Characterization of small defects using ultrasonic arrays: a scattering coefficient matrix approach

The ultrasonic scattering coefficient matrix defines the far field scattering behaviour of a defect as a function of incident and scattering angles. An ultrasonic array with a large aperture can then be used to interrogate a defect over a range of angles and hence extract parts of this matrix. Experimental results are presented using a commercial 64-element, 5-MHz array on two aluminium test samples that contain a number of simulated defects including machined slots and side-drilled circular holes. A sub-array approach is used in which a group of 8 elements are excited in order interrogate the defect from a given angle as well as providing spatial discrimination. In this way a neighbouring circular hole and slot can be spatially resolved and clearly distinguished by their different scattering coefficients over a range of incident angles and scattering angles. Using signals reflected from a known-size side-drilled circular hole as the reference, scattering coefficients of a defect can be normalized. These measured scattering coefficients show a good agreement with scattering coefficients predicted from finite element modelling. For example, the orientations of defects directly below the array are extracted to an accuracy of a few degrees and their lengths were determined with an error of ±10%.