Session: 18: Student poster competition

Paper Number: 108638

108638 - Optimising Ultrasonic Wavelength for Defect Sizing Using a Geometrically Focussed Emat Array 

Guided ultrasonic waves can be used for sample inspection and defect detection in a variety of materials. Often the constraints of such inspections require that no couplant is used between the transducer and the sample. In the case of electrically conductive materials, electromagnetic acoustic transducers (EMATs) offer a viable solution as they are non-contact and do not require couplant. They can be manufactured to generate and detect specific types of ultrasonic wave, including Rayleigh or Lamb waves, and tuned to generate or be sensitive to a specific frequency range.

This work presents a novel technique utilising an array of geometrically focussed EMATs, generating Lamb waves to inspect a sample. This combines the advantages of Lamb waves’ ability to interrogate the entire thickness of a sample, along with the higher-resolution, point-by-point analysis possible when using a focussed detection technique. There remains a challenge in identifying the optimal balance between wavelength and resolution, particularly when analysing thicker materials (4 mm and above). Conventional wisdom suggests that larger wavelengths result in a lower resolution for defect detection. However, the wavelength must be sufficiently large that Lamb waves are generated, rather than bulk waves, and hence retain their ability to interrogate the entire sample thickness, and to obtain a suitable focal spot size; choice of wavelength must therefore be made carefully. The optimal wavelength for detection of a 10 x 10 mm square defect and a round defect with diameter of 10 mm is identified for different thickness samples. The fundamental antisymmetric mode (A0) and the fundamental symmetric mode (S0) have both been utilised and their respective benefits assessed. The impacts of wavelength and defect size are detailed, identifying the optimal measurement parameters for inspecting different samples.

Presenting Author: Joseph Cregeen University of Warwick

Presenting Author Biography: Joseph Cregeen is a PhD student in the Ultrasound Group at the University of Warwick, UK. His work involves the design and implementation of focussed arrays of electromagnetic acoustic transducers (EMATs) for the detection and sizing of defects within aluminium, along with novel data processing and data fusion methods.