Thermoelastic Forum Vol.1, No.9, March.
1997
First Words
R&D Side
New Products
Tech Tips
University Corner
Events
Tid Bits
TSA Primer
In this issue of the Thermoelastic Forum the temperature rises in the
"R&D Side" as Jon Lesniak describes simple measures to keep
the DeltaTherm1000 working at peak performance while applying TSA at temperatures
as high as 1000°C! The "University Corner"
highlights TSA experimentation on a new class of CMCs by T.J. Mackin and
M.C. Roberts of the University of Illinois at Urbana-Champaign. Mike Zickel
"puts Krylon to the test" as he rates the emissivity of six different
types of flat spray paint coatings in the "Tech
Tips" section. The "Events"
column lists four conferences that are sure to interest TSA users. You can
see a live DeltaTherm demonstration and try out the latest version of DeltaVision software for control, acquisition and post processing of TSA data at two of the four conferences. Get a "bit" of information about SP's latest SBIR and STTR contracts in the "Tid
Bits" section of this newsletter.
If you're still hungry for TSA information after reading this issue of the
Thermoelastic Forum (or if you just want to see cool TSA data images), join
the thousands who have already visited the Stress Photonics WWW site at
http://www.StressPhotonics.com. The Stress Photonics home page is continuously
updated so, even if you have already paid us a visit, be sure to stop by
again for the latest in example applications, accessories, products and
new Thermoelastic Stress Analysis
developments.
Elevated Temperature Camera Operation
by Jon Lesniak
As a result of high-temperature testing under the current Air Force SBIR contract, Stress Photonics has had the opportunity to gain more experience applying TSA at temperatures as high as 1000°C. Not every engineer needs to test under these extreme conditions, but most engineers do run into situations, such as a transmission case, that require testing a heated component. In this article, I will describe the simple measures that will keep the DeltaTherm1000
operating at peak performance at any temperature without saturating the
camera. (There are other calibration issues that will be discussed in future
articles.)
The number of photons emitted from a warm body increases with temperature.
At 50°C the flux in the 3-5 mm band nearly doubles. At 100°C it
is nearly an order of magnitude higher. To understand the effects that this
may have on the camera we need to understand the basic design of the IR
camera head. The camera collects electrons generated from each photon strike
and stores them in a capacitor known as the quantum well. The depth of this
well is about 40 million photons. When the capacity of this well is saturated
it is not possible to measure a differential thermal signal and all is lost.
The camera empties these wells at the framing rate of 434 Hz. or about every
2 ms. As seen in figure 1, DeltaTherm is set up to handle temperature excursions
of up to 50°C without saturating. At temperatures above 50°C, an
attenuator must be used to deal with the abundance of photons.
There are several options available to avoid camera saturation. Similar
to a photographic camera, the amount of light striking the film or detector
can be attenuated by changing the aperture stop, using a faster shutter
speed, or using a neutral density filter. The impact of apertures and neutral
density filters are dictated by a very important difference between visible
optics and infrared optics. A physical aperture or filter emits IR radiation
by the virtue of its own absolute temperature. The radiation it emits will
not carry useful signals but will still contribute to measurement noise.
By decreasing the exposure time or the integration time, one can control
the abundance of photons without introducing non-signal photons. With the
electronic iris feature the sample integration time can be controlled from
2.3 ms to 36 ms.
As exemplified in figure 1, a 100°C component temperature would require
an aperture attenuating all but 5% of the photon flux. Roughly half the
well will be filled by the flux emanating from the filter itself, so only
half the well capacity is left to be utilized by the component flux. At
extreme temperatures, filters and apertures can be applied if the electronic
iris is first reduced. This is because the emissions from the aperture are
reduced to an insignificant level by the electronic iris.
If you would like more information about elevated temperature testing, please
contact Stress Photonics Inc.
Elevated Temperature Accessories
Stress Photonics is introducing a new line of DeltaTherm1000
accessories to assist elevated temperature applications. Depending on your
application and temperature range, several products are available including
screw in apertures, filters, and an electronic iris or "shutter speed
control" (as described in the R&D side
article). These products will improve moderately elevated temperature
work and facilitate work at temperatures in excess of 1000°C.*
The screw in filter holder mounts to the back of most standard IR lenses
supplied with the DeltaTherm and can be used to hold neutral density filters
as well as physical apertures. The holder kit comes with several apertures,
with neutral density filters available for an additional cost.
