Waveguide-based in-process ultrasound inspection and imaging in harsh environments: advances and challenges

Process engineering often requires continuous monitoring of structural health in components exposed to chemically aggressive environments or high temperatures. Examples include thermal solar power plants, where liquid-metal heat-transfer fluids must be monitored to ensure safe and efficient operation, and lithium or next-generation batteries, where liquid-metal electrodes or electrolytes may develop uneven flow, instabilities, or hotspots detectable in real time using ultrasound. Conventional contact ultrasound imaging, however, is limited in these applications because transducers are highly sensitive to harsh conditions, since harsh environment already include elevated temperatures, often resulting in device failure. Waveguides provide a promising alternative by shielding transducers from aggressive environments. Single-mode waveguides (SMWGs) have been widely studied for ultrasound testing, offering protection from thermal damage but requiring slow point-by-point scanning for imaging. Multimode waveguides (MMWGs) have therefore been proposed for ultrasound imaging at high temperatures, though challenges remain regarding wettability, geometry, boundary conditions and the development of aberration correction algorithms to address wavefront distortion. This paper presents a systematic review of ultrasound waveguide transducers for structural health monitoring under high-temperature conditions. It evaluates key designs, highlighting their benefits, drawbacks and contributions to knowledge and surveys aberration correction methods and modelling approaches applicable in harsh environments. The review also traces the evolution from SMWGs to advanced MMWGs that exploit multiple wave modes, discusses opportunities in computational ultrasound imaging, and outlines future research directions. These findings support the optimization of waveguide-based ultrasound imaging for high-temperature liquids in solar power and battery applications, with potential transferability to improve medical ultrasound imaging through aberrative medium (e.g skull).