Jürgen Nuffer

Damage evolution in ferroelectric PZT induced by bipolar electric cycling

Abstract The fatigue behaviour of commercial bulk Lead Zirconate Titanate (PZT) induced by bipolar cycling was investigated. Polarisation and strain hysteresis loops as well as acoustic emissions (AE) were monitored. Higher cycling fields (2× E c ) yield stronger fatigue, higher AE energy values and lower threshold values for the onset of AE events at high cycle numbers. AE amplitudes and energies were found to be a measure for the degree of domain clamping. Cycling at 2 E c leads to an increasing asymmetry of the strain hysteresis, which is explained by an offset polarisation. Corrosion paths were observed by SEM, most numerously in the centre of a strongly fatigued sample. A damage scenario based on domain wall clamping due to the coalescence of point defects is presented.

Microstructural modifications of ferroelectric lead zirconate titanate ceramics due to bipolar electric fatigue

Fatigue in ferroelectric ceramics is the result of an intricate interplay of electrical, mechanical and electrochemical processes, each of which has been claimed responsible for fatigue. We present experimental results interrelating the different scales of the fatigue mechanism showing that all forms of fatigue occur. Electrochemical mechanisms lead to point defect agglomeration. The agglomerates are rendered visible as etch grooves in strongly etched surfaces by scanning electron microscopy. The overall length of the etch grooves strongly increases during cycling. Micro- and macrocracking were both observed and quantified using electron and optical microscopy. Their number similarly increases as a result of fatigue. Furthermore, a shrinkage of the unit cell volume was found by XRD.

Stability of pinning centers in fatigued lead–zirconate–titanate

After bipolar electric fatigue of bulk lead–zirconate–titanate ceramics, the well known reduced remnant polarization values are observed as well as an asymmetric strain hysteresis reflecting a macroscopic offset polarization. Thermal treatment and high electric fields each liberate the domain system from the microscopic defects responsible for the macroscopic offsets. High electric fields (4 Ec) only allow one to remove the type of pinning center inducing an offset polarization, while thermal treatment also removes more stable agglomerates and rejuvenates the material to almost its original properties.

Microcrack Clouds in Fatigued Electrostrictive 9.5/65/35 PLZT

Abstract The formation of microcracks during cyclic bipolar fatigue is shown for purely dielectric electrostrictive 9.5/65/35 transparent lead–lanthanum–zirconate–titanate (PLZT). The optically transparent material allows for the direct observation of broken grain facets. The maximum reached polarisation decreases by 30% after 3×10 6 cycles. The formation of microcrack clouds at the specimen edges and at macroscopic cracks are shown. No cracks were found in sample regions without obvious flaws present. By estimating the cracked volume, comparisons with FEM from previous studys allow to calculate the influence of the observed cracking on the polarization. As a consequence, the observed microcracks are considered to be mainly responsible for polarisation degradation.

Negligible oxygen liberation during bipolar electric cycling of ferroelectric lead zirconate titanate ceramics

The long-standing question whether or not oxygen gas is liberated from the perovskite lattice during fatigue of ferroelectric lead zirconate titanate ceramics is addressed. During cycling, the surrounding gas atmosphere of the sample was analyzed continuously by a mass spectrometer. It was found that, in contrast to previous statements in the literature, the oxygen detected originates from air dissolved in the surrounding liquid. An upper limit for oxygen liberation from the sample is given. We conclude that all mechanistical approaches for ferroelectric fatigue based on oxygen liberation are in need of revision.

Acoustic emission in PZT under bipolar electric driving and uniaxial mechanical stress

Abstract The acoustic emission (AE) from ferroelectrics has at least two possible sources, switching and microcracking. Both components have to release a distinct amount of elastic energy in a brief time span to generate the burst like AE. Whether ferroelectric switching on its own can generate AE is not fully understood. Furthermore the internal discharges in pores or microcracks can be an AE source. An easy way to suppress the ferroelastic switching component is to apply uniaxial mechanical stress parallel to the switching direction. This publication deals with the acoustic emission behavior of commercial ferroelectric-ferroelastic PZT ceramics under constant uniaxial load and concurrent electric hysteresis measurements. Both the coercive field and the remanent polarization decrease under compressive mechanical stress. Mechanical loading causes a stronger decrease in the AE activity and the dissipated AE-energy than in the remanent polarization. The threshold polarization for the occurrence of AE decreases with the mechanical load. Discharges in pores and microcracks can be discarded as AE sources.

Experimental sensitivity analysis for robustness studies of a controlled system

Active and adaptive systems consist of various components with different functionalities. As the complexity of the systems increases, reliability and robustness studies become a more complicated task. Sensitivity analysis helps system designers to understand interactions between the system components and to identify the important parameters with significant overall influences on the system performance. To analyze the complex interactions of the components and parameters of an active system with respect to system performance, a framework structure with active vibration damping in a lab scale test rig was set up. With this test rig an experimental sensitivity analysis was performed to investigate the influences of the system components and parameters on the vibration reduction. A higher robustness of the active system was achieved by using adaptive control.

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.

Adaptronik — ein technischer Ansatz zur Lösung bionischer Aufgaben

Adaptronische Systeme bieten dem Maschinenbau vollig neue Gestaltungsmoglichkeiten fur Maschinen und Konstruktionselemente. Wesentlich und grundlegend neu ist hierbei, dass die bisher passive Struktur sich mit Hilfe geeigneter Aktor-, Sensor- und Regelelemente aktiv an die jeweilige Betriebsbedingung anpasst. Eine solche „intelligente“ Struktur arbeitet effizienter, okonomischer und mit erhohter Lebensdauer. Die Natur lebt uns solche adaptiven Systeme vor, z.B. passt ein Vogel seine Flugel standig den Stromungsverhaltnissen an. Die Bionik beschaftigt sich mit der Aufgabe, von der Natur zu lernen und diese Erkenntnisse in neue Konstruktionsprinzipien umzusetzen. Der vorliegende Artikel verdeutlicht das enorme Potential der Adaptronik zur Losung dieser Aufgabe.

Approaches to Sensitivity Analysis for System Reliability Study of Smart Structures for Active Vibration Reduction

The modern engineering products must fulfill the increasing requirements to the vibroacoustical behavior of the components and the system. For many applications such as automotive engineering, where light-weight design is desired, passive measures for noise and vibration reduction have reached certain limits. For this reason, active techniques for structural vibration reduction are becoming increasingly important in this field of applications. Commonly, actively or adaptively controlled structural systems consist of a large number of components with various functionalities. As the complexity of the systems increases, reliability and robustness studies become a more complicated task. The knowledge of parameter effects and their interactions is important for the reliability study and the design optimization of such systems. Sensitivity analysis can help the system designers to understand interactions between the system components and identify the important parameters with significant overall influences on the system performance. In this paper, several approaches to sensitivity analysis are applied for a smart structure system with active vibration control. Through these analyses, the influences of the system parameters and control algorithms on the performance of active vibration reduction are investigated. An improvement of the robustness of the active system by using adaptive control will be shown.