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3D Printing and Structural

Analysis: Is There an Alternative

to FE Analysis for Quick Design

Info & for FEM Validation?

• FW Palmieri, Ph.D.

• 3/24/2014

Copyright © 2014 Raytheon Company. All rights reserved.

Customer Success Is Our Mission is a trademark of Raytheon Company

AIAA Orange County Chapter

11th ASAT Conference

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In spite of the advances we have made in recent

years using computer technology to improve the

accuracy of stresses predicted to occur in our

aerospace structures, and in the process improve

structural efficiency and hence reduce weight, we

have not really gained much in terms of the lead time

associated therein with completion of the analyses. In

fact, in many instances our lead time from receipt of

the design/CAD details to output of information to the

designers has actually increased along with the

associated analysis costs.

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This has been justified by the fact that we have

increased the aforementioned structural

optimization (i.e., less weight).

Yet: "... recent studies have shown that,

surprisingly enough, modern methods do not do a

better job of predicting failure of the resulting

designs, as shown by recent Air Force data." This

means that, although we may have reduced

weight, we have not improved our capability for

predicting failures.

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There exists an idea for a process improvement

that would potentially produce a shorter interval

between receipt of the design and evaluation of

its structural integrity, at least as far as strength

under static or dynamic loads or stiffness is

concerned.

What is that idea?

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This idea is to simply utilize the 3D Printing design specimen

as a basis for modern photoelastic evaluation techniques.

These specimens are currently being developed as standard

practice in many industries today, including the aerospace

industry.

The PhotoStress® sheet material is cast on a flat surface and

then applied to any flat, single or doubly contoured surface of

a test item and bonded thereupon. The test item can then be

subjected to static and even dynamic applied loads and

modern photoelasticity techniques can permit the accurate

evaluation of stress fields arising in these plastic, metallic or

composite specimens through optical interference patterns

and stroboscopic technology.

PhotoStress® is a registered trademark of Vishay Precision Group

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The following material regarding the definition of 3D

Printing, the various processes and the materials that are

involved therein have been obtained with permission from

the web site*:

http://www.additive3d.com/rp_int.htm

entitled: “A Brief Tutorial – What is Rapid Prototyping”

*Worldwide Guide to Rapid Prototyping web-site

(C) Copyright Castle Island Co., All rights reserved.

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3D Printing is now the most common name given to a host of

related technologies that are used to fabricate physical objects

directly from CAD data sources. These methods are unique in

that they add and bond materials in layers to form objects. While

3D printing has displaced rapid prototyping for top honors, that

term is still quite popular. Such systems are also known by the

names: additive manufacturing, additive fabrication, solid

freeform fabrication (SFF) and layered manufacturing and many

others. Today's additive technologies offer advantages in many

applications compared to classical subtractive fabrication

methods such as milling or turning:

• Objects can be formed with any geometric complexity or intricacy without

the need for elaborate machine setup or final assembly;

• Rapid prototyping systems reduce the construction of complex objects to a

manageable, straightforward, and relatively fast process.

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This has resulted in their wide use by engineers as

a way to reduce time to market in manufacturing, to

better understand and communicate product

designs, and to make rapid tooling to manufacture

those products. Surgeons, architects, artists and

individuals from many other disciplines also

routinely use the technology. With the advent of

low-cost and open-source systems hobbyists and

consumers are also now using additive

technologies in substantial numbers.

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Additive methods aren’t a solution to every part

fabrication problem. After all, CNC technology is economical,

widely understood and available, offers wide material selection and

excellent accuracy. However, if the requirement involves producing a part

or object of even moderately complex geometry, and doing so quickly - RP

has the advantage. It's very easy to look at extreme cases and make a

determination of which technology route to pursue, CNC or RP. For many

other less extreme cases the selection crossover line is hazy, moves all

the time, and depends on a number of variably-weighted, case-dependent

factors. While the accuracy of rapid prototyping isn't generally as good as

CNC, it's adequate today for a wide range of exacting applications.

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The materials used in rapid prototyping are

limited and dependent on the method chosen.

However, the range and properties available

are growing quickly. Numerous plastics,

ceramics, metals ranging from aluminum,

stainless steel to titanium, and wood-like paper

are available. At any rate, numerous secondary

processes are available to convert patterns

made in a rapid prototyping process to final

materials or tools.

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The names of specific processes themselves are also often used

as synonyms for the entire field. Among these are stereolithography

(SLA for stereolithography apparatus), selective laser sintering (SLS),

fused deposition modeling (FDM), laminated object manufacturing

(LOM), inkjet systems and three dimensional printing (3DP). Each of

these technologies - and many others - has its singular strengths and

weaknesses.

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What is the ‘Modern’ PhotoStress® Methodology?

I have taken the liberty of extracting some of the

information presented on Vishay Precision Group’s

web site with their approval and included it in this

presentation in the following slides*:

(see: http://www.vishaypg.com/micro-

measurements/photo-stress-plus/category/photo-

stress-analysis-system/?subCategory=main-

benefits)

* Courtesy of Micro-Measurements, Raleigh, NC, USA

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So, what I have presented in the material we have just reviewed is a combination of:

1) the creation of rapid prototyping models, 2) using modern PhotoStress®

techniques, and 3) structural tests to obtain quick turn-around information on the

structural integrity of potential designs.

Of course, one could argue that it would take time and money to design and build a

test fixture and this would offset the potential savings. So, this trade-off would have

to be evaluated for each potential use.

It would be interesting to investigate the feasibility of the application of this technique

to the evaluation of the structural capability of a typical aerospace component and

compare the results with the cost and time that would be required for the structural

analysis by our conventional finite element method (as well as, perhaps, the

accuracy of the predictions).

It could also be useful for obtaining rapid validation of our analytic models (as shown

in one of the previous slides), something that ordinarily does not happen until after

the design is finalized, hardware produced and environmental tests conducted.

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I also received the following interesting comment from

Vishay’s Product Marketing Engineer, David England:

“What you didn’t mention, and may find interesting is that at

least two rapid prototype materials (clearvue and Accura 60)

are strain sensitive and bi-refringent as produced.

Unfortunately their K value or optical sensitivity isn’t enough

to be useful in practical applications. What would be ideal

would be to print an object, spray the “far side” with an

aluminized paint and use a reflection polariscope to study

the loaded structure. This would save the user from the

craftsmanship required in coating preparation.”

In the future, this method may prove to greater simplify the

whole stress/strain visualization process.

David England

Product Marketing Engineer