Anton Erhard

Piezoelectric and electrostrictive effects in ferroelectret ultrasonic transducers

Electromechanical response of polypropylene ferroelectret transducers under application of high-voltage pulses was measured by laser Doppler vibrometry and compared with results of ultrasonic through-air transmission between two ferroelectret transducers. The electromechanical response was completely explained by piezoelectric and electrostrictive effects. The electrostrictive effect dominates at high voltages and provides significant enlargement of the transducer constant, up to factor of 2.5. The induced strain of 1.7% was achieved at −2000 V. The nonlinear ultrasonic transmission was shown to be well described by the piezoelectric and electrostrictive response of transmitter, except in the range of high negative exciting voltages where some limitation of the transmitted signal was observed. This limitation seems not to be a fundamental one and does not abolish the advantages of high-voltage excitation of polypropylene ferroelectret transducers.

Air-Coupled Ultrasonic Applications of Ferroelectrets

Improvement of ferroelectret transducers for air-coupled ultrasonic applications was demonstrated on the base of optimization of the ferroelectret electromechanical properties and adjustment of the ferroelectret transducers to the ultrasonic system. Controlled inflation of the polypropylene ferroelectret films results in an increase of the coupling factor in two and the figure of merit in ten times. Integration of a pre-amplifier with high impedance matching increases the ferroelectret transducer sensitivity in ∼ 40 dB. Ferroelectret transducers are characterized by the broad transmission band with smooth amplitude-frequency dependence and provide a well shaped and short pulse transmitted through the air.

Non-destructive Evaluation

Non-destructive testing (NDT) methods are employed in the technical field as a precautionary to avoid accidence or emergency situations for the human and the environment. A risk situation in the technical field could happen if the load of a component is increasing and the wall thickness due to defect growth is decreasing until a critical defect situation. The result of such scenarios could be catastrophic breakdown of the component with sometimes severe aftermath for the human.

Aging management for long-term interim storage casks

Abstract The aging management system for the mechanical components of nuclear power plants (NPPs) must be established and used by the licensee in such a way that the quality of safety relevant components is guaranteed for the completely designed lifetime of the NPP. This demands an extensive plant life management with special emphases on the knowledge of the degradation in material properties. The basic safety concept in Germany observes this circumstance. Lifetime extension of the German NPPs is an aim of the current valid coalition agreement of the German government. Operational extension of interim storage facilities requires, in comparison to the aging management system for NPP, an aging management system adapted to the special circumstances of spent fuel storage casks. Extension of interim storage periods for spent fuel casks beyond the designed lifetime requires, in comparison to the components of an NPP, an increasing knowledge of material degradation with potential impact on cask integrity, e.g. leak tightness. Dry interim storage in Germany has been approved for 40 years. After that time, according to the present strategy, a final repository should be available. However, until now, such a final facility still does not exist, and the German exploration and licensing process is heavily delayed. Currently, discussions are continuing regarding further exploration of the Gorleben salt mine. There is willingness to overcome this situation that is clearly described in the available coalition agreement of the federal government. Anyway, however, the prediction is viewed; a repository for heat generating radioactive waste in Germany will not be available in the near future and may not be available when first storage facilities and casks reach their 40 years of approved lifetime, which will occur in ∼25 years starting from now. Therefore, the question must be asked: what has to be done with the existing storage casks in the interim facilities? May these casks be fit for continued use, with an extension of the storage period? One option is to have an aging management system, which creates enough information about the technical condition of safety relevant cask properties. This is the basis for safety evaluation for extended storage periods. In the present paper, possible aging mechanisms for high level waste storage casks are discussed, as well as the influence of the time dependent changes of the component properties.

Enhanced electromechanical response of ferroelectret ultrasonic transducers under high voltage excitation

Abstract Abstract 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.

Viscoelastic properties of cellular polypropylene ferroelectrets

Viscoelastic properties of cellular polypropylene ferroelectrets (PP FEs) were studied at low frequencies (0.3–33 Hz) by dynamic mechanical analysis and at high frequencies (250 kHz) by laser Doppler vibrometry. Relaxation behavior of the in-plane Youngs modulus ( Y11′ ∼ 1500 MPa at room temperature) was observed and attributed to the viscoelastic response of polypropylene matrix. The out-of-plane Youngs modulus is very small ( Y33′ ≈ 0.1 MPa) at low frequencies, frequency- and stress-dependent, evidencing nonlinear viscoelastic response of PP FEs. The high-frequency mechanical response of PP FEs is shown to be linear viscoelastic with Y33′ ≈ 0.8 MPa. It is described by thickness vibration mode and modeled as a damped harmonic oscillator with one degree of freedom. Frequency dependence of Y33* in the large dynamic strain regime is described by the broad Cole-Cole relaxation with a mean frequency in kHz range attributed to the dynamics of the air flow between partially closed air-filled voids in PP FEs. ...

