Update from the Terahertz Working Group
Within BINDT, a very active group has been set up to explore terahertz imaging as an emerging non-destructive testing (NDT) method. Firstly, is it a method, technology or technique? Exploring the internet for information, it became apparent that the current NDT applications are not the mainstream ones that BINDT colleagues are accustomed to.
Some of the information relating to this method includes the following: it is termed terahertz or tremendously high frequency (less used); it is abbreviated as THz or THF (less used); it relates to frequencies between 300-3000 GHz; and it involves wavelengths of 1-0.1 mm. THz radiation occupies a middle ground where the ranges of microwaves and infrared lightwaves overlap.
It is being used for experimental medical imaging to replace X-rays, which does not come under the remit of industrial NDT but may have spin-offs. In the industrial sector, there are high-speed linear THz scanning systems used for conveyor-style applications for NDT and quality control of various materials in an industrial environment. One of the key benefits of THz imaging technology is its high sensitivity to multiple features and inclusions in the volume of
non-metallic materials (ceramics, plastics, wood). One of the most notable uses of the conveyor-type applications is the evaluation of density variations in pressed powders (pills, caplets and tablets in the pharmaceutical industry and humidity in ‘green tiles’ and kiln-fired tiles in the ceramic industry, etc).
Advantages of the THz imaging system include an extremely high image acquisition rate (up to 5000 fps) and the fact that it is non-destructive (no ionising radiation) and relatively low cost. The technology shows great promise for applications such as the detection of disrupted steel (and nylon) reinforcements, foreign inclusions in rubber parts and the detection of corrosion in coated metal parts. Some of the related applications include the inspection of paint and coating layers, material research, gas sensing, hydration monitoring and security.
The BINDT working group has a strong association with the marine composites sector, especially the Royal National Lifeboat Institution (RNLI) lifeboats. It is said that polymers and THz are a perfect match, with THz imaging revealing hidden air bubbles in polymer materials. Many plastic materials are transparent for THz waves and pulsed THz systems can work for non-contact thickness measurements. Pulsed THz radiation provides information on the thickness of an object, even in multi-layered samples, via time-of-flight techniques: each layer interface reflects a part of the incident pulse and the time elapsed between the arrivals of pulse ‘echoes’ from either side is directly proportional to the optical thickness of that layer.
The potential applications of THz radiation for plastic inspection are not limited to thickness measurements. Scanning a component with the help of a THz beam turns a one-dimensional profile into a three-dimensional image that allows delamination, subsurface cracks and voids to be located. Spectroscopic techniques are able to determine the absorption coefficient and refractive index of compound materials and reveal both their composition and internal structure.
Another application is in tyre manufacture, where a harmless, safe and effective solution is required for inspecting the steel cords embedded inside rubber tyres. Automobile tyres are made of 1-2 mm-thick
rubber sections that are pressed together in a sandwich manner to form a robust tyre layer. The NDT check is for the continuity and/or availability of the embedded metal wires.
THz imaging can be used for the detection of corrosion in metal parts. Many metal parts are exposed to corrosion, but it is often difficult to determine corrosion due to the parts being coated or painted. Uncorroded metal parts reflect the THz signal, whereas corroded parts will reflect the signal much less. This makes it easy to interpret the images produced and locate any corroded metal.
There are very few standards that have been published relating to THz and the most pertinent is BS EN ISO 2808:2019 ‘Paints and varnishes –Determination of film thickness’. The THz method has been added as method 11 in this standard as a viable option. It is still an evolving technology with a lot of potential and reduced safety issues when compared to radiography.
Typically, when introducing third-party certification such as PCN, a technology is more mature, with published standards available that help to generate the syllabus. To include this method there will need to be a course or courses generated with corresponding examinations and, for that, minimum equipment criteria will have to be agreed on along with specimen types for both the training and the examinations. Question banks, trainers and examiners all need approving and it takes time to create a robust system that industry will want and accept, not forgetting the willingness of the training schools to invest in it. It is an exciting time for THz imaging and this is most probably why the group is well attended.
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