Stefan J. Rupitsch

Acoustic Microscopy Technique to Precisely Locate Layer Delamination

In this paper, the synthetic aperture focusing technique (SAFT) is applied to extend the depth of focus for spherically focused ultrasound transducers. This technique uses a virtual source element in the geometrical focus of the transducer. Initial experiments with a 90-mum copper wire are conducted to investigate the efficacy of SAFT processing for positive and negative defocus. Furthermore, delamination of two glued together transparent Perspex plates is investigated. Compared to the common B-mode ultrasonic imaging that can only detect the presence of layer delamination, the proposed SAFT can accurately locate the position of delamination and visualize its extension.

Complete characterization of piezoceramic materials by means of two block-shaped test samples

We present an approach enabling entire characterization of piezoceramic materials. Contrary to the IEEE/ CENELEC Standard on Piezoelectricity, which is commonly applied for material characterization, the so-called inverse method requires only two block-shaped test samples. The method is based on a comparison of numerical simulations and measurement results for the frequency-resolved electrical impedance. Thereby, the aimed material parameters are iteratively updated so that simulations match measurements as well as possible. We utilize the identification procedure to characterize the piezoceramic material PIC255 as well as PIC155 from PI Ceramic, both of crystal class 6mm. In contrast to the parameters provided by the manufacturer, the identified data set leads to accurate simulation results for electrical and mechanical quantities of piezoceramic materials. This also holds if one predicts the behavior of geometrical shapes (e.g., disk) that are not considered within the inverse method. Moreover, we exploit the identification procedure to determine temperature dependences of the material parameters in the temperature range of -35°C to 130°C. To some extent, the parameters of PIC255 and PIC155 strongly depend on temperature. Nevertheless, the resulting electromechanical coupling factors for both materials remain nearly constant in the investigated temperature range.

Ultrasound transducers based on ferroelectret materials

Ferroelectret materials can be utilized to set up electroacoustic transducers. The materials offer both, a rather large bandwidth and a high piezoelectric strain constant. Due to its cellular structure, the material is flexible and exhibits an excellent matching to air. Therefore, this polymer is appropriate for many sound as well as ultrasound transducer applications. Our research is concentrated on the simulation based design and characterization of ultrasound transducers. In this contribution, we present a finite element based modeling of the cellular structured ferroelectret materials. In particular, a microscopic as well as a macroscopic model is discussed. We fabricate single element and array transducers based on ferroelectret materials, namely the so-called EMFi (Electro- Mechanical-Film) material. To show the applicability of ferroelectret materials for ultrasound transducers, different applications in air and water are presented. An emitter-receiver-unit is introduced which is utilized in an artificial bat head and allows the functional reproduction of the biosonar system found in bats. Moreover, a robust sensor array consisting of 16 single elements (4x4) is studied. With the aid of this sensor array, cavitation effects in ultrasonic cleaning systems can be investigated on the specimens surface, which is not possible with common ultrasound sensors, e.g., hydrophones.

A modified Preisach hysteresis operator for the modeling of temperature dependent magnetic material behavior

In this paper, we present a model for temperature dependent hysteretic nonlinearities with nonlocal memories. This model can be applied to describe hysteretic material behavior. Common applications are ferromagnetic or magnetostrictive materials. Our model consists mainly of a Preisach operator with a continuous Preisach weight function. We choose a weight function which shows a strong correlation between the function’s parameters and certain properties of the hysteresis curve. As a new approach, the weight function is written as a function of temperature. The model parameters are customized to a set of symmetric hysteresis curves. We verify our model for magnetic materials with differently shaped hysteresis curves, different temperatures and magnetic field amplitudes.

Estimation of the surface normal velocity of high frequency ultrasound transducers

This paper is concerned with the characterization of the true locally resolved surface normal velocity of an assumed piston-type ultrasonic transducer. Instead of involving a very complicated direct pointwise measurement of the velocity distribution, an inverse problem is solved which yields a spatially discretized weighting vector for the surface normal velocity of the transducer. The study deals with a spherically focused high frequency transducer, which is driven in pulse-echo mode. As a means of posing the inverse problem, the active transducer surface is divided into annuli of equal surface so that for each annulus the spatial impulse response can be calculated. An acrylic glass plate acts as a simple structured target. The resulting ill-posed nonlinear inverse problem is solved with an iterative regularized Gauss-Newton algorithm. The solution of the inverse problem yields an estimated weight for the surface normal velocity for each annulus. Experimental results for a thin copper wire target are compared to simulation results for both uniform and estimated surface normal velocities.

Influence of the fabrication process on the functionality of piezoceramic patch transducers embedded in aluminum die castings

Piezoceramic patch transducers are integrated into aluminum components using high-pressure die casting. Expanded metal has proven suitable as a supporting structure for placing the patch transducers inside the die cavity and for stabilization during the injection of molten metal. However, difficulties arise when the transducers are positioned off the neutral axis within the wall of the casting. Numerical simulations of the die filling are performed to analyse the evolution of the integration process. The asymmetric infiltration of the supporting structure is identified as the major factor contributing to the formation of cracks and perforations inside the piezoceramic transducer. By means of measurements and numerical calculations of the electrical impedance of the transducer, a close relation is established between mechanical damage patterns observed in radiographs of the patch transducers and loss of performance.

Impedance-Based Temperature Sensing With Piezoceramic Devices

This paper deals with the temperature dependence of the electrical impedance of piezoceramic transducers. The behavior under thermal loads is investigated for bulk ceramics and composite structures where the piezoceramics are integrated into passive materials. According to these identified dependencies, temperature sensing through the piezoceramics impedance is realized using a polynomial fitting method. A creep operator and a lead filter are introduced to consider the time-dependent behavior of the electrical impedance. The presented approach is applied to a commercially available air ultrasound transducer. The accuracy of the measurement method is investigated for an arbitrary temperature profile yielding

Determination of Dynamic Material Properties of Silicone Rubber Using One-Point Measurements and Finite Element Simulations

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Fibre-reinforced composite structures based on thermoplastic matrices with embedded piezoceramic modules

. The presented method can be utilized for additionally sensing temperature in devices with integrated piezoceramic transducers, especially ultrasound transducers or in structural health-monitoring applications.

Estimation of material parameters for piezoelectric actuators using electrical and mechanical quantities

The dynamic Youngs modulus, Poissons ratio, and the damping factor of silicone rubber are determined from a laser triangulation measurement of the top surface motion of a flat cylindrical sample excited by a shaker. These material parameters are estimated on the basis of an Inverse Method that minimizes the difference between measured data and a prediction from a finite-element model (FEM), in which the sought-after material data are the adjustable parameters. The results are presented for measurements within the 10-400-Hz frequency range under atmospheric pressure and temperature conditions. At first, the measured data are compared with FEM predictions using constant material parameters to show the material behavior in principle. Afterward, the frequency dependence of the moduli and Poissons ratios are determined by matching measurements with simulations within small frequency ranges. Finally, the material parameters determined are given as functions versus frequency. A sensitivity analysis shows the accuracy of the presented method. This paper is motivated by the need for a precise description of vocal fold models, commonly manufactured from silicone rubber.