Performance Control: Nondestructive Testing and Reliability Evaluation

The performance of materials – as constituents of the components of engineering systems – is essential for the functionality of engineering systems in all branches of technology and industry. Instrumental for characterizing the performance of materials are 1. methods to study and assess the basic damage mechanisms that detrimentally influence the proper functioning of materials, such as materials fatigue and fracture (Chap. 7), corrosion (Chap. 12), friction and wear (Chap. 13), biogenic impact (Chap. 14), materials–environment interactions (Chap. 15) 2. methods to study and assess the performance of materials in engineering applications and to support condition monitoring of materialsʼ functional behavior. In this chapter the following experimental and theoretical methods for performance control and condition monitoring are compiled. Nondestructive evaluation (NDE) methods Methods of industrial radiology Methods of computed tomography (CT) Embedded sensors techniques to monitor structural health and to assess materials performance in situ under application conditions Methods to characterize the reliability of materials with statistical tools and test strategies for structural components and complex engineering systems.

Air-Coupled Ultrasonic Transducers Based on Cellular Polypropylene Ferroelectret Films

Analysis of the dielectric and electromechanical properties of the cellular polypropylene ferroelectret films (EMFIT) evidences their high potential for the application in the air-coupled ultrasonic transducers. Simple prototype transducers, based on the EMFIT films, were developed and their high sensitivity was proved in the air-coupled ultrasonic experiment. Amplitude and delay time scanned images of the polyethylene step wedge with holes, obtained in both pulse-echo and transmission modes, demonstrate that non-contact ultrasonic imaging with EMFIT transducers is possible.

Ferroelektret-Prüfköpfe für die zerstörungsfreie Prüfung mit Luftultraschall

Kurzfassung Für die luftgekoppelte Ultraschall-Prüfung wird der Prototyp eines neuartigen Prüfkopfes vorgestellt. Statt eines piezokeramischen Ultraschall-Wandlers mit einer λ/4-Anpassschicht wird eine Ferroelektret-Folie aus schaumförmigem Polypropylen eingesetzt. Durch die außerordentlich geringe akustische Impedanz der Folie reduzieren sich die Reflexionsverluste an den Grenzschichten zur Luft so stark, dass bei diesem Prüfkopf auf eine λ/4-Anpassschicht verzichtet werden kann. Außerdem ist die Folie hochspannungsfest. Sie konnte mit Spannungen von mehr als 3500 V angesteuert werden und erlaubte eine Vorspannung am Ultraschall-Wandler des Empfangsprüfkopfes von 2000 V. Mithilfe der hohen Anregungsspannung stieg die Amplitude des akustischen Prüfimpulses auf das 11,2-Fache. Die Empfindlichkeit des Empfängers erhöhte sich durch die Vorspannung weiter um den Faktor 4,3. Insgesamt steigt die Amplitude am Empfänger also auf das 48-Fache, bei gleichbleibendem Rauschpegel. Ein abschließender Vergleich mit kommerziellen Luftultraschall-Prüfköpfen belegt die Überlegenheit der Folien-Prüfköpfe. Damit rückt der Einsatz einer vollwertigen Impuls-Echo-Technik auch bei der luftgekoppelten Ultraschallprüfung deutlich näher.

Vibrations- und Stoßbelastungen an IBCs und Tankfahrzeugen

Kurzfassung Die unterschiedlichen Stoß- und Vibrationsbelastungen bei der Fahrt auf Straßen oder im freien Gelände dürften den meisten Lesern aus dem täglichen Leben bekannt sein. Je nach Ausführung und Zustand des Feder-/Dämpfungssystems eines Fahrzeuges spürt der menschliche Körper unterschiedliche Belastungen, die die Schmerzgrenze erreichen können. Diese dynamischen Belastungen werden auch auf Ladung, in unserem Fall auf die Gefahrgutumschließungen und auf das Gefahrgut, übertragen. Derartige Belastungen können durch entsprechende Stapelung und Sicherung der Ladung in ihren Auswirkungen gemindert werden. Für andere Belastungen, z.B. Stoß durch einen Auffahrunfall, sind Schutzsysteme am Heck des Fahrzeugs gefordert. Bei Tankfahrzeugen kann eine Beschädigung des Tanks durch einen solchen Auffahrschutz gemindert werden. Im vorliegenden Beitrag werden Ergebnisse von Vibrationsmessungen an Lastkraftwagen mit den Ergebnissen des Vibrationstests an einem Intermediate Bulk Container (IBC) verglichen. Berechnungen zum Auffahrschutz an Tankfahrzeugen werden Messergebnissen gegenübergestellt.