Modeling of Ultrasonic Nondestructive Testing of Surface-breaking Cracks

Paper in proceeding, 2012

A complete model of ultrasonic nondestructive testing of defects in thick-walled components has been developed since more than a decade and has resulted in the computer program UTDefect. The program includes models of transmitting ultrasonic probes, and through a reciprocity argument also recieving probes. A number of simply shaped defects are included. Here the focus is on surface-breaking strip-like cracks, and a comparison of these with strip-like interior cracks. Both scattering problems are solved with a hyper-singular integral equation method, where the unknown crack-opening displacement (COD) is expanded into sets of Chebyshev functions. These expansions are different for the two cracks; whereas the interior crack has a COD that behaves as a square root at the tips, the surface-breaking COD has a finite value at the crack mouth, and these behaviours must be taken into account. Numerical comparisons are made between the two cracks, and it seems that the interior crack pushed to the surface behaves very much like the surface-breaking one, except at very low frequencies (crack size less than half a wavelength). Favourable comparisons with experiments are also made.