Fig. 5: Pulse shape (top) and spectrum (bottom) for a 2 MHz immersion probe with PZT (left) and composite transducer (right)
Such broadbanded probes with high signal amplitude are especially well suited for the nondestructive testing of plastic materials, composite structures, cast iron, austenitic steel and other highly attenuating materials. Depending on the application, composite probes will provide from 3 to 20 dB more sensitivity combined with a distinctly shorter pulse when compared to standard transducers. Due to their low acoustic impedance, composite transducers will especially improve the features of probes with plastic delay lines, e.g. dual or angle beam probes, and immersion probes. Piezocomposite probes are of exceptional advantage for automatic testing machines as here the combination of high sensitivity and extreme resolution is often required. For arrays and paintbrush probes the construction with piezocomposite transducers becomes substantially easier as here only a light backing is needed to produce high resolution probes. This light backing will reduce the mass of the probe thereby making the probe more resistant against thermal and mechanical stress which typically occurs in testing machine applications.
Today, composite transducers can be produced at acceptable costs when the necessary technological equipment is installed. The higher costs of the transducer are partially compensated by reduced labor needed to finish the complete probe. Therefore, the composite transducer will replace the conventional piezoceramic crystals where the technical advantages are clearly visible. However, not all probes need to be broadbanded. On the contrary, the flaw evaluation using DGS method requires narrow banded probes, i.e. the probes with conventional ceramic will maintain their field of application in future.
Dr. Gerhard Splitt , born 1943, studied physics at theUniversity of Hamburg and received there his PhD onsolid state physics in 1975. From 1969 until 1975 he workedas a scientist at the Institute of Applied Physics of theUniversity of Hamburg. Since 1976 he is in charge of thedepartment for probe development at Krautkramer GmbH inHuerth, Germany.Rolf Diederichs 1. Juni 1996, info@ndt.net