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- Cellular polypropylene (3)
- Magnetic flux leakage (3)
- Magnetic stray field (3)
- Non-destructive testing (3)
- Reconstruction (3)
- Active thermography (2)
- Crack sizing (2)
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Organisationseinheit der BAM
- 5 Werkstofftechnik (2)
- 5.2 Metallische Hochtemperaturwerkstoffe (2)
- 8 Zerstörungsfreie Prüfung (2)
- 8.4 Akustische und elektromagnetische Verfahren (2)
- 9 Komponentensicherheit (2)
- 5.1 Mikrostruktur Design und Degradation (1)
- 7 Bauwerkssicherheit (1)
- 7.1 Baustoffe (1)
- 8.5 Röntgenbildgebung (1)
- 9.2 Versuchsanlagen und Prüftechnik (1)
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.
Fast defect parameter estimation based on magnetic flux leakage measurements with GMR sensors
(2011)
We present a fast inverse scheme that is capable of simultaneously estimating the parameters depth, opening and length for rectangular 3D geometries of surface-breaking defects. The parameter estimation is realized by an iterative least-squares minimization using the trust-region reflective algorithm. A semi-analytic magnetic dipole model that allows the sensor characteristics to be incorporated is used for predicting the stray magnetic fields. Giant magneto-resistance (GMR) measurements were carried out on a test specimen that includes a series of artificial defects. For the estimation of the defect depths relative errors between 0.6% and 15.9% have been obtained. Due to its very low computational costs, the inverse scheme can suitably be employed in automated production environments.
Active thermography is a nowadays widely used NDT method making use of thermal material properties for defect detection. Basically, the sample is heated and the resulting surface temperature is recorded by an IR camera. For laser thermography a laser is used to heat the sample locally. The resulting spherical heat flow allows the detection of voids in arbitrary orientation. In this work, a method is presented which is suitable for the quantitative characterization of depth and angle of surface cracks. The main idea is to evaluate the crack-caused asymmetries of the laser's thermal footprint. The heat is introduced at fixed reference positions relative to the crack. In this paper a data analysis procedure is presented which allows the crack depth and angle to be described by only two characteristic scalar parameters. By investigating artificial test specimens with spark eroded notches, the feasibility of this method is validated. Furthermore, the behavior of the characteristic parameters with variations of crack angle, depth and experimental conditions is studied systematically by FEM simulations, showing that these parameters are well behaved.
The structures and properties of silver selenide samples strongly depend on the method of preparation. Thin films prepared by pulsed laser deposition (PLD) contain lamellae of two low-temperature modifications of Ag2Se whose structures were analyzed in detail by electron microscopy techniques. Besides the stable and known Naumannite-type phase (LT1), a metastable phase was observed (LT2), and the phase transformation was examined in situ. Via cooling and annealing, LT2 becomes dominant in thin films, thus enabling solving the structure of LT2 by electron diffraction. These experiments and HRTEM point to an Acanthite-type (stable low-temperature modification of Ag2S) structure of LT2. Rapid-quenching procedures were applied for the first synthesis of LT2 as bulk sample. Depending on the conditions of melt spinning experiments, well-defined products were formed. In one series, we found a separation of LT1 and LT2 into distinct morphologies, i.e. microspheres and dendrites, respectively. The results on the magnetoresistance of the rapidly quenched materials support previous work.
Impulse and lock-in thermography have been applied to detect delaminations of prototype solder joints, similar to those to be produced between Cu shunts and Cu busbar stabilisers at the Large Hadron Collider (LHC) at CERN. Two infrared cameras with different detector materials and with different spectral ranges and two excitation techniques have been tested and compared for their ability to detect delaminations behind 2 and 3 mm thick Cu shunts. We have analyzed the signal to noise ratio (SNR) for each detected defect and are able to detect defects down to a nominal edge length of 4 mm behind 2 mm thick Cu shunts by using fast impulse thermography and a camera with a microbolometer array. For the 3 mm thick Cu shunt, on the other hand, the nominal 4 mm defect is only visible in the lock-in thermography phase images and the highest SNR has been achieved with a cooled InSb-based camera. In addition, numerical simulations show the influence of the minimum detectable defect size on the shunt thickness and that the developed on-site testing technique is sufficient to find all defects that are detectable theoretically.
