18

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

Colleagues:

Joy to all, and thank you for your awesome support of our Optomechanical Engineering 2017 Conference in August.  Just wait ’till you see the program.

Now, back to business.  Fractures in glass optics seem ubiquitous.

So… the optomechanical engineer has a problem.  He’s nominally responsible for protecting the glass elements in the optical systems he designs.  But there’s very little (if any) information available on the structural properties of the glasses that optical designers specify.  And the guidance for the engineer on using the available data is all-over-the-map, from the incomprehensible to the impossible.  No wonder most engineers use simple “rules-of-thumb.”

That’s where I started, “Keep the tensile stresses under 2,000 psi.”  But then the glass broke anyway!  So I started testing the glass objects to 4,000 psi.  I broke a few in testing but those that went into service are still in service, as far as I know.  I didn’t get to make many, they were too big and heavy.  A colleague who specialized in space-based ISR systems confided to me that he kept the stresses under 500 psi!  That’s when another colleague gave me a copy of “Reconnaissance and Surveillance Window Design Handbook” (AFAL-TR-75-200).

Voila!  Section 7.3.1 is the perfect introduction for the engineer to “Allowable Stresses in Glass.”  It covers fabrication process controls, slow crack growth through stress corrosion (from moisture) and estimating the service life by integrating the stress corrosion equations for eight glasses.  The Wizard’s green curtain is drawn back disclosing all of his secrets and Dorothy dances down the yellow brick road and back to safety in Kansas.

Every optomechanical engineer needs a copy of that Handbook to help him protect the optical glass that has been entrusted to him by the optical designer.  It guides the design, analysis and fabrication of glass optical elements.  Perhaps the engineer should enter the required structural properties, including the fracture toughness and stress corrosion constants, on the lens drawings (think the yellow brick road).  The Handbook’s drawback is that only eight glasses are treated and some of those have since been re-formulated to remove toxic elements.

The glass suppliers also need copies of the Handbook so they’ll know what the engineer is requesting and, maybe someday, put the information in their glass catalogs and data sheets.

Joy!  Spring is just around the corner.  Ahh… Kansas in Springtime!

Al H.
3-10-17

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 – Bridging the Chasm

Colleagues:

Ah yes, there is a great deal to optomechanics!

And so much of it is on the hairy edges of, slightly beyond, or in-between the academic disciplines that we study at the university.  The engineering challenge is to make it work anyway.  In my practice I refer to it as “Bridging the Chasm” or B/C.  It involves surveying both sides of the gorge between disciplines before building on them.  That’s just good engineering practice.  It almost falls into Systems Engineering… almost.

One example of B/C is a dialog I had a while ago with a dear friend who was concerned about the fracture of glass lenses mounted in metal compression rings.  Structural engineers were not interested in glasses.  The opticians were disinterested in elastic analysis.  I had to sit down and solve the differential equations of the problem myself (Proc. SPIE, 7424-09. 2009).

They showed that the actual tensile stresses were four orders of magnitude smaller than those predicted by the traditional method (Delgado and Hallinan, Opt. Eng., 1975).  That “bridge” between the disciplines for metals and glasses has saved my clients’ buttons more than once.

Another B/C example was a modification to an unstable EO weapon sight… but that’s enough about me

“The time has come,” the Walrus said, “to talk of many things…”

Yes, the great Optics+Photonics 2017 Roadshow (Symposium) is returning to San Diego this coming August and the Optomechanical Technical Group is organizing an exciting two-or-more day conference to highlight your accomplishments this past year.

You all have your own experiences and stories.  Bring them to your Optomechanical Engineering 2017 Conference.  It’ll be a great show!

“…of shoes and ships and sealing wax, of cabbages and Kings.”

Here’s hoping I’ll see all of you in San Diego come August, at least those who survive the Great Pumpkin.

Al H.
10-24-16