Ultrasonic wavefield imaging in structural health monitoring: A review

This chapter reviews ultrasonic wavefield imaging techniques and their pivotal role in the structural health monitoring (SHM) sector. The chapter begins with an introduction to contact transducer-based ultrasonic wavefield imaging, discussing various approaches, including theoretical solution-based imaging, numerical solver-based imaging, and model-based wavefield inversion. It then transitions to examining the domain of noncontact ultrasonic wavefield imaging, which includes an in-depth analysis of noncontact full wavefield imaging systems such as laser ultrasound, air-coupled ultrasound, shearography, and digital image correlation (DIC), each dissected in terms of their unique methodologies and system integrations. A significant part of the chapter is dedicated to discussing wavefield representations and processing, including a detailed list of notations and a thorough examination of time-space, frequency-space, and frequency-wavenumber domain representations, as well as advanced filtering techniques such as wavelet-aided and frequency-wavenumber filtering. The chapter also addresses various imaging conditions, ranging from amplitude-based and energy-based imaging conditions to phase-based and spatial gradient and curvature-based imaging conditions, culminating in an exploration of wavenumber-based imaging. Special attention is given to the application of these noncontact field measurement techniques in the aerospace industry. It covers in detail dispersion characterization, wave phenomenon studies, and the inspection of various materials and structures such as metallic plates, composite laminates, honeycomb composite panels, and foam-core sandwich composites, detailing their specific challenges such as corrosion, thinning, and fatigue cracks. The chapter concludes with a critical evaluation of the challenges and opportunities in applying these advanced techniques to more complex structures and environments, emphasizing the need for practicality in SHM applications. This review serves as a resource for researchers and practitioners in the field, offering a detailed and current overview of ultrasonic wavefield imaging techniques in the context of structural health monitoring.