Time-of-flight diffraction tomography for NDT applications

An ultrasonic imaging system based on a direct time-space reconstruction algorithm has been developed. The radio-frequency signals are collected along a linear synthetic aperture with point sources and receivers in separate positions. This allows the investigation of defects with different incident angles and the final image is less sensitive to the defect characteristics of size, shape and orientation. Theoretical formulae for the lateral and the axial resolution are derived and the actual system performance is evaluated with a computer testing program. Theoretical models of ultrasonic scattering from simple artificial defects embedded in metals, based on the Born approximation and the geometrical theory of diffraction, are employed to simulate signals from some simplified objects covering a broad ka range, with ka up to about 5. Comparisons between theoretical, simulated and experimental lateral and axial resolution are reported, with lateral resolutions found by experiment of 1.34λ, by simulation λ compared with the theoretical limit of 0.36λ.