Application of advanced ultrasonic techniques to the inspection of pressure vessel components

After some years of experimental work, some ultrasonic techniques for non-destructive testing of thick steel components have been developed by CISE, and put to an in-field working stage (+). In particular, four methods of inspection have been investigated: short pulse wave technique (SPWT); time of flight (TOF); multifrequency acoustic holography (mfah); synthetic tomography (ST); spsw consist in a mathematical algorithm applied to waveforms produced by pulse echo technique in order to size planar defects of known orientation from the deconvolved echo time duration. Tof carries out a similar analysis, taking advantage by the use of two ultrasound transducers, one transmitting and the other receiving the diffracted signals. MFAH is a more sophisticated technique, based on principles yet established in the environment of optics, so giving an image of defects detected by pulse echo techniques during a scanning, the use of multifrequency ultrasounds improves the axial resolution often poor in monochromatic holography. ST is new technique based on multi-element piezoelectric transducers which allows to deflect and focalize the acoustic beam ST provides in few minutes an image of the defects, under the form of a sector scanning. All these techniques are devoted to location and characterization of flaws. The first two are well suited for a quantitative sizing of the defects, not affected by the characteristics of the equipment and of the transducers and are successfully employed where geometry prevents the use of the imaging methods. Holography gives three-dimensional characterization of defects, providing the best spatial resolution, but demands a simple geometry in order to make the scanning possible. Synthetic tomography provides two-dimensional images of the flaws, with lower problems of inspectability of the component, thanks to the limited size and no need of mechanical scanning