Hello again! I want to shift away slightly from machine learning and focus on a nondestructive testing method that is not discussed widely, terahertz (THz) testing. THz waves can range from 100 GHz to 30 THz and they fall within the electromagnetic spectrum [1]. Some researcher describe that THz testing lie in between the infrared and millimeter wavelengths [2]. THz wave are technically classified as non-ionizing radiation making it not harmful to biological tissue. There have been natural sources of THz waves reported and researched. THz waves or radiation is discharged from various phenomena such as cosmic sources from celestial bodies, the Earth’s atmospheric emissions in for of thermal radiation from clouds, and even some biological process from reactions and molecular vibrations in living organisms.
Historically generating and measuring THz waves have been challenge. But with the continual advances in ultrafast laser and ultra-micro machining innovations that date back to the 1980s have closed the gap and provided limited applications for THz waves [1]. One of the applications happens to be NDT technology, where THz testing is emerging in composite inspection. THz testing for composite materials grants deeper penetration and higher resolutions than other NDT methodologies.
The composite materials that are ideal for THz strong penetrating power are non-metallic, non-polar (typically hydrophobic) materials like composites, rubber, paint, resin, glass, ceramic and foams (to name a few) [1]. A THz testing system comprises of various components that can detect, analyze, and evaluate a material internal network. First THz short waves are introduced into a material and interact with its internal structures. The reflected or transmitted THz waves are then analyzed to make an assessment on the data about the material’s internal features. This reveals whether the material has different phases, inclusions, defects or damage.
Two main systems of the THz testing technique classified by how data is measured: THz time-domain spectroscopy (THz-TDS) and continuous wave (THz-CW) systems [1]. The THz-TDS system utilizes short pulsed THz waves that are traditionally generated by laser-excited photoconductive antenna. THz-TDS systems allow for time dependent THz electric field measurements to determine the phase information in composite materials. Alternatively, the THz-CW systems generate high-power THz waves using either gas or quantum cascade lasers that can measure the phase information through the average intensity of the overall electromagnetic field.
Commercialized THz NDT systems have inspection areas ranging from 0.01 to 4 m2 [1]. Imaging resolution can range from 220×160 pixels to 2560×1920. Lastly, the imaging rate or the speed at which images are captured and process vary in hertz (Hz) can range from 10 – 30. All of the parameters listed all depend on the manufacture of the THz testing equipment. One must remember that THz testing is still considered an emerging technology. So there may be some parameters that are not listed from manufacturers and contacting them to understand their system may be required.
Like all NDT techniques there are advantages and disadvantages. Understanding the limitations of this method will help insight to future research directions and strategies. Some of the limitations of THz are a lack of speed during inspection, costly infrastructure, and inspection only being available to non-conductive materials [1]. In addition, THz waves are highly sensitive to the inspected materials surface conditions. THz waves can further be absorbed if there is any moisture present on the surface of the material, which will reduce the effectiveness of the inspection technique.
In conclusion, THz testing offers a viable solution for composite materials. THz testing operate in the “terahertz gap” between 100 GHz to 30 THz, where the waves can interact with the internal structures of the material. The THz wave reflections and transmissions reveal the material’s internal features. However, THz testing is still an emerging technology where there is limited amount of commercially available systems and the number of limitations that may plague the method. The two main types of systems are categorized based on how the THz waves are generated and measured. It will be interesting to see how the research progresses over the years for THz testing. Hopefully some exciting advances will be made in reducing the overall cost of the method. This can either be manufacturing smaller THz sources and detectors or developing alternative technology for generating and measuring THz waves.
-DB PhD
References:
[1] B. Wang, S. Zhong, T.-L. Lee, K. S. Fancey, and J. Mi, “Non-destructive testing and evaluation of composite materials/structures: A state-of-the-art review,” Adv. Mech. Eng., vol. 12, no. 4, p. 168781402091376, Apr. 2020, doi: 10.1177/1687814020913761.
[2] X. Li, J. Li, Y. Li, A. Ozcan, and M. Jarrahi, “High-throughput terahertz imaging: progress and challenges,” Light Sci. Appl., vol. 12, no. 1, p. 233, Sep. 2023, doi: 10.1038/s41377-023-01278-0.