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Eingeladener Vortrag
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The spatial resolution in ultrasonic testing as a wave phenomenon in nature is limited to a
certain fraction of the wavelength, usually defined to be close to the Rayleigh criterion. In case of
complicated reflector surfaces – such as stress corrosion cracking - this limitation prevents an exact
visualization of the defect shape. There exist a few approaches to improve the spatial resolution, whose
reconstruction quality all in common also depend on the achieved signal-to-noise ratio of the raw data. In this
work we present a specific SAFT analysis, in which a high number of different angles of incidence produce a
sufficiently high number of different reflections at the crack edges resulting in an improved SNR. In doing
so, we reconstruct a coherent crack structure. First investigations were made at artificially simulated crack
configurations with different contours and curvatures in flat and cylindrical test blocks. The measurements
results visualized by representative scans – show details of crack design and crack orientation. We also will
present a comparison of the SAFT analysis between modelling and phased array measurements.
Polypropylene based ferroelectret films exhibit a strong electromechanical activity and provide a promising solution for the air coupled ultrasonic (ACUS) transducers. Ultrasonic transmission between two air coupled ferroelectret transducers in dependence on the amplitude and polarity of the high voltage exciting pulse revealed a strongly non-linear electromechanical response of the ferroelectret transmitter which provides an increase in the transmitter efficiency. The authors present a simple model describing both promotion and competition of the piezoelectric and electrostriction contributions, as well as increase in the transducer constant under high voltage excitation. Enlargement of the inverse transducer constant of the polypropylene ferroelectret film by a factor of 4 was demonstrated. The non-linear properties of the polypropylene ferroelectrets result in a strong increase in their ACUS figure of merit under the high voltage excitation, which exceeds the results of their technological optimisation. Consequently, enhancement of the ACUS system transmission by 12 dB and signal to noise ratio by 32 dB was achieved.
Quantitative NDE methods play a key role when it comes to inspect components,
which requires high operational safety. UT-SAFT is one of the well-known reconstruction tools,
which provides information about the defect size. In this work we studied the use of phased array
technique in combination with the SAFT algorithm to inspect power plant components. As a first
example we inspected a real-sized mock-up model representing a part of a reactor pressure vessel with
a 180 mm-thick ferritic base material followed by a 6 mm-thick austenitic cladding layer. The phased
array probe was coupled at the outer ferritic surface. We detected and sized fatigue cracks within the
cladding with a depth ranging from 4 mm to 10 mm. Secondly, we investigated a mock-up model
resembling a nozzle including a thermo sleeve inlet and a maximum wall thickness of about 37 mm.
Artificially inserted notches with a depth of 3 mm could be detected and sized, where the thermo
sleeve is welded at the inside of the nozzle.
Stress corrosion cracking is a transcrystalline or intercrystalline crack formation in materials which
occurs under the influence of static tensile stress or residual stress and a specific aggressive
medium such as chloride containing substances.
This special sort of crack formation is involved in complex crack configurations, which may
only insufficiently be captured by conventional ultrasonic probes with fixed angles of incidence.
Only a number of different beam angles produce sufficient reflection to reconstruct the complete
defect shape from measured ultrasonic data. The SAFT algorithm, which was recently successfully
used in many industrial NDT-applications, is a promising tool for the reconstruction process. In
addition, the combination of phased array technique and SAFT was developed in several projects
by BAM, Berlin. Investigations using phased array equipment were performed to look into crack
configurations in test blocks with different surface curvatures. UT-SAFT has been used for the
analysis of spark-eroded notches simulating stress corrosion cracks at the thermo sleeve weld of a
nozzle. For comparison, UT-SAFT has also been applied for the analysis of real reflectors at the
same position in a nozzle used in a power plant, which was repaired later on. SAFT-scans received
from reconstructed ultrasonic measurement data confirm the practical usefulness of the SAFTalgorithm
developed by BAM.
We present a generalized analytical model of thermo-acoustic sound generation based on the analysis of thermally induced energy density fluctuations and their propagation into the adjacent matter. The model provides exact analytical prediction of the sound pressure generated in fluids and solids; consequently, it can be applied to arbitrary thermal power sources such as thermophones, plasma firings, laser beams, and chemical reactions. Unlike existing approaches, our description also includes acoustic near-field effects and sound-field attenuation. Analytical results are compared with measurements of sound pressures generated by thermo-acoustic transducers in air for frequencies up to 1 MHz. The tested transducers consist of titanium and indium tin oxide coatings on quartz glass and polycarbonate substrates. The model reveals that thermo-acoustic efficiency increases linearly with the supplied thermal power and quadratically with thermal excitation frequency. Comparison of the efficiency of our thermo-acoustic transducers with those of piezoelectric-based airborne ultrasound transducers using impulse excitation showed comparable sound pressure values. The present results show that thermo-acoustic transducers can be applied as broadband, non-resonant, high-performance ultrasound sources.
In this contribution we present a novel thermo-acoustic approach for the generation of broad band airborne ultrasound and investigate the applicability of resonance-free thermo-acoustic emitters for very short high pressure airborne ultrasound pulses. We report on measurements of thermo-acoustic emitter consisting of a 30 nm thin metallic film on a usual soda-lime glass substrate, generating sound pressure values of more than 140 dB at 60 mm distance from the transducer and compare the results with conventional piezoelectric airborne ultrasound transducers. Our experimental investigations show that such thermo-acoustic devices can be used as broad band emitters using pulse excitation.