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In the electromagnetic spectrum, Terahertz (THz) radiation sits between the infrared and microwave regions with frequencies between 300GHz and 3000GHz (0.3-3THz), and corresponding wavelengths from 1mm to 0.1mm. Historically, difficulties in generating and detecting THz radiation limited research into its interactions with matter1. However, recent advances now mean that it is possible to generate and detect 0.1-3THz frequencies.

Applications that benefit from Terahertz technology include aircrafts and fiber-reinforced composites. Image Credits: Frank_peters/shutterstock.com

THz radiation can penetrate a wide variety of non-conducting materials (such as cardboard, plastic, ceramics, clothing, wood, masonry, and paper.)). Penetration depth is relatively large (although usually less than that of microwave radiation), making THz-radiation suitable for probing the inner structure of samples in search of defects and inclusions2. Many chemicals have defining fingerprints at THz-frequencies so THz spectroscopy can be used to identify and characterize substances such as drugs and explosives.

THz radiation penetrates many materials (more than infrared, for example) and, as an imaging technique, also exhibits good spatial resolution (better than microwaves, for example)2. This higher resolution means it is well-suited for analyzing complex systems with a low contrast range where established methods (like an ultrasound) may fail.

As THz pulses carry very low energies (compared to X-ray or UV radiation), they do not ionize and damage the material under study. This makes them particularly suitable for applications such as non-invasive imaging and non-destructive quality control. THz technology can be used where ultrasonic and other inspections cannot, such as when physical contact is not possible or when sensitive materials require examination (such as lightweight aircraft composites) or where materials do not conduct ultrasound1.

Low energies mean this type of radiation is also safe for people to use; there are no associated health or safety risks.

Terahertz technology provides highly accurate, reliable and repeatable inspection data.

Pulsed terahertz imaging can provide a range of information about a sample. It can measure the thickness of a coating and study how well it is bonded, and give highly precise multi-layer thickness measurements in one go3.   It can also detect the presence of internal voids or a specific gas, and detail the density distribution in a layer of foam4. Its major advantage is that can be used with the most sensitive of materials and testing environments.

Applications include NDT in aircraft and fiber-reinforced composites; ceramic coating thickness measurements; inspection and repair of pipelines; evaluation of seams; detection of voids; inspection of radomes (enclosures that protects radar antennae) and automotive fuel tanks; security applications such as checking for explosives in airports; and imaging of internal structures1.

TeraMetrix Inc has designed robust compact pulsed terahertz measurement systems for use in an industrial environment. Sensor heads are coupled by an optical fibre to a rack mount package containing the electro-optical acquisition hardware. This gives complete flexibility as sensor heads can be replaced and configured to a user’s specific requirements. Cutting edge research can be performed quickly and easily as the T-Ray 5000 systems can be adapted to handle a large number of experiments thanks to their modular construction.

T-Ray 5000 series Control Units can be combined with a handheld gauge or  line scanner for nondestructive testing4. The Single Point Gauge (SPG) determines the thickness of several coating layers with unparalleled precision, and the Line Scan Gauge (LSG) produces images of structures under the surface. Measurements with the SPG can be taken either straight-on or at right-angle depending on which tip is selected. While essentially non-contact, a measurement tip helps the user to position the object at the focus of the THz beam when using a hand-held tool. Measurement configuration and mode of operation are selected using the touchscreen display. The system is capable of working on metallic or composite substrates.

Applications areas include aerospace where the system is used to measure coating thickness and panel alignment and identify defects; marine (coating thickness and corrosion under paint); building products (seam inspection and void detection); and petrochemical (steel pipe coatings and pipe repair inspection).

The sensor head allows the  

T-Ray 5000 systems to work in flammable atmospheres, such as paint spray booths or coating facilities, as the transmitter and receiver are housed within sealed stainless steel and the lens is coated with Teflon to withstand solvents. It can also be mounted on a robot. While its working distance is set to 115 mm, this can be changed if needed.

A good example is the F-35 Joint Strike Fighter aircraft. Its exterior is coated with specialty chemicals that are particularly challenging to measure. However, THz pulses have proved useful in measuring coating thickness soon after application once the coatings have dried. A handheld THz scanner can then provide the information necessary to assemble the airframe ensuring that parts are properly aligned.

Once in use, the aircraft needs to be inspected and serviced regularly. THz systems can be used to evaluate external coatings, as well as helping with paint removal when coatings need removing. In this case, they can provide information to control laser power and cutting depth.

NASA uses terahertz systems to examine the sprayed-on-foam insulation on the exterior of spacecraft fuel tanks. The Previous model T-Ray 4000 was also used to inspect the tiles of the shuttle heat shield as corrosion can occur underneath. By measuring the layers which attached the tiles to the orbiter, NASA could decide if tiles need replacing.

NASA is also investigating terahertz inspection of heat shields, thermal blankets and ultra-high pressure tanks as it develops the new Ares launch platform; as well as for checking ultra-high pressure gas tanks used for attitude adjustment rockets. Terahertz imaging can monitor the tanks’ Kevlar fiber coatings to check for broken fibers. Any rupture would result in the loss of the orbiter.

THz technology can be used to detect contamination in packaged products such as powdered antibiotics while the product remains inside its packaging. It can also test the weight of tablets in blister packs and powdered antibiotics can be non-destructively examined inside their packages.

Pipelines need regular monitoring for corrosion and leaks and THz technology can be used to measure the thickness of multi-layer coatings and monitor pipe repairs. Usually, fiberglass composite wraps are used as patches and THz imaging can check these patches for degradation. The advantage of THz technology is that it can probe below the patch to the surface of the pipe and check for problems with the pipe surface as well as the patch.

This information has been sourced, reviewed and adapted from materials provided by Baugh and Weedon NDT.

For more information on this source, please visit Baugh and Weedon NDT.

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