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 1982 ; Zoughi 2000 ; Feinstein 1967 ; Ash 1973 In ; Auld 1981 , 149-154; Case 2017 ). However, sustained activities in this field date back to the early 1980s (Zoughi 1995 , 71-74; Zoughi 2018 , 1051-1057; Kharkovsky 2007 , 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 2018 , 1051-1057; Schull 2002 ). 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 2017 ; Bakhtiari 1994 . , 389-395; Bakhtiari 1993 , 740-743; Bakhtiari 1993 , 19-24; Ganchev 1995 , 326-328; Bois 1999 , 1131-1140; Ghasr 2009 , 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 2017 ). 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'.