By Eric Wachter
ORNL has developed a new laser-based spectral shift method for detecting early signs of thermal damage in high-tech composite materials used in advanced aircraft.
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| Fig. 1. Eric Wachter (right) and Walt Fisher show the composite damage imager they developed for imaging thermal damage in structural components of aircraft. |
Unfortunately, my colleagues told me, these great materials have a vulnerability. When subjected to high temperatures (e.g., during a fire on the tarmac, from a lightning strike, or from misdirected jet exhaust during takeoff and landing), composite materials become brittle and prone to failure. This damage was completely undetectable until it became so severe that the material was weakened well beyond the danger point. Muhs showed me some old research papers on these materials that implied that a common analytical method— fluorescence spectroscopy—might make it possible to detect early damage so that a part could be repaired or replaced before it endangers human lives. Because my group has experience with similar spectroscopic methods and because I always try to solve complex technical problems using light, I agreed to take a look at his samples.
Learning
from Failure
The first thing we tried was to reproduce the methods
described in papers that Muhs found in his literature search. This approach
proved to be a complete dead end, or so we thought. When we looked at the
initial results, we observed absolutely no correlation between fluorescence
signature and degree of damage. By shining ultraviolet laser light of the
right wavelength on composite samples, we were able to excite the samples
into emitting light with a specific fluorescence signature. The problem
was that each sample exhibited the same, strong fluorescence, regardless
of damage level; we failed to excite a separate fluorescence signature
indicating damage.
Nevertheless, undaunted by failure, I convinced Muhs to let me try another favorite spectroscopic method in my toolbox—Raman scattering. I told him that Raman offers a powerful way to probe thermal damage at a chemical level. Unfortunately, all we got from the Raman tests was more fluorescence. We tried blue lasers, green lasers, red lasers, near-infrared lasers—every light source I could find—and still all we got was a huge fluorescence interference. It was like driving home from ORNL late at night and looking for the reflections from the eyes of deer but seeing only the blinding headlights of approaching traffic. The challenge was to differentiate the dim green reflections from the bright white headlights.
The epoxy in these composite materials is a very complex chemical polymer. We found it fluoresces when excited with light any color of the rainbow. But then we noticed something strange. When we plotted the intensity of these "interferences" against thermal damage, we got a nice correlation. Fluorescence would work after all—we just needed to use a different excitation method. Talk about starting with a lemon and ending up with lemonade. I learned long ago in school that failure in the lab is as important as success—as long as you learn something from each failure. Recognizing that our "failure" using Raman scattering could be useful gave us the insight we needed to solve our problem.
Trying
Another Tool
We probed Muhs’ set of test samples over and over
using our Raman spectrometer to collect fluorescence spectra, enabling
us to write a nice report for the sponsor—a consortium of U.S. Air Force
and U.S. Navy parties. But I was troubled by a subtle inconsistency in
our new method. Under some conditions, it was virtually impossible to distinguish
very damaged components from undamaged ones because the fluorescence response
appeared very similar. Also, some samples seemed to have local "hot spots"
of fluorescence, which could complicate interpretations of results.
To this point in my career I had always worked on bulk properties, but it was now obvious that, to understand these complications, I would have to think differently. I would have to understand in detail what goes on at the surface. So I sought the help of Bruce Warmack, a researcher in the Molecular Imaging Group of ORNL’s Life Sciences Division, knowing that his microscopy tools could complement my spectroscopy tools.
Warmack loaned me a far-field microscope—a device similar to a telescope that is used for much closer and smaller objects and that provides far higher magnification. This microscope proved to be incredibly important. By illuminating the samples uniformly with my laser, I could use the microscope to look at fluorescence emission from the surface on a micron scale.
What I saw was amazing! Not only did the intensity of emission change as a function of thermal damage—just as I’d seen with my Raman system —but the color did as well. I hadn’t detected the color change with the Raman system because I couldn’t observe the range of wavelengths necessary to notice a change in spectral response. Moreover, the "hot spots" were clearly identifiable as microscopic particles of residual hardener awash in a sea of epoxy. While I have often told colleagues that it is necessary to step away from a problem to see the big picture, with my initial fix I had completely missed the big picture—a spectral shift in fluorescence that correlates directly with thermal damage and a strategy for eliminating interference from the residual hardener.
About this time, Walt Fisher joined my group (the former Measurement Systems Research Group), and I asked him to help me work on this new spectral shift method for detection of thermal damage. He collected complete fluorescence emission spectra for each of Muhs’ original test samples at a variety of laser wavelengths ranging from the ultraviolet to the near infrared. This huge data set allowed us to pick optimal excitation conditions that provided the most useful information about thermal damage while minimizing interference from the hardener particles. Finally, we had eliminated the inconsistent results that had made it difficult to distinguish between non-damaged and extremely damaged composites.
Laziness
Leads To Ingenuity
Satisfied with our new and improved method, Fisher
and I were faced with a big task. Our sponsor liked the new spectral shift
method and wanted us to use it to map localized damage on large composite
panels. We did some quick back-of-the-envelope calculations—the most important
kind of calculations for experimentalists like us. We realized that, by
illuminating and collecting data on small panel sections one by one, it
would take more than a day of laborious work to map a single panel.
Fisher and I share a common personal and professional trait—we’re both basically lazy. We would rather spend time thinking up an easy way to do something rather than spend a lot of effort doing it the obvious way. So we went into the lab, looked at the samples, looked at our laser, looked at the samples, looked at each other, and looked at the samples some more. Then he said, "I think we could illuminate the entire sample at one time and use our detector to capture an image of fluorescence." As these words poured out of his mouth, I immediately said "... and we could use a tunable filter to collect those images at various emission wavelengths." In those few moments we had developed fluorescence imaging of composites. It would be fast—about 1 second per sample, instead of more than a day. And in our laziness we had come up with a practical way to quickly screen entire aircraft for thermal damage (see Fig. 1).
Now, several years later, Fisher and I continue to improve upon this method. Recently, by developing software for our computerized system, we can automatically collect and process spectral data for each pixel in our image, allowing us to quantify thermal damage pixel by pixel. Once again, this improvement was motivated by the desire to make our job easier through technology.
Seeing
the Light
Our sponsor has asked us to detect even more subtle
forms of thermal damage. So we have begun working with other research groups
specializing in nondestructive testing in an effort to incorporate our
approach into an overall solution to the unique problems of testing composites.
I hope that our work will play a role in enabling wider use of composites
in the commercial transportation industry to help reduce its energy consumption
and operating costs. Such progress can only help make a large world smaller
for the people traveling in the faster composite planes of the future.
And it further proves my conviction that many problems can be solved using
light, especially if you are willing to learn from failure and if you are
lazy enough to seek a more clever solution. Now, if I could only find a
way to use that interference from the hardener particles . . . .
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