Aging aircraft are an increasing concern. Commercial and government vehicles are being flown past their originally intended service life in order to save money. The answer, however, isn’t merely to build more aircraft. NASA’s Aircraft Aging and Durability (AAD) Project is taking a proactive approach to extending the life of aircraft even before age is a concern. The project targets young aircraft, and develops technologies and materials to mitigate the effects of aging as early — and for as long — as possible.

Infrared thermography reveals differences in temperature that can indicate flaws or damage in a material — in this case, the white spots along this wall. (Image courtesy of Paul Cox)
Working with innovative materials means lots of testing, and in the case of large structures like an aircraft wing, it means developing methods to scan components without taking them apart. While there are many ways to perform nondestructive testing (NDT), NASA has developed special methods for using thermography, or infrared imaging, to obtain the best results from scanning composite aircraft structures.

Thermography works by detecting infrared radiation emitted by objects; it measures how hot something is. While thermographic cameras can only see the surface of objects, the rate at which different parts of a surface emit heat can vary based on what lies beneath. For example, thermographic imaging can easily show water leaks behind normal-looking walls, because the water keeps an area of wall cooler than its surroundings. Similarly, defects throughout the material composing an aircraft wing can be exposed by measuring the heat given off along its surface.

Thermography can be passive or active. Passive thermography simply measures surface temperatures as they are. Active thermography involves applying or removing heat from a system or material and then observing how its temperatures change over time following the intervention. Active thermography works best to reveal any hidden flaws on large structures, but it is difficult to capture high-quality data quickly. The camera must be positioned to detect one part of the wing at a high resolution. Heat is applied, the subsequent temperature shifts are recorded, and then the camera is moved to repeat the whole process until a full picture can be stitched together.

MISTRAS’s scanning cart can rapidly analyze a surface. LST technology allows it to take infrared measurements much faster than more traditional methods.
To make this process more efficient, NASA developed line scanning thermography (LST). In LST, a moving device applies heat to the material’s surface in a thin line, while an infrared camera above moves in tandem with the heat source, capturing each part of the surface before, during, and after the application of heat. In the 1990s, NASA filed a patent on the system, with an emphasis on the algorithm for generating data from the measurements taken by the LST camera.

NASA was approached by ThermTech Services, a company that specializes in inspecting boiler water-wall tubing at power stations. ThermTech licensed the technology in 1999, and developed equipment for applying the method to industrial applications. The company’s work caught the eye of MISTRAS Group (Princeton Junction, NJ), which specializes in NDT. MISTRAS decided to expand into thermography in order to broaden its capabilities. MISTRAS acquired ThermTech, and has since applied LST in a variety of applications in power stations, manufacturing plants, and even in scanning large sections of ships constructed from composite materials.

In the case of boilers in power stations, any unplanned downtime can quickly become expensive, as the electricity that the plant fails to generate while components are offline must be purchased instead. LST allows MISTRAS to scan large structures within planned downtime, minimizing the cost, while still providing an accurate picture of the health of components.

Using LST on aircraft, MISTRAS is also able to get a sense of how close an older vehicle is to structural failures. The NASA-developed technology plays a crucial role in making that assessment.

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