https://link-springer-com-443.webvpn.jmu.edu.cn/content/pdf/10.1007%2F978-3-319-30050-4_35-2.pdf

Terahertz Techniques in NDE

Terahertz (THz) technology is a competent non-destructive evaluation (NDE) technique, particularly for advanced materials such as Fibre Reinforced Polymer (FRP) composites due to its ability to penetrate most non-metallic and nonpolar substances. Typically, THz NDE studies are carried out using expensive and broadband pulsed THz systems limiting their widespread use in practical applications. In contrast, Continuous wave (CW) THz systems can potentially be a narrowband, cost-effective and scalable solution for NDE applications. However, conventional CW THz systems employ a coherent detection scheme which results in large acquisition time per pixel thus limiting their real-time applicability. In this paper, a CW THz system with incoherent detection scheme using Schottky receiver along with spatial adaptive sampling technique is employed to achieve rapid THz imaging of Glass Fibre Reinforced Polymer (GFRP) composite with artificial defects. Here, an initial coarse scan of 2 mm step size has been done, and gradient based thresholding criterion is used for identifying the regions of interest to progressively scan the sample with finer resolution down to a step size of 0.5 mm. Results demonstrate a total reduction in the image acquisition time by a factor of 50 compared to the coherent CW THz imaging. Further, the THz image acquired through adaptive sampling shows excellent correlation with that of the traditional uniformly sampled THz image with 0.5 mm step size.

Rapid terahertz imaging for non-destructive evaluation applications using Schottky receivers and spatial adaptive sampling

Terahertz (THz) imaging is an attractive alternate to ultrasonic based Non-destructive Evaluation (NDE) especially for Fiber Reinforced Polymers (FRPs) such as Glass FRP (GFRP) composites as the latter demands proximity and additional coupling medium for the best performance. Typically, THz imaging system uses a single emitter-detector configuration employing raster scan method for image acquisition. The image acquisition speed is greatly limited by the speed of the mechanical stages and hence its usage in real-time industrial NDT applications such as in-line quality control has been limited. Alternatively, having an array of detectors will significantly increase the system cost. As an optimal compromise for speed and cost, line scanners are highly desirable. In this work, rapid imaging performance of a THz line scanner has been studied by imaging closely spaced defects in GFRP composites using a 100 GHz source. The total acquisition time for imaging the GFRP sample of dimensions 55× 35 mm2 is 10 s, which is >100 times faster compared to a conventional raster scanning technique. In addition, image deconvolution techniques such as Lucy Richardson and Weiner deconvolution have been adopted to improve the quality of the acquired THz images. The results show that the THz line scanners can successfully be employed for rapid defect detection in GFRP composites.

Rapid nondestructive evaluation of defects in GFRP composites using terahertz line scanner

In recent years, terahertz radiationTerahertz radiation (THz = 1012 Hz) has attracted much attention due to its exceptional non-invasive and non-ionizing sensing capabilities. The sub-THz band (0.1–0.3 THz) and the THz band (0.3–10 THz) lie between millimeter waves (mm-waves) and light waves with the ability to harness their advantages. The capacity for these sub-THz and THz waves to penetrate deeply into dielectric materials combined with their high spatial resolution makes them well suited for biomedical applications, including in-vivo and ex-vivo experiments. The purpose of this chapter is to discuss how the sensors based on these frequency spectra can be used in various biomedical applications, classified into three major domains, i.e., diagnostics, imaging, and treatment, where they provide many advantages over the existing devices. Next, we will discuss the appropriateness of using photonics and electronics THz instruments in THz applications and the suitability of using electronics in the sub-THz regime. Finally, we'll look at artificial intelligence's function in enhancing the technology's versatility. 

Terahertz Spectrum in Biomedical Engineering

Engineering of novel structures with high strength to weight ratio for applications in aerospace, renewable energy and naval industries has resulted in an increased popularity of sandwich structured composites. A sandwich-structured composite is fabricated by bonding a thick lightweight core between two stiff, thin skins such as Glass Fiber Reinforced Plastic (GFRP). Balsawood is a type of homogeneous core which is widely used for renewable energy structures, such as wind turbine blades. In this paper, a GFRP-balsawood sandwich structure is evaluated non-destructively for internal defects such as holes, using a CW Terahertz system in transmission mode. Internal defects will give rise to differential THz transmission and hence can be identified using THz imaging. The imaging studies are carried out with a central frequency of 0.35 THz and the sample is raster scanned using 2-D translational stages controlled by high precision stepper motors in x-y directions to obtain the THz image. The image acquired using CW THz system clearly identifies the defects in the GFRP-balsawood composite structure with good contrast demonstrating the potential of THz imaging for non-destructive testing of sandwich composite structures.

Harikrishnan Brajesh Kaimal

Papers

Non-destructive evaluation of GFRP-wood sandwich structure composite using terahertz imaging

Rapid terahertz imaging for non-destructive evaluation applications using Schottky receivers and spatial adaptive sampling