ND T Abstracts

Imaging vertically oriented defects using the Synthetic Aperture Focusing Technique (SAFI') requires special consideration. When the faces of the defect are reasonably smooth, diffraction energy is low and the indirect reflections (waves reflected at front and/or back- wall) must be used for proper imaging. In this paper we present a technique that makes use of one fixed source, illuminating a defined region of interest, and a scanning receiver. The position of the source and the receiver aperture have to he chosen carefully, as will be shown with the aid of modeled results. Taking into account direct and indirect wavepaths implies the use of multiple synthetic apertures. Therefore, the imaging method is referred to as Multi-SAFT. To avoid mode-conversion effects reducing the signal amplitudes, a simple modification in the set-up wil l be proposed. Finally, one of the first results of applying Multi-SAFT to a real dataset wil l be shown.

49895 Li, Y.; Newhouse, V.L.; Shankar, P.M.; Karpur, P. Speckle reduction in ultrasonic synthetic aper ture images Ultrasonics, Vol. 30, No. 4, pp. 223-237 (1992)

The ultrasonic synthetic aperture imaging technique has been used for non-destructive testing since it gives very good lateral resolution in all regions of interest. However, when this technique is used on coarse-grained materials, the images show a significant amount of speckle. The performance of two techniques for speckle reduction in synthetic aperture images is evaluated: the split spectrum processing technique with polarity thresholding algorithm and the interlaced array technique. Experimental results show significant speckle reduction in these images after the application of these techniques. The signal-to- noise ratios of the processed and unprocessed images have been calculated to quantitatively measure the reduction of speckle.

49393 Lord, W.; Hsu, D.; Ghorayeb, S.; Minachi, A.; You, Z. Finite element applications in NDE engineering Ultrasonics International 91, Le Touquet (France), 1-4 Jul. 1991. pp. 463-466. Butterworth Heinemann (1991).

Domain methods of modeling ultrasonic wave propagation give a full displacement field solution which can be useful in studying physical phenomena associated with the nondestructive inspection of materials. A number of applications have been studied which make use of this attribute including the analysis of oblique echoes in thick composites, the simulation of SAFT reconstructions, the validation of a Gauss- Hermite beam model and the sensitivity of diffraction tomography algorithms to transducer aperture variations and auisotropy.

48925 Mak, D.K. Match-scan - a new acoustical imaging method Review of Progress in Quantitative Nondestructive Evaluation, La Jolla, California (United States), 15-20 Jul. 1990. Vol. 10A, pp. 613-619. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1991). ISBN 0-306-43903-4.

A new method of acoustical imaging is described which differs in operation from the common methods of synthetic aperture focussing, circular backpmjection and amplitude and transit time locus curves (ALOK). Known as Match-Scan, it uses two transducers which both act as transmitters and receivers as well as transducer pairs. Three sets of data are collected at each tranaducer-palr-scau position. Travel time and amplitude of the signal peaks, and f'wst arrival tinw~ are stored for each set. hnpmvad accuracy, compared to the other methods mentioned, is achieved by using a Dirac delta function for the theoretical point spread function. Disadvantages include the need for two tnmsducers and difficulties imaging near-surface defects.

48907 Thompson, R.B.; Thompson, D.O.; Holger, D.K.; Hsu, D.K.; Hughes, M.S.; Papadakis, E.P. Tsai, Y.-M.; Zachary, L.W. Ultrasonic NDE of thick composite Enhancing Analysis Techniques for Composite Materials, Atlanta, Georgia (United States), 1-6 Dec. 1991. pp. 43-57. Edited by L. Schwer, J.N. Reddy and A. Mal. NDE-Vol. 10. ASME (1991)

A series of ultrasonic techniques being developed for the characterization of thick composites are described. Techniques for the in-situ measurements of elastic constants on thick-walled cyfinders based on the times of a variety of ultrasonic modes of propagation am fwst presented, followed by discussion of the determination of the porosity from measurements of the frequency depo~dence of the attenuation. Two techniques for imaging delamination are then discussed. The fwst involves inferring size from plots of reflected signal amplitude versus lateral position of the transducer with a model for beam propagation in anisotropic media used to deconvolve the effects of the distant and direction dependem beam width. The second makes use of synthetic aperture techniques.

