Publications

Magnetoactive elastomers (MAEs) are a special type of smart materials consisting of an elastic matrix with embedded microsized particles that are made of ferromagnetic materials with high or low coercivity. Due to their composition, such elastomers possess unique magnetic field-dependent material properties. The present paper compiles the results of investigations on MAEs towards an approach of their potential application as vibrating sensor elements with adaptable sensitivity. Starting with the model-based and experimental studies of the free vibrational behavior displayed by cantilevers made of MAEs, it is shown that the first bending eigenfrequency of the cantilevers depends strongly on the strength of an applied uniform magnetic field. The investigations of the forced vibration response of MAE beams subjected to inplane kinematic excitation confirm the possibility of active magnetic control of the amplitude-frequency characteristics. With change of the uniform field strength, the MAE beam reveals different steady-state responses for the same excitation, and the resonance may occur at various ranges of the excitation frequency. Nonlinear dependencies of the amplification ratio on the excitation frequency are obtained for different magnitudes of the applied field. Furthermore, it is shown that the steady-state vibrations of MAE beams can be detected based on the magnetic field distortion. The field difference, which is measured simultaneously on the sides of a vibrating MAE beam, provides a signal with the same frequency as the excitation and an amplitude proportional to the amplitude of resulting vibrations. The presented prototype of the MAE-based vibrating unit with the field-controlled "configuration" can be implemented for realization of acceleration sensor systems with adaptable sensitivity. The ongoing research on MAEs is oriented to the use of other geometrical forms along with beams, e.g. two-dimensional structures such as membranes.

In developing prototypes, one fundamental activity is to model appropriate systems which mimic fundamental features of (biological) paradigms. In this way, we set up different models for the investigation of natural frequencies. The aim is to detect object contacts of technical sensors in observing their vibration behavior. For this, we compare the range and the shift of natural frequencies determined from the analysis of the arising two-point boundary-value problems. In particular, we found two systems with complementary spectra of eigenvalues. Considering boundary damping we analyzed these eigenvalues in the first octant of the complex plane. The fundamental result is that these two systems offer no common eigenvalue, they are alternative. This is an interesting and unique observation.

Compliant mechanisms are widely applied in precision engineering, measurement technology and microtechnology, due to their potential for the reduction of mass and assembly effort through the integration of functions into fewer parts and an increasing motion repeatability through less backlash and wear, if designed appropriately. However, a challenge during the design process is the handling of the multitude of geometric parameters and the complex relations between loads, deformations and strains. Furthermore, some tasks such as the dimensioning by means of optimization or the modeling for a controller design require a high number of analysis calculations. From this arises the need for sufficient computational analysis models with low calculation time. Existing studies of analysis models are mostly based on selected load cases, which may limits their general validity. The scope of this article is the comparison of models for the analysis of corner-filleted flexure hinges under various loads, to determine their advantages, disadvantages and application fields. The underlying methods of the study can further be used to evaluate future models based on a broad selection of possible load cases.

A pendulum with an attached permanent magnet swinging in the vicinity of a conductor is a typical experiment for the demonstration of electromagnetic braking and Lenz law of induction. When the conductor itself moves, it can transfer energy to the pendulum. An exact analytical model of such an electromagnetic interaction is possible for a flat conducting plate. The eddy currents induced in the plate by a moving magnetic dipole and the resulting force and torque are known analytically in the quasistatic limit, i.e., when the magnetic diffusivity is sufficiently high to ensure an equilibrium of magnetic field advection and diffusion. This allows us to study a simple pendulum with a magnetic dipole moment in the presence of a horizontal plate oscillating in vertical direction. Equilibrium of the pendulum in the vertical position can be realized in three cases considered, i.e., when the magnetic moment is parallel to the rotation axis, or otherwise, its projection onto the plane of motion is either horizontal or vertical. The stability problem is described by a differential equation of Mathieu type with a damping term. Instability is only possible when the vibration amplitude and the distance between plate and magnet satisfy certain constraints related to the simultaneous excitation and damping effects of the plate. The nonlinear motion is studied numerically for the case when the magnetic moment and rotation axis are parallel. Chaotic behavior is found when the eigenfrequency is sufficiently small compared to the excitation frequency. The plate oscillation typically has a stabilizing effect on the inverted pendulum.

Die zerstörungsfreie Prüfung ist von besonderer Bedeutung im Lebenszyklus technischer Produkte. Insbesondere bei sicherheitsrelevanten Komponenten in Flugzeugen, Zügen, Pipelines oder Kraftwerken helfen Verfahren der zerstörungsfreien Prüfung, Qualitätsmängel bereits während des Herstellungsprozesses oder in regelmäßigen Wartungskontrollen zu erkennen, ohne deren Funktion zu beeinträchtigen. Heute erfordern steigende Qualitäts- und Sicherheitsanforderungen sowie die Entwicklung neuer Materialien für moderne Leichtbaukonstruktionen immer zuverlässigere Prüfverfahren, um den ordnungsgemäßen Betrieb von Bauteilen und technischen Anlagen zu gewährleisten. Für die Untersuchung von Bauteilen aus metallischen Werkstoffen kommen häufig Wirbelstromprüfverfahren zum Einsatz. Im Gegensatz zu klassischen Induktionsverfahren werden die Wirbelströme bei der bewegungsinduzierten Wirbelstromprüfung durch die Relativbewegung zwischen einer Magnetfeldquelle und einem elektrisch leitfähigen Prüfkörper hervorgerufen. Die dabei auftretenden physikalischen Effekte ermöglichen die Detektion von Fehlern im Prüfkörper. Gegenstand dieser Arbeit ist die Entwicklung eines Prinzips für ein portables System zur bewegungsinduzierten Wirbelstromprüfung, welches für den Einsatz als handgeführtes Prüfgerät zur Untersuchung von Bauteilen an ihrem Einsatzort geeignet ist. Nach einer Einführung in die zerstörungsfreie Prüfung und die in der Industrie am häufigsten eingesetzten Prüfverfahren, werden die bekannten Verfahren der bewegungsinduzierten Wirbelstromprüfung vorgestellt. Basierend auf den Erkenntnissen vorangegangener Studien im Bereich der bewegungsinduzierten Wirbelstromprüfung, erfolgt die Präzisierung der Anforderungen an das zu entwickelnde Sensorsystem und die Diskussion möglicher Lösungsvarianten. Im Rahmen dieser Ideenfindung werden zwei favorisierte Lösungsvarianten beschrieben, für die jeweils ein Messsystem zur experimentellen Validierung entwickelt und aufgebaut wird. Außerdem werden die angewendeten Verfahren der Messdatenauswertung erläutert. Das für den Anwendungszweck besser geeignete Prinzip wird anhand eines Variantenvergleichs ermittelt. Weiterführende Experimente mit dem ausgewählten Sensorsystem dienen dazu, Handlungsempfehlungen für dessen Einsatz als handgeführtes Prüfgerät abzuleiten. Darüber hinaus werden verschiedene Möglichkeiten zur Darstellung und Interpretation von Prüfergebnissen aufgezeigt.