Terahertz nondestructive evaluation

Terahertz nondestructive evaluation

Terahertz nondestructive evaluation pertains to devices, and techniques of analysis occurring in the terahertz domain of electromagnetic radiation. These devices and techniques evaluate the properties of a material, component or system without causing damage.[1]

After the Shuttle Columbia accident in 2003, Columbia Accident Investigation Board recommendation R3.2.1 stated “Initiate an aggressive program to eliminate all External Tank Thermal Protection System debris-shedding at the source….” To support this recommendation, inspection methods for flaws in foam are being evaluated, developed, and refined at NASA.[1][2][3]

Contents

Overview

During the STS-114 flight in July of 2005, significant foam shedding was still observed. The ability to nondestructively detect and characterize crushed foam after that flight became a significant priority when it was believed that the staff processing the tank had crushed foam by walking on it or from hail damage when the shuttle is on the launch pad and in preparation for launch.

Additionally, density variations in the foam are also potential points of flaw initiation causing foam shedding. The innovation described below answered the call to develop a nondestructive, totally non-contact, non-liquid-coupled method that could simultaneously and precisely characterize thickness variation (from crushed foam due to worker handling and hail damage) and density variation in foam materials. It was critical to have a method that did not require fluid (water) coupling; i.e; ultrasonic inspection methods require water coupling.

There are millions of dollars of ultrasonic equipment in the field and on the market that are used as thickness gauges and density meters. This technology, when fully commercialized into a more portable form, and as the technology gets less expensive, will be able to replace the ultrasonic instruments for structural plastic, ceramic, and foam materials. The new instruments will not require liquid coupling thereby enhancing their usefulness in field applications and possibly for high-temperature in-situ applications where liquid coupling is not possible. A potential new market segment can be developed with this technology.[2][3]

Terahertz imaging

Terahertz imaging is an emerging and significant nondestructive evaluation (NDE) technique used for dielectric (nonconducting, i.e., an insulator) materials analysis and quality control in the pharmaceutical, biomedical, security, materials characterization, and aerospace industries. This new method can play a significant role in those industries for materials characterization applications where precision thickness mapping (to assure product dimensional tolerances within product and from product-to-product) and density mapping (to assure product quality within product and from product-to-product) are required.[2]

Nondestructive evaluation

Sensors and instruments are employed in the 0.1 to the 10 THz range for nondestructive evaluation, which includes detection.[2][3]

Terahertz Density Thickness Imager

The Terahertz Density Thickness Imager is a nondestructive inspection method uses terahertz energy for density and thickness mapping in dielectric, ceramic, and composite materials.

This non-contact, single-sided terahertz electromagnetic measurement and imaging method characterizes micro-structural (e.g., spatially-lateral density) and thickness variation in dielectric (insulating) materials. This method was demonstrated for space shuttle external tank sprayed-on foam insulation and has been designed for use as an inspection method for current and future NASA thermal protection systems and other dielectric material inspection applications where no contact can be made with the sample due to fragility and it is impractical to use ultrasonic methods.[2]

Rotational spectroscopy

Rotational spectroscopy uses electromagnetic radiation in the wavelength range from 0.1 to 0.5 terahertz (THz). This range includes millimeter-range energies and is particularly sensitive to chemical molecules. The resulting THz absorption produces a unique and reproducible spectral pattern that identifies the material.

Detect trace amounts of explosives - THz spectroscopy can detect trace amounts of explosives in less than one second. Because explosives continually emit trace amounts of vapor, it should be possible to use these methods to detect concealed explosives from a distance.[3]

THz-wave radar

THz-wave radar can sense gas leaks, chemicals and nuclear materials. In field tests, THz-wave radar detected chemicals at the 10-ppm level from 60 meters away. This method can be used in a fenceline or aircraft mounted system that works day or night in any weather.

It can locate and track chemical and radioactive plumes. THz-wave radar can sense radioactive plumes from nuclear plants have been detected several kilometers away based on radiation-induced ionization effects in air.[3]

Passive imaging techniques

Passive detection and imaging techniques can be used to perform medical imaging. Because THz radiation is biologically safe, it can be used in high resolution imaging to detect.[3]

See also

References

  1. ^ a b Anastasi, RF, et al. (May 2007) (Chapter title:). Terahertz NDE for aerospace applications. Book title: Ultrasonic and advanced methods for nondestructive testing and material characterization. World Scientific Publishing. pp. 279–303. ISBN 978-981-270-409-2. http://books.google.com/?id=lKUaF6RWR5sC&pg=PA279&dq=terahertz+NDE+for+aerospace+applications#v=onepage&q=terahertz%20NDE%20for%20aerospace%20applications&f=false. 
  2. ^ a b c d e Nondestructive inspection method uses terahertz energy.
  3. ^ a b c d e f Sensors and Instrumentation and Nondestructive Evaluation - Please see above links

Further reading


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