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Frontiers of Mechanical Engineering

Front. Mech. Eng.    2019, Vol. 14 Issue (3) : 273-281
Progress in terahertz nondestructive testing: A review
Shuncong ZHONG()
Laboratory of Optics, Terahertz and Non-Destructive Testing, School of Mechanical Engineering and Automation, Fuzhou University, Fuzhou 350116, China; School of Mechatronic Engineering and Automation, Shanghai University, Shanghai 200072, China
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Terahertz (THz) waves, whose frequencies range between microwave and infrared, are part of the electromagnetic spectrum. A gap exists in THz literature because investigating THz waves is difficult due to the weak characteristics of the waves and the lack of suitable THz sources and detectors. Recently, THz nondestructive testing (NDT) technology has become an interesting topic. This review outlines several typical THz devices and systems and engineering applications of THz NDT techniques in composite materials, thermal barrier coatings, car paint films, marine protective coatings, and pharmaceutical tablet coatings. THz imaging has higher resolution but lower penetration than ultrasound imaging. This review presents the significance and advantages provided by the emerging THz NDT technique.

Keywords terahertz pulsed imaging (TPI)      nondestructive testing (NDT)      composite material      thermal barrier coating     
Corresponding Author(s): Shuncong ZHONG   
Just Accepted Date: 19 April 2018   Online First Date: 21 May 2018    Issue Date: 24 July 2019
 Cite this article:   
Shuncong ZHONG. Progress in terahertz nondestructive testing: A review[J]. Front. Mech. Eng., 2019, 14(3): 273-281.
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Fig.1  Experimental setup of typical transmission THz pulsed spectroscopy (TPS) system [28]
Fig.2  Typical biased THz emitter [30]
Fig.3  Terahertz pulsed imaging (TPI) system [33]
Fig.4  Typical THz waveform measured from single-layer-coated structure [11]
Fig.5  Measured THz pulsed waveforms. (a) time-domain waveform [38]; (b) frequency amplitude spectra
Fig.6  THz NDT measurements of glass fiber composite material [39]. (a) Irregular fiber orientation (C-scan); (b) regular fiber orientation (C-scan); (c) C-scan of inclusion; (d) B-scan of inclusion; (e) C-scan of delamination; (f) B-scan of delamination
Fig.7  Fiber composite imaged by (a) THz and (b) ultrasound [40]
Fig.8  Thickness measurement of TBC topcoat using THz waves [41]. (a) Schematic diagram of the terahertz propagation path; (b) waveform of the terahertz wave measured
Fig.9  Comparison of averaged TGO thickness (SEM method) and delay time (THz method) [44]
Fig.10  TPI reflected waveform of multilayered car paint on carbon fiber substrate [48]. (a) B-scan map measured along the y-direction. (b) time-domain reflection waveform of a single pixel
Fig.11  Comparisons of SWT detail coefficients for intact and defected marine protective coatings [52]. A defect with radius of 12 mm and thickness of 0.18 mm was embedded inside (a) the three antifouling paint layers and (b) the anticorrosive paint layers
Fig.12  3D coating thickness images on two sides and center band of coated biconvex tablet [55]
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