48620 You, Z.; Lord, W. Finite aper ture and anisotropy effects in ultrasonic imaging Review of Progress in Quantitative Nondestructive Evaluation, La Jolla, California (United States), 15-20 Jul. 1990. Vol. 10A, pp. 1019-1026. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1991). ISBN 0-306-43903-4.

This paper is intended to evaluate the effects of the finite aperture by varying the aperture size of the transmitter. Other assumptions of diffraction tomography are also considered. Previous study shows that the use of the normal component of the displacement dons not degenerate the quality of the image even though the derivation of the algorithm uses the acoustic potential instead. The possible use of the same algorithm in anisotropic material, however, has never been explored. This paper shows the effects of material anisntropy and attempts to improve the quality of reconstructed images by pmprocessing the raw data used for reconstruction. This paper continues to show the usefulness of the finite element test bed for exercising inverse algorithms.

48619 Ghorayeb, S.R.; Hughes, M.S.; Holger, D.K.; Zachary, L.W. A prel iminary application of SAFT on composites Review of Progress in Quantitative Nondestructive Evaluation, La Jolla, California (United States), 15-20 Jul. 1990. Vol. 10A, pp. 1027-1034. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1991). ISBN 0-306-439034.

One major assumption required by SAFT, is that the sample under test be homogeneous and isotropic in nature. As a result, techniques based on the SAFT process already in use have been unavailable for application on anisotmpic composite materials. In this paper, we present results of a comparison of reconstructed images of flat bottom holes, side drilled holes and flat bottom slots in both Plexiglass and Graphite/Epoxy specimens. From the given results, it appea~ that SAFT has been quite successful at providing a reasonably good resolution when applied to anisotropie media.

48171 Schmitz, V. Ultrasonic imaging methods - principles, rcalisations and applications Proceedings of the 10th International Conference on NDE in Nuclear and Pressure Vessel Industries, Glasgow (Scotland), 11-14 Jun. 1990. pp. 289-295. Edited by MJ . Whittle, J.E. Doherty and K. Iida. ASM International, (1990)

Ultrasonic methods for the evaluation of welding defects which are able to specify flaw position, flaw depth, inclination and shape are known as imaging or reconstruction methods. Systems like ALOK, Comson, P- Scan, SAFT, SuperSAFTZlP-Scau use either high-frequency or rectified data. Their technical realizations differ due to aspects like application as a searching method or as an analyzing method or concerning data reduction questions, off-line, on-line or realtime processing, easy handling or price of the equipment. This paper tries to compare systematically the different concepts and the implementation especially with respect to data acquisition and pixel oriented imaging. Examples of the different methods should allow to gain a deeper understanding of ultrasonic imaging methods.

48129 McNab, A.; Hayward, G.; Cochran, A. Monolithic ultrasonic arrays for accurate flaw sizing Proceedings of the 10th International Conference on NDE in the Nuclear and Pressure Vessel Industries, Glasgow (Scotland), 11-14 Jun. 1990. pp. 707-714. Edited by M.J. Whittle, J.E. Doherty and K. Iida. ASM International, (1990)

Monolithic array structures can be used to accurately size flaws such as fatigue cracks, and then to monitor their subsequent growth for input to a fracture mechanics assessment. The arrays, consisting of a continuous piezoelectric plate with an electrode pattern to define the elements, are easily manufactured and at low-cost. To interrogate a flaw, each element of the array is fired sequentially in pulse echo mode and the data recorded. Since the elements have an accurate spatial separation, defined by the array pattern, a number of sizing algorithms can be applied to this data. This includes the formation of time-of-flight curves in a B-scan presentation, or the imaging of the crack-tips using SAFT.

