Ultrasonic inspection of highly scattering materials

Abstract

Ultrasonic Non-Destructive Evaluation (NDE) relies on the scattering of waves from discontinuities, such as fractures or voids, to probe media otherwise invisible to the naked eye. Whilst this has been industrially exploited for several decades within acoustically transparent materials, many materials maintain a microstructure that causes scattering of the propagating waves. This undermines the aforementioned premise as it becomes exceedingly difficult to discern the features of interest from the scattering inherent to microstructural features, thereby limiting the range of materials which can be reliably inspected, non-destructively. Experimental investigations confirm the challenges and significant shortcomings for the inspection of future industrial components where such microstructures are desirable for their mechanical properties. It is demonstrated that the rapid increases in scattering with the insonifying frequency severely limit the achievable sensitivity of conventional ultrasound techniques. A review of the latest advances in ultrasound technology, including signal processing and imaging algorithms, explore the opportunities to exceed current limitations and advance the capability of ultrasonic NDE. Establishing these advances, and those of future approaches, requires a rigorous definition of performance. In contrast to commonly adopted strategies, a novel strategy which considers the probabilities of detection and false alarms is proposed as a valuable benchmark that can be used to make objective comparisons in terms of performance between competing algorithms. Future progress will also rely on a better scientific understanding of scattering, which can be provided by powerful modelling tools. Here, Finite Element modelling is established to be very useful; it captures the complex scattering physics and allows an investigative flexibility which can provide extremely useful insights. Whereas previous studies have often been restricted to weak scattering assumptions, the present FE modelling capability now enables the study of more complex, highly scattering environments. This is demonstrated by investigating ultrasonic arrays, where through optimising their engineering, especially in terms of their configuration, significant performance enhancements are shown to be possible. These important scientific tools have enabled the assessment of the latest imaging algorithms, the optimisation of inspection configurations, and increased our understanding of scattering phenomena. Their use in the future enables wide possibilities towards further pursuing the ultrasonic inspection of highly scattering materials.