Investigation on the use of metalens for ultrasound imaging in non-destructive testing

Abstract

Non-destructive testing (NDT) such as ultrasonic testing is one of the quality control and quality assurance management tools that determines the internal structure of an object using frequency sound waves. Most NDT methods can be used to test the materials to find defects or discontinuities on the surface without causing any damage to the specimens. However, the scattering effects will cause the highest resolution of an imaging system to be restricted by half of its wavelength. Therefore, holey structured metamaterials lens will be used to allow high frequencies and evanescent waves to be amplified to overcome the resolution limit.

This project aims to determine if using a higher frequency in A-scan or using metamaterials lenses will improve the image resolution and distinguish the distance for small defects from 4mm, 2mm, 1mm, 0.5mm, 0.4mm, 0.3mm and 0.2mm. Using the pulse-echo method for the A-scan machine, the author will be comparing 4MHz and 20MHz transducers to verify the two distinct peaks for each set of defects. Next, using the immersion method for the C-scan machine, the author will determine if the image resolution has improved and check if the machine can detect the defects when the defects distance is getting closer together with the use of different diameters and lengths of metamaterial lenses and without metamaterial lenses.

The results show that metamaterials lens 2 with the D6L40mm gave a better image resolution compared to the other metamaterial lenses. However, as the defect distance gets closer to 0.5mm to 0.2mm, it is unable to identify two holes and distinguish the distance apart. Whereas, using 20MHz without metamaterial lenses were able to identify the two holes for 0.5mm to 0.2mm but not able to distinguish the distance apart accurately. This report will discuss the experimental result, possible experimental errors, and further improvement of this project.