Thermoelastic Forum Vol.1, No.8, Oct.
1996
First Words
The R&D Side
Tech Tips
UW Corner
Events
Tid Bits
TSA Primer
In the May issue of the Thermoelastic Forum, "First Words"
opened with "Things have been very busy at Stress Photonics this year."
When we consider everything that is going on now, we realize that last May
was rather relaxed. In August, Stress Photonics moved to a new, larger location
(refer to the "Tid Bits" column for our new address). We have
also recently begun a Phase II SBIR contract with the Federal Highway Administration
concentrating on detecting flaws in large steel structures (e.g. bridges).
This issue of the Thermoelastic Forum will fill you in on what's new at
Stress Photonics as well as what's new in Thermoelastic Stress Analysis
research and the latest in Thermoelastic Stress Analysis techniques. Be
sure to read our feature article on SP's new website! Then surf over for
a visit. Dan Bazile has some helpful hints for surface preparation when
using Thermoelastic Stress Analysis in the "Tech Tips" column.
In the "R&D Side," Jon Lesniak describes a new two-position
zoom lens arrangement for the DeltaTherm 1000 system. The "University
Corner" highlights TSA work using a DeltaTherm system by Professor
Rowlands and Brian Bauman at the University of Wisconsin-Madison. Read the
"Events" column to locate technical presentations on TSA and other
thermal methods. The "Events" column also gives details regarding
the upcoming Technology 2006 Conference and Exhibition in Anaheim, CA on
October 29-31 where you can see a live demonstration of the DeltaTherm 1000
system.
By Jon R. Lesniak
No matter what experimental stress analysis technique is discussed- moiré,
photoelasticity, strain gages or Thermoelastic Stress Analysis- all stress
engineers run into an application that pushes the limits of spatial resolution.
A materials researcher may be examining local stress distributions at small
damage sights. Designers might need to know the stress concentration at
a small fillet in a critical component.
All techniques have some limiting aspect whether it is gage size, coating
thickness or simply the practicality of applying necessary procedures at
a small scale. TSA has an advantage in that the only necessary preparation
is the application of a thin coat of spray paint. Because TSA is a simple
optical imaging technique it is able to image tight places like the edges
of small holes and inside corner fillets and notches. TSA's key limiting
factor is the effect of heat conduction; however, this can be controlled
by increasing the load frequency.
The big problem with most magnification techniques is that they are difficult
to use. "What am I looking at?" and "Is it in focus?"
are two questions often asked. To answer the demand for high resolution
optics yet maintain ease-of-use, Stress Photonics has designed a two position
zoom. A two position zoom is the simplest and the least costly zoom lens
design. In a two position zoom system, the optical system is effectively
flipped by moving a single element, hence, the reciprocal magnification
is achieved. What does this mean? Lets start with the fundamental concept
of an optical system. For each object to detector distance there is a position
that focuses an image of the specimen on the detector. By looking at the
undeviated ray that passes through the center of the lens, it is easy to
see from similar triangles that the magnification is equal to the ratio
of the distance to the detector from the lens divided by the distance from
the object to the lens. In this example, the magnification is 0.5.
For each object to detector distance, a given lens has only one other position
at which it can focus an image on the detector. If the lens is moved to
be a distance D from the specimen, it will be focused on the detector at
a distance 2D. The total distance S does not change but the similar triangles
have been flipped. Now by comparing similar triangles it is clear that the
magnification is 2.0. It is clear that by flipping the lens around the center
axes of the optical system, we can flip from a magnification of m to 1/m.
The detector size in a DeltaTherm 1000 is 6.4 mm, which means that the image
will be 12.8 mm x 12.8 mm in the wide position and 3.2 mm x 3.2 mm in the
zoom position. Considering the array has 128 x 128 detectors, the pixel
resolution in the zoom position is about 25 mm.
Stress Photonics has the opportunity to develop such a system with Air Force
SBIR funding out of Wright Patterson AFB. The optical system has been tested
on the bench and is now being packaged as a commercial product. The wide
angle view can be used to target the area of interest and to focus. With
the flip of a switch, an in-focus high-resolution image is available for
capture.
By Dan Bazile
All thermographic techniques are dependent on the optical and thermal properties
of the target surface. When TSA is considered, quality surface preparation
is the key to ensuring quality data. Usually a coating is employed to improve
the data collection. The coating should be
Although flat black paint is the most commonly used preparation, it is not
the only paint used for TSA. It is not the color so much as the emissivity
which affects the image quality. A flat paint, no matter what the color,
is superior to other finishes or bare metal. Most SPATE and DeltaTherm users
are quite familiar with Krylon Ultra Flat Black, which we recommend as a
coating for TSA imaging. There are several other coatings in use including
other manufacturers' flat black paints, ceramic sprays, and oxides. Water
soluble tempera paints have also been used successfully with only small
losses in image quality. Water soluble, environmentally friendly coatings
are becoming more important as regulations on the use of spray paints become
stricter.
