Optics & Photonics News

the counter-rotating beams. Of course, the effect wasn’t caused by ether, but simply the fact that one of the light beams had slightly farther to travel than the other.

This “Sagnac effect,” which wasn’t exploited until the era of lasers and fiber optics, is the basis for modern interferometers and ring laser gyroscopes. The latter are able to detect rotation because they have two counter-propagating beams over the same path in order to detect rotation; interference between the beams indicates shifts in the standing wave pattern and hence rotation. Ring laser gyroscopes are now key components of the inertial guidance systems of the Boeing 777 and Airbus A320, as well as many military aircraft.

move in tandem down a track placed next to the target of inspection.

Understanding the exact mechanisms of composite degradation as a function of time, environment and load is still an active area of materials science, Cramer said. His team focuses on the measurement and characterization of those properties.

Another commercially available
aircraft inspection technique is called
laser shearography. These systems start by
recording the laser speckle reflected from
a surface of interest. Then the inspectors apply a small stress
load to the aircraft component and take a second image of the
specular reflections. By subtracting one image from the other
on a computer, someone can see the characteristic fringes that
represent surface strain. “Originally a lot of the systems used
a birefringent crystal that would provide a sheared image,”
Cramer said. “Now they can do that electronically.”
According to a recent issue of National Geographic, NASA
recently unveiled a “personal air vehicle” prototype called the
Puffin that may someday enable one-person flight. While the
Puffin may or may not allow humans to fly like Icarus did, one
thing is for sure: Light-based technology will surely play a major
role in 21st century aviation. t

Scientists are developing micro-and nano-textured surfaces that would be laser-etched onto the external surfaces of aircraft.

Nondestructive testing of materials

How can inspectors tell when part of the structure of an airplane has been weakened and should be replaced? Not just by looking—the flaws may be too small to detect with the naked eye. And pilots can’t exactly pull their vehicles off to the side of the road to check a wavering wing.

Thermal imaging is one method used for aircraft inspection as well as for quantitative characterization of metallic and composite materials. Researchers can measure changes in the density or thickness of corroding metals, or the fiber volume fraction of composites. They can also assess changes in density that may be due to porosity, entrapped voids in the material, or delaminations or microcrack density in the material. (In delamination, stress causes the layers of a carbon-fiber-based composite material to separate. As in metal fatigue, the early stages of the process occur below the surface of the material.) All these flaws could be precursors to potentially catastrophic material failure.

Beyond merely looking for defects, researchers want to relate their material property measurement to the remaining useful life or the residual strength in the material, so that they can understand how it will perform in service, said Cramer at NASA Langley.

Today, composites are used in aircraft control surfaces such as rudders, ailerons and elevators—although they may compose much larger percentages of tomorrow’s airplanes. Infrared thermography is one of the methods that can be used to look for water entrapped in those materials or to look for delaminations.

To examine an aircraft part, whether aluminum or composite, inspectors inject a small heat flux into the part—its temperature needs to be raised only one or two degrees Celsius above ambient levels. A commercially available infrared camera records the way the heat spreads out within the material. For checking aluminum fuselages, inspectors might use a cryogenically cooled HgCd Te detector working at wavelengths of 8 to 12 µm. The heat source and infrared camera

Patricia Daukantas ( patd@nasw.org) is a freelance writer specializing in optics and photonics.

[ References and Resources ]

>> American Airlines. “Fuel Smart,” online at www.aa.com/i18n/ amrcorp/newsroom/fuel-smart.jsp.

>> A.D. Kersey et al. “Fiber optic gyroscope technology,” Opt. News 15( 11), 12 (Nov. 1989).

>> K.E. Cramer and W.P. Winfree. “Thermal characterization of defects in aircraft structures via spatially controlled heat application,” Proc. SPIE 2766, 202 (1996).

>> K.E. Cramer. “NASA thermographic inspection of advanced composite materials,” keynote address, 7th International Conference on Quantitative Infrared Thermography (QIRT; Rhode-St-Genèse, Belgium, 2004).

>> A. Helfrick. Electronics and the Evolution of Flight (College Station: Texas A&M University Press, 2004).

>> M. Naeem. “Advancement in Laser Drilling for Aerospace Gas Turbines,” in Proceedings of the 3rd Pacific International Conference on Applications of Lasers and Optics (PICALO; Beijing, China, 2008), paper 401.

>> R.E. Bailey and J. Arthur. “Head-Worn Displays for Commercial and Business Aircraft,” SPIE Newsroom, May 14, 2009; online at spie. org/x34989.xml?ArticleID=x34989.

>> R. Vandel and E.F. Weener. “Head-Up Guidance System Technology—A Clear Path to Increasing Flight Safety” (Flight Safety Foundation report, November 2009).