Review of advances in microwave and millimetre-wave NDT&E: principles and applications

Microwave and millimetre-wave non-destructive testing and evaluation (NDT&E) has a long history dating back to the late 1950s (Bahr 1982Microwave non-destructive testing methods; Zoughi 2000Microwave Non-destructive testing and evaluation principles; Feinstein 1967Surface crack detection by microwave methods; Ash 1973 In3rd European Microwave Conference; Auld 1981Phys. Technol.12, 149-154; Case 2017Mater. Eval.75). However, sustained activities in this field date back to the early 1980s (Zoughi 1995Res. Nondestr. Eval.7, 71-74; Zoughi 2018Mater. Eval.76, 1051-1057; Kharkovsky 2007IEEE Instrumentation & Measurement Magazine10, 26-38). Owing to various limitations associated with using microwaves and millimetre waves for NDT&E, these techniques did not see much utility in the early days. However, with the advent and prevalence of composite materials and structures, in a wide range of applications, and technological advances in high-frequency component design and availability, these techniques are no longer considered as 'emerging techniques' (Zoughi 2018Mater. Eval.76, 1051-1057; Schull 2002Nondestructive evaluation: theory, techniques, and applications). Currently, microwave and millimetre-wave NDT&E is a rapidly growing field and has been more widely acknowledged and accepted by practitioners over the last 25+ years (Case 2017Mater. Eval.75; Bakhtiari 1994IEEE Trans. Microwave Theory Tech.42, 389-395; Bakhtiari 1993Mater. Eval.51, 740-743; Bakhtiari 1993IEEE Trans. Instrum. Meas.42, 19-24; Ganchev 1995IEEE Trans. Instrum. Meas.44, 326-328; Bois 1999IEEE Trans. Instrum. Meas.48, 1131-1140; Ghasr 2009IEEE Trans. Instrum. Meas.58, 1505-1513). Microwave non-destructive testing was recently recognized and designated by the American Society for Nondestructive Testing (ASNT) as a 'Method' on its own (Case 2017Mater. Eval.75). These techniques are well suited for materials characterization; layered composite inspection for thickness, disbond, delamination and corrosion under coatings; surface-breaking crack detection and evaluation; and cure-state monitoring in concrete and resin-rich composites, to name a few. This work reviews recent advances in four major areas of microwave and millimetre-wave NDT&E, namely materials characterization, surface crack detection, imaging and sensors. The techniques, principles and some of the applications in each of these areas are discussed. This article is part of the theme issue 'Advanced electromagnetic non-destructive evaluation and smart monitoring'.