Papers by Wolfgang Sachse
IEEE 1992 Ultrasonics Symposium Proceedings
Some novel applications of the pointlike source and pointlike detector for ultrasonic waves in th... more Some novel applications of the pointlike source and pointlike detector for ultrasonic waves in the investigation of various physical phenomena are described. The pointlike sources are generated by the fracture of a glass capillary, a focused laser beam and small lead zirconate titanate (PZT) transducers, while the pointlike detectors include small PZT and capacitive transducers. The topics discussed are observation
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Die Kunst of Phonons, 1994
Recently there have been a number of ultrasonics experiments performed that elicit detailed infor... more Recently there have been a number of ultrasonics experiments performed that elicit detailed information on the elastodynamic response of anisotropic solids to transient point-like forces. In this lecture I will be describing methods for computing the dynamic Green’s functions of anisotropic solids, and showing how these response functions are able to account for the observations. In these experiments small-aperture transducers or acoustic lenses are used to achieve wide-angle radiation and detection sensitivity as well as high spatial resolution. Many salient aspects of thermal phonon imaging, such as phonon focusing, polarization selectivity and mode conversion at surfaces also feature in these ultrasonic experiments, but there are neverthless significant differences. Unlike thermal phonons, the ultrasonic waves are coherent, and because their characteristic wavelengths are several orders of magnitude larger than those of phonons, the far field condition is not as well satisfied, and diffraction effects feature prominently. Various modes of excitation and detection exist, both broadband and monochromatic, some of which tend to disguise these diffraction effects, while others reveal them clearly.
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Ultrasonics, 1987
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Journal of Applied Physics, 1989
This paper describes a novel approach for investigating the fracture of brittle solids by combini... more This paper describes a novel approach for investigating the fracture of brittle solids by combining a theory of acoustic emission (AE) with the Dugdale–Barenblatt model. An example is provided with indentation-produced, half-penny-shaped cracks that were generated by pressing a Vickers indenter onto the surface of a glass plate and that produced AE signals. These were detected using four capacitive displacement transducers, one at epicenter and the other three at off-epicentral positions. Assuming that a mode I type of crack was generated and using indentation fracture mechanics, a mode I stress intensity factor KIc=7.44×105 N/m3/2 was obtained. Both dipole strength and source-time function associated with crack formation were determined from the AE theory. Assuming that an effective gauge length can be interpreted equal to a dipole linkage distance and using the Dugdale–Barenblatt model for the fracture of brittle solids, it is shown that all the other important parameters on crack...
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Journal of Applied Physics, 1996
This article reports measurements of the group velocity surface of silicon in the region of the 〈... more This article reports measurements of the group velocity surface of silicon in the region of the 〈100〉 axis. Pulsed ultrasonic beams generated by a piezoelectric longitudinal mode transducer are transmitted through water and focused onto a small spot on the surface of a (001) oriented silicon single crystal disk. This gives rise to transient elastic waves which propagate in a wide range of directions through the specimen and they are detected by a small piezoelectric longitudinal mode transducer fixed at the center of the opposite face. The imaging of the group velocity surface about the 〈100〉 axis is accomplished by scanning the focused beam in various directions along lines that intersect at epicenter. The scanned images reveal complex foldings of the slow transverse sheet of the ray surface about the 〈100〉 axis, in general agreement with the predictions of ray acoustics. However, the observed image manifests pronounced wave motions well beyond the cuspidal edges predicted by ray a...
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Journal of Applied Physics, 1994
A direct determination of group velocity surfaces of zinc from the group velocities measured in v... more A direct determination of group velocity surfaces of zinc from the group velocities measured in various directions within and without a cuspidal region is described. A zinc crystal disk oriented in the symmetry direction [0001] is used as a specimen. Quasilongitudinal (QL) and quasitransverse (QT) elastic pulses propagating in a wide range of directions are generated by a broadband pointlike source activated by the fracture of a tiny glass capillary on the surface of the specimen and they are detected at epicentral and off-epicentral positions on the opposite face by a miniature capacitive transducer which senses a displacement normal to the surface. The group velocities associated with QL and QT modes are directly measured from the detected signals. The QL mode arrival is easily identified as the first point from which the signal jumps from the noise level. In the cuspidal region, there are three branches of QT group velocity: fast QT (FQT) ray branch, intermediate speed QT (IQT) r...
