Terahertz nondestructive evaluation pertains to devices, and techniques of analysis occurring in the terahertz domain of

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...

. These devices and techniques evaluate the properties of a material, component or system without causing damage.

Terahertz imaging

Terahertz imaging is an emerging and significant

nondestructive evaluation Nondestructive testing (NDT) is any of a wide group of analysis techniques used in science and technology industry to evaluate the properties of a material, component or system without causing damage. The terms nondestructive examination (NDE), n ...

(NDE) technique used for

dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...

(nonconducting, i.e., an insulator) materials analysis and quality control in the pharmaceutical,

biomedical Biomedicine (also referred to as Western medicine, mainstream medicine or conventional medicine)

, security, materials characterization, and

aerospace Aerospace is a term used to collectively refer to the atmosphere and outer space. Aerospace activity is very diverse, with a multitude of commercial, industrial and military applications. Aerospace engineering consists of aeronautics and astrona ...

industries. It has proved to be effective in the inspection of layers in paints and coatings, detecting structural defects in

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...

and

composite materials A composite material (also called a composition material or shortened to composite, which is the common name) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or ...

and

imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of an image). Imaging technology is the application of materials and methods to create, preserve, or duplicate images. ...

the physical structure of paintings and manuscripts. The use of THz waves for non-destructive evaluation enables inspection of multi-layered structures and can identify abnormalities from foreign material inclusions, disbond and delamination, mechanical impact damage, heat damage, and water or hydraulic fluid ingression. This new method can play a significant role in a number of industries for materials characterization applications where precision thickness mapping (to assure product dimensional tolerances within a product and from product-to-product) and density mapping (to assure product quality within a product and from product-to-product) are required.

Nondestructive evaluation

Sensors and instruments are employed in the 0.1 to the 10 THz range for

, which includes detection.

Terahertz Density Thickness Imager

The Terahertz Density Thickness Imager is a nondestructive inspection method that employs terahertz energy for density and thickness mapping in

, and

composite material A composite material (also called a composition material or shortened to composite, which is the common name) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or ...

s. This non-contact, single-sided terahertz

electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...

measurement and

method characterizes micro-structure and

thickness Thickness may refer to: * Thickness (graph theory) * Thickness (geology), the distance across a layer of rock * Thickness (meteorology), the difference in height between two atmospheric pressure levels * Thickness planer a woodworking machine * O ...

variation in dielectric ( insulating) materials. This method was demonstrated for the

Space Shuttle external tank The Space Shuttle external tank (ET) was the component of the Space Shuttle launch vehicle that contained the liquid hydrogen fuel and liquid oxygen oxidizer. During lift-off and ascent it supplied the fuel and oxidizer under pressure to the ...

sprayed-on foam insulation and has been designed for use as an inspection method for current and future NASA

thermal A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...

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 Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...

methods. Nondestructive inspection method uses terahertz energy. * *John H. Glenn Research Center * *

Rotational spectroscopy

Rotational spectroscopy uses electromagnetic radiation in the frequency range from 0.1 to 4 terahertz (THz). This range includes millimeter-range wavelengths and is particularly sensitive to chemical molecules. The resulting THz absorption produces a unique and reproducible spectral pattern that identifies the material. 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.

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 fence line or aircraft mounted system that works day or night in any weather. It can locate and track chemical and radioactive plumes. THz-wave radar that can sense radioactive plumes from nuclear plants have detected plumes several kilometers away based on radiation-induced ionization effects in air.

THz tomography

THz tomography techniques are nondestructive methods that can use THz pulsed beam or millimeter-range sources to locate objects in 3D. These techniques include tomography, tomosynthesis, synthetic aperture radar and time of flight. Such techniques can resolve details on scales of less than one millimeter in objects that are several tens of centimeters in size.

Passive/active imaging techniques

Security imaging is currently being done by both active and passive methods. Active systems illuminate the subject with THz radiation whereas passive systems merely view the naturally occurring radiation from the subject. Evidently passive systems are inherently safe, whereas an argument can be made that any form of "irradiation" of a person is undesirable. In technical and scientific terms, however, the active illumination schemes are safe according to all current legislation and standards. The purpose of using active illumination sources is primarily to make the signal-to-noise ratio better. This is analogous to using a flash on a standard optical light camera when the ambient lighting level is too low. For security imaging purposes the operating frequencies are typically in the range 0.1 THz to 0.8 THz (100 GHz to 800 GHz). In this range skin is not transparent so the imaging systems can look through clothing and hair, but not inside the body. There are privacy issues associated with such activities, especially surrounding the active systems since the active systems, with their higher quality images, can show very detailed anatomical features. Active systems such as the L3 Provision and the Smiths eqo are actually mm-wave imaging systems rather than Terahertz imaging systems like Millitech systems. These widely deployed systems do not display images, avoiding any privacy issues. Instead they display generic "mannequin" outlines with any anomalous regions highlighted. Since security screening is looking for anomalous images, items like false legs, false arms, colostomy bags, body-worn urinals, body-worn insulin pumps, and external breast augmentations will show up. Note that breast implants, being under the skin, will not be revealed. Active imaging techniques can be used to perform medical imaging. Because THz radiation is biologically safe (non ionisant), it can be used in high resolution imaging to detect skin cancer. Sensors and Instrumentation and Nondestructive Evaluation
Argonne test facility

Nondestructive Evaluation
* – Please see above links

Space Shuttle inspections

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program na ...

inspections are an example of this technology's application. After the

Shuttle Columbia Space Shuttle ''Columbia'' (OV-102) was a Space Shuttle orbiter manufactured by Rockwell International and operated by NASA. Named after the first American ship to circumnavigate the upper North American Pacific coast and the female personifi ...

accident in 2003,

Columbia Accident Investigation Board The ''Columbia'' Accident Investigation Board (CAIB) was an internal commission convened by NASA to investigate the destruction of the Space Shuttle ''Columbia'' during STS-107 upon atmospheric re-entry on February 1, 2003. The panel determin ...

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.

STS-114 STS-114 was the first "Return to Flight" Space Shuttle mission following the Space Shuttle Columbia disaster, Space Shuttle ''Columbia'' disaster. Space Shuttle Discovery, ''Discovery'' launched at 10:39 Eastern Time Zone, EDT (14:39 Coordinated ...

employed Space Shuttle ''Discovery'', and was the first "Return to Flight"

mission following the Space Shuttle ''Columbia'' disaster. It launched at 10:39 EDT, 26 July 2005. During the

flight significant

foam shedding Foams are materials formed by trapping pockets of gas in a liquid or solid. A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the r ...

was observed. Therefore, 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 was on the

launch pad A launch pad is an above-ground facility from which a rocket-powered missile or space vehicle is vertically launched. The term ''launch pad'' can be used to describe just the central launch platform (mobile launcher platform), or the entire ...

or during other preparations for launch. Additionally,

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

variations in the foam were 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 testing Ultrasonic testing (UT) is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse-waves with center frequencies ...

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 meter A density meter, also known as a densimeter, is a device that measures the density. Density is usually abbreviated as either \rho or D . Typically, density either has the units of '' kg/m^3 '' or '' lb/ft^3 ''. The most basic principle of how d ...

s. When ''terahertz nondestructive evaluation'' is fully commercialized into a more portable form, and becomes less expensive it will be able to replace the ultrasonic instruments for structural

plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...

, and

foam Foams are materials formed by trapping pockets of gas in a liquid or solid. A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the reg ...

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.