High resolution in non-destructive testing : A review

Abstract

Since the beginning of the applications of non-destructive testing/evaluation (NDT/NDE) techniques, efforts have been made consistently to improve their detection sensitivity and resolution. In the present paper, factors governing lateral resolution in three major NDT techniques, viz., ultrasonic testing (UT), x-ray radiographic testing (XRT), and eddy current testing (ECT) are presented. Furthermore, a review of recent advances in these NDT techniques to reach the theoretically achievable resolution limit or even surpassing the same using alternate approaches is also discussed. For example, resolution in UT is theoretically limited to half the wavelength by the Rayleigh limit; however, subwavelength resolutions have been achieved through the applications of near field methods by capturing the evanescent field. On the other hand, the resolution achieved in XRT is primarily limited to half the source/focal spot size, which is many orders of magnitude larger than the wavelength. Over the years, the reduction in the focal spot from macro-focus to micro-focus and now to nano-focus has led to improvement in the resolution to a few nanometers, of course, in combination with suitable magnification required due to detectors with limited pixel size (a few μm to a few 10 s of μm). Similarly, innovations in electromagnetic/magnetic sensors have significantly improved the resolution achieved in ECT. Atomic force microscopy, metamaterials, and artificial neural network-based methods have been employed for obtaining high-resolution NDE images. At the end, authors' perspective toward possible directions for high-resolution NDT is presented.