47943 Reiboid, R.; Kazys, R. Radiation of a rectangular strip-like focussing t ransducer Par t 1: harmonic excitation Ultrasonics, Vol. 30, No. 1, pp. 49-55 (1992)

The radiation of an ultrasonic transducer intended for use in synthezised aperture focussing technique (SAFT) applications is analysed. The transducer is a narrow rectangular strip (15 x 3 ram) with a radiating surface cylindrically concave along its length. In order to investigate focussing

158 N D T & E I n t e r n a t i o n a l V o l u m e 26 N u m b e r 3 1 9 9 3

effects of the sound field for different particle velocity distributions, the transducer is subdivided into strip-like elementary transducers. The fields of the elementary transducers are obtained by means of the impulse response method proposed by Lockwood and Willette. The errors arising due to the transducer subdivision into elementary strips are discussed. The results of numerical modelling are presented in terms of three- dimensional plots, it turned out that in the case of harmonic excitation the ultrasonic waves are effectively focussed only in one plane and spread into the perpendicular plane. The side lobes can be effectively reduced using non-uniform particle velocity distributions on the radiating surface. The results are of particular significance for acoustic imaging applications.

47942 Reibold, R.; Kazys, R. Radiation of a rectangular strip-like focussing t ransducer Par t 2: t ransient excitation Ultrasonics, Vol. 30, No. 1, pp. 56-59 (1992)

The transient radiation of an ultrasonic transducer intended for use in synthesized aperture focussing technique (SAFT) applications is analysed. The transducer is a narrow rectangular strip (15 x 3 ram) with a radiating surface cylindrical concave along its side length. The pulsed sound fields are calculated using the impulse response method according to Lockwood and Willette. The influence of various particle velocity distributions on the spatial-temporal structure of the sound field is evaluated as in Part 1 of this paper. The results obtained show that due to the strip-like shape of the transducer in the transient mode focussing is observed only in one plane. In the perpendicular plane the ultrasonic beams spreads, rather as it does in the case of harmonic excitation. The shortest duration of an impulse is obtained in the focal region. The spurious side lobe impulses can be suppressed effectively using a non-uniform particle velocity distribution monotonously decreasing to zero toward the rim.

47936 Muller, W.; Kreutter, T.; Thumser, D.; Schmitz, V. Ultrasonic turbine shaft inspection using the synthetic aper ture focussing technique (SAFT) , Nuclear Engineering and Design, Vol. 130, No. 3, pp. 339-346 (1991)

Ultrasonic imaging techniques developed for turbine shaft inspection result in a two-dimensional image of the defect distribution inside the specimen. It is shown, how the effect of reflectors outside of the image plane lead to misinterpretation of the reflector positions. The expansion of two-dimensional imaging technique to three-dimensions will help to overcome these problems.

47886 Behravesh, M. Application of electromagnetic acoustic t ransducers to coarse- grained material Electric Power Research Institute, Palo Alto, California (United States), EPRI NP-7438, Final Report, 32 pp. (Jul. 1991)

Investigators conducted research to determine which ultrasonic wave mode has the lowest attenuation and the least scattering from grains at a given frequency. After determining that the shear horizontal mode met that criteria, they designed a 500-H-Iz EMAT to focus the wave into the interior of centrifugally cast stainless steel. Because EMATs require no couplant fluid, investigators demonstrated the ease of scanning over the surface of a part by using synthetic aperture focusing techniques to enhance the detectability of small defects. Finally they conducted laboratory tests on fine-grained material to test EMAT theory and on coarse-grained material to determine EMAT effectiveness

46917 Doctor, S.R.; Bowey, R.E.; Hutton, P.H.; Kurtz, R.I.; Schuster, G.I. Advanced NDE technologies and characterizat ion of RPV flaw distribution Proceedings of the 18th Water Reactor Safety Information Meeting, Rockville, Maryland (United States), 22-24 Oct. 1990. NUREG/CP-0114. Vol. 3, pp. 137-155. Office of Nuclear Regulatory Commission, Washington, DC 20555 (Apr. 1991)

This paper is a review of several NRC programs. The fast program involves the final stages of the development and acceptance of two advanced NDE technologies: acoustic emission (AE) for continuous monitoring of light water reactor components and synthetic aperture focusing technique for ultrasonic testing (SAFT-UT). The second program involves the development of a program for the characterization of fabrication defects in U.S. reactor pressure vessels (RPVs).