The electronic iris is part of an electronics upgrade package available for the DeltaTherm1000. The electronic iris allows more efficient use of the sensor's photon wells (as discussed in "R&D Side") and has been integrated into the DeltaVision control, acquisition, and post processing software as a simple slide bar setting. The electronic iris will allow TSA measurements up to 300°C without optically limiting the flux. Example applications include testing of drivetrain components in operation and material testing at elevated temperatures.
With the use of the electronic iris, filters and apertures, the DeltaTherm
is capable of taking TSA images at temperatures upwards of 1000°C (see
cover article).
For questions or comments about these products please contact Stress
Photonics.
*Extreme temperature work may require the use of Stress Photonics' Stealth Furnace.
Coatings
by Mike Zickel
Choosing and applying the proper coating for thermographic analysis (TSA or standard thermography) is
a simple but important step in acquiring accurate and reliable data. In
the last issue of the Thermoelastic
Forum (Vol. 1, No. 8, Oct. '96), Dan Bazile described general characteristics
of good thermographic coatings and provided some tips on applying a coating.
Flat black spray paint has been mentioned as one of the best types of coatings
for thermographic analysis because it is highly emissive (non-reflective)
and easy to apply. TSA users prefer to use Krylon Ultra Flat Black over
any other kind of flat black spray, claiming that its "ultra"
low reflectivity makes a difference when measuring small temperature changes
on an oscillating sample. We decided to put Krylon to the test.
An aluminum bar was prepared with various types of flat spray paints and
then uniformly heated to a few degrees above room temperature. A room temperature
image was subtracted from the "hot" image and the resulting image
(fig. 1) represents a measurement of the relative emissivities of the different
paints. Each type of paint is labeled in the image. A thin region of bare
aluminum was left between each stripe to set them apart and to provide a
highly reflective surface to contrast the low reflectivity paints.
A profile plot along a line drawn vertically down the image produces the
graph shown in fig. 2. The plot indicates that all of the paints are relatively
close in emissivity, with Krylon and Tempera having the highest response.
In a standard thermographic test any of these coatings (excluding the Rust)
would probably be sufficient. In a thermoelastic test, where the sample
is oscillating, it is very important to use the coating with the lowest
reflectivity (i.e. greatest emissivity; where emissivity + reflectivity
= 1), because it is more difficult to account for oscillating reflections
that might not be apparent in a static test. Estimating the average values
for each coating and doing a simple analysis reveals that TrueTest, Rusteolum
Black and Model Black are at least 11% more reflective than the Tempera
and Krylon. This may not seem significant, but when you consider that temperature
changes on the order of 0.001°C are being detected in a typical TSA
test, then even the most innocuous seeming reflection can corrupt your results.
The relative emissivity of the Tempera paint is a surprise. It has the advantage
of being non-toxic, water soluble and easily removable; however, it is more
difficult to spray onto a sample. Just the right amount of water needs to
be added to make the paint sprayable, but not so much that it is too watery,
causing drips.
by T. J. Mackin and M. C. Roberts of the Department of Mechanical and
Industrial Engineering, University of Illinois at Urbana-Champaign
A new generation of relatively "ductile" Ceramic Matrix Composites
(CMCs) is being developed to meet the demands of increased operating temperatures
in energy conversion systems. These new materials rely on inelastic mechanisms
such as interface failure, matrix cracking, fiber failure, and fiber pullout
to redistribute stress away from locations of stress concentration. Experiments
at The University of Illinois at Urbana-Champaign are being carried out
to determine the mechanistic underpinnings and the absolute extent of stress
redistribution in this new class of CMCs.
