Ultraschallprüfmethoden für den Fehlernachweis im Plattierungsbereich D
G. Brekow during his lecture

Austenitic cladded test blocks with wall thicknesses of 170 mm, 260 mm and 50 mm and with different artificial crack like reflectors in the cladding region were used for measurements to study improvements for cladding crack detection. The claddings showed thicknesses of 6 and 8 mm in the case of great wall dimensions. Claddings with thicknesses of 8, 10 and 12 mm existed on the test blocks with small wall dimensions. Probes with different frequencies, transducer dimensions and;angles of incidence were coupled at the ferritic side of the test blocks to detect the reflectors in the half skip distance.

The crack like reflectors were electrically eroded or machined notches in the cladding with surface or interface breaking position and additionally in a position in the cladding between surface and interface. Further artificial flaws were also formed as underclad notches. To optimize flaw detection, all the measurements were done with a great variety of different inspection parameters to find out parameter combinations with best signal to noise ratios. Using a phased array probe for shear waves with a frequency of 1.5 MHz the angle of incidence was changed between 35° and 57°. Probes for shear waves with different transducer dimensions, with an angle of incidence of 45° and with frequencies of 1, 1.5, 2 and 4 MHz were also applied to see the influence of inspection parameters.

Reflector indications produced by reflexion or crack tip diffraction were analysed to show the potential of prediction about cladding integrity. Reflector detection in the case of depths as it is reqired in common rules is nearly always possible. But the inspection methods under investigation do not give a discrimination between a surface breaking flaw and a flaw only penetrating the interface between base material and cladding and still having a ligament to the surface. In the case of great wall thicknesses cracks only damaging the interface or underclad cracks show two indications in the A-scan caused by sound beam reflexions at the surface and the interface. But only one indication can be seen in the A-scan when the crack starts in the cladding and reaches the surface. Thin walled cladded components can be inspected with frequencies up to 4 MHz in contrast to thick walled, where already 1.5 MHz is the upper limit. An estimation of crack size can be realized for those cracks extending into the base material, because there are indications from upper crack tip diffraction and from reflexion at the interface.

Because of the unknown dendritic structure orientation in the cladding always both directions of incidence are necessary for flaw detection. This must be demanded because of the direction dependent detection behaviour of the probes, which is decisively dependent on the orientation of the dendritic crystals, and because of differences in the signal to noise ratio up to 8 dB, which can be seen between both opposite directions of incidence. Regarding all the measurements a region of distinguished angles of incidence with a maximum of signal to noise ratio was not observed.