Thermoelastic Forum Vol.1, No.6, April
1995
First Words
The R&D Side
Tech Tips
Events
Tid Bits
There are a number of exciting news items to bring to your attention
in this issue of the Thermoelastic Forum. First and foremost, Stress Photonics'
new array based thermoelastic stress analysis camera is now being demonstrated
and offered for sale. The DeltaTherm 1000 is a high frame rate infrared
camera integrated with special signal processing electronics to produce
near-instantaneous thermoelastic stress images. The DeltaTherm system will
be on display at the spring SEM conference in Grand Rapids, MI the week
of June 12. Within the next few months, you will be receiving a brochure
explaining the capabilities of the DeltaTherm 1000.
As many of you know, the DeltaTherm system was developed by Stress Photonics
under SBIR funding from NASA-Langley's NDE Sciences Branch. NASA has joined
"forces" with Wright Patterson's NASP Joint Program Office to
fund continued development of the DeltaTherm system. The new Phase II SBIR
contract includes the development of a number of key applications: elevated
temperature, fracture mechanics, variable and impact loading, and composite
damage assessment.
Stress Photonics is offering a spring course on Thermoelastic Stress Analysis
(TSA) May 8-9. The "Tid Bits" column in this newsletter provides
further details, as well as highlights from last fall's course.
Also in this edition of the Thermoelastic Forum, the "R&D Side"
features an in-depth article on Stress Photonics' new DeltaTherm 1000, "Tech
Tips" shows you how to eliminate the "Zig Zags" from your
SPATE scans, and the "Events" column highlights last fall's SEM
conference and notes other upcoming conferences.
By Jon Lesniak
Several years ago, Stress Photonics was awarded a phase I, and subsequently
a phase II, SBIR contract from NASA-Langley to develop array-based Thermoelastic
Stress Analysis (TSA) technology. Stress Photonics is pleased to introduce
the fruits of this effort, the DeltaTherm 1000. This new technology has
provided image acquisition times of approximately 10 seconds while minimizing
the system's size, weight, and complexity. The short stress imaging time
is accomplished through a combination of array detector technology and high-speed
digital processing. The system provides near-instantaneous stress images
under most common structural loading.
The DeltaTherm 1000 can display near-instantaneous stress images, near-
instantaneous DC images, time-averaged stress images or time averaged DC
images on a standard video monitor. Key images can be transferred to a lap-top
control computer for storage and simple analysis. The images can be studied
in depth at a workstation with DeltaVision software. The rapid feedback
associated with the system's "video camera-like" ease of operation
will drastically improve operator efficiency as well as minimize training
time. The camera is focused by viewing the "live" DC image. The
desired image area is determined while monitoring the near-instantaneous
stress images. The final image is time-averaged and sent to the computer.
There are no sensitivities to set and phase can be set automatically.
The near-instantaneous images immediately reflect set-up errors before testing
time is lost. The DC images provide excellent documentation of the test
set-up as well as serving many other useful functions, such as thermal NDE
methods or standard IR camera operations.
Designing and building a TSA system is quite a challenge; approximately
100,000 background photons are emitted for every signal photon created by
the thermoelasic effect. Only the exquisite sensitivity of modern detectors
combined with sophisticated electronics and signal processing has made it
possible to design imaging systems that provide a useful level of thermal,
and hence, stress resolution.
The DeltaTherm 1000 employs an InSb focal plane array (FPA), which has the
best performance to cost ratio. The camera has 16,384 (128x128) detectors
working in parallel to reduce noise by staring at their respective sample
areas for the entire acquisition time, approximately 10 seconds. Special
high-speed digital electronics were developed to process the incoming signals
at frame rates up to 1000 frames/second assuring that the advantage of the
multiplicity of detectors is not lost in a signal processing bottleneck.
The array camera correlates the IR signals digitally described by the relation
where S is the amplitude of the signal, yn is the sampled signal, N is
the number of samples, and sinb(n) is a reference.
To increase dynamic range, a high-speed offset technique is implemented
where the offsets are calculated on a per detector basis and are updated
continuously.
The noise of the end measurement S as described by Eq. 1 is related to the
scan time by
This means that the image quality improves with time as described in
Fig. 1. The stress image is immediately apparent; the operator need only
wait the short time necessary to obtain the level of quality desired for
the job.
Fig. 1 Resolvable temp. vs time
A common question about the use of an array camera is what effect does the
detector to detector variance have on the stress image. Because the electronics
system has a map of the detector sensitivities, the variance can be eliminated
in the image processors. Detectors with substandard performance can be compensated
for with values from the nearest neighbors.
The DeltaTherm system is very robust; it is solid-state requiring no optomechanical
devices. The few optical elements required for a staring array make the
best use of the available IR radiation and makes adapting the optics for
special purposes easy. There already exists a wide variety of lenses for
the DeltaTherm system.
DeltaTherm is already proving great usefulness for product testing, fracture
mechanics and aging aircraft studies. Stress intensity measurements have
been demonstrated at both room and high-temperature.
TSA will take a giant step forward as a result of this new array-detector
technology becomes available. We think the capability to make near-instantaneous
full-field stress maps efficiently will make TSA an essential engineering
tool.
For more information about the DeltaTherm 1000, contact
Stress Photonics.
The latest release of the SPATE 9000 software has a new feature: unidirectional
scanning. Unidirectional scanning eliminates the jaggedness of the vertical
edges, common to most SPATE scans. The jagged appearance is caused by mirror
motor hysteresis.