46885 Grating lobe reduction in ultrasonic synthetic focusing Electronic Letters, Vol. 27, No. 14, pp. 1225-1227 (4 Jul. 1991)

ND T Abstracts

Two novel schemes of data acquisition in synthetic aperture ultrasonic imaging systems using a linear array transducer are presented to solve the troublesome grating lobe problem. Simulation and experimental results show that in both schemes the grating lobes are significantly reduced without appreciable degradation of the main lobe characteristics.

46160 Mayer, K.; Marklein, R.; Langenberg, K.I.; Kreutter, T. An ultrasonic imaging system for three-dimensional high- resolution defect imaging Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), 23-28 Jul. 1989. Vol. 9A, pp. 967- 974. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (I 990)

A quantitative ultrasonic imaging system is described which utilizes a diffraction tx~nography nondestructive testing technique. This technique is known as Fourier Transform Synthetic Aperture Focusing (FT-SAFT) and uses an army processor complementary Golay sequences to improve the signal-to-noise ratio and vice deconvolution, the axial resolution. A personnel computer is used for data acquisition, storage, processing and sophisticated graphics display.

45902 MacDonald, D.E. Images of flaws in generator retaining rings using SAFT reconstruction of TOFD data Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), 23-28 Jul. 1989. Vol. 9A, pp. 765- 772. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1990)

Retaining rings are assembles onto each end of electric generator rotors to support the rotor end-turn windings against rotational forces. These same geometric features to discriminate form real damage. The capability of SAFr processing of TOFD data to discriminate IGSCC from geometric reflectors was investigated using several retired retaining rings assembled to mandrels simulating the generator rotor.

45404 Hutchins, D.A.; Kramer, S.M.; Saleh, C. SAFT processing of non-contact ultrasonic NDE data IF_,BE 1989 Ultrasonics Symposium, Montreal, Quebec (Canada), 3-6 Oct. 1989 Vol. 1, pp. 661-664 Edited by B.R. McAvoy. (1989)

Ultrasonic data has been obtained by scanning a pulsed laser source and EMAT detector over the surface of an aluminium sample containing artificial defects. The annular EMAT was concentric with the source to imitate pulse-echo systems. Reflected data was processed using SAFr methods, to give images of the defects. Details are given for modifications to the usual SAFr approach, to compensate for the finite EMAT aperture.

45400 Kovalev, A.V.; Kozlov, V.N.; Samokrutov, A.A.; Shevaldykin, V.G.: Yakovlev, N.N. Pulse echo in concrete monitoring: interference and spectral selection Soviet Journal of Nondestructive Testing, Vol. 26, No. 2, pp. 110-118 (Oct. 1990)

The physical basis is given for focusing aperture synthesis for spatial selection of reflectors in inhomogeneous materials in ultrasonic echo monitoring. The conclusions and recommendations are based on acoustic studies on concrete as the ~nost characteristic representative of such materials. Estimates have been made on the signal/noise ratios for initial realizations in the presence of spatial correlation for the structural noise, simulation, and experiments on a visualisation system with a focusing synthetic aperture and combined probing.

45183 Silk, M.G. The capabilities and limitations of NDT: Part 5 - Ultrasonic testing special techniques British Institute of NDT, 31 pp (1989), ISBN 0903132109

In early 1970's the common criticism of ultrasonics, which was voiced regularly, was of the poor accuracy of ultrasonic techniques when used to size defects. The accuracy and reliability of ultrasonic defect location and sizing has been improved substantially for the majority of inspection tasks. The initial work concentrated on improvement of the accuracy of estimating defect length or width (e.g. in C-scans), followed later by techniques to estimate defect height. Most recently the problems raised by materials characterisation" the inspection of difficult materials and the determination of stress have been considered. Some of the results of these developments in ultrasonics are described in the following sections.

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