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Ultrasonics, 2018
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Journal of Polymer Science: Polymer Physics Edition, 1982
Hydrostatic pressure usually increases the glass transition temperature Tg of a polymer glass by ... more Hydrostatic pressure usually increases the glass transition temperature Tg of a polymer glass by decreasing its free volume; if the pressurizing environment is soluble in the polymer, however, one might expect an initial decrease in Tg with pressure as the polymer is plasticized by the environment. Just such a minimum in the Tg of polystyrene (PS) is observed as the pressure of CO2 gas is increased over the range 0.1–105 MPa from both ultrasonic (1 MHz) measurements of Young's modulus E and static measurements of the creep compliance J. A time‐temperature‐pressure superposition law is obeyed by PS which allows a master curve for the compliance to be constructed and shift factors to be determined. A master curve for E is then obtained by using the Boltzmann superposition principle. The compliance J reaches a maximum, and E and Tg reach minima, at a CO2 pressure of ca. 20 MPa at both 34 and 45°C, which are above the critical temperature (31°C) of CO2. At the minimum, Tg is 41 at 4...
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Nondestructive Characterization of Materials IV, 1991
We describe in this paper the use of transient ultrasonic waves to image wave anisotropy effects ... more We describe in this paper the use of transient ultrasonic waves to image wave anisotropy effects in single crystal specimens of silicon. The measurements rely on the point-source/point-receiver (PS/PR) technique with a Nd: YAG laser as a scanned source and a point piezoelectric transducer as a detector. The image is based on the measurement of the signal arrival times and corresponding amplitudes of wave groups along various propagation directions in the single crystal specimens. It is found that along some directions there are strong focusing effects. An interpretation of the measured images based on an analysis of elastic waves in anisotropic solids is given. A potential application of this imaging procedure as a materials characterization tool is also shown.
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Springer Series in Synergetics, 1997
In the previous chapter on the adaptive modeling of natural laws it was stated that tasks associa... more In the previous chapter on the adaptive modeling of natural laws it was stated that tasks associated with such modeling included the estimation and storage of the probability distribution, as well as the development of a method for its effective application. The fundamental problem related to the application of a natural law is the formulation of a method by which some unknown property of Nature can be predicted on the basis of information obtained by partial observation and a known natural law. If a natural law which is expressed as a functional relationship between various physical variables is to be applied, the task is to find the values of some variables from the given values of others by arithmetic procedures. If a natural law is represented by a model contained in a certain physical system then this task corresponds to the projection of a set of inputs into a set of outputs. However, this concept must be generalized when a natural law is represented by a probability density. To do this, we use some of the fundamental concepts of prediction theory which will be briefly reviewed in the next part of this section. The main goal of this chapter is to demonstrate the solution of several problems related with an optimal application of empirical information, stored as a set of representative points in a discrete memory of a modeler.
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Ultrasonics International 83, 1983
A normal mode theory is used to compute the signals in the far-field of an axis-symmetric AE sour... more A normal mode theory is used to compute the signals in the far-field of an axis-symmetric AE source in a flat plate. Results are shown for plates of mild steel and a broad range of source-receiver separations. The computed impulse response signals are convolved with various ...
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Ultrasonics International 93, 1993
This paper describes results of an experimental investigation of acoustic emission (AE) source ch... more This paper describes results of an experimental investigation of acoustic emission (AE) source characterization in terms of location and strength from strain gage signals detected on a two–dimensional frame–like structure. The signals are analyzed using two different smart signal processing algorithms. One is a feed forward neural network (FFNN) that was trained by a modified back-propagation algorithm and the second is a linear system called an auto-associative processor (AAP). The common feature of these algorithms is the use of a set of pre-processed, measured prototype signals to develop a system memory. This memory is subsequently employed to process the detected signals to determine the location and strength of the AE source.
