EMAT application: Corrosion detection with guided waves in rods, pipes and plates

Corrosion on tubular structures and the targeted use of dispersive ultrasound wave modes offers the possibility to detect corrosion-related wall thickness reductions along the path of the ultrasonic wave considering ultrasonic transmission measurements. The effect measured is based on the fact that both phase- and group velocity of the ultrasonic wave are a dispersive mode that varies with wall thickness. Through analysis of phase position and /or travel time of specific modes, information resulting from the wall thickness reduction, along the acoustic wave path can be accessed. This is made possible through Electromagnetic Acoustic Transducers (EMATs), which allow a pure mode excitation of a guided ultrasonic wave to be generated, such that this excitation as well as receiving configuration allows an exact evaluation of the phase position to be obtained. Furthermore, a targeted impression of a desired trace wavelength is possible. Due to the EMAT principle, generation and recording of an ultrasonic signal is done without direct contact with the specimen. Fraunhofer IZFP's newly developed EMATs not only allow the variation of ultrasonic modes, but also a change in the trace wavelength. Hence, by analysis of the phase in different specific modes and locations on the component to be inspected, important information for the quantitative error determination can be found. Particularly, this approach can be used to detect shallow wall thickness changes, which do not generate measurable reflection signals.