Coating should be uniform and as thin as possible. Whatever coating is used,
don't apply the paint any thicker than required to get a complete and uniform
coating. Thick coatings can attenuate the TSA signal, thereby reducing the
quality of your results.
No runs, no drips, good data. Do not allow paint drips to form. Remember
you are trying to apply a thin uniform coat; if drips form you are applying
too much paint.
Spray from a distance. When done correctly, it is almost as if you are dusting
your specimen, allowing the paint to
partially dry in the air on the way to the specimen.
Take care in preparing the surface. Quality results don't just depend on
the coating itself. Dirt, corrosion and other contaminants left on the part
can seriously change the results of TSA images. Care should be taken in
cleaning, ensuring that previous coatings are not prohibitively thick and
are securely bonded.
Use caution in spraying one coating over another. Often times, the coating
you are applying acts as a solvent to the coating already on the object.
This can result in a poor coating which must be reapplied. If possible,
check compatibility on an extra part or an area you will not be imaging.
One last tip. There is some variance from batch to batch of any coating.
Small changes can effect the emissivity and reflectivity of the coating.
Look for changes in coating appearance by using a comparison bar or running
a quick test.
One simple test is to wave your hand in front of the specimen so that it
will reflect into the camera. If using a DeltaTherm, there should be no
change in the AC mode if the coating is acceptable. A flashlight or other
incandescent source can also be used to look for changes in the "Live
" mode. For SPATE users, the same test can be run by watching the detector
output on an oscilloscope and looking for changes in the output of the lock-in
amplifier.
By Professor Bob Rowlands and Brian Bauman of the University of WI
Mechanical Engineering researchers (Brian Bauman and Bob Rowlands) at the
University of Wisconsin-Madison, in conjunction with the USDA Forest Products
Laboratory (Ron Wolfe), are using Thermoelastic Stress Analysis to measure
the stresses in metal plate connectors such as those commonly found in wood
frame trusses. The following figures show a 3x5 in. commercial connector
plate butt-joining two standard 2x4's. The joint was incrementally loaded
longitudinally at 2 Hz to failure and the stresses were recorded with a
Stress Photonics' DeltaTherm 1000. Total scan time per image was approximately
5 minutes. Figure 1 shows the plate connector cycled between 500 and 2500
lb, whereas Fig. 2 shows the same metal connector near failure when loaded
from 1000 to 4000 lb tension. Most truss failures occur at a joint. Objectives
of this study are to develop improved design methods and to optimize such
connections, thereby saving both metal and wood.
The Seventh National Technology Transfer Conference and Exposition sponsored
by NASA , NASA Tech Briefs and the Technology Utilization Foundation will
be held in Anaheim, CA October 29-31. Mike Zickel and Dan Bazile will represent
Stress Photonics with a live table-top demonstration of the DeltaTherm 1000
in the SBIR Pavilion. The exhibit will feature all of the latest enhancements
to the DeltaTherm system and DeltaVision software.
A technical conference entitled Nondestructive Evaluation Techniques for
Aging Infrastructure & Manufacturing will be held in Scottsdale, AZ,
December 2-5 at the Double Tree Paradise Valley Resort. Stress Photonics
will participate in the technical session, "Nondestructive Evaluation
of Bridges and Highways," which will be in the Flagstaff Room on Wednesday
and in the Chaparral Room on Thursday. Jon Lesniak will present a paper
at the session entitled, "Structural Integrity Assessment via Coating
Tolerant Thermography" on Thursday at 4:50 pm. Phil Fish of the Wisconsin
DOT will present a paper in the "NDE of Steel Bridges" session,
and Conference Chairman Steve Chase of the Federal Highway Administration
will be giving the opening and closing remarks.
The 15th International Modal Analysis Conference will be held at the Sheraton
World Resort in Orlando, FL February 3-6, 1997. The conference will feature
a new theme entitled, "New Horizons for Damage Detection." IMAC-XV
has broadened its scope to include many interesting topics in Applied Modal
Analysis, Civil Engineering Structures and Damage Detection. The traditional
focus on automotive testing and analysis will be expanded to include the
aircraft/aerospace and marine industries as well as bridges, buildings and
other large civil structures. The conference, which is sponsored by SEM,
will mark Stress Photonics' first appearance in the Exhibition Hall at IMAC.
Brad Boyce and Brian Bartel plan on attending. Visit the SP booth and see
a live DeltaTherm 1000 demonstration.
We've Moved!
After doubling in size over the past two years, the Stress Photonics
office started to get pretty crowded! Our new expanded location is complete
with an optics lab, a shop, and an expanded production area. Be sure to
record our new information:
Stress Photonics Inc.
3002 Progress Rd. · Madison, WI 53716
ph 608-224-1230 · fx 608-224-1233
info@StressPhotonics.com
http://www.StressPhotonics.com
We look forward to serving you from our new facility.
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.
