Detection and characterisation of defects in highly scattering materials using ultrasonic arrays

Coarse grained materials are widely used in the power generation industry and their inspection is critical for the safe operation of nuclear power plants. Ultrasonic array imaging is an important non-destructive testing method which is capable of detecting and characterising defects in a component. However, the performance of ultrasonic inspection can be severely affected by the existence of grains in a polycrystalline material which cause ultrasonic attenuation and backscatter. In this paper, we aim to explore the effect of two important parameters, frequency and inspection depth, on the characterisation accuracy of small side- drilled holes through experiments. The average grain size of the Inconel specimen is 750μm, and 2mm holes (in diameter) are found to be detectable when the frequency does not exceed 2 MHz even at large distances from the array (i.e. 60mm). Moreover, it is shown that reliable sizing of these small holes is achievable using the scattering matrix.