Ultrasonic inspection of near surface defects with additive manufactured metasurface lens

Near surface defects in engineering encompass flaws, cracks, or imperfections that occur in close proximity to the surface of a material or component. These defects can arise from various factors, including manufacturing processes, environmental conditions, or mechanical stress. Scratch marks, pits, cracks, and delamination are examples of near surface defects found in materials such as metals, ceramics, and composites. Their presence significantly affects the mechanical properties and durability of the material, thereby impacting its performance in diverse applications. Consequently, the development of reliable and effective strategies to mitigate these surface defects is highly desirable.

In engineering, non-destructive testing techniques such as ultrasonic testing, eddy current testing, X-ray radiography, and magnetic particle inspection are commonly employed to detect and characterize near surface defects. These techniques are vital for ensuring material and component quality and reliability. They can identify defects that may not be visible to the naked eye, thus preventing catastrophic failures. Ultrasonic inspection, for instance, has gained attention due to its flexibility, efficiency, and relatively low cost. However, it currently faces limitations in detecting near surface defects using bulky waves, primarily due to the presence of destructive and constructive interference and the effects of electronic transmission noise. Similarly, alternative methods like surface waves and phased arrays encounter their own drawbacks.

In recent times, metamaterial-assisted inspection has emerged as a promising solution, driven by advancements in additive manufacturing technology. Metamaterials possess properties that are highly advantageous for numerous applications. Acoustic metasurfaces, in particular, can enhance the compactness of acoustic systems and amplify harmonics generated by defect nonlinearities. Though there have been a few studies on the inspection possibilities of metamaterials integrated with commercial transducers, further research is needed to validate these claims.

A study mainly conducted by Dr. Gianluca Memoli from the University of Sussex, and Dr. Qi Zhu from Shanghai University, explored the use of an acoustic metasurface lens (MSL) for nondestructive detection, inspection, and characterization of near surface defects using longitudinal waves. The researchers established the design limits and parameters, including material selection and delay design, for an MSL operating at 100 kHz. They assembled a lens by optimizing a library of labyrinthine cells for transmission and simulated its performance in a hole inspection process. The study, titled “Metasurface Lens for Nondestructive Detection, Inspection, and Characterization of Near Surface Defects with Longitudinal Waves,” was published in the peer-reviewed journal Frontiers in Materials.

The authors demonstrated that by combining additive manufacturing and time/frequency domain analysis, it was possible to transform a traditional transducer into a reliable focusing transducer using an MSL at a relatively lower cost. The feasibility and functionality of the MSL transducer design were experimentally validated using a UTR9000-based MSL on an acrylic sample. Simulations indicated increased interface and scattering effects, resulting in a significant drop in peak amplitude from 20% without MSL to 70% with MSL, thanks to the focusing behavior.

Additionally, the presence of extra transmission peaks facilitated the calculation of defect diameter with improved accuracy, mitigating the effects of near surface interference. The ring scheme employed served several critical functions: separating the incident plane wave into 10 channels, enabling free beam control design, mitigating near field effects, and controlling the phase delay profile to achieve convergence of the ultrasonic beam near the surface.

In summary, this study is the first to demonstrate the application of a metasurface focusing lens for comprehensive defect inspection and nondestructive testing purposes. The proposed lens design proved effective in detecting near surface defects. Professor Gianluca Memoli, in a statement to Advances in Engineering, explained that this new study represents initial steps towards developing flexible beam control systems using a single transducer.