Texts on structural and mechanical analysis are numerous, and indeed, this
entire text is based on the pioneering works of others in the field. This book,
therefore, draws on those texts and presumes a working knowledge of the
strength of materials [see J. W. Pepi, Strength Properties of Glass and
Ceramics, SPIE Press (2014)]. With that foundation, we apply those
engineering principles to opto-structural analysis. In the precision world of
optics, we are often concerned with displacements and deformations of very
small values, from fractions of a wavelength of light to the micron and
nanometer (millionths of an inch) order. Furthermore, optical systems
designed for flight are often required to be of very light weight. While the
analytical techniques in any case are the same as on the macro-level, careful
analysis is required when moving the decimal point so far to the left.
In preparing to write this book, some thought went into the title. Before
selecting the term “opto-structural analysis,” an alternative term “optomechanical
analysis” was considered for the title. However, several excellent
texts under that latter title are available. It is certainly not the intent to replace
those worthy sources but rather to supplement them. To this end, the title
contains the term “opto-structural,” perhaps because the author is a structural
engineer, but more so, to point out the “static” nature of the topic. If
structural analysis is defined as applying to things that don’t move once they
are deformed, mechanical analysis as applying to things that move (such as
mechanisms), and dynamic analysis as applying to things that move slightly,
the title selection becomes more clear (although these latter topics are
discussed in the book).
This book is written with the intent to understand basic structural
deformation and stress analysis as applied to optical systems. It provides the
tools for first-order analyses required in the design concept phase before
entering into the intricate details of a full-up design. Ever-increasing computer
technology has allowed former tedious and unwieldy problems to be solved in
a fraction of the time by using finite element analysis. Unfortunately, reliance
on such fast methods without hand analysis backup can lead to unsuspected
errors. Thus, first-order calculations are an excellent way to complement the
current state of the industry that relies more on computational design
techniques. These calculations accelerate the design process by allowing an
understanding of the critical governing parameters and allowing accelerated
design trades and sensitivity studies to be performed that decrease schedule
and cost. The insights gained from these techniques can then be used to guide
the development of appropriate finite element models, including model
fidelity, and details focusing on the critical and most sensitive design
parameters. These models, in turn, are more efficient and provide the optostructural
engineer a comprehensive and insightful design approach. This
approach can then inform the roadmap for risk reduction and environmental
testing.
While finite element analysis is paramount to a successful design, the
purpose of this text is not to use finite element analysis to validate the hand
analysis but rather to use hand analysis to validate the finite element models.
The hand analysis forces a discipline that aids tremendously in the
understanding of structural behavior. It is the intent, then, not to forget such
techniques.
“Forsan et haec olim meminisse iuvabit.”1
1. From Virgil’s The Aeneid [translation: “Perhaps, someday, we will look back fondly on these
things.”]
John W. Pepi
October 2018