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Ultrasonics International 93, 1993
This paper reports on the results of the application of ultrasonic techniques that can be used to... more This paper reports on the results of the application of ultrasonic techniques that can be used to characterize thin structures, such as one- or two-ply, unidirectional laminates whose thicknesses are as small as 140 μm. The measurements rely on the application of the ultrasonic point-source/point-receiver technique (PS/PR) using either a pulsed laser beam operating as a dipole source or a small aperture, piezoceramic shear transducer serving as a monopole source. Detection of the signals is with a piezoceramic sensor that directly responds to the lateral or shear motions of the specimen surface. The detected time signals represent the detailed spatial and temporal characteristics of the elastic wave field. The experimental group velocity data is analyzed using a simple plane wave, plane stress model that describes the propagation of quasi-longitudinal and quasi-transverse membrane waves in the plane of the plate. Appropriate inversion procedures permit the recovery of the entire stiffness tensor of the laminates from in-situ measurements. The measurement system is also used to map out the group velocities in branches comprising the cuspidal region of the quasi-transverse group velocity curve.
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Ultrasonics International 91, 1991
We study the diffusion of multiply-scattered ultrasound in materials and its connection to micros... more We study the diffusion of multiply-scattered ultrasound in materials and its connection to microstructural grain size. It has been argued that diffusing ultrasonic energy density, conceived as a random walk process, evolves according to a modified heat equation. It is shown here that diffuse fields do so evolve and that measurements in specimens of brass and steel give ultrasonic diffusivities which vary with the grain size of the material and with the frequency of the ultrasonic signals, in agreement with theory. The method is shown to have potential application to nondestructive materials characterization.
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Ultrasonics International 91, 1991
We describe in this paper the design principles and fabrication details of a microhammer which ca... more We describe in this paper the design principles and fabrication details of a microhammer which can be used as a compact ultrasonic point source. Quantitative ultrasonic measurements were used to obtain a calibration of this source. It is shown that the impact duration may be controlled, but the peak force relative to the impact duration is fixed for a given material. The results of operating the microhammer on a variety of materials, including composites, are described.
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Ultrasonics, 1996
To characterize 5–50 nm poly(vinyl-2-pyridine) (PVP) films, we generate ultra-high frequency long... more To characterize 5–50 nm poly(vinyl-2-pyridine) (PVP) films, we generate ultra-high frequency longitudinal acoustic waves (30–150 GHz) in Al using femtosecond laser pulses in a time delayed ‘pump’ and ‘probe’ beam configuration. For polymer film thicknesses of 12 nm we find an enhanced longitudinal wavespeed of 3858 m s−1. The acoustic response from a PVP film with a thickness of 5
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Review of Scientific Instruments, 1975
We describe in this paper a simple device which facilitates the measurement and recording of the ... more We describe in this paper a simple device which facilitates the measurement and recording of the response curve of vibrating systems. The method utilizes a digital counter to measure the period and a peak detector to measure the amplitude of vibration. Applications of the method to record the steady-state and transient response of linear and nonlinear vibrating systems are given.
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The Journal of the Acoustical Society of America, 1993
The essential elements of a point–source/point–receiver ultrasonic system include a broad bandwid... more The essential elements of a point–source/point–receiver ultrasonic system include a broad bandwidth, small-aperture source generating elastic waves with a broad angular spectrum and a receiver with similar characteristics to detect them. By moving the source or receiver and stacking the received waveforms, one generates a scan image that provides a view of the complete elastic wave field in a test specimen. Wave arrivals are related to the speeds of propagation of various wave modes in various directions and the signal amplitudes reflect the propagational characteristics of the material. An interpretation of the scan images requires an understanding of the propagation of transient elastic waves in a bounded structure. Recent developments on both the forward and inverse problems are summarized and the full solution of the forward problem for the response of a general axisymmetric anisotropic viscoelastic plate is reported. The computed results are compared to measured waveforms in vi...
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The Journal of the Acoustical Society of America, 1987
A small non-colinear transducer array for omnidirectional acoustic emission monitoring is disclos... more A small non-colinear transducer array for omnidirectional acoustic emission monitoring is disclosed. The small transducer array consists of four piezoelectric sensor elements of sufficiently small diameter as to function as essentially point receivers and of sufficient frequency response as to be sensitive to the signals to be detected. The sensor elements of the transducer array are close together and are non-colinear so that the signals received by them can be used for determining the group velocity of acoustic waves on solid plates and on plate-like structures such as shells and pipes, and to permit determination of both the source direction and distance. The array is designed to monitor the region exterior to the area enclosed by the array, and the ratio of the smallest distance between sensors to the radius of a single transducer element is relatively large in order to minimize measurement errors introduced by undertainties in transducer element positions caused by large transducer sizes with respect to the acoustic waves being received.
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Papers by Wolfgang Sachse