Development of a Super Resolution Imaging Technique Based on Ultrasonic Guided Waves for Non-destructive Testing

Abstract

Guided ultrasonic waves testing is a well-established inspection technique for structures such as plates, pipes, and rail tracks. The method is very useful for inspecting buried or otherwise inaccessible sections of a structure from a remote location. Guided ultrasonic wave, like any other wave, is diffraction limited in that features smaller than half of the working wavelength cannot be resolved under normal circumstances. Defects lying within half a wavelength of design features such as welds and supports may go undetected. Due to the low frequencies used in guided wave testing the resolution capability is relatively poor. Therefore, it is used for screening purposes to inspect and identify areas of concern in a structure. A secondary more sensitive method, such as bulk ultrasonic waves testing, is then deployed to explore in detail the identified areas of concern. Not all areas of structures are easily accessible for purposes of secondary close contact, high-resolution inspection. It was therefore of practical interest to develop a method for detection and characterization of defects beyond the half-wavelength limitation from a remote location. The approach proposed in this research utilized fabricated metamaterials to capture and amplify evanescent waves which are generated at defect boundaries. Both simulations and experiments were carried out in this research. Simulations were done using commercial finite element analysis software, Abaqus, to determine the viability of the method and to optimize the dimensions of the metamaterials. Some of the variables investigated included metamaterial parameters, defect types, size and spacing. Crack-like linear defects and corrosion-like rounded defects in a plate were considered. Subwavelength resolution was achieved in both categories of defects with λ/72 resolution attained for rounded defects and λ/6 for linear defects. The method developed in this research improved the resolution of guided ultrasonic wave inspection to the same sensitivity level as that of bulk ultrasonics while retaining its application over a long range. The results of this research have much promise for remote nondestructive inspection in hazardous environments such as radiation contamination zones as well as petrochemical industry, utilities, pipelines, railways and in biomedicine. In the design of the metamaterials used in this wok it is recommended that the channel holes should be reduced to smaller than a tenth of the probing wavelength and the channel lengths to remain integer multiples of half a wavelength.