point scatterer positioned at a location with coordinate 2r, which means that the spherical ball reflector can be used for the evaluation of PSF of a focusing system and gives the same spatial dependence although twice compressed in the spatial domain as an ideal point scatterer.
Sidkey, M.A.: EI Malawany, R.: Khafagy, A.: Afztl. H. Mechanical properties of molybdenum-tellurite glasses National Institute for Standards, Cairo, Egypt
Experimental results and discussion. This method was successfully applied for the evaluation of the PSF of factory-made acoustic lenses (Leica, FRG) (200 MHz and I GHz centre frequencies) as well as laboratory made focusing system with an acoustic knife edge (AKE) allowing continuous modification of lens geometry (70 MHz working frequency) Sorted glass spheres of 50-200 I'm radius as well as air micro bubbles were used as spherical reflectors.
Longitudinal and shear ultrasonic velocities and densities of tellurite glasses containing different concentrations of molybdenum oxide were measured at room temperature using the pulse-echo method. Elastic constants (longitudinal, shear. Young's, bulk moduli and Poisson's ratio) as well as internal friction were calculated. Results showed that all the elastic moduli depend strongly upon the composition of the prepared glasses. This finding was attributed to the compactness of the glass network. Detailed discussion is reported.
It was shown that the PSF of a standard I GHz acoustic lens, supposed to be axially symmetrical, is basically asymmetric with an elongation in the focal plane reaching 200% and a shape significantly changing with variation of working frequency. This behaviour can be explained by positioning the lens surface in the near-field zone of a piezotransducer. At the same time a 200 MHz lens demonstrated a much more symmetrical PSF.
Sidkey, M.A., £1 Malawany, R., Khafagy. A.; Ajiji, H. Ultrasonic attenuation of molybdenum-tellurite glasses National Institute for Standards, Cairo, Egypt Mnofia University, FacuIty of Science, Physics Department
In an AKE setup a segment of a circular lens aperture is covered by an opaque knife edge, which enables detection of anisotropy and scattered waves but also unavoidably changes the PSF of the acoustic system. It was demonstrated that focal plane PSF became wider in the direction perpendicular to the AKE border with an approximately two-fold resolution loss for 50% covering of the lens half-aperture. At this covering sidelobes were insignificant, whereas further covering caused their profound growth with emphasized PSF oscillation structure in the direction perpendicular to the border with an essential resolution drop in this direction. It is interesting to point out that for every covering by a knife edge the PSF revealed a plane of symmetry perpendicular to the edge border. Conclusion. The proposed method of PSF evaluation makes it possible to receive the same information as the point scatterer technique while possessing two valuable advantages: (a) essentially high signal amplitude and (b) difficult criteria of small scatterer size and absence of support interference are replaced by the much more practical condition of spherical surface perfectness. The method can be effectively applied for the evaluation of commercially available reflection focused ultrasonic imaging systems as well as for newly designed laboratory made ones,
References Bertoni, H.L. and Somekh, M.G. 'Ray optical analysis of spherically focusing transducers for acoustic microscopy' IEEE 1985 Ultrasonics Symposium (1985) pp 715-719 2 Kolosov, O.V. and Yamanaka, K. 'Adjustable acoustic knife edge for anisotropic and dark field acoustic imaging' Ultrasonics Electronics '92, November 3 I-December 2 1992, Senday, Japan, to be published
Rover, D.; Casula, 0.; Fink, M. A broadband imaging system of acoustic fields using a heterodyne interferometer Laboratoire Ondes et Acoustique de l'Universite Paris 7, ESPCJ, 10 rue Vauquelin, 75231 Paris Cedex 05, France We have built an optical imaging system for acoustic fields transmitted in water by high frequency piezoelectric transducers. This system includes a thin membrane immersed in front of the transducer and a broadband, compact heterodyne interferometer. The probe beam is focused on a gold reflecting coating on the membrane and reflected. The motion of the membrane, induced by the acoustic pressure, modulates the phase of the optical wave. Beating with a reference wave on a photodiode and a coherent demodulation of the photocurrent provide an electrical signal proportional to the mechanical displacement. As the optical heterodyne process makes the measurements insensitive to environmental disturbances, such as low frequency motions of water, the minimum detectable displacement (I A) is as small as in air. To record the acoustic field, the transducer is moved along the X and Y directions by stepping motors. The digitized signals were transferred to a computer for subsequent calculation. Pulsed ultrasonic fields transmitted in water by piezoelectric transducers have been investigated. Experiments carried out with 5 to 15 MHz focused or planar transducers show the capability of the system to map acoustic fields both in the time and spatial domains. In a first experiment, the acoustic energy, computed from the sampled signal, was plotted in the focal plane of a 15 MHz focused transducer. The spatial resolution and the dynamic range, respectively 50 I'm and 60 dB, are sufficient to prove the absence of sidelobes as expected in the transient regime. In the time domain, the large bandwidth (30 MHz) of the optical detection allows quantitative measurements of the diffraction impulse response of focused transducers. In another experiment, B-scans of the acoustic fields transmitted by planar transducers clearly show the occurrence of plane and edge waves.
