Abstract-Non-Destructive Evaluation of Hidden Defects Beneath the Multilayer Organic Protective Coatings Based on Terahertz Technology

Wanli Tu, Shuncong Zhong, Manting Luo, Qiukun Zhang, 

https://www.frontiersin.org/articles/10.3389/fphy.2021.676851/full

An organic protective coating system plays an important role for the corrosion protection of offshore metallic structures. It is of practical importance to detect possible coating defects and evaluate coating performance for corrosion degradation monitoring. Reliable defect identification can provide timely and effective maintenance to avoid serious consequences. This work investigated various multilayer organic coating systems by terahertz pulse imaging technology and aimed to explore the inspection ability of terahertz non-destructive testing (THz NDT) of protective coatings. Several types of defective coating samples were measured. The comparison of measurements obtained for sites with and without defects was provided. The changes in the signals caused by the presence of defects were explained. Structural analysis, quantitative evaluation, and defect identification were carried out in detail. The results of measurements showed that corrosion defects, paint bulge defects, and paint detachment defects can be distinguished and identified in combination with the causes of defects. It can provide effective technical guidance for terahertz technology to be gradually extended to engineering applications.

Abstract-Defect feature extraction of marine protective coatings by terahertz pulsed imaging

Wanli Tu. Shuncong Zhong, Atilla Incecik, Xibin Fu

https://www.sciencedirect.com/science/article/pii/S0029801818300337

Feature extraction of marine protective coatings using Hilbert transform (HT) and wavelet transform modulus maximum (WTMM) on terahertz pulsed imaging (TPI) of the coatings was proposed. For TPI-based marine protective coating detection, it is difficult to locate exactly the reflected echoes form internal structure of coating system by the faint feature in the time domain due to the interference of background noise. However, those faint features were usually caused by the interface between two different medium layers whose refractive indices are very similar or caused by micro defects beneath the coating. The proposed algorithm was validated by simulated and experimental TPI waveform obtained from marine paint samples with or without defects. To extract the structure feature more clearly and intuitively, Hilbert transform procedure was carried out on detected terahertz signal to get Hilbert envelope for further processing. Subsequently, the modulus maximum of the stationary wavelet transform approximation coefficients were employed as the criteria for feature extraction of internal interfaces and defect features, according to the relationship between WTMM and signal singularity. The results demonstrated that the combination of HT and WTMM algorithms could be used to exactly extract the structure feature and to evaluate the position of defects beneath coatings.

Evaluating Semiconductor Quality With Femtosecond Lasers

Ultrashort laser pulses could help identify high quality wide-gap semiconductor materials that can be used as power devices.

http://www.asianscientist.com/2015/11/in-the-lab/osaka-gallium-nitride-next-terahertz-laser-emission-microscope/

 AsianScientist (Nov. 9, 2015) - In a study published in Scientific Reports, researchers have developed a laser-based method to visualize defects in semiconductor materials such as gallium nitride (GaN). 

Wide-gap semiconductors such as GaN are widely used for optical devices such as blue LED and are also anticipated as materials for next-generation energy saving power devices and solar cells. 

However, the quality of GaN crystals does not match the standards of conventional semiconductor materials such as silicon, preventing GaN from being used for power devices. For that reason, new technology for producing high-quality crystals with fewer defects and rearrangement is expected, and the development of evaluation technologies are crucial. 

A group of researchers led by Iwao Kawayama, an associate professor of the Institute of Laser Engineering at Osaka University, in cooperation with Screen Holdings Co., Ltd., succeeded in visualizing changes in defect density on the surface of GaN through the laser terahertz emission microscope (LTEM) which measures terahertz (THz) waves generated by laser emission. 

This group's discovery shows that LTEM is useful as a new method for evaluating the quality of wide-gap semiconductors and it is also expected that LTEM will bring a breakthrough in the development of next-generation optical devices, super high frequency devices, and energy devices. 

The group examined the intensity distribution of THz generated by radiating ultraviolet femtosecond laser pulses on the surface of GaN crystal through LTEM. They found that there were regions with high intensity of THz emission and ones with low intensity of THz emission. 

Additionally, when the LTEM image was compared with the image obtained through photoluminescence using a conventional method, it was found that there was a strong correlation between the distribution of emission intensity due to lattice defects and the intensity distribution of THz wave emission. 

Furthermore, from results measurement through modification of excited lasers, it was confirmed that THz emission needs excitation light with larger energy than the band gap energy. 

The article can be found at: Sakai et al. (2015) Visualization of GaN Surface Potential Using Terahertz Emission Enhanced by Local Defects. ——— Source: Osaka University. Read more from Asian Scientist Magazine at: http://www.asianscientist.com/2015/11/in-the-lab/osaka-gallium-nitride-next-terahertz-laser-emission-microscope/

TeTechS Blog-Hidden Object Detection with Terahertz Sensors

Hidden Object Detection with Terahertz Sensors

Posted by Daniel Hailu on Monday, 29 September 2014 in Terahertz

http://www.tetechs.com/blog/latest

After the Columbia disaster in 2003, NASA grounded the space shuttles for more than a year as it worked on new safety protocols to ensure that such a tragedy would not happen again. As part of the preparations for the Return to Flight mission, the Agency required a method for detecting potentially hazardous defects in the external tank’s sprayed-on insulating foam prior to launch. The solution NASA Langley Research Center scientists suspected was a new imaging technology called terahertz imaging that had the potential to accurately find flaws in the foam on the external tank. Terahertz wavelengths can be used to see through many materials and reveal defects like cracks, voids, and density variations. They can be used to image or as an anomaly detector, or both at the same time. Hidden object or defects inside opaque structures can be seen using Terahertz waves. For example a hidden glue gel inside a plastic enclosure is observed to change through absorption the Terahertz wave generated and detected by Terahertz sensors. As seen in this video, the hidden gel that is behind an opaque barrier and can not be seen by a visible nor IR camera becomes visible in the Terahertz frequency range.

There has been intense interest in the use of millimetre wave and terahertz technology for the detection of concealed weapons, hidden metallic as well as non-metallic objects, explosives and other threats. Radiation at these frequencies is safe, penetrates barriers and has short enough wavelengths to allow discrimination between objects. In addition, many solids including explosives have characteristic spectroscopic signatures at terahertz wavelengths which can be used to identify them.

In many cases, real-time or video rate images with sub-millimeter resolution are required to see small defects or hidden objects. Strong Terahertz sources are required for these imaging applications, which are currently expensive and bulky. While low-cost and room temprature detectors are required for imaging and sensing. These challenges have to be overcome in order for Terahertz technology to be widely adapted by industry.