Enhanced Capabilities for Sub-Wavelength Defect Detection using Focussed Lamb Waves

Abstract Electromagnetic acoustic transducers (EMATs) are commonly used for non-destructive testing and the location of defects within conductive materials. EMATs can be designed to generate guided waves, such as Lamb waves. Lamb waves are typically used for screening for defects as they can travel long distances through a sample with little attenuation. However, this requires large wavelengths which are less sensitive to small defects. As such, defects smaller than the wavelength may not be detected and are typically challenging to resolve. This work presents a method for the focussing of Lamb waves using curved, geometrically focussed EMATs. The Lamb waves were excited in 1 mm thick aluminium sheets. The size of the focal spot is shown to be dependent on the wavelength of the generation signal. At the wavelength chosen for defect detection, the focal spot is shown to be approximately 3 mm × 4 mm in size. An array of focussed EMATs was used to detect the perturbation of the generation pulse, along with reflections, from 0.5 mm deep square defects located in the plate. B-scans are presented and a method for defect width measurement is proposed, which is shown to be accurate to within ± 0.73 mm for a 10 mm × 10 mm square defect. C-scans are also presented which show the location and geometry of defects with a very good agreement to the known location and size of the machined defects.