Materials characterization £l-Ma//awany, R. Ultrasonic properties of tellurite glasses Physics Department, Faculty of Science, Menofia University, Egypt Measurements of the longitudinal wave velocity and attenuation for binary tellurite glasses have been made at the frequencies 2-8 MHz in the temperature range 120-300 K. The ultrasonic properties have been found to be dependent on frequency, composition and temperature. The activation energies in the various glasses were found to be proportional to the mean force constant.
Longitudinal ultrasonic attenuation measurements in tellurite glasses containing molybdenum oxide with different concentrations have been made by making use of the pulse-echo technique at 2.4,6 and 10 MHz frequencies in the temperature range - 160 to 2Ye. The results showed the presence of a well defined peak which shifts its position to higher temperature with increasing frequency. This suggests some sort of relaxation process and the activation energies as well as relaxation frequencies were calculated. It was found that the activation energy decreases with increasing molybdenum oxide concentration. Discussion of the results obtained has been reported.
Baker, Richard A.*; Netravali, A.N.t; Sachse, Wolfgang* Fibre strength characterization from an acoustic emission based single fibre composite test *Department of Theoretical and Applied Mechanics, Cornell University, Ithaca, New York 14853, USA t Fiber Science Program, Department of Textiles and Apparel This paper describes how acoustic emission (AE) is used to determine fibre strength characteristics at very short gauge lengths. Our method is based on single fibre composite (SFC) techniques used to determine interfacial properties of composite constituents. In the conventional version of the SFC test, a fibre embedded in a polymer matrix is loaded in tension. which results in fragmentation of the fibre. The final fragmentation length distribution combined with a micro-mechanical model determines the average fibre matrix interfacial shear strength. In this paper, we present three specific improvements to the SFC test that relies on acoustic emission measurements to detect the fibre fractures and to locate the fibre breaks while the specimen is loaded in tension. These improvements are: (I) determination of the fragmentation history of the fibre; (2) a novel procedure for the in situ measurement of the wavespeed of the first wave arrival that is based on a pulsed thermoelastic source; and (3) an adaptive signal processing procedure which permits a high-resolution determination of the arrival time of signals detected at the sensors, which is robust for a broad range of wave shapes. Using AE monitoring enabled us to go beyond determining only the interfacial shear strength (IFSS). to determining the fibre strength/length relationship at short fibre gauge lengths. Previously fibre strength has been determined by hundreds of individual fibre tension tests. These tests are limited to gauge lengths of greater than one centimetre and results are extrapolated to the lengths of interest around one millimetre. By tracking both the fibre failure stress and the fibre fragmentation length for each break during the test we are able to determine the fibre statistical strength parameters. We observed that the in situ fibre strength values obtained from such SFC tests were slightly higher and had lower variability than those obtained from conventional tensile tests. The higher stress values can be accounted for by the presence of a debond region along the fibre/matrix interface or a bridging of pre-existing cracks in the fibre. The lower variability may be a consequence of the single fibre being tested rather than the large number that is required to carry out the conventional tensile tests. The AE data also revealed a change in Wei bull shape parameter at a particular fragment length, which may indicate different flaw popUlations for fibres longer and shorter than this length. Additional measurements using this approach are being conducted to permit a more precise evaluation of the errors associated with the determination of the fibre fragment length and the in situ fibre failure stress. For accurate source location, we must know the effective wave speed of the first wave arrival over a broad range of specimen strains. To determine this wave speed we have used a small thermoelastic source mounted on the specimen. A small strain gauge glued to the side of the specimen was pulsed, simulating an AE event, and a piezoelectric sensor was scanned along the length. Thus, we determined an effective wave speed of first wave arrival for an epoxy specimen with significant damping as a function of distance from the source. Integration of this wave speed calibration into the source location algorithm of the automated version of the test is currently under way. In our initial application of AE measurements to determine the locations of the fibre breaks, we processed every digitized waveform manually to determine the difference in arrival time of two signals. Because background noise causes numerous false triggers when set for maximum sensitivity, the by hand approach is not practical. When the imbedded fibres are large in diameter or possess high fracture strength, the breaks are accompanied by large amplitude emissions which are easily discerned from the smaller-amplitude noise. For such cases, the normal threshold triggering circuitry in commercially available counters is adequate for reliably determining the difference in arrival times. In measurement situations when the above conditions are not met, difficulties may arise in obtaining reliable arrival time data. For this reason, we implemented a processing algorithm for determining the first wave arrivals in software. Because the time involved in
NDT & E International Volume 25 Number 4/5 1992