The SPATE software directs the mirror motor to a certain point. Because
of the mirror motor hysteresis, the location of that point varies dependent
on the previous location of the mirror. When a mirror originating on the
left is commanded to go to the center, it settles slightly left of center.
Conversely, if a mirror originating on the right is commanded to go to the
center, then it settles slightly right of center. This phenomenon, coupled
with the SPATE's default bidirectional horizontal raster pattern, results
in vertical edges with a zigzag presentation.
The amount of mirror motor hysteresis often depends on the age of the equipment.
The prominence of the jagged edges is a result of the size and fineness
of your scan, whether or not you regularly smooth your scans and, of course,
how much hysteresis your horizontal mirror motor has.
The most palatable solution to the problem is unidirectional scanning, that
is, using a scanning raster pattern that always approaches from the same
side. Ometron can upgrade your software to the latest version, which would
allow you to collect all of your future scans with the unidirectional scan
feature. The alternative is to replace the mirror motors. This solution
is, however, quite expensive (approximately $9200.00).
If you would like to eliminate the jagged edges from scans that you have
already collected, we can help. Stress Photonics can write software that
will offset every other row of a bidirectional scanned image by a specific
number of DAC steps.
If any of these solutions sound appealing, call Stress Photonics and we
will be happy to suggest how they might be done or implement them for you.
"Integration of FEA and Structural Testing with Rapid Prototyping"
was the title for last fall's SEM conference. Stress Photonics exhibited
DeltaTherm images and DeltaVision software.
To encourage more industry participation, papers were not required for presentations.
There were, however, two presentations that dealt with thermoelasticity.
"Damage Tolerance Design Verification via Thermoelastic Stress Analysis"
by J.R. Lesniak, Stress Photonics Inc., A. Nagar, Wright-Patterson AFB and
B.R. Boyce, Stress Photonics Inc.
Lesniak presented a method of using Thermoelastic Stress Analysis to find
the stress intensity factor, KI. The technique can then be used to evaluate
how critical a crack is.
"Rapid Stress Analysis with Rapid Prototyping" by N.F. Enke, Ford.
Enke's work involved using TSA on a Rapid-Prototyped dogbone specimen. This
work is significant because a TSA design check of a Rapid-Prototyped component
could greatly simplify the design cycle.
On April 10-14, 1995, The American Institute of Aeronautics and Astronautics (AIAA) will be having its 36th Structures, Structural Dynamics, and Materials Conference at the Sheraton New Orleans Hotel. Stress Photonics will be demonstrating the DeltaTherm 1000 during a Technology Exhibit/Demonstration session. For more information contact AIAA, 370 L'Enfant Promenade, SW Washington, DC 20024-2518.
(part of SPIE's AeroSense '95)
The International Society of Optical Engineering (SPIE) is holding its Thermosense
conference April 17-21 at Marriott's Orlando World Resort and Convention
Center in Orlando, FL.
At the conference, Lesniak will be presenting "Differential Thermography
Applied to Structural Integrity Assessment" by Lesniak and Boyce for
the Thermosense working group.
Forced-Diffusion Thermography (FDT) is a nondestructive evaluation technique
that detects cracks and subsurface delaminations.
The DeltaTherm system is used for FDT much like it is used for TSA, but
in FDT the cyclic temperature change is caused by a projected heat pattern
instead of the thermoelastic effect.
The 1995 spring SEM conference, "1st International Conference on
Mechanics of Time Dependent Materials," will be held at the Amway Grand
Plaza Hotel in Grand Rapids, MI, June 12-15. As part of the Thermal Methods
session, two papers of
particular interest will be presented.
"The Use of Thermoelasticity in the Analysis of Fatigue Cracking in
the Web Gap of Steel Bridges" by M.J. Fleming, University of Wisconsin-Madison
and J.R. Lesniak, Stress Photonics Inc.
"Thermoelastic Analysis of Stress Concentrations in Thin Cylindrical
Tubes" by J.M. Dulieu-Smith, P. Panutas, University of Liverpool, P.
Stanley and C. Garroch, University of Manchester.
Don't miss the Thermal Methods Round Table jointly sponsored by the Thermal
Methods division of SEM and the SPATE Applications Working Group (SAWG).
Monday, 6/12, 8:30 - 9:50 a.m.
While you're at SEM, make sure to visit the Stress Photonics booth on the
trade show floor to see the latest in Thermoelastic Stress Analysis.
Stress Photonics TSA Course
In October 1994, Stress Photonics hosted a course entitled "Using Thermoelastic
Stress Analysis Effectively." The University of Wisconsin-Madison campus
was the site for the course, which offered classroom lectures, as well as
hands-on laboratory time.
The course was designed for both experienced and inexperienced SPATE* technicians
and engineers. Attendees learned ways to improve SPATE data, such as using
out-of-phase images to gage data quality. Advanced applications were also
discussed, including fracture mechanics and high-temperature work.
Stress Photonics will host a spring TSA course at the same location May
8-9. This course will be geared towards SPATE users as well. For those new
to TSA, an introductory course is in the works.
Those who took the course in the fall gave it rave reviews. Representatives
from Pratt-Canada, Association of American Railroads, Bell Helicopter Textron,
GM-Luxury Car, GM-North American Truck, Pratt-FL, Rockwell International
and Ometron all attended. Bill Shust of AAR said, "It was the best
course I've ever taken."
Come join us in Madison. We'll help you improve your SPATE'n skills, discuss
your toughest SPATE problems, and have fun showing you "MadTown."
Contact Stress Photonics if you would like to attend, and we'll send you
detailed course information.
