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Visualization of sound propagation with electrodynamic probes

When dealing in ultrasonic testing with inhomogeneous material structure
data interpretation can be rather difficult. This is especially the case when using
anisotropic dissimilar welds made from austenitic steel or nickel based alloys, which are
currently used for modern power plant concepts. For better understanding of the
complex interaction between the sound field and the component under test, the
visualization of sound propagation in solids is a substantial task to increase the
probability of detection of relevant defects. However, there exist only a small number of
appropriate techniques published today, such as scanning laser interferometer,
piezoelectric and optical approaches in case of transparent solids. In this work we
present an electrodynamic technique providing a simple use and a high signal to noise
ratio. By detecting the grazing beam with an electrodynamic probe with a size smaller
than 10 mm, we measured the particle displacement as a function of time with a spatial
resolution in the order of 1 mm. Adapting the electrodynamic probe and its coil
alignment allows for measuring the displacement components in all three dimensions.
This comprises the detection of the horizontal and vertical particle displacement with
respect to the surface and thus also the transformation from longitudinal waves into
transversal waves and vice versa is possible. A SNR of higher than 36 dB could be
achieved within ferromagnetic and high conductive chrome steel when using a
transversal wave generated by an angled beam transducer. We report on measurements
of the sound field in complex weld joints. One example shows a 10 mm thick narrowgap
weld joining a nickel alloy with a chrome steel yielding a substantial anisotropy of
the weld structure. The test system enables us to visualize the wave propagation within
the weld and indicates the reflection scenario and the energy losses due to both the
anisotropic structure and material defects.