A DeltaTherm1000 system is being utilized to assess damage initiation and evolution in several cement-based and oxide-based ceramic fiber reinforced composite systems. The cement based systems are aimed at intermediate temperature applications (up to 600°C) while the oxide composites are envisioned for high temperature engine applications (>1000°C). Tensile specimens are fabricated with edge notches, figure 1, and pulled to various percentages of the sample ultimate strength. Thermoelastic stress maps are made of the specimen at each damage level, and used to qualitatively assess the extent of non-linearity and stress redistribution in the samples. Figure 2 shows a sequence of three damage states in an alumina fiber reinforced alumina matrix composite. This figure reveals shear bands propagating perpendicular to the notch plane, effectively blunting the stress concentration at the notch tip. Line scans between the notches are normalized by the far field TSA signal and used to quantitatively assess the change in stress concentration factor at the notch root, figure 2. These data are useful in determining the operative damage mechanism (in this instance, shear bands) and in measuring the effect of that damage mechanism.
AIAA Structures Conference
The 38th AIAA Structures, Structural Dynamics, and Materials Conference
and Exhibition will take place in Orlando at the Orlando Hyatt Resort, April
7-10. Visit the SP booth in the exhibition hall to see a live DeltaTherm demonstration and try out the latest version of DeltaVision software for acquisition and post processing of TSA
data.
ASCE Structures
Congress XV
The American Society of Civil Engineers is sponsoring the 15th Structures
Congress to be held in Portland, April 14-16 at the Portland Hilton Hotel.
Jon Lesniak has been invited to present a paper describing Stress Photonics'
progress in Forced Diffusion Thermography of structures.
Thermosense XIX
Thermosense XIX: An International Conference on Thermal Sensing and Imaging
Diagnostic Applications is set within SPIE's Aerosense '97 Symposium, and
takes place in Orlando, April 20-25. Stress Photonics will be presenting
in the NDE technical session. The conference site is Marriott's Orlando
World Center.
SEM Spring Conference
The 1997 Spring Conference and Exhibition will be held in Bellevue, WA,
June 2-4 at the Red Lion Hotel. Lesniak with co-authors Bazile and Zickel
anticipate presenting papers in the Thermal Methods Technical Division and
in Applied Photoelasticity. Brad Boyce will chair the Thermal Methods Round
Table Discussion.
The SP booth in the exhibit hall will feature the latest instrumentation and software advancements SP has to offer.
Stress Photonics has been awarded two new research contracts for new
product development. The first contract is a Phase II Small Business Innovative
Research (SBIR) grant from the Federal Highway Administration (FHWA). The
second is a Phase II Small Business Technology Transfer (STTR) grant from
NASA Langley. Each contract has a two year duration closing in early 1999.
The Phase II SBIR with the FHWA began in late September of 1996 and is aimed
at producing an easy to use bridge inspection system for use on the many
aging steel bridges in the US. The research will focus on implementing a
thermal method based on Forced Diffusion Thermography to inspect bridges
and other large steel structures efficiently. The envisioned single operator
unit will be hand held with an easy interface into current bridge inspection
reporting systems.
The Phase II STTR is a contract from NASA to SP and The College of William
& Mary. The STTR program, introduced by Congress in 1994, differs from
the SBIR program in that offerors must be teams of small businesses and
research institutions who will conduct joint research. The goal of the program
is to transfer technology developed by universities and federal labs into
the private marketplace through the entrepreneurship of a small business.
In the spirit of the STTR, Stress Photonics and W&M will jointly carry out the development of new research which will focus on combining TSA with photoelasticity for full field stress imaging. W&M will concentrate on new coatings useful for TSA and photoelasticity.
If you need more information, or if you would like to keep up with these
and other new developments, check out the SP WWW site over the coming months
at http://www.StressPhotonics.com.
Thermoelastic Stress Analysis (TSA) produces a full-field stress map by imaging temperature changes with a sensitive infrared camera. All materials, whether solid, liquid or gas, change temperature when compressed or expanded. In solids, stresses cause small temperature changes described by the thermoelastic equation
To provide accurate measurements, the temperature changes induced by
the thermoelastic effect are repeated and time-averaged during a continuous
dynamic loading, usually provided by a closed-loop hydraulic load frame.
A special infrared camera, known as a differential thermographic system,
correlates the load-induced IR signals with the reference signal from the
load system. This allows a thermal resolution of 1.0mK, which translates
to the following stress resolutions:
Stress sensitivity is similar to that of a common strain gage.