Spot welding is one of the most important technologies for joining sheet metal. While there are lot of approaches to non-destructive testing, quality assurance still mainly relies on welding parameter monitoring and destructive testing, leading to significant failure rates. In this paper an approach to spot weld testing using flash thermography is presented. The main focus of attention is on the identification of two typical error classes: stick welds and welds at the splash limit. Besides investigating the principal feasibility of thermography for zinc plated samples the results of a series test of spot welds joining 1 mm thick TRIP steel are shown. Based upon these results a statistical criterion is developed which allows a reliable classification of the named error classes.
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.
Epitaxial thin films of nitrogenated La0.65Sr0.30MnO3 were grown on MgO(100) substrates by pulsed laser deposition (PLD). The nitrogenation was achieved by a continuous nitrogen flow in the PLD chamber with pressures of up to 0.12 mbar. The chemical analysis of the samples regarding the exchange of oxygen by nitrogen was achieved by time of flight secondary ion mass spectrometry, sputtered neutral mass spectrometry (SNMS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and yielded a content of incorporated nitrogen ranging from 0.6% to 3.8%. Without nitrogenation the electrical resistivity of La0.65Sr0.30MnO3 exhibited a metalinsulator (MI) transition at about 180 K. The magnetoresistance (MR) effect (ΔR/R(0)) was about -50% at the transition temperature. Our nitrogen contents affected the MI transition so as to completely disappear and resulted in a resistivity increase of more than three orders of magnitude as well. By carefully reoxidizing the samples with subsequent heat treatments in air the MI transition reappeared at lower temperatures and we found a continuously enhanced MR ratio for decreasing temperatures. MR ratios of more than -99% were observed for a magnetic field of 10 T. The results are interpreted as a percolation phenomenon of ferromagneticmetallic domains within an antiferromagneticsemiconducting matrix.
Aufgrund eines anhaltenden Trends der Miniaturisierung von Bauteilen sowie eines gesteigerten Qualitätsanspruches in Fertigung und Instandhaltung besteht ein wachsender Bedarf für hochauflösende zerstörungsfreie Prüfverfahren. Ein derzeit vielversprechender Lösungsansatz in der elektromagnetischen Prüfung wird durch die magnetischen Mikrosysteme eröffnet. Insbesondere die jüngst mit dem Physik-Nobelpreis ausgezeichnete GMR-Technologie bietet neben einer hohen Feldempfindlichkeit zugleich hervorragende Ortsauflösungen mit Schichtabmessungen bis in den unteren µm-Bereich. Hierdurch werden magnetische Feldverteilungen bei der Wirbelstromprüfung oder der Streuflussprüfung mit einem gegenüber Spulensystemen erhöhten Signal/Rausch-Verhältnis (SNR) und einer verbesserten Ortsauflösung gemessen. Im vorliegenden Beitrag wird das Potenzial der MR-Technologie anhand der Wirbelstromprüfung von Drähten verdeutlicht. Hierbei wird ein kreuzförmiges GMR-Sensor-Array um den zu untersuchenden Draht positioniert. Jeder einzelne GMR-Sensor misst dabei mit einer Feldempfindlichkeit von etwa 200pT/√Hz und einer Ortsauflösung von 100µm. Der Nachweis von im Durchmesser etwa 200µm großen Defekten in einer Tiefenlage von 200µm unterhalb der Drahtoberfläche gelingt mit einem SNR von über 400. Oberflächendefekte ähnlicher Größenordnung können mit einem SNR von nahezu 104 detektiert werden. Ein derart hohes SNR birgt in Kombination mit der hohen örtlichen Auflösung Potenzial für eine 3D-Fehlerlokalisierung. Unter Verwendung eines analytischen Ansatzes sowie auf FEM beruhende Dateninversionsalgorithmen können Defekte mit einer Abmessung von größer 200µm auf einige 10µm exakt lokalisiert werden.
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.