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Eingeladener Vortrag
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Air-coupled ferroelectret ultrasonic transducers applied to testing of fiber-reinforced polymers
(2013)
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.
Defect sizing is required for a quantitative assessment of the quality and reliability of safety relevant components and materials using ultrasonic non-destructive testing. The SAFT (Synthetic Aperture Focussing Technique) and the TOFD technique (Time Of Flight Diffraction) are such promising sizing candidates, extracting more information from the raw ultrasound echo data and the corresponding crack tip response. In this work the phased array technique is used to inspect a clad mock-up model of a pressure vessel section. The full scale model contains artificial test reflectors which are located in the weld and in the cladding region as well. The defects—representing typical flaws at a very early stage—are analysed with different frequencies, beam angles and directions of incidence.
For the reconstruction of reflector indications a SAFT algorithm is applied to the phased array measurement results. Additionally the reflectors are analysed by means of the TOFD technique, using different beam angles at the same time. Both analysis methods are performed using different directions of incidence considering the complex cladding structure underneath the inner surface of the mock-up model. A direct comparison of the SAFT and TOFD techniques shows that, besides the clarity of the results, the detection and sizing capabilities of SAFT are far better.
NDT is a multidisciplinary research area fusing the disciplines of natural Science and engineering. Particularly in the early development stages of a new testing method, feasibility investigations focus on the fundamental physical interaction between the sensing mechanism and specimen. Research activities during subsequent phases of modular prototype development, technical System Integration and Validation primarily attempt to solve the practical engineering and real-life aspects of a testing method. We show several examples of new methods in different stages of development and highlight the potential for further industrial use. The first section deals with Steel surfaces of lightweight components which are
inspected with pm-sized magneto resistive sensor arrays. This quantitative magnetic stray field approach combines high spatial resolution and very high sensitivity, enabling the user to detect even very small micro-defects. Tuming to CFRP components we also show new developments in airbome ultrasonic testing. Here we use ferroelectret foils to reduce the acoustical mismatch between transducer and air, which enhances the signal-to-noise ratio. A further example illuminates the use of metallic nano layers as a broadband thermo acoustical emitter. Their use provides both high sound pressures and very broadband Signal characteristics. When it comes to CFRP components and other anisotropic and highly Scattering materials, ultrasonic matrix arrays in conjunction with many angles of incidences per transducer position can help to enhance the statistics of a testing problem. In the field of conventional water-coupled UT testing we also show the use of matrix array transducers
providing a tomographic-like 3D-image of impact damage